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IMPACT OF CLIMATE CHANGE ON

DESIGN WIND SPEEDS IN

CYCLONIC REGIONS

J.D. Holmes

June 2008


CONTENTS

Page


Executive summary ………………………………………………………………… . 2

1. Introduction .…………………………………………………………………….. 4

2. History of cyclonic design wind speeds and regional system in AS/NZS1170.2 ... 8

3. Simulation methods for prediction of cyclonic wind speeds …………………… 16

4. Vertical profiles in tropical cyclones and hurricanes …………………………... 21

5. Inland weakening and penetration of tropical cyclones ……………………….. 27

6. Observed effects of climate change on tropical cyclones worldwide ………… . 33

7. Observed trends in tropical cyclone activity in the Australian region ….……… 38

8. Predicted effects of climate change on tropical cyclones in the Australian region 42

9. Observations and projections for the Northern Territory ……………………… . 46

10. Conclusions and recommendations…………………………………………… 51

Appendix A - Maximum recorded wind gusts from tropical cyclones at selected

locations in Australia ……………………………………………………………….. 55

Acknowledgements ………………………………………………………………… 58

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Executive Summary

The objective of the study, carried out by JDH Consulting on behalf of the Australian

Building Codes Board, is to determine whether there is sufficient information and

justification to change design wind speeds in cyclonic regions of Australia in the

Australian Standard for Wind Actions (AS/NZS1170.2): a) with reference to

currently available wind data, and b) with particular reference to climate change.

The history of the regional wind speed system for the tropical-cyclone affected

coastlines of Australia, developed since the 1970s, is described, and the probabilistic

simulation approach used for hurricane wind speed prediction in the United States,

and used in the past for guidance in Australia, is outlined. Available data on the

vertical profile of cyclonic wind gusts with height is reviewed, and the background to

the gust profiles in the Standard is given. The inland weakening and penetration of

tropical cyclones is reviewed; in comparison with U.S. data, on average the current

regional widths in AS/NZS1170.2 appear to be adequate.

Recent studies of climate change effects on tropical cyclones are reviewed. These

indicate that in the Australian Region, the total number of cyclones has diminished.

This can be related to the preponderance of El Nino events affected Australia’s

climate during the last few decades. However, there is evidence that the number of

the more severe events has increased. Simulations of future climate, with projected

increases in CO 2 concentrations, indicates further increases in the more severe tropical

cyclones and a southerly drift in the genesis region on the Queensland coast. This

indicates a greater risk of a severe cyclone affecting Brisbane and South-east

Queensland than was assumed in the past.

A review of the recent cyclones affecting the Northern Territory indicates that the risk

to the northern coastal strip and offshore islands may also be greater than currently

implied by the Standard. However, upgrading of Darwin to Region D is not justified

on present evidence.

A number of recommendations have been made both with respect to the Standard,

AS/NZS1170.2, and more generally with regard to the database and measurements of

the Bureau of Meteorology are made. The main recommendations are as follows:

• Extensions of the current Region D boundaries on the coasts of Western

Australia and Northern Territory.

• Extension of the southerly limit of Region C on the Queensland coast from

25 o S to 27 o S.

• Introduction of a general cyclone ‘uncertainty’ factor of 1.1 applicable to

Regions B, C and D. This factor should be reviewed periodically.

• Ongoing monitoring of predictions emerging from global climate models as

their resolution and reliability improves.

• More emphasis should be given to predictions for Western Australia and the

Northern Territory.

• Rectification of errors and removal of inconsistencies in the assessment of

wind speed in the tropical cyclone database maintained by the Bureau of

Meteorology are required.

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• More resources should be devoted to the instrumentation of cyclonic wind

speeds in Australia, including the adoption of proven technologies such as

dropwindsondes and mobile tower arrays. Some effort should also be made

in better understand the response to cyclonic gusts of past and present

anemometers operated by the Bureau of Meteorology.

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1. Introduction

1.1 Project scope

Buildings and many other structures in Australia are designed for wind loads using the

Australian/New Zealand Standard ‘Structural design actions, Part 2: wind actions’

(Standards Australia 2002a). This document is called up by the Building Code of

Australia (BCA) administered by the Australian Building Codes Board (ABCB).

The Commentary to AS/NZS1170.2 (Standards Australia 2002b) states: “The

Standard does not attempt to predict the effects of possible future climatic changes, as

the evidence for changes in wind speeds is inconclusive.” With evidence from

climate scientists that global warming is occurring with apparent effects on the

intensity of windstorms, it is clear that the above statement cannot continue to be

made in the Standard.

The Australian Building Codes Board has particular concerns about possible

increased cyclonic wind speeds for the northern coastline of Australia, and has

specified the following scope for the study which is the subject of this report.

• Objective

The objective of the study is to determine whether there is sufficient information and

justification to change design wind speeds in cyclonic regions of Australia a) with

reference to currently available wind data, and b) with particular reference to climate

change.

• Work Plan

To achieve the above objective, the study should include two parts:

o A review of available cyclonic wind speed data and other relevant data to

determine whether the current definitions of cyclonic regions and wind

speeds are appropriate including the boundaries of the cyclonic regions.

o An assessment on how climate change would impact on design wind

speeds. This should include

− A review of probable scenarios for climate change

− A review of cyclonic wind models

− An analysis of how climate change would impact on design wind

speeds in cyclonic regions including the possibility of extending

the cyclone region boundaries

• Reference Sources

The study is to take into account any known relevant information sources

including but not limited to the following:

o AS1170.2-1989

o AS/NZS1170.2:2002 + Amendments

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o Bureau of Meteorology data on cyclonic regions

o Cyclone Testing Station Report on Cyclone George

o Nicholls report (Community Group for the ‘Review of NT Cyclone Risks’)

o CSIRO Research Reports on Climate Change

o BRANZ Report ‘An assessment of the need to adapt buildings for

unavoidable consequences of climate change’

o Data from Indian Ocean Climate Change Initiative

o Data from Queensland Climate Change Centre for Excellence

1.2 Australian Tropical Cyclone database

The National Climate Centre of the Bureau of Meteorology maintains a database of

information on tropical cyclones from 1906 onwards recorded in the Australian

Region, which is defined as the region south of the Equator between 105 o and 160 o E

longitude. The database has extensive column fields for parameters about the tropical

cyclones. The database lists daily observations or estimations of many variables.

The following are relevant to the present report: latitude and longitude of the centre of

the storm, central pressure, outer radius, eye diameter, maximum wind speed and

direction, direction and speed of storm movement.

Since about 1970 satellite information has been available. This has enabled virtually

all cyclones in the Australian Region to be identified and their position to be located

fairly accurately. Using a technique devised by Dvorak (1984) satellite images have

also been used to estimate the strength of cyclones. The cloud patterns in the

vicinity of the eye are used to estimate the maximum wind speed (assumed to be a

sustained wind speed at about 3000 metres), and hence through a formula, the central

pressure. Data derived in this way are generally those found in the database, and

very rarely are reliable direct observations available in the Australian Region. This

contrasts with the Atlantic Ocean where, in most cases, aircraft flights have been

made to determine the storm strength with accuracy. It has been suggested that

changes to satellite images interpreted by the Dvorak technique have resulted in false

trends in intensity estimates over the last 30-40 years in the Australian database.

It is also known that there are many errors in the Tropical Cyclone database, and the

Bureau of Meteorology currently has a programme of work to correct these.

However, extensive corrections have already been made to the Western Australian

section of the database funded by Woodside Offshore Petroleum.

1.3 Occurrence of tropical cyclones in the Australian Region

Figure 1.1 shows the average number of occurrences of tropical cyclones in 2 o × 2 o

squares in Australian waters based on data from the tropical cyclone database

maintained by the Bureau of Meteorology, (Abbs et al, 2006).

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Figure 1.1. Average number of days per year that tropical cyclones occurred in

Australian waters in 1970-2000 (from Abbs et al, 2006).

Figure 1.1 shows four regions of significant tropical cyclone occurrence: the northwest

coastline of Western Australia near Broome, the Timor Sea off the Northern

Territory coast, the Gulf of Carpentaria, and the Coral Sea offshore from Queensland.

It is noted that cyclonic activity is closest to the coast line offshore from the Western

Australian coastline near Broome.

The regions of cyclonic activity shown in Figure 1.1 are largely reflected in the

present regional zoning system for wind speeds in the Australian Standard

AS/NZS1170.2:2002, although consideration of cyclonic wind speeds are not

explicitly required by the Standard for Perth in Western Australia, or for the coastline

of Northern New South Wales. It is noted that Cyclone ‘Alby’ produced significant

wind speeds (up to 75 knots) in the Perth area in 1978.

1.4 Structure of this report

In the following Chapter 2, the history of the development of design wind speeds from

tropical cyclones, and the associated regional boundaries in AS/NZS1170.2:2002 and

its predecessors, is outlined. In Chapter 3, probabilistic simulation methods that are

currently used to determine hurricane design wind speeds in the United States, and

have been used for guidance in Australia in the past, are discussed. Vertical profiles

of wind speeds in tropical cyclones are discussed in Chapter 4. This is relevant, not

only for conversion of design wind speeds at the standard reference height of 10

metres in open over-land terrain to other heights and other terrains, but also to convert

upper-level wind speeds from simulation models and those given in the Bureau of

Meteorology database, to surface conditions. Chapter 5 reviews recent work on the

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inland penetration and weakening of tropical cyclones and hurricanes and compares it

with the weakening implied by the regional system in Australia.

Climate change issues are discussed in Chapters 6-8. Chapter 6 reviews observations

of global changes in tropical cyclone activity in relation to increased greenhouse gas

concentrations and sea surface temperatures. In Chapter 7, studies of changes in

cyclonic activity in the Australian Region during the last few decades are reviewed.

Recent simulations of future changes in tropical-cyclone activity in the Australian

Region are discussed in Chapter 8. Chapter 9 reviews some recent work from

Darwin concerning cyclone risk specific to the Northern Territory. Chapter 10

contains conclusions and recommendations, including some proposed changes to the

cyclonic regional boundaries in the Australian Standard for Wind Actions

AS/NZS1170.2:2002 (Standards Australia 2000a).

References

D.J. Abbs, S. Aryal, E. Campbell, J. McGregor, K. Nguyen, M. Palmer, T. Rafter, I.

Watterson and B. Bates (2006), Projections of extreme rainfall and cyclones, Report

to the Australian Greenhouse Office, CSIRO.

V.F. Dvorak (1984), Tropical cyclone intensity analysis using satellite data, NOAA

Technical Report NESDIS-11, National Oceanic and Atmospheric Administration,

Silver Spring, Maryland.

Standards Australia (2002a), Structural design actions. Part 2: Wind actions. AS/NZS

1170.2:2002.

Standards Australia (2002b), Structural design actions-Wind actions-Commentary.

(Supplement to AS/NZS 1170.2:2002).

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2. History of cyclonic design wind speeds and regional system in AS/NZS1170.2

2.1 Whittingham’s analyses

The first analysis of extreme gust wind speeds in Australia was carried out by

Whittingham (1964). This work included analysis of recorded annual maximum

wind gust data from a number of stations in tropical-cyclone-affected Northern

Australia: Broome(WA), Cairns (Qld.), Carnarvon (WA), Darwin (NT), Groote

Eylandte (NT), Karumba (Qld.), Onslow (WA), Port Hedland (WA), Rockhampton

(Qld.), Townsville (Qld.) and Willis Island (Qld). Whittingham carried out extreme

value analyses for each station and then drew contour maps for extremes at various

return periods up to 100 years; however he analyzed annual maximum gusts at each

station irrespective of their source, and did not specifically extract those produced by

tropical cyclones.

It is interesting to compare Whittingham’s predictions, which were based on quite

short record lengths in most cases, with current values in AS/NZS1170.2 for the wind

gust with an average recurrence interval of 100 years – i.e. V 100 . This is done in

Table 2.1, in which Whttingham’s values have been converted from knots to metres

per second. The values from AS/NZS1170.2 include the F C and F D ‘uncertainty’

factors.

Station

Table 2.1 Comparison of design gust wind speeds

Whittingham’s values versus AS/NZS1170.2:2002

Whittingham

(1964)

V 100 (m/s)

AS/NZS1170.2:

2002

V 100 (m/s)

Ratio

Broome, WA 50.4 58.8 1.17

Cairns, Qld 43.7 58.8 1.35

Carnarvon, Qld 65.8 72.6 1.10

Darwin, NT 55.0 58.8 1.07

Groote Eylandte, NT 48.4 58.8 1.21

Karumba, Qld 31.9 58.8 1.84

Onslow, WA 70.0 72.6 1.04

Port Hedland, WA 64.3 72.6 1.13

Rockhampton, Qld 48.4 58.8 1.21

Townsville, Qld 44.2 58.8 1.33

Willis Is., Qld * 75.1 58.8 0.78

*

Willis Island has been assumed to be in Region C, although this is not explicit in

AS/NZS1170.2

The ratios of the values in the 2002 Standard to Whittingham’s values for these

stations are shown in the extreme right column in Table 2.1. All these values except

one are greater than 1.0, in some cases significantly so. The exception is Willis

Island which is several hundred kilometres offshore from Queensland.

Part of the reason for the ratios being greater than 1.0 in Table I can be attributed to

the F C and F D uncertainty factors used in AS/NZS1170.2:2002. However the main

reason for the large differences from 1.0 is the fact that Whittingham attempted to

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make predictions with so few years of maximum gusts; the maximum number

available to Whittingham for any station in Table 2.1 was 21, and in one case as few

as 7 years were available. It is significant that the V 100 values in several of the

stations were exceeded significantly within a few years of Whittingham’s analyses –

for example at Townsville during Cyclone ‘Althea’ (1971), at Darwin during Cyclone

‘Tracy’ (1974), and at Karumba during Cyclone ‘Ted’ (1976).

2.2 Development of cyclonic design wind speeds in the Australian Standard

2.2.1 1952 to 1973

The first loading standard in Australia was a simplified structural loading Standard,

known as SAA Interim 350 (1952). Part II of this document gave information on

wind loads. There was no zoning or contour map but a simple table with six numbers

gave advice on design wind speeds. For the coastline (definition not given) north of

Latitude 25 o S, a value of 110 mph (49.2 m/s) was specified for all structures up to

300 feet (91m) in flat country.

The first ‘real’ stand-alone wind loading standard was CA34 Part II -1971 (Standards

Australia 1971). This contained a table of regional basic design wind velocities in

miles per hour for about fifty locations, including many on the cyclone-affected

coastlines and all except one of those in Table 2.1. Values were given for return

periods of 5, 25, 50 and 100 years, and were corrected to the standard height of 33

feet (10m) in flat open terrain (defined in the Standard as ‘Terrain Category 2’). It

also appears that a re-analysis of the annual maximum wind gusts was carried out,

using the extra data available since the work of Whittingham (1964), as the values

differ somewhat from those listed in Table 2.1.

A metric version of the Standard, AS 1170, Part 2-1973 (Standards Australia 1973)

was published shortly after, with the same design wind speeds as in CA34 converted

to metres per second and rounded. Table 2.2 summarizes the values in both CA34

and AS 1170.2-1973 in comparison with the current values.

Table 2.2. Comparison of design gust wind speeds

CA34.2-1971 and AS1170.2-1973 versus AS/NZS1170.2:2002

Station

AS1170.2-1973

V 100 (m/s)

AS/NZS1170.2:

2002

V 100 (m/s)

Ratio

Broome, WA 48 58.8 1.23

Cairns, Qld 49 58.8 1.20

Carnarvon, Qld 49 72.6 1.48

Darwin, NT 53 58.8 1.11

Karumba, Qld 31 58.8 1.90

Onslow, WA 67 72.6 1.08

Port Hedland, WA 48 72.6 1.51

Rockhampton, Qld 53 58.8 1.11

Townsville, Qld 51 58.8 1.15

Willis Is., Qld 63 58.8 0.93

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The ratios of the current (2002) values of V 100 to the 1971-3 in Table 2.2 are similar to

those in Table 2.1. Cyclone ‘Althea’ had occurred in Townsville in late 1971, and

produced a maximum recorded gust of 55 m/s at Townsville Airport, but this had not

been included in the 1971 re-analysis, and hence in the values in the 1973 edition.

However the recorded value exceeded both V 50 and V 100 in both editions of the

Standard.

Both the 1971 and 1973 Standards had a contour map showing values of V 50 . That in

AS1170.2-1973 is shown in Figure 2.1. In AS1170.2-1973 these ranged up to 60 m/s

in the north of Western Australia, and near Willis Island off Queensland. In the

Northern Territory near Darwin, the value of V 50 reached 50 m/s.

Also in both the 1971 and 1973 Standards, a ‘cyclone’ factor of 1.15 to be applied to

the V 50 wind speeds in an area up to 50 kilometres inland from the coastline north of

Latitude 27 o S was specified. This was regarded as an additional load factor for

cyclonic wind loads to reduce the risk of exceedence of the final design wind loads on

engineered buildings. Load factors are an important part of the specification of

design wind loads.

Figure 2.1. Regional basic wind speeds V 50 (m/s) – AS1170.2-1973

2.2.2 1975 to 1983

The occurrence of Cyclone ‘Tracy’ in Darwin in 1974 produced an immediate

reaction in the wind loading Standard. Although the actual peak gust at Darwin

Airport was not recorded, it has been estimated to be about 70 m/s. This greatly

exceeded the value of V 100 of 53 m/s - the highest value shown in AS1170.2-1973.

10


The response in AS1170.2-1975 was the specification of a single cyclonic strip within

50 kilometres of the ‘smoothed’ coastline north of Latitude 27 o S, with a value of V 50

of 55m/s. This value reduced by 5m/s over each of another two 10 kilometre strips

(Figure 2.2).

Figure 2.2. Regional basic wind speeds V 50 (m/s) – AS1170.2-1975 to AS1170.2-

1983

The map shown in Figure 2.2 was retained in the 1981 and 1983 editions of the

Standard. The ‘Cyclone Factor’ of 1.15 was also retained in the 1975, 1981 and

1983 editions. Tabulated values at return periods of 5, 25, 50 and 100 years were

also given for all the stations shown in Table 2.2. For all the stations except Onslow

and Willis Island the value of V 100 was specified as 60 m/s. These locations retained

the same values given in AS1170.2-1973; thus for Onslow the value of V 100 was given

as 67 m/s, and for Willis Island 63 m/s was specified.

Table 2.3 shows the values of V 100 specified between 1975 and 1983 compared to the

present values. The ratios are now much closer to 1, with the exceptions being

Carnarvon and Port Hedland which are now rated much higher than previously

(together with Onslow they are now zoned in the higher Region D).

11


Station

Table 2.3. Comparison of design gust wind speeds

AS1170.2-1975 to -1983 versus AS/NZS1170.2:2002

AS1170.2-1973

V 100 (m/s)

AS/NZS1170.2:

2002

V 100 (m/s)

Ratio

Broome, WA 60 58.8 0.98

Cairns, Qld 60 58.8 0.98

Carnarvon, Qld 60 72.6 1.21

Darwin, NT 60 58.8 0.98

Karumba, Qld 60 58.8 0.98

Onslow, WA 67 72.6 1.08

Port Hedland, WA 60 72.6 1.21

Rockhampton, Qld 60 58.8 0.98

Townsville, Qld 60 58.8 0.98

Willis Is., Qld 63 58.8 0.93

2.2.2 1989 to present

New cyclonic regional boundaries were introduced in the 1989 edition of AS1170.2,

at the same time that contours were discontinued and a regional system introduced for

the whole of Australia (Figure 2.3). Also with the introduction of limit states design

into Australian structural standards, separate wind speeds were designated for ultimate

and serviceability limit states. In the former case, an annual risk of 1/1000 was

specified for V u (i.e. V u was V 1000 for all structures in current terminology). The

Cyclone Factor of 1.15 was discontinued, being effectively incorporated in the values

for V u given for the cyclonic regions C and D.

Regions C and D were essentially the same as those in the current (2002) Standard.

There was no extensive re-analysis of cyclonic wind speeds, but the justification for

the values given were described by Holmes, Melbourne and Walker (1990). Values

of V 1000 of 70 m/s and 85 m/s were specified for Regions C and D respectively and

represent approximately the middle of the Category 4 and 5 ranges of the Australian

version of the Saffir-Simpson scale.

The only changes in the 2002 Standard (AS/NZS1170.2:2002) from the 1989 edition

were the additions of two small areas of Region B: firstly a strip between 100 and 150

kilometres from the coastline between 20 o and 25 o S near the Western Australian

coastline, and secondly east of 142 o E and north of 11 o S covering the Torres Strait

Islands. The regional map in the 2002 Standard is reproduced in Figure 2.4.

12


Figure 2.3. Regional boundaries and basic wind speeds (m/s) – AS1170.2-1989

Figure 2.4. Regional boundaries – AS/NZS1170.2:2002

13


2.3 Summary and Discussion

The changes in the cyclonic wind speeds and regional boundaries in the Australian

Standards since 1971 have come about from the realization that extreme value

analyses of recorded wind speeds in regions affected by tropical cyclones will give

large uncertainties when tropical cyclones only rarely occur, and only short records of

wind data are available in relation to the interval between direct strikes by cyclones at

a particular location. This reaction was primarily provoked by the occurrence of

Cyclone ‘Tracy’ at Darwin in 1974 which produced maximum gusts greatly

exceeding those given in the Standard at the time for building design. Specification

of design wind speeds in cyclonic onwards from 1975 onwards was mainly guided by

computer-based simulation approaches (see Chapter 3), although these have not been

used greatly in Australia since the early 1980s.

The introduction of limit states design in the 1980s focused on the need for estimates

of design wind speeds at high return periods of 500 to 1000 years – these represent

risk of exceedence of 0.05 to 0.10 for typical buildings with design lives of 50 years.

It is noted that the Commentary to AS/NZS1170.2 (Standards Australia 2002b) states:

“The Standard does not attempt to predict the effects of possible future climatic

changes, as the evidence for changes in wind speeds is inconclusive.” As noted in

the Introduction to this report, this statement is now becoming unsustainable as

significant changes in wind speeds, due to climate change, are expected within the

lifetime of buildings currently in the design or construction phases.

References

J.D. Holmes, W.H. Melbourne and G.R.Walker, A commentary on the Australian

Standard for Wind Loads, Australian Wind Engineering Society, 1990.

Standards Association of Australia (1952), Minimum design loads on structures, SAA

Int. 350.

Standards Association of Australia (1971), SAA Loading Code. Part II – Wind forces,

AS CA34, Part II - 1971.

Standards Association of Australia (1973), SAA Loading Code. Part 2 – Wind forces,

AS 1170.2 - 1973.

Standards Association of Australia (1975), SAA Loading Code. Part 2 – Wind forces,

AS 1170.2 - 1975.

Standards Association of Australia (1981), SAA Loading Code. Part 2 – Wind forces,

AS 1170.2 - 1981.

Standards Association of Australia (1983), SAA Loading Code. Part 2 – Wind forces,

AS 1170.2 - 1983.

14


Standards Association of Australia (1989), SAA Loading Code. Part 2 – Wind loads,

AS 1170.2 - 1989.

Standards Australia (2002a), Structural design actions. Part 2: Wind actions. AS/NZS

1170.2:2002.

Standards Australia (2002b), Structural design actions-Wind actions-Commentary.

(Supplement to AS/NZS 1170.2:2002).

H.E. Whittingham (1964), Extreme wind gusts in Australia, Bureau of Meteorology,

Bulletin No. 46.

15


3. Simulation methods for prediction of cyclonic wind speeds

3.1 Introduction

This Chapter reviews probabilistic methods for prediction of extreme wind speeds due

to tropical cyclones. These methods have developed due to the shortcomings of

methods based on the analysis of historical recorded data on gust wind speeds due to

tropical cyclones from anemometer stations. The methods aim to make use of all

information available on the tropical cyclones in the vicinity of a site. Thus track

information, such as heading and translation speeds, as well as information on central

pressures and storm size, as indicated by the radius to maximum winds, are used.

This data is used with empirical wind field models to make predictions of extreme

wind speeds; the latter normally give upper level mean speeds (gradient winds), and

assumed factors must be used to convert these to a gust wind speed near the surface

must be applied.

Methods described Vickery et al (2000a, 2000b), have been used to directly develop

design wind speeds for hurricane regions of the United States in ASCE 7 (the

equivalent of AS/NZS1170 in Australia). These approaches are described in Section

3.3.

Some similar work by Harper (1999) for Australia is described in Section 3.4.

3.2 History of simulation approaches

3.2.1 Work in the United States

Vickery and Twisdale (1995a) describe the development of simulation methodologies

for the hurricane-prone coastline of the United States. The first of these was

implemented for the Texas coastline by Russell (1971). Shortly after Russell and

Schueller (1974), Tryggvason et al (1976), Georgiou et al (1983) and Twisdale and

Dunn (1983) used similar approaches for portions of the United States coastline.

Batts et al (1980) were the first to apply the methodology to the entire U.S. eastern

and southern coastlines affected by hurricanes; these predictions were the basis for the

wind speed contours in ASCE 7-1988. Improved estimates were made by Vickery

and Twisdale (1995a, 1995b) and Vickery et al (2000a, 2000b) and these are the basis

for the hurricane wind speed contours in the present American Loading Standard

ASCE 7-05 (ASCE, 2005).

In all the approaches, probability distributions are developed for the central pressure

difference (∆p), translation speed, c, and radius of maximum winds, R max , for

approaching hurricanes in the historical databases. Most of the approaches

mentioned above model hurricanes in a circular sub-region centred on the site of

interest. However, Batts et al (1980) and Twisdale and Dunn (1983) used a coast

crossing technique to derive the basic probability distributions.

16


3.2.2 Work in Australia

In the late 1970s and early 1980s, Australia was active in developing probabilistic

simulation methods for tropical cyclone wind speeds through the work of Gomes and

Vickery (1976), Martin and Bubb (1976) and Tryggvason (1979). Probably partly as

a result of criticism of the approach by Dorman (1984), and partly because of the

demise of publicly-funded research into wind loading generally, there was little work

of this type after the mid 1980s.

However more recently there has been an interest in such predictions from the

insurance industry. The work described by Harper (1999) is an example of this; the

latter work is discussed in Section 3.4.

3.3 Methods by Vickery et al (1995-2000)

The methodology used by Vickery and Twisdale (1995a) for predictions for the U.S.

coastline is as follows. The statistical distributions of central pressure difference,

translation velocity of the hurricane, angle of approach, and the minimum distance of

the centre of the hurricane from the site are derived from data obtained from the

National Climatic Data Center of the U.S. These distributions are site-specific and

vary significantly with location along the coastline.

Using site-specific probability distributions of ∆p, c, θ and d min , in conjuction with a

wind-field model, thousands of hurricanes were simulated. The distributions used for

∆p, c, θ and d min were Weibull, Lognormal, Bi-Normal and uniform, respectively.

The wind-field model used was that of Shapiro (1983). Each simulated hurricane

was assumed to travel along a straight line path throughout the simulation region

defined using the sampled values of d min and θ . The sampled value of ∆p was held

constant up to landfall, after which it was reduced using the filling model described by

Vickery and Twisdale (1995b).

The radius of maximum wind, R max , was assumed to have some correlation with the

central pressure difference for storms south of 30 o N, and in the case of northern

storms with the latitude. R max was modeled with a Lognormal distribution, with the

mean value a function of central pressure difference or latitude.

Using the above methodology, Vickery and Twisdale (1995a) made predictions of 50,

1000 and 2000-year return period fastest mile wind speeds at 30 coastal and 16 inland

locations. These differed significantly for many stations from the earlier predictions

of Batts et al (1980) – this was attributed mainly to the use of the Shapiro windfield

model and a new filling model (Vickery and Twisdale 1995b).

As well as a number of sensitivity studies, Vickery and Twisdale (1995a) compared

area and point exceedence probabilities – thus for a single point in the Miami area a

fastest mile wind speed of 65m/s was associated with a return period of about 300

years. However, the return period of this wind speed for anywhere in Dade County,

Florida, is only about 100 years.

The Shapiro wind field model described by Vickery and Twisdale (1995b) consisted

of numerical solutions to the vertically integrated equations in coordinates moving

17


with the hurricane. An imposed pressure distribution for the vortex was used.

Comparisons with measurements in five hurricanes were favourable and considerably

better agreement was shown than for the wind field model of Batts et al (1980). The

filling model used by Vickery and Twisdale (1995b) was an earlier version of that

described by Vickery (2005) and discussed in Section 5.4 of this report.

A new technique for modelling hurricane wind risk in the United States was described

by Vickery et al (2000a). A storm track modelling approach was adopted in which

the entire track of a hurricane as it crosses the ocean and makes landfall was

modelled. The advantage of this approach over the circular region approach used

previously resulted from not having to assume uniform climatology over the

subregion. Using the empirical storm track modelling approach, storm intensities

change with time and storms change both direction and speed as they pass by a site.

This is more realistic compared to earlier methods. This new technique, together

with the new wind field model described by Vickery et al (2000b), was used as the

basis for the revised wind speed contours for the hurricane coastlines in ASCE 7-02

and ASCE 7-05. The wind field model showed excellent agreement with recorded

mean and gust time histories for numerous locations in several hurricanes.

The work described by Vickery et al has been very well documented and uses soundly

based wind field and filling models, and is calibrated with the comprehensive data set

of measurements available in the United States from both aircraft flights and surface

measurements. However, the predictions for wind speeds in the future are still

dependent on the characteristics of past hurricanes being representative of those in the

future. At present future climate change effects are not included. Also, no doubt the

predictions would be significantly changed for some locations, following the large

number of landfalling hurricanes in the United States in the 2004 and 2005 seasons.

3.4 Work by Harper (1999)

Harper (1999) describes a similar numerical simulation approach to the prediction of

wind speeds due to tropical cyclones in Australia, and of insurance losses in a city

from a single event. There is little detail given of the methodology used in terms of

the probability distributions used for relevant parameters etc, except that apparently a

circular sub-region approach (see Section 3.2.1) with a radius of 500 kilometres.

However, examples are given of predicted wind speeds for Cairns, Townsville,

Mackay and Brisbane in Queensland. This apparently used cyclone track information

from 1959 onwards. Reasonable agreement of the simulated predictions with those

from the recorded gusts due to tropical cyclones for these locations was found,

although the latter may not, in themselves, be representative of the long-term climate.

3.5 Work by Arthur et al (2008)

At the time of writing, Geoscience Australia is developing a deterministicprobabilistic

Tropical Cyclone Risk Model for wind speeds (Arthur et al 2008a). It

has been calibrated against Cyclone Tracy (Arthur et al, 2008b).

The current work has generated 500-year gust wind speeds for the northern Australian

region based on 5000 years of simulated tropical cyclones (Arthur et al, 2008a).

While generally looking quite realistic for the main cyclonic regions of the Pilbara

18


and the Queensland coast, the predicted wind speeds appear to be unrealistically high

south of 25 o S on the Western Australian coast. This may be the result of insufficient

weakening of the cyclones as they move south in the Indian Ocean being incorporated

in the modelling. On the other hand, the predicted wind speeds north of 12 o on the

Northern Territory, coast seem low considering the occurrence of three strong

cyclones there in the 1998-2006 period (see also Chapter 9).

However, this model is at early stages of development and improvements in the

predictions can be expected in the near future (R. Cechet – personal communication).

The model will eventually be publicly available (see also Section 8.4).

3.6 Summary and Conclusions

This Chapter has outlined the development in probabilistic simulation methods for

prediction of wind speeds from hurricanes or tropical cyclones. This work has

primarily been done in the United States where the results are used directly for design

wind speed specification in ASCE 7 - the equivalent of AS/NZS1170.2.

There is a clear need for more work of this type to be done in Australia, and for it to

be kept up-to-date with modern techniques and an improved historical database.

However, Australia suffers from a lack of aircraft flights into tropical cyclones

leading to an inaccurate cyclone database, and also a lack of surface measurements

required for calibration of wind field and filling models. This means the potential

accuracy of probabilistic simulation models could not be expected to match that in the

U.S.

The simulation work needs to be integrated with larger scale climate modelling (in

Australia and elsewhere), incorporating climate change effects. Although Australia

has been actively involved in the development of global climate models for the

simulation of tropical cyclones on a synoptic scale, the next step of coupling with

wind field models for prediction of wind speeds at defined sites has not yet been

undertaken.

References

American Society of Civil Engineers (2005), Minimum design loads for buildings and

other structures, ASCE Standard ASCE/SEI 7-05.

W.C. Arthur, A. Schofield, R. P. Cechet and L. A. Sanabria (2008a), Return period

cyclonic wind hazard in the Australian Region. 28th AMS Conference on Hurricanes

and Tropical Meteorology, 28 April - 2 May 2008, Orlando, FL, USA.

W. C. Arthur, A. Schofield and R. Cechet (2008b), Severe wind hazard assessment of

Cyclone Tracy using a parametric tropical cyclone model, 15 th National Australian

Meteorological and Oceanographic Society (AMOS) Conference, Geelong, January

29-February 1, 2008.

M.E. Batts, M.R. Cordes, L.R. Russell, J.R. Shaver and E. Simiu (1980), Hurricane

windspeeds in the United States, Report No. BSS-124, National Bureau of Standards,

U.S. Dept. of Commerce, Washington D.C.

19


C.M.L. Dorman (1984), Tropical cyclone wind speeds in Australia, Civil

Engineering Transactions, Institution of Engineers, Australia, Vol.26, pp132-139.

L. Gomes and B.J. Vickery (1976), Tropical cyclone gust speeds along the north

Australian coast, Civil Engineering Transactions, Institution of Engineers, Australia,

Vol.18, pp40-48.

B.A. Harper (1999), Numerical modelling of extreme tropical cyclone winds,

Journal of Wind Engineering & Industrial Aerodynamics, Vol.83, pp35-47.

G.S. Martin and C.T.J. Bubb (1976), Discussion of ‘Tropical cyclone wind speeds

along the North Australian coast’, Civil Engineering Transactions, Institution of

Engineers, Australia, Vol.18, pp48-49.

L.R. Russell (1971), Probability distributions for hurricane effects, Journal of

Waterways, Harbors and Coastal Engineering, ASCE, Vol.97, pp139-154.

L.R. Russell and G.F. Schueller (1974), Probabilistic models for Texas Gulf Coast

hurricane occurrences, Journal of Petroleum Technology, pp279-288.

L.J. Shapiro (1983), The asymmetric boundary layer flow under a translating

hurricane, Journal of Atmospheric Sciences, Vol.40, pp1984-1998.

B.V. Tryggvason, D. Surry and A.G. Davenport (1976), Predicting wind-induced

response in hurricane zones, Journal of Structural Division, ASCE, Vol.102, pp2333-

2350.

B.V. Tryggvason (1979), Computer simulation of tropical cyclone wind effects for

Australia, James Cook University, Wind Engineering Report 2/79, April.

L.A. Twisdale and W.L. Dunn (1983), Extreme wind risk analysis of the Indian Point

Nuclear Generation Station, Final Rep. 44T-2491, Research Triangle Institute, North

Carolina, U.S.A.

P.J. Vickery (2005), Simple empirical models for estimating the increase in central

pressure of tropical cyclones after landfall along the coastline of the United States,

Journal of Applied Meteorology, Vol. 44, pp1807-1826.

P.J. Vickery and L.A. Twisdale (1995a), Prediction of hurricane wind speeds in the

United States, J. Struct.Engg.,Vol.121, pp1691-1699.

P.J. Vickery and L.A. Twisdale (1995b), Wind-field and filling models for hurricane

wind-speed predictions, J. Struct.Engg.,Vol.121, pp 1700-1709.

P.J. Vickery, P.F. Skerlj, A.C. Steckley and L.A. Twisdale (2000a), Hurricane wind

field model for use in hurricane simulations, J. Struct.Engg.,Vol.126, pp1203-1221.

P.J. Vickery, P.F. Skerlj, and L.A. Twisdale (2000b), Simulation of hurricane risk in

the U.S. using empirical track model, J. Struct.Engg.,Vol.126, pp1222-1227.

20


4. Vertical profiles in tropical cyclones and hurricanes

4.1 Introduction

The Australian Standard specifies a Regional wind speed as a 3-second gust at a

height of 10 metres in open country terrain (Terrain Category 2). For buildings of

greater or lesser height than 10 metres, or located in a different terrain type, this wind

speed must be adjusted. This is accomplished by the ‘Terrain-height Multiplier’,

M z,Cat . Currently in the Standard a different set of Terrain-height Multipliers is

specified for Tropical Cyclone Regions C and D in Table 4.1(B) of the Standard.

Clearly the values specified for these Multipliers in the Standard for buildings that are

not close to 10m in height are very significant in determination of the wind loads in

tropical cyclone regions. The ratio between upper level (gradient) winds and surface

level winds is also important when simulation methods are used to predict wind

speeds due to tropical cyclones (Section 3).

In the following reviews are given of the original tower measurements at N.W. Cape

that were the basis of the gust profiles (M z,Cat ) used in AS/NZS1170.2, and of the

recent dropwindsonde measurements that have been carried out in Atlantic hurricanes.

4.2 Tower measurements at NW Cape by Wilson

Some of the few available tower measurements of vertical wind profiles in tropical

cyclone, hurricane or typhoon, were made by Wilson (1979a, 1979b) at the North-

West Cape near Exmouth in Western Australia. Observations were made from

anemometers mounted on guyed tower at heights of 60m, 191m, 279m and 390m

(with anemometers operating at all heights only during one cyclone). Another

anemometer was mounted at 9m height on a pole about 350m away from the main

tower. The fetch was open water with only a short land fetch of less than 5

kilometres for a large range of wind directions from NW through N to S.

In a period of four and half years the anemometers were able to record velocities from

four tropical cyclones during the nineteen-seventies: ‘Beryl’ (1973), ‘Trixie’ (1975),

‘Beverley’ (1975) and ‘Karen’ (1977). The highest 10-minute wind speed at the top

anemometer was about 57 m/s during Cyclone ‘Beverley’.

A significant feature of several of the profiles recorded was the high values of wind

speed recorded on the 60 metre anemometer compared to those at both the 9m and

191m heights. The surface roughness lengths for the open water fetch and for the

land surrounding the tower were estimated to be similar in magnitude (1 to 3 mm –

corresponding to Terrain Category 1 in AS/NZS1170.2), and it was concluded that the

inner boundary layer resulting from the water to land transition had little effect on the

measured wind profiles. This appeared to be confirmed by the measurements which

showed little variation with the changing land-water fetch of different wind azimuths.

However the shear (i.e. the difference in wind speeds) between 9 and 60m was

significantly underestimated by the logarithmic law with a roughness length of 1-3

mm. It would have to be concluded that the boundary layer wind flow was not in

equilibrium with the underlying terrain, or was not neutrally stable.

21


In the absence of other comparable tower measurements in tropical cyclones,

hurricanes and typhoons, profiles of M z, Cat based on the NW Cape measurements

were incorporated into the 1989 edition of the Australian Standard on Wind Actions

(AS1170.2-1989) for both Regions C and D, and the gust profile was continued

without modification in the 2002 edition.

Figure 4.1 shows the average maximum gust profiles derived from the 10-minute

mean windspeeds and gust factors obtained for the four recorded cyclones at NW

Cape. The profile in the Standard is assumed to increase monotonically up to 100m

and above that height takes a constant value of 1.40 for all Terrain Categories. The

Standard is conservative with respect to the averaged measurements above 100 m.

However, these profiles need reviewing in the light of the more recent dropwindsonde

measurements in U.S. hurricanes described in the following section.

Mz cat comparisons

500

Height (m)

400

300

200

100

0

0.0 0.5 1.0 1.5

Mz,Cat1

Avg. measured -

NW Cape

AS/NZS1170.2

Figure 4.1 Comparison of average measured maximum gust profiles for four

cyclones (1973-7) at North-west Cape and M z,Cat 1 from AS/NZS1170.2:2002

4.3 Dropwindsonde measurements

The dropwindsonde (Figure 4.2) is a probe containing sensors and a GPS satellite

receiver that enables profiles of various atmospheric variables, including wind speed,

to be monitored as it falls after being dropped from an aircraft.

Since 1997 the National Oceanic and Atmospheric Administration (NOAA) and the

U.S. Air Force have been deploying GPS-based dropwindsondes into hurricanes in the

Atlantic and eastern North Pacific oceans. These have generated vertical profiles of

wind and thermodynamic parameters from flight level (typically about 3000 metres)

down to sea level. It should be noted that dropwindsondes are not normally deployed

over land.

The development of the dropwindsonde has been described by Hock and Franklin

(1999). Detailed analyses of wind profiles from 1997-1999 have been described by

Franklin et al (2003); the main purpose was to obtain a ratio between surface (10

22


metres) and flight level wind speeds for forecasting purposes. Wind profiles were

averaged separately for the eyewalls of hurricanes, and for the outer vortex regions.

Powell et al (2003) analysed similar data and fitted logarithmic profiles for various

speed ranges. In contradiction to previous extrapolations for wind over the ocean,

they found that the surface drag coefficient and roughness length did not continue to

increase beyond U 10 equal to 30 m/s, in fact, the roughness length decreased from

about 3 mm to 1mm between 33 and 50 m/s.

Figure 4.2 Schematic of a dropwindsonde probe

4.3.1 Nature of dropwindsonde profiles

The horizontal wind speed is determined from the position of the sonde sampled every

0.5 seconds, and assumes that the sonde is travelling at the wind speed. However,

corrections are made based on the horizontal and vertical accelerations as described

by Hock and Franklin (1999).

Since the probes translate circumferentially through the hurricane, as well as fall

vertically, the individual dropwindsonde profiles reflect the larger scale turbulent

eddies. Smaller scale turbulence as would be observed by a high-response

anemometer on a fixed mast, is not included due to the sampling interval, the response

of the probe and its movement in ‘following the eddies’. However, ensemble

23


averaging of many individual normalized profiles, as undertaken by Franklin et al and

Powell et al, will give expected mean vertical velocity profiles of horizontal velocity.

There is some debate about the validity of the corrections to the profiles when the

probe horizontal speed is different to the wind speed – for example as the sonde falls

it will tend to have a higher wind speed associated with a greater height. However,

the corrections are already quite small, and improvements to the correction method

will be unlikely to significantly change the profiles, and have negligible effect on the

ensemble-averaged mean profiles. (It is noted that due to the vertical falling motion,

the relative wind speed to the sonde will normally be vertical and parallel to the

cylinder axis. A relative horizontal wind speed will deflect the total wind vector, but

the cylinder will then tend to align itself with the new wind vector direction).

4.3.2 Results from dropwindsonde profiles in hurricanes

From a engineering design point of view, the analysis of Powell et al and Franklin et

al has produced the following significant results :

• A logarithmic law for the mean wind speed appears to hold in the eyewall

region for heights above the surface up to about 3000 metres. Above that

height, the rate of increase is much smaller with a maximum mean velocity at

about 500 metres. Above 500 metres, the velocity falls significantly in the

eyewall region.

• For mean velocities at 10 metres height above water surface of about 33 m/s,

the roughness length reaches a maximum value of about 3 mm, falling to 1

mm for velocities of 50 m/s (Powell et al 2003).

Franklin et al (1999) gives velocity ratios, ⎺U z /⎺U 10 for the eyewall region. These

values are reproduced in Table 4.1, and compared with a logarithmic law with z 0

equal to 0.0001 m (0.1 mm).

Although there is an apparent discrepancy between the roughness lengths quoted by

Powell et al, and implied by Franklin et al (i.e. a factor of 10), in both cases, the

values are significantly lower than those currently used for design of buildings for offwater

winds from hurricanes or tropical cyclones.

Since dropwindsondes are only deployed over the ocean, there are no profiles

available for wind over land from hurricanes, as required for the design of most

structures. It also should be noted that the dropwindsonde profiles do not give any

useful information on turbulence intensities, and hence on peak gust envelope profiles

as used in many design codes or standards, (such as ASCE 7 or AS/NZS1170.2).

However it would not be expected that the latter would differ significantly from the

mean velocity profiles, (and may have an even lower slope.)

24


Table 4.1. Mean velocity profile in the hurricane eyewall from Franklin et al (2003)

(compared with a logarithmic law with z 0 = 0.1 mm)

Height (m) ⎺U z /⎺U 10 log e (z/0.0001)/log e (10/0.0001)

10 1.000 1.000

15 1.027 1.035

20 1.048 1.060

30 1.081 1.095

50 1.128 1.140

75 1.169 1.175

100 1.198 1.200

150 1.229 1.235

200 1.261 1.260

250 1.288 1.280

300 1.305 1.295

4.4 Summary and Discussion

The Australia / New Zealand Standard AS/NZS1170.2 since 1989 has had a gust

profile for cyclonic regions based on measured tropical cyclone data from the late

1970s and early 1980s from the NW Cape mast near Exmouth Western Australia,

described in Section 4.2. These showed a sharp increase in wind speed from the

lowest anemometer (9 m height) to 60 metres, and then relatively uniform wind

speeds to the highest anemometer on the mast. Although above 100m, the measured

profiles from the NW Cape and the dropwindsonde profiles are near-uniform, the high

shear below 100m is not seen by the dropwindsondes, as discussed in Section 4.3. It

is possible that the lowest (9m) anemometer reading from the NW Cape mast was

heavily influenced by the inner boundary layer as the surface changed from water to

land, although this possibility was discounted at the time of the measurements

(Wilson, 1979b).

The Australian Standard also assumes increasing roughness length with increasing

wind speed over water – this has been shown to be an incorrect assumption over 30

m/s in hurricanes by the dropwindsonde data.

Unfortunately the measurement programme of the Bureau of Meteorology at NW

Cape (Section 4.2) is now discontinued. However, with many high rise buildings

constructed, or under construction, in centres such as Cairns, Darwin and Townsville,

there is a clear need to improve the information available to designers and the

Standard (AS/NZS1170.2) on wind gust profiles in tropical cyclones in Australia.

There should be financial support for renewed measurement programmes in Australia,

including the deployment of dropwindsondes (Section 4.3)

References

J. L. Franklin, M.L. Black and K.Valde (2003), GPS dropwindsonde wind profiles in

hurricanes and their operational implications, Weather and Forecasting, Vol.18,

pp32-34.

25


T.F. Hock and J.L. Franklin (1999), The NCAR GPS dropwindsonde, Bull. Amer.

Met. Soc., Vol. 80, pp 407-420.

M.D. Powell, P.J. Vickery and T.A. Reinhold (2003), Reduced drag coefficient for

high wind speeds in tropical cyclones, Nature, Vol. 422, pp 279-283, 20 March 2003.

K.J. Wilson (1979a), Wind observations from an instrumented tower during Tropical

Cyclone Karen, 1977, 12 th Technical Conference on Hurricanes and Tropical

Meteorology, New Orleans, April 1979.

K.J. Wilson (1979b), Characteristics of the subcloud layer wind structure in tropical

cyclones, International Conference on Tropical Cyclones, Perth, W.A., November

1979.

26


5. Inland weakening and penetration of tropical cyclones

5.1 Introduction

As the high convection regions of a tropical cyclone cross a coastline from a warm

ocean, the storm loses its source of energy and starts to weaken. This results in

maximum wind speeds falling progressively with distance from the coastline. This is

reflected in AS/NZS1170.2 by 50 kilometre wide strips that designate the wind

regions in the Standard. At present, the width of these strips is the same on the

Western Australian coastline as on the more topographically complex east

(Queensland) coast. The validity of this has been questioned and will be addressed

in this report.

This Chapter reviews recent studies of the inland penetration of tropical cyclones and

hurricanes, with consideration of the relevance to the current regional zoning system.

The current Standard defines the regional strips with respect to the ‘smoothed’

coastline without defining it. Chapter 10 of this report addresses this point and

attempts to establish a workable definition.

5.2 Observations in Cyclone ‘George’ (2007)

Boughton and Falck (2007) have surveyed the available information on wind gusts,

for up to 120 kilometres inland, from Tropical Cyclone ‘George’ which crossed the

Pilbara coast of Western Australia east of Port Hedland in March 2007.

Unfortunately for this event, there was a lack of anemometer data, and few simple

road signs, that have been used in past cyclones as an indicator of upper and lower

limits of wind gust speeds, were available. Hence Boughton and Falck used tree

damage and damage to vehicles, buildings and masts at isolated mining camps to

establish wind speeds at various distances from the coastline. The authors claim

accuracy of +/- 10% for estimates off gust wind speeds made from tree damage and of

+/-5% for predictions made from structures and vehicles.

Maximum gust speeds at Port Hedland on the coast were estimated at 55m/s from the

failure of road signs. However the township was located 60 to 70km west of the

storm track and outside of the eye wall of the cyclone. Tree damage in Port Hedland

was subsequently used to calibrate observed damage at other parts of the track.

Estimates of 200 to 270 km/hour (55 to 75 m/s) at locations along the storm track

about 50 kilometres from the coast were made, but these are quite wide limits, and it

is difficult to judge the weakening of the storm without also having wind speeds at

landfall.

Calculations from the failure of the top of a radio mast at Strelley about 50 km inland

along the track gave a peak gust of 64 to 78 m/s. However, the authors apparently did

not consider possible resonant vibrations of this slender structure which would have

amplified the stresses. Ignoring this possibility may have resulted in overestimates of

the peak gusts.

27


A road-sign failure near the eye wall about 90 kilometres along the track after

landfall, but about 50 kilometres from the coastline indicated a lower bound of 55 m/s

and a probable maximum gust of 60 m/s. This observation is significant, as although

this location is in Region C in AS/NZS1170.2, it would have been in Region B had

the cyclone travelled at right angles to the coastline. For Region B, V 500 is 57 m/s,

less than the probable maximum gust in this event.

A lower limit of 50 m/s was obtained for the FMG Rail Camp based on overturning of

a piling rig and a toilet block, although the values assumed for force and pressure

coefficients were not stated. This site is about 90 km from the smoothed coastline,

but about 140 km from landfall along the track of the cyclone. This wind speed

exceeds estimates made independently based on the movement and overturning of

buildings within the camp.

Boughton and Falck stated that without the current F C and F D factors in AS/NZS, their

estimated maximum gust wind speeds would have been exceeded in Cyclone

‘George’ based on distances measured along the cyclone track from landfall, but this

was not the case for the actual regional location of the sites based on the shortest

distance from the smoothed coastline, for which the current Standard appeared to be

quite adequate.

5.3 Studies by Kaplan and DeMaria (1995, 2001)

Kaplan and DeMaria (1995, 2001) developed a simple empirical model for predicting

the decay of tropical cyclone winds after landfall. This work was done primarily for

evacuation and emergency management application.

The model is based on a least squares fit to the maximum sustained (1-minute) surface

wind estimates made by the National Hurricane Center to land-falling tropical storm

and hurricanes in the United States. The equation for maximum sustained wind as a

function of time takes the form.

V (t) = V b

+ (RV 0

−V b

)e −αt (5.1)

where V b is a ‘background’ wind speed

V 0 is the maximum wind speed just before landfall

R is a reduction factor to account for the immediate effect of surface

roughness immediately on landfall

α is a decay parameter

For storms making landfall closer to the Equator than 37 o , Kaplan and DeMaria

established values for V b , α and R of 26.7 knots, 0.095 hour -1 and 0.9, respectively.

For storms land-falling on the Atlantic coast north of 37 o N the values obtained were:

V b = 29.6 knots, α = 0.187 hour -1 and R = 0.9.

5.4 Studies by Vickery (2005)

For application to hurricane simulation models, Vickery (2005) studied the decay of

the central pressure rather than wind speed, after landfall. An exponential decay

function of the form of Equation (5.2) was used.

28


∆p(t) = ∆p 0

e −at (5.2)

where ∆p(t) is the difference between the central pressure and the far field

atmospheric pressure at time t after landfall, ∆p 0 is the pressure difference at the time

of landfall, and a is a filling constant.

The filling constant, a, was modeled as:

a = a 0

+ a 1

⎛ ⎜

∆p 0

.c ⎞ ⎟ (5.3)

⎝ RMW ⎠

where c is the translation speed of the storm at the time of landfall, and RMW is the

radius to maximum winds.

The characteristics of up to 57 storms which made landfall were studied and grouped

according to the sections of the U.S. coastline where they made landfall. Best fits to

the observed decay of central pressure for up to 36 hours after landfall were made

using Equation (5.3). For example, for the Florida Peninsula, the following

expression fitted the data with a correlation coefficient (r 2 ) of 0.84.

a = 0.0225 + 0.00167 ⎛ ⎜ ∆p 0.c ⎞ ⎟ (5.4)

⎝ RMW ⎠

5.5 Application to decay with distance, and widths of regional boundaries

The results of Kaplan and Maria, as summarized in Section 5.3, are intended be used

to predict the decay of wind speeds tropical cyclones with respect to time after

landfall. However, they can be converted to the decay rate with distance from the

coastline by multiplying by an average or representative storm translation speed.

Based on these decay rates, appropriate regional boundary widths may also be

determined.

Equation (5.1) can be modified in terms of decay with distance instead of time, as

follows:

V (x) = V b + (RV 0 −V b )e −βx

V (w) = V b + (RV 0 −V b )e −(β / cosθ )w (5.5)

where β is a decay constant based on the distance inland x

w is the shortest distance from the smoothed coastline equal to x cos θ

θ is the angle of the cyclone track from the normal to the smoothed coastline

β can be estimated from the decay constant of Kaplan and Maria as follows:

29


α

β = cav

(5.6)

where c av is an average translation speed of the storm.

An average value of the angle to the normal can be calculated as follows:

+π / 2

(cosθ ) av = ⎜ ⎛ 1 ⎞


⎝ π ⎠ ∫ cosθ dθ = 2

π

−π / 2

Hence substituting in Equation (5.5),

V (w) = V b + (RV 0 −V b )e −(παw / 2c av )

(5.7)

(5.8)

Then the values found by Kaplan and DeMaria for V b and α were applied, and

average values used for storm translation speed from Vickery (2005) of 6.2 m/s and

10.6 m/s for storms in the United States less than and greater than 37 o latitude,

respectively. R was taken as 0.9 and V 0 as 70 m/s. A factor of 1.3 was used to

convert the ‘1-minute maximum sustained wind speeds’ to the maximum 3-second

gust used in the Australian Standard.

The curves given by Equation (5.8), using parameters found by Kaplan and DeMaria

for U.S. hurricanes, for latitudes less than and greater than 37 o , are plotted in Figures

5.1 and 5.2 respectively, and compared with the step changes resulting from the

regional boundaries in AS/NZS1170.2 between 20 o S and 25 o S in Western Australia.

The differences in the decay rates for the hurricanes north of 37 o N in the U.S.

(including the more topographically complex New England region) and those south of

37 o S are small. However, it is noted that decay rates with time are higher for the

northern storms (Kaplan and DeMaria, 2001), but that the average translation speeds

are also higher (Vickery, 2005); these effects tend to compensate and give a similar

rate of decay with distance after landfall as the southern storms.

Figures 5.1 and 5.2 indicate that the current boundaries in AS/NZS1170.2 are

conservative with respect to the average decays found in the U.S. hurricanes, and

appear to be quite satisfactory. However, the lines shown from the U.S. hurricanes

are averages, and individual hurricanes will, of course indicate faster or slower

decays.

5.6 Summary

Research from the United States on the decay in wind speeds in hurricanes after

landfall indicates that the current regional boundaries are generally conservative with

respect to the average decay of both low latitude (less than 37 o ) and high latitude

(greater than 37 o ) storms. However to account for cyclones that decay less faster

than the average, the current boundaries appear to be adequate.

The difference in decay rate with distance for the more northerly storms in the U.S.

with topographically complex terrain after landfall, is little different to those in the

south which cross the coast at the flat coastline of Florida, Louisiana and Texas.

Hence, there is currently little evidence to differentiate the current regional boundaries

30


etween Queensland and Western Australia, as some have suggested. However, an

analysis of the Australian tropical cyclone database (Section 1.2) could be usefully

carried out to determine whether there is a significant difference in the decay rates

between the Queensland and Western Australian coastlines.

1.0

0.8

Decay in windspeed

Decay

0.6

0.4

0.2

0.0

37deg

AS1170.2:2002

(WA)

0 50 100 150 200

Normal distance from coastline (km)

Figure 5.2. Decay in gust wind speed for U.S. hurricanes north of 37 o N compared to

the Australian Standard

31


References

G.N. Boughton and D. Falck (2007), Tropical Cyclone George – Wind penetration

inland, Cyclone Testing Station, James Cook University, Draft Technical Report

No.53, August 2007.

J. Kaplan and M. DeMaria (1995), A simple empirical model for predicting the decay

of tropical cyclone winds after landfall, Journal of Applied Meteorology, Vol. 34,

pp2499-2512.

J. Kaplan and M. DeMaria (2001), On the decay of tropical cyclone winds after

landfall in the New England area, Journal of Applied Meteorology, Vol. 40, pp280-

286.

P.J. Vickery (2005), Simple empirical models for estimating the increase in central

pressure of tropical cyclones after landfall along the coastline of the United States,

Journal of Applied Meteorology, Vol. 44, pp1807-1826.

32


6. Observed effects of climate change on tropical cyclones worldwide

6.1 Introduction

Recent international literature concerning observed trends in global tropical cyclone

activity and correlation with increasing sea surface temperature is reviewed in the

following. Several of the papers are reviewed in the following. In Section 6.6

consensus statements by expert groups are reproduced in an endeavour to give a

balanced view of opinions on this somewhat controversial topic.

6.2 Interpretations by Webster et al (2005)

Webster et al (2005) conducted a comprehensive analysis of global tropical cyclone

statistics for the satellite era (1970-2004) in each tropical ocean basin in which they

occur. They found significant increasing trends in sea surface temperature (SST) in

each of the ocean basins except for the South Pacific. Since it is well established that

a SST of 26 0 C is required for tropical cyclone formation in the current climate, it

might be expected that there would be an increase in the number of tropical cyclones.

However there was no significant trend in global cyclones of all strengths.

The North Atlantic region did show a statistically significant increase since 1995 in

tropical cyclones of hurricane strength (defined by Webster et al as having wind

speeds greater than 33 m/s). However an attribution of the increase to increasing

SST is not supported because of the lack of correlation of the number of tropical

cyclones of this strength in other basins.

When Webster et al examined the number of hurricanes by allocated category (using

the Saffir-Simpson scale), they found a significant increasing trend in the numbers of

Category 4-5 storms, and found that the numbers of these strongest storms increased

from about 50 globally per 5-year period in the 1970s to nearly 90 per five-year

period in the decade 1995-2004 (see Figure 6.1). This conclusion has been

controversial and is further discussed in the following. Also as seen in Figure 1

there has been no trend from the 1990-1994 period to the 2000-2004 period (as

pointed out by Klotzbach (2006)).

Figure 6.1 Apparent increase in number of Category 4 and 5 hurricanes world-wide

(from Webster et al (2005))

33


6.3 Interpretations by Emanuel (2005)

Emanuel’s interpretations of the effects of increasing sea surface temperatures were

expressed in terms of a ‘power dissipation index’ (PDI) defined as follows.

τ

PDI = ∫ V max 3 dt

0

where V max is the maximum sustained wind speed at 10 metres height, and τ is the

lifetime of a storm.

On the assumption that the economic loss in windstorms varies as the cube of the

wind speed, the PDI is assumed to represent the ‘destructiveness’ of a tropical

cyclone.

Emanuel summed the PDIs over all storms in each calendar year for the North

Atlantic and North-west Pacific Basins, and found significant increases from about

1990 and 1980 respectively. Significant apparent correlations with changing sea

surface temperatures were found. However, thermodynamic considerations would

indicate only a 6-9% increase in PDI for the 0.5 o C observed increase in sea surface

temperature, whereas the observed changes in PDI, with about a 50% increase, greatly

exceeded this range in both basins. Emanuel therefore concluded that only part of the

observed apparent increase in PDI can be attributed to increased sea surface

temperatures.

6.4 Interpretations by Klotzbach (2006)

Klotzbach (2006) extended the analysis to all basins with tropical cyclone activity,

and excluded data before 1986 on the basis that, before the mid 1980s, only visible

satellite information was available and hence nighttime observations were excluded;

also the quality and resolution of satellite imagery had improved greatly by the later

period.

Klotzbach used an ‘Accumulated Cyclone Energy Index’ (ACE) as an indicator of

trends. The ACE is similar to the PDI but incorporates the square instead of the cube

of the maximum surface wind speeds.

Only in the North Atlantic Basin was a significant increasing trend in ACE found. In

fact, decreasing trends were found in the Northeast, Northwest and Southwest Pacific

Basins. Klotzbach’s analysis, using the more recent (and more reliable) data, found

only a small increase in Category 4-5 hurricanes in the North Atlantic and Northwest

Pacific during the 20-year study period. Klotzbach’s findings were stated to be

‘..contradictory to those of Emanuel (2005) and Webster et al (2005)’.

6.5 Interpretations by Kossin et al (2007)

To eliminate the variability in global hurricane intensity records due to improvements

in satellite technology, Kossin et al (2007) constructed a more homogeneous data by

34


first constructing a consistently analyzed satellite database for 1983 to 2005, and then

applying a new objective algorithm to form hurricane intensity estimates.

Although an increasing trend in PDI, and frequency and percentage of strongest

storms was found for the North Atlantic in agreement with Emanuel (2005) and

Webster et al (2005), this was not the case for any other basin. In the South Indian

Ocean (including tropical cyclones affecting the WA coastline) no significant trends

were found in the reanalyzed homogeneous data set; in the South Pacific, a

decreasing trend was observed for the period 1983-2005 (Figure 6.2).

The combined global records re-analyzed by Kossin et al showed no consistent longterm

trend in PDI, number of Category 4-5 storms or in the percentage of strongest

storms. This was in contradiction to conclusions by Webster et al (2005) and

Emanuel (2005).

Figure 6.2 Trend in PDI for Southern Indian Ocean (left) and South Pacific Ocean

(right). Red lines from uncorrected database; blue lines from corrected

homogeneous database (from Kossin et al (2007))

6.6 Consensus statements by IPCC and IWTC

The following statements from the International Panel on Climate Change (IPCC),

and an expert group at the International Workshop on Tropical Cyclones, summarize

consensus expert scientific opinion on the possible global impact of climate change,

particularly sea surface temperature increases on tropical cyclones.

IPCC (2001)

‘... there is some evidence that regional frequencies of tropical cyclones may change

but none that their locations may change. There is also evidence that the peak

intensity may change by 5% to 10% .... ’

35


IWTC (2006)

‘The scientific debate … is not as to whether global warming can cause a trend in

tropical cyclone intensities. The more relevant question is how large a trend: a

relatively small one decades into the future or large changes occurring today.

Currently published theory and numerical modeling results suggest the former, which

is inconsistent with the observational studies of … Webster et al (2005) by a factor of

5… .’ (McBride et al, 2006).

IPCC (2007)

‘There is observational evidence for an increase in intense tropical cyclone activity in

the North Atlantic since about 1970, correlated with increases in sea surface

temperatures. There are also suggestions of increased intense tropical cyclone

activity in some other regions where concerns over data quality are greater. … There

is no clear trend in the annual numbers of tropical cyclones.’

6.7 Summary and Conclusions

There has been some speculation that the frequency of severe tropical cyclones worldwide

has already been increasing as a result of global warming. This view has been

promoted by two well-known publications in 2005.

The main problem appears to be observational errors, in both the numbers of tropical

cyclones observed and their intensity, prior to about 1980. Before the advent of

regular satellite observations in about 1970, many storms that did not cross a coastline

were not observed or not recorded. It is only since 1980 that satellite observations

have been regular enough and of sufficient quality to produce reasonable and

consistent estimates of storm strengths. Currently it is only in the North Atlantic

basin, that the strengths of hurricanes are being regularly observed by means of

aircraft penetration flights and then only for those storms that potentially may cross

the coastline of the United States. In other parts of the world where frequent tropical

cyclones occur a technique based on a qualitative interpretation of satellite images has

been used. This may have resulted in some mis-classification of tropical cyclones.

References

K. Emanuel (2005), Increasing destructiveness of tropical cyclones over the past thirty

years, Nature, Vol. 436, pp686-688.

P.J. Webster, G.J. Holland, J.A. Curry and H.R. Chang (2005), Changes in tropical

cyclone number, duration and intensity in a warming environment, Science, Vol. 309,

pp1844-1846.

CSIRO (2007). Climate change in Australia. Technical Report. CSIRO.

P.J. Klotzbach (2006), Trends in global tropical cyclone activity in the last twenty

years (1986-2005), Geophysical Research Letters, Vol. 33, L10805

36


J.P. Kossin, K.R. Knapp, D.J. Vimont, R.J. Murnane, B.A. Harper (2007), A globally

consistent reanalysis of hurricane variability and trends, Geophysical Research

Letters, Vol. 34, L04815.

J. McBride, J. Kepert, J. Chen, J. Heming, G. Holland, K. Emanuel, T. Knutson, H.

Willoughby and C. Landsea (2006), Statement on tropical cyclones and climate

change, 6 th WMO International Workshop on Tropical Cyclones (IWTC-VI), Costa

Rica, November.

37


7. Observed trends in tropical cyclone activity in the Australian Region

7.1 Introduction

This Chapter reviews trends in tropical cyclonic activity in the Australian Region

during the last thirty years, and possible explanations.

7.2 Observations by Nicholls et al (1998)

A review of cyclonic activity in Australia from 1969/70 to 1995/6 was given by

Nicholls et al (1998). A downward trend in the total number of tropical cyclones

observed in the Australian Region was observed. However, there was a slight

upward trend in the number of intense cyclones, defined as those with a central

pressure of 970hPa or less, as shown in Figure 7.1.

Nicholls et al noted that at least part of the downward trend in overall cyclone

numbers can be explained by changes in the way storms are classified as cyclones.

For example Cyclone ‘Wanda’ in 1974 had a central pressure of 1000 hPa, and would

not have been classified as a cyclone in later years. This has an effect on the trend in

the number of cyclones with central pressures higher than 990hPa, but negligible

effect on stronger ones.

The downward trend in overall numbers was primarily explained by the negative

trend in the Southern Oscillation Index (SOI) – otherwise known as the El Nino

phenomenon – over the time period in question. The SOI is the difference in mean

sea level atmospheric pressure between Tahiti and Darwin, standardized to a mean of

zero and standard deviation of 10, and low values are associated with droughts in

Australia, as well as fewer tropical cyclones. No explanation was given for the

increase in more intense cyclones, although this trend is also predicted by climate

models for the east coast of Australia as a consequence of increasing greenhouse gas

concentrations (see Section 8.4).

Figure 7.1 Trend in number of cyclones with central pressures of 970 hPa or less in

the Australian Region during 1969-96 (from Nicholls et al, 1998).

38


7.3 Study by Ramsay et al (2007)

A recent detailed study Ramsay et al (2007) provided evidence that the sea surface

temperature in the east and central Pacific Ocean is the main contributing factor to

cyclonic activity in the Australian Region. In fact, the average sea surface

temperature in North Australian waters is only weakly correlated with tropical

cyclone activity near Australia. Cyclone numbers are also affected by the monsoonal

trough and by vertical shear in the atmosphere.

Thus warming of the central Pacific Ocean (coinciding with El Nino conditions) is

unfavourable for tropical cyclone generation in the Australian Region. The

connection between sea surface temperatures in the central Pacific Ocean and

conditions in the Australian Region are illustrated in Figure 7.2. Thus it is possible

that increased ocean temperatures outside of the Australian Region, produced by

greenhouse effects, may result in fewer tropical cyclones affecting Australia.

However, Ramsay et al note that the numbers of severe tropical cyclones (with central

pressure less than 965 hPa) are much less influenced by El Nino conditions, and

explain the more frequent occurrence of severe cyclones off north-west of Western

Australia by this.

Figure 7.2. Schematic showing warmer sea surface temperatures in the central

Pacific Ocean, associated with the El Nino phenomenon, and conditions associated

with reduced cyclone activity in the Australian Region (from Ramsay et al, 2007)

7.4 Study by Kuleshov et al (2008)

Kuleshov et al studied tropical cyclone activity in the Southern Hemisphere between

1981/2 and 2005/6, and its dependence on the El Nino Southern Oscillation

phenomenon. As observed previously by others, they found that fewer tropical

39


cyclones occurred in both the South Pacific and South Indian Oceans during El Nino

than during La Nina years. On average 25 and 29 tropical cyclones occurred in El

Nino and La Nina years respectively in the Southern Hemisphere. Kuleshov et al

also found that an area of cyclogenesis located between 60 o E and 85 o E in El Nino

years in the Indian Ocean shifted eastwards (i.e. closer to Australia) in La Nina years.

In contrast, the focus for cyclogenesis in the South Pacific shifted eastwards (away

from Australia) in El Nino years. These two effects explain the predominance of

tropical cyclones in the Australian Region during La Nina years. The decline in

total number of tropical cyclones affecting Australia in recent years can be explained

by the fewer La Nina events compared with El Nino periods.

As shown in Figure 7.3, taken from their paper, Kuleshov et al identified statistically

significant increasing trends in severe tropical cyclones, defined as those with a

central pressure less than 945 hPa in the Southern Hemisphere (SH), southern Indian

Ocean (SIO), and in the south Pacific Ocean (SPO). This observation confirms the

earlier one by Nicholls et al (Figure 7.1), and is also consistent with predictions of the

effect of CO 2 induced global warming discussed in the next chapter.

Figure 7.3 Occurrences of tropical cyclones with central pressures of 945 hPa or less

in the Southern Hemisphere between 1981/2 and 2005/6 (from Kuleshov et al, 2007).

7.5 Summary and Conclusions

Studies of tropical cyclone activity in Australian waters have found that the numbers

of tropical cyclones are negatively affected by the Southern-Oscillation Index – El

Nino phenomenon. Fewer cyclones occur in the Australian Region when higher sea

surface temperatures occur in the central and east Pacific Ocean. However the more

intense cyclones (i.e. those of interest for structural design) are much less influenced

by the El Nino phenomenon, and there is evidence of increasing trends in the stronger

(Category 3-5) events during the last 30-40 years. However, these conclusions

should be re-visited in the future to ensure that no fictitious increase in intensity has

occurred in the tropical cyclone database (Section 1.2) due to changes in interpretation

of satellite data over the years (see also Section 6.5)

To the extent that global warming will affect the sea surface temperature in the east

and central Pacific and hence the El Nino-La Nina phenomena, it may be expected

40


that there will be changes to the number and intensities of cyclones in the Australian

Region. Recent predictions are discussed in the following Chapter.

References

Y. Kuleshov, L. Qi, R. Fawcett and D. Jones (2008), On tropical cyclone activity in

the Southern Hemisphere: trends and the ENSO connection, Geophysical Research

Letters, Vol. 35, L14S08.

N. Nicholls, C. Landsea and J. Gill (1998), Recent trends in Australian Region

tropical cyclone activity, Meteorological and Atmospheric Physics, Vol. 65, pp 197-

205.

H.A. Ramsay, L.M. Leslie, P.J. Lamb, M.B. Richman and M. Leplastrier (2007),

Interannual variability of tropical cyclones in the Australian Region: Role of largescale

environment, submitted to Journal of Climate.

41


8. Predicted effects of climate change on tropical cyclones in the Australian

region

8.1 Introduction

A number of simulation studies have been carried out in recent years to try to

determine the effects of enhanced greenhouse gas concentrations on the occurrence

and intensities of tropical cyclones in the Australian region. These are based on

Global Climate (computer) Models (GCM) developed originally for weather

forecasting purposes, and hence have a relatively coarse resolution. The smallest

grid size used in the studies reviewed here is 15 kilometres. This, of course, is

insufficient to resolve tropical cyclones (with eye diameters of the order of 50

kilometres) in detail, and the conclusions of these studies must be taken as

preliminary.

These studies have identified ‘tropical-cyclone like vortices’ in the output of GCM

models, and drawn conclusions about actual cyclones in the real world. The

advantage of such studies is that enhanced greenhouse gas concentrations in the upper

atmosphere can be simulated.

8.2 Simulations by Walsh et al (2000-4)

Walsh and Ryan (2000), published in 2000, used a regional climate model (125km

resolution nesting to 30 km) of the Australian region, inserted idealized tropical

cyclones and examined their intensity evolution under current and enhanced CO 2

concentrations. Small increases in intensity were observed – for example after 2

days, the average central pressure reduced from about 970 hPa to 965 hPa. However

the error bars representing the standard deviations of the observed changes exceeded

the predicted changes. Improved modelling in subsequent years has improved the

predictions of these effects.

Walsh et al (2004) used 30-kilometre horizontal resolution climate models to simulate

the current climate (1967-96) and with a climate with enhanced (3×) carbon dioxide

concentrations in the upper atmosphere. Under the enhanced greenhouse conditions

the numbers of tropical cyclones formed and their region of formation do not change

much. However there was a 26% increase in the number of storms with central

pressures less than 970 hPa, and an increase in the number of intense storms occurring

south of 30 o S latitude. There was no discussion of any change in the number coast

crossings of the eastern Australian coast, however.

8.2 Simulations by Abbs et al (2006)

Simulations by Abbs et al (2006) are the only ones that covered the Indian Ocean off

north-west of Western Australia, and the Timor Sea region, as well as the Coral Sea

off the Queensland coast. These simulations indicated a significant reduction in the

number of cyclones off Western Australia, little change off the Northern Territory

coast, but some increase off the North Queensland coast by 2070. Increases in storm

strength for the most severe storms were found by this model.

42


Simulations by this group are continuing and improved predictions can be expected in

the near future.

8.3 Simulations by Leslie et al (2007)

The study described by Leslie et al (2007) is the most recent of the simulations

published at the time of writing, but is limited by a relatively coarse grid spacing of 50

kilometres. It was funded by the Insurance Australia Group (IAG).

The model was used to simulate the climate in the south-west Pacific Ocean in a

‘control’ period of 1970-2000. A favourable agreement was achieved between the

‘tropical-cyclone-like vortices’ in the simulated outputs from the GCM model and the

observed cyclones, with respect to the number of tropical cyclones in various intensity

categories, lifetime of storm, monthly distribution, and distribution with latitude.

The model was then applied to the 2000-2050 period with both current and enhanced

greenhouse gas concentrations. No significant change was found in the total tropical

cyclone numbers in the south-west Pacific during 2000-2050. However, there was a

marked increase (about 22%) in the number of Category 3-5 storms in response to

increasing greenhouse gases. A southerly shift of over 2 degrees of latitude in the

tropical-cyclone genesis region was found.

An interesting finding of the simulation was a prediction of a Category 3 Cyclone

directly hitting the Brisbane area in 2047. Such precision in dates is questionable, but

the possibility of such an event occurring within the next fifty years should be

considered. The paper also concluded that there is a potential for tropical cyclones to

develop during the next fifty years that are more intense that any so far recorded in the

south-west Pacific, including ‘super cyclones’ with central pressures below 900hPa.

The latter would produce extreme winds considerably in excess of those specified for

building design for any return period up to 2000 years along the Queensland coast

(Regions B and C in AS/NZS 1170.2).

8.4 Other research in progress in Australia

Geoscience Australia is currently undertaking a study for the Garnaut Review of the

future impacts of tropical cyclones in the Australian region under a range of climate

change scenarios. The Garnaut Climate Change Review is an independent study by

Professor Ross Garnaut, which was commissioned by the Commonwealth, State and

Territory Governments. The Review is examining the impacts of climate change on

the Australian economy, and plans to recommend medium to long-term policies and

policy frameworks to improve the prospects for sustainable prosperity.

The impact of climate change on tropical cyclone risk for the states of Queensland,

and Western Australia and also the Northern Territory has been examined utilizing

global climate models available from the Intergovernmental Panel on Climate Change

(IPCC) Fourth Assessment Report. The model output has been used to estimate the

influence of climate change on tropical cyclone activity following the technique

described in Vecchi and Soden (2007). The current generation of models lacks the

horizontal resolution necessary to resolve the intense inner core of tropical cyclones

so a thermodynamic approach aimed at identifying changes in large-scale

43


environmental factors (that are known to affect cyclone development/strength) is

employed. The maximum potential intensity (MPI) of a tropical cyclone was utilised

to determine changes in intensity. The MPI sets a theoretical upper limit for the

distribution of tropical cyclone intensity at a given point, given a vertical temperature

and humidity profile (Holland, 1997; Emanuel, 1999). A range of climate change

scenarios have been employed to assess the wind and storm surge hazard and risk

compared to current climate levels. The overall trend in MPI for the climate change

simulations is one of increased levels across northern Australia, resulting in higher

maximum wind speeds and elevated storm surge levels. This work will be reported in

September 2008 and published on the Garnaut Review website

http://www.garnautreview.org.au/ .

Geoscience Australia has also been contracted by the Federal Department of Climate

Change to conduct a similar study to that Knutson et al (2008) in the Australian

region. The regional modelling is being undertaken by K. Emanuel (MIT) utilising the

IPCC Fourth Assessment Report simulations. A tropical cyclone climatology

(cyclone tracks with associated intensity information) is produced for each IPCC

model simulation considered. The climatology is utilized by the Geoscience Australia

Tropical Cyclone Risk Model (TRCM) (Arthur et al, 2008) through the synthetic

track generation module which can be seeded with either historical data or

alternatively the tracks of tropical cyclone-like vortices (TCLV’s) extracted from

climate simulations. In this way and utilizing a statistical sampling process (Monte-

Carlo simulation), the model can rapidly increase the catalogue of TC events under

future climate regimes. Geoscience Australia’s 2008-9 work program includes

activities to determine the changes in cyclonic wind hazard under a range of climate

change scenarios using TCRM for the Australian region. This work will be reported in

early 2009

8.5 Summary

Although no simulation study up to now has used climate models with sufficient

spatial resolution to identify tropical cyclones, they are able to generate ‘tropicalcyclone-like

vortices’ with most of the characteristics of genuine tropical cyclones.

By comparing and calibrating the simulations with the historical record of cyclone

occurrences in number and intensity over the last 30 years or so, reasonable

predictions of events in a warmer climate may be made for the Australian region.

Although only one of the studies to date has included the Indian Ocean basin off the

north-west of Western Australia, and the Timor Sea region, near Darwin, there have

been several studies of the south-west Pacific and Coral Sea regions off Queensland.

In relation to the SW Pacific simulations, some consensus has emerged, with the

following general conclusions:

• The overall number of tropical cyclones in the SW Pacific is not expected to

change significantly in a warmer world over the next fifty years or so.

• There is expected to be an increase in the number and frequency of the

strongest storms in the next fifty years – i.e. in the Category 3 to 5 storms

that are interest in structural design of buildings and other structures.

• It is expected that a southward shift of 2-3 0 in the genesis region and track

locations will occur in the next 50 years.

44


• A storm of Category 3 strength in the Brisbane area is quite likely in the next

50 years.

However, the quality of these predictions is low given the current resolution of the

prediction models. It is expected that the resolution and hence the quality of the

predictions will improve over the next few years. The only study of the Western

Australian region has indicated significantly fewer cyclones in that basin. However

this requires confirmation by other independent studies.

References

D.J. Abbs, S. Aryal, E. Campbell, J. McGregor, K. Nguyen, M. Palmer, T. Rafter, I.

Watterson and B. Bates (2006), Projections of extreme rainfall and cyclones, Report

to the Australian Greenhouse Office, CSIRO.

W.C. Arthur, A. Schofield, R. P. Cechet and L. A. Sanabria (2008), Return period

cyclonic wind hazard in the Australian Region. 28th AMS Conference on Hurricanes

and Tropical Meteorology, 28 April - 2 May 2008, Orlando, FL, USA.

K.A. Emanuel (1999), Thermodynamic control of hurricane intensity, Nature,

Vol.401, pp665-669.

G.J. Holland (1997), The maximum potential intensity of tropical cyclones, Journal

of Atmospheric Sciences, Vol.54, pp2519-2541.

T.R. Knutson, J. J. Sirutis, S. T. Garner, G. A. Vecchi, and I. M. Held (2008),

Simulated reduction in Atlantic hurricane frequency under twenty-first-century

warming conditions. Nature Geoscience, Vol.1, pp359-364.

L.M. Leslie, D.J. Karoly, M. Leplastrier, and B.W. Buckley (2007), Variability of

tropical cyclones over the Southwest Pacific Ocean using a high-resolution climate

model, Meteorology and Atmospheric Physics, Vol. 97, pp171-180.

K.C. Nguyen and K.J.E. Walsh (2001), Interannual and decadal and transient

greenhouse simulation of tropical cyclone-like vortices in a regional climate model of

the South Pacific, Journal of Climate, Vol.14, pp 3043-3054.

G.A. Vecchi, and B. J. Soden, (2007), Increased tropical Atlantic wind shear in model

projections of global warming. Geophysical Research Letters, Vol.34, L08702.

K.J.E. Walsh and J.J. Katzfey (2000), The impact of climate change on the poleward

movement of tropical-cyclone like vortices in a regional climate model, Journal of

Climate, Vol. 13, pp1116-1132.

K.J.E. Walsh and B.F. Ryan (2000), Tropical cyclone intensity increase near Australia

as a result of climate change, Journal of Climate, Vol. 13, pp3029-3036.

K.J.E. Walsh, K.C. Nguyen and J.L. McGregor (2004), Finer-resolution regional

climate model simulations of the impact of climate change on tropical cyclones near

Australia, Climate Dynamics, Vol. 22, pp47-56.

45


9. Observations and projections for the Northern Territory

9.1 Introduction

This Chapter reviews some studies by individuals and groups for cyclonic wind

speeds in Darwin and the Northern Territory. Comments are made about the

appropriate zoning of Darwin and other parts of the Northern Territory, and the need

for more studies are discussed.

9.2 Reports by M. Nicholls

A community Group for the ‘Review of NT Cyclone Risks’ was formed in 2005 and

received a grant from Emergency Management Australia in 2006 to study cyclone

risks in the Northern Territory. Mike Nicholls was the Secretary of the Group and

also the subcontractor for most of the work. The work was completed in early 2007

and a web site (www.cyclone.org.au) contains the report of the work consisting of a

main report and fifteen lengthy Appendices (Nicholls et al, 2007).

A two-page summary of the report was supplied to Sub-Committee BD006-02 of

Standards Australia in August 2007. The following comments relate primarily to that

summary.

The findings of the report were summarized as follows (with wording only slightly

altered):

a) Three of the most intense cyclones that have been observed in Australian waters

since satellite observations began in 1960 (‘Thelma’ 1998, ‘Ingrid’ 2005 and

‘Monica’ 2006) all came within 350 km of Darwin when they were at maximum

intensity and within a nine-year period.

b) These ‘TIM’ cyclones may have been the result of global warming or they may

signal the return of a more active period similar to one that appears to have existed in

the first 90 years of (European) NT settlement.

c) NT buildings should probably be designed for wind loads that are at least 60%

higher than the minimum loads permitted under the current AS/NZS1170.2.

a) is generally not disputed, although the exact strength of the three storms is subject

to speculation. All three were rated Category 5 at some point in their lives by the

Bureau of Meteorology, but so also was Cyclone ‘Larry’ in Queensland in 2006.

However, on landfall at Innisfail, the latter produced damage commensurate with a

high Category 3 or a low Category 4 event.

b) The first 90 years of European settlement produced a number of severe cyclones

but without the benefit of modern satellites and ground-based instrumentation it is

difficult to be categorical about their intensities. It is possible that the intensity of

TIM cyclones was related to global warming.

c) This conclusion is justified by some ‘ball-park’ probabilistic estimates. These

are discussed in the following.

46


Estimates of the exceedence probability for given wind speeds in Darwin are stated to

be the product of the probabilities of three independent events:

a) the ‘time probability’ of a cyclone having at least that gust speed occurring in any

one year (within 500 kilometres in Darwin),

b) the ‘spatial’ probability of the region of maximum winds of the cyclone

enveloping a building site,

c) an ‘intensity’ probability that the cyclone will maintain its intensity ‘all the way to

Darwin’ (this probability was assumed have a value of 0.5).

Based on this approach, and the occurrence of the TIM cyclones in a 9-year period,

the Nicholls’ group derives a relationship between wind gust speed and average

recurrence interval (return period) which approximates Region D in AS/NZS1170.2.

Darwin is currently located in Region C in the Standard.

The methodology is, in fact, a highly simplified version of the probabilistic simulation

approaches described in Chapter 3. It has some validity in principle but has many

shortcomings in its implementation. Some of these are as follows.

• Estimated (from satellite images) upper wind speeds given in the Bureau of

Meteorology database may have been converted to gust wind speeds at 10

metres height, instead of using reported surface values (see further

discussion in the next section). If a factor of 0.8 was applied to the

predicted gust wind speeds by the Nicholls group, the line for Darwin would

approximate Region C not D.

• In the case of the TIM cyclones, most of the 500 kilometres between

Darwin and the centre of the cyclones was over land, not water.

• The spatial of ‘geometric’ probability (b) should be based on area, not

radius, as used by Nicholls. That is the probability of intersection of a

point (building site) with the footprint area of maximum winds is required.

• The intensity factor of (c) is very over simplified. As discussed in Chapter

5, the weakening in intensity depends on the distance travelled by a storm

over land (see Chapter 5). Thus this factor would be very different for a

storm approaching Darwin overland compared to those approaching from

the sea.

The method does not take account of the preferred tracks of cyclones affecting the

Northern Territory coastline. In fact ‘Thelma’ and ‘Ingrid’ followed a generally east

to west track along the NT coast and were all well north of Darwin. As shown in

Section 9.4 the observed gust wind speeds at Darwin from ‘Thelma’ and ‘Ingrid’ were

all quite low. ‘Monica’ (2006) also produced quite a low gust at Darwin.

The highest gust speed listed by the Nicholls group of 357 km/h (99 m/s) is well

above anything that has ever been measured near ground level anywhere in the

Australian region.

47


9.3 Report by Cook

G.D. Cook (2007), in an unpublished manuscript, makes predictions of wind gust

speed as a function of return period for three coastal locations in the Northern

Territory: Darwin, Maningrida and Nhulunbuy. These appear to have been made

partly on historical experience based on the cyclone database of the Bureau of

Meteorology, and partly using a simulated database generated by a U.S. group

WindRiskTech. Cook concluded that the cyclone risk to Northern Territory stations

are better described by the Region D line AS/NZS1170.2 rather than that for Region

C. Surprisingly given the cyclones of recent years (i.e. the TIM cyclones), Cook

found Darwin to have a slightly higher risk than Maningrida and Nhulunbuy.

Cook makes the valid points that the Arafura Sea has shallower depths and warmer

temperatures than the North-West Shelf (WA) and Coral Sea (Queensland). Also

quoting references based on data from Atlantic hurricanes, wind fields are more

peaked with smaller radii of maximum winds in lower latitudes, so that maximum

gust speeds will be higher for a given central pressure differential. These factors

need to be included in simulation models to predict tropical cyclone wind speeds for

the Northern Territory.

In the introduction to his paper, Cook gives ‘estimated’ maximum gust speeds of 87,

91 and 99 m/s for Cyclones ‘Thelma’, ‘Ingrid’ and ‘Monica’. These values are not in

the Bureau of Meteorology database, which gives estimates of upper level sustained

wind speeds indirectly made using satellite images, and some reported surface

readings. The reported surface values reported in the database are significantly lower

than those used by Cook. For example, for Cyclone ‘Thelma’, the database gives a

maximum wind speed of 75 m/s, not 87 m/s. However, it appears that these

overestimated gust values may have been used to make predictions by both Cook and

M. Nicholls.

The probabilistic method used by Cook to predict wind speed is similar to that used

by Nicholls as described in the previous section; the associated criticisms of this

approach given above are valid.

9.4 Observed wind speeds and overview of cyclone risk for Darwin and the NT

The predictions of significantly higher wind speeds by Nicholls and Cook for Darwin,

than those that the Standard AS/NZS1170.2:2002 predicts, are not supported on

strong probabilistic arguments, and are based on overestimates of gust wind speeds

from recent cyclones. Also, as noted by Nicholls, his group’s predictions contradict

significantly earlier predictions for Darwin by Georgiou (2000) and Harper (2005),

(although these earlier studies did not include the full effect of the TIM cyclones).

The recent study by Geoscience Australia (Arthur et al 2008a) shows a similar risk for

Darwin as for the North Queensland coast, but significantly lower risk than for the

Port Hedland-Onslow coastal strip in Western Australia.

It is of interest to consider the observed gust wind speeds at Darwin Airport due to

tropical cyclones. This is shown in Figure 9.1 for the period 1960 to 2005.

Maximum wind gusts from the daily database during all cyclones in the Bureau of

Meteorology cyclone database are shown. The ‘official’ value of 61 m/s for Cyclone

48


‘Tracy’ (1974) shown in the daily database for Darwin Airport, is shown although it is

widely believed that the true maximum is greater (the anemometer failed during this

event).

Gust speed (m/s)

70

60

50

40

30

20

10

0

1960

Tracy 1974

Cyclonic wind speeds - Darwin 1960-2005

1964

1965

1968

1971

1974

1980

1981

Year

1985

1987

1990

Thelma 1998

1995

1998

Ingrid 2005

2002

Figure 9.1. Maximum wind gusts speeds at Darwin Airport due to cyclones

1960-2005

The gust speed for ‘Tracy’ is clearly much larger than all others recorded. Those for

‘Thelma’ (1998) and ‘Ingrid’ (2005) at Darwin are both no more than 20 m/s. Figure

9.1 can be compared with similar figures for Onslow and Port Hedland (Region D)

shown in Appendix A. With seven cyclones producing gusts greater than 40 m/s,

and three events producing 50 m/s over a similar period, Port Hedland is clearly

historically much more ‘active’ than Darwin. Onslow had two events producing 60

m/s and four above 50 m/s during a similar period. However Learmonth WA, also

currently in Region D, has a similar chart to Darwin with only one large event

(Cyclone ‘Vance’ in 1999).

Similar data to that in Figure 9.1 are not available for stations on the northern

coastline and offshore islands of the Northern Territory, but it is possible that such

charts if they were available, would be more similar to those for Port Hedland or

Onlsow, based on recent cyclonic activity.

9.5 Summary and Conclusions

Recent studies by Nicholls and Cook in Darwin have cast doubt on the current

(Region C) zoning for Darwin in AS/NZS1170.2. These studies have been criticized

in previous forums and again in this report. Apparently Nicholls and Cook

significantly over-estimated the gust speeds near ground level, by using inaccurate

upper level wind speed estimates, instead of the reported surface wind gust speeds, in

the Bureau of Meteorology database for Cyclones ‘Thelma’, ‘Ingrid’ and ‘Monica’

49


(the ‘TIM’ cyclones). The latter events had little effect on Darwin, but a significant

effect on the northern coastline and islands of the Northern Territory, and a stronger

case can be made for upgrading the latter locations to Region D.

However, there is scope for new probability-based simulations to determine wind

speeds for the Northern Territory, incorporating recent information on cyclone tracks,

such as the recent TIM events.

The climate change studies for Australia using global climate simulation models

described in Chapter 8 have, up to now, concentrated on the Queensland coastline

(perhaps understandably given the population density). It would be desirable for

these to be extended to the Northern Territory and Western Australian coasts,

especially given the rapid development of resource projects (oil, gas, minerals) in

these areas.

Uncertainty in the future number and strength of tropical cyclones resulting from

climate change for Darwin, and other locations, can also be handled by increases in

the ‘Uncertainty’ Factor F C , and recommendations for this are given in the following

chapter.

References

W.C. Arthur, A. Schofield, R. P. Cechet and L. A. Sanabria (2008), Return period

cyclonic wind hazard in the Australian Region. 28th AMS Conference on Hurricanes

and Tropical Meteorology, 28 April - 2 May 2008, Orlando, FL, USA

G.D. Cook (2007), Has the hazard from tropical cyclone gusts been underestimated

for northern Australia? personal communication (M.J. Syme), CSIRO Sustainable

Ecosystems.

P.N. Georgiou, (2000), On the probability of Darwin being struck by a category 5

cyclone. A report to the Northern Territory Department of Transport and Works and

Colless & O'Neill Pty Ltd. Environment and Climate Risk Assessment, Pymble,

NSW.

B.A. Harper, (2005), Darwin TCWC Northern Region Storm Tide Prediction System.

System Development Technical Report, pp. 112 + appendices. Systems Engineering

Australia Pty Ltd, Bridgeman Downs, Qld.

M. Nicholls et al (2007), Review of NT cyclone risks, Report by the Community

Group for the review of NT cyclone risks, April 2007. (available on CD-ROM and at

www.cyclone.org.au).

50


10. Conclusions and recommendations

10.1 Conclusions

The Australian Standard for Wind Actions (Standards Australia, 2002a) sets wind

speeds for structural design for return periods specified by the Building Code of

Australia. The Standard has designated special regions for design wind speeds for

tropical cyclones since 1975. However, possible effects of climate change due to

global warming have, up to now, been deliberately excluded (Standards Australia

2002b). Now may be an appropriate time to incorporate, at least partially, current

projections for building design wind speeds in cyclone regions.

JDH Consulting has reviewed scientific literature and other sources, from both

Australia and overseas, relevant to the problem of predicting cyclonic wind speeds in

a warming climate. The trends in the last thirty years in which satellite images are

available and consistent are still somewhat inconclusive. The following most recent

statement by the International Panel on Climate Change (IPCC) summarizes the

global situation: ‘There is observational evidence for an increase in intense tropical

cyclone activity in the North Atlantic since about 1970, correlated with increases in

sea surface temperatures. There are also suggestions of increased intense tropical

cyclone activity in some other regions where concerns over data quality are greater.

… There is no clear trend in the annual numbers of tropical cyclones.’

In the Australian Region there has been a fall in the total number of tropical cyclones

during the last thirty years. This is correlated with the Southern Oscillation Index and

the well-known El Nino phenomenon. The latter also manifests itself in the increase

in sea surface temperature in the central Pacific Ocean, and may be a consequence of

global warming.

There is, however, evidence that the number of more intense cyclones (Category 3

and above) has increased slightly in the last thirty years. These more intense storms

are those of relevance for ultimate limits states design of buildings and other

structures. The apparent trend may be affected, at least partly, by changes in the

observational practices of the Bureau of Meteorology, and reflected in the tropical

cyclones database maintained by the National Climate Centre of the BoM.

However, an increase in more intense cyclones due to global warming, for the Coral

Sea region off the Queensland coast, is predicted by global climate models by several

independent studies. Two studies also predict 2-3 o southward shift in average tropical

cyclone occurrences off the Queensland coast. One study predicts a Category 3

cyclone could directly hit South-east Queensland, including Brisbane, in the middle of

the century. The possibility of such a scenario should be considered in future

revisions of AS/NZS1170.2.

No comparable climate change studies have apparently yet been made for the Indian

Ocean (Western Australian coast) or the Arafura Sea (Northern Territory).

Studies of the average inland decay of hurricanes in the United States and for Cyclone

‘George’ (2007) in Australia, indicate the current widths of the regional boundaries

51


are slightly conservative but adequately specified. However a clearer definition of

‘smoothed coastline’ needs to be given in the Standard.

Recommendations for changes to the design wind speeds for Regions B, C and D in

AS/NZS1170.2 are given in the following section. Other related recommendations

are listed in Section 10.3.

10.2 Recommendations for the Australian Standard and BCA

1. The current regional boundary for Region D should be extended north east along

the Western Australian coast to 15 o S. This will incorporate Broome and Derby. This

change reflects the occurrence zone of tropical cyclones (see Figure 1.1), and is also

recommended in the report by the Cyclone Testing Station on Cyclone ‘George’

(Boughton and Falck, 2007).

2. Region D should also be extended to incorporate the Northern Territory coastline

north of 12 o S. This is justified based on the observed severe tropical cyclones that

have affected this coastline during the last ten years (‘Thelma’ 1998, ‘Ingrid’ 2005,

and ‘Monica’ 2006). However, on the basis of existing information, it is not

recommended that Darwin be included in Region D.

3. Region C should be extended south on the Queensland coast to 27 o S. This is

justified based on the recent simulation studies on the effects of climate change, and

restores the boundary to that in the Standard between 1975 and 1989. It is also

consistent with the boundary of Region C on the Western Australian coast.

4. The existing factor for Region C, F C , should be increased to 1.10, and a factor of

1.10 for Region B should also be considered. These changes are primarily in response

to predictions of increased Category 3-5 cyclones in Queensland waters by climate

model simulations. It is noted that a F B of 1.25 would be required to convert the

current V 50 for Region B of 44 m/s to the peak gust associated with a Category 3

cyclone (about 55 m/s), so that a factor of 1.10 is only a partial adjustment. It is

recommended that these factors be reviewed in the future as predictions of global

warming effects improve with higher resolution models.

A single ‘uncertainty factor’ for the cyclonic regions B, C and D might be more

conveniently handled as a single factor (e.g. M c ) of 1.10.

5. The ‘smoothed’ coastline may be defined by applying a ‘moving average’ filter

with a length of 50 kilometres to the actual coastline. This value is characteristic of

the eye diameter of a typical cyclone, and ensures that minor features that will not

affect the storm characteristics, are not included. A project should be initiated to

provide a map of Australia with this ‘smoothed’ coastline on an internet web site, for

ease of use by designers and local authorities.

6. The vertical profiles of wind gust speed for Regions C and D in AS/NZS1170.2

have been derived from measured data for a particular location, 3-5 kilometres from

the coastline, and incorporate a sea-land transition that may not be appropriate further

inland. The wind profiles (M z,cat ) for tropical cyclones need to be re-considered

taking account of recent dropwindsonde measurements in U.S. hurricanes.

52


7. The Standard currently recommends that Terrain Category 2 (roughness length =

0.02 m) should be used for off-ocean winds for both cyclonic and non-cyclonic

regions for ultimate limit states design. However the recent dropwindsonde

measurements in hurricanes off the United States (discussed in Chapter 4) indicate

that the water surface is much smoother at high wind speeds near the radius of

maximum winds. It is therefore recommended that Terrain Category 1 be specified

for off-water winds for coastal locations in Regions C and D.

10.3 Other Recommendations

1. Up to the present time, global climate model simulations of global warming

effects have concentrated on the Queensland coastline. They should be extended to

Western Australia and the Northern Territory. Oil, gas and mineral interests in those

areas should have a strong interest in such work.

2. Global climate models (such as those described by Leslie et al 2007) should be

extended with surface wind field models to give predictions of extreme gust wind

speeds at 10 metres height for structural design.

3. Ongoing funding should be available for support and ongoing maintenance of

probabilistic models for cyclonic wind speed prediction in Australia. These should

be integrated with global climate models to enable continuous updating of cyclonic

design wind speeds. Such support would bring Australia into line with state-of-the

art technology for hurricane wind speed prediction currently practised in the United

States. Work along these lines has commenced at Geoscience Australia.

4. As climate change effects on tropical cyclone activity is the subject of ongoing

studies by several groups in Australia and overseas, a similar review to the one carried

out for this report should be undertaken every 3 to 5 years. Suitable bodies for

undertaking the independent reviews would be the Bureau of Meteorology or the

Cyclone Testing Station, James Cook University.

5. The historic tropical cyclone database maintained by the Bureau of Meteorology

is most important for the prediction of cyclonic wind speeds and other effects. It is

known to have significant errors and is currently being revised by the Bureau. There

also appear to be systematic errors in the wind speeds listed in the database derived

from satellite images by the Dvorak technique. These values are unrealistically high

and inconsistent with surface observations, including some of those listed elsewhere

in the database. These errors have led to inaccurate conclusions to be drawn by

some users of the database, and it is recommended that the methodology used to

derive the wind speeds be reviewed.

6. The quality and quantity of observations of the strength and effects of tropical

cyclones in the Australian Region is vastly inferior to that available in the United

States. Accepting that resources will not allow the same level of instrumentation to

be deployed in Australia, some increased effort should be made in this country in the

following areas:

53


• Improving the understanding of the response characteristics of present and

previous anemometers used by the Bureau of Meteorology to record past

cyclonic wind speeds.

• Deployment of mobile anemometers before land-falling cyclones to obtain

accurate spatial wind speed information from future cyclones. (This is

currently practiced by several groups in the U.S.; however in Australia damage

and post-event surveys have had to rely on wind damage assessment from

structures such as road signs, and even from tree damage).

• Deployment of dropwindsondes (see Chapter 4) in Australia, including the

possibility of dropping them over remote land sites (deployment over land is

currently not permitted in the United States)

References

G.N. Boughton and D. Falck (2007), Tropical Cyclone George – Wind penetration

inland, Cyclone Testing Station, James Cook University, Draft Technical Report

No.53, August 2007.

CSIRO (2007). Climate change in Australia. Technical Report. CSIRO.

L.M. Leslie, D.J. Karoly, M. Leplastrier, and B.W. Buckley (2007), Variability of

tropical cyclones over the Southwest Pacific Ocean using a high-resolution climate

model, Meteorology and Atmospheric Physics, Vol. 97, pp171-180.

Standards Australia (2002a), Structural design actions. Part 2: Wind actions. AS/NZS

1170.2:2002.

Standards Australia (2002b), Structural design actions-Wind actions-Commentary.

(Supplement to AS/NZS 1170.2:2002).

54


APPENDIX A

MAXIMUM RECORDED WIND GUSTS FROM TROPICAL CYCLONES AT

SELECTED LOCATIONS IN AUSTRALIA

The following charts show gusts primarily recorded by Dines anemometers.

However some later values may have been recorded by cup anemometers associated

with Automatic Weather Stations. Note that these instruments do not necessarily

respond in the same way to fluctuating wind gusts. In a few cases at Onslow and

Port Hedland, modeled values have been substituted when the anemometer has failed.

Cyclonic wind speeds - Broome 1957-2007

70

Wind gust speed (m/s)

60

50

40

30

20

10

0

70

Wind gust speed (m/s)

60

50

40

30

20

10

0

1956

1959

1961

1969

1957

1961

1963

1970

1973

1975

Broome (Region C) has been fortunate in not experiencing the full brunt of a tropical

cyclone although it is located close to the most active part of the Australian region.

Cyclonic wind speeds - Cairns 1956-2001

1971

1976

1977

1979

1981

1983

1986

1997

2000

1977

1980

1983

1986

1988

1993

1997

1999

2001

2004

2007

Cairns (Region C) has not received a direct strike by a tropical cyclone since 1956.

Cyclone ‘Winifred’ in 1986 did significant damage to Innisfail to the south of Cairns

but only produced a 30 m/s gust at Cairns Airport.

55


Cyclonic wind speeds - Darwin 1960-2005

70

60

50

40

30

20

10

0

1960

1964

1965

1968

1971

1974

1980

1981

1985

1987

1990

Gust speed (m/s)

1995

1998

2002

Year

Darwin (Region C) was directly hit by Cyclone ‘Tracy’ in 1974, but no other direct

strikes between 1959 and 2005.

Cyclonic wind speeds - Learmonth 1979-2006

Wind gust speed (m/s)

80

70

60

50

40

30

20

10

0

1979

1981

1982

1983

1984

1985

1986

1986

1987

1989

1991

1995

1995

1996

1997

2000

2006

Learmonth/Exmouth (Region D) experienced the largest gust in Australia recorded by

a Dines anemometer during Cyclone ‘Vance’ in 1999 (74 m/s)

Year

56


Cyclonic wind speeds - Onslow 1958-2004

80

Wind gust speed (m/s)

70

60

50

40

30

20

10

Cyclonic wind speeds - Port Hedland 1958-2004

70

60

50

40

30

20

10

0

1958

1960

1962

1963

1968

1970

1973

1974

1975

1979

1981

1983

1984

1987

1991

1996

1998

2000

2002

2004

1958

1960

1962

1963

1964

1966

1968

1970

1971

1973

1975

1976

1980

1981

1984

1986

1988

1990

1995

1996

1999

2000

2004

0

Onslow (Region D) has experienced several strikes between 1959 and 2004 with a

highest gust of 68.5 m/s from Cyclone ‘Trixie’ in 1975.

Wind gust speed (m/s)

Port Hedland (Region D) experienced several high cyclonic gusts between 1960 and

2004 including those from Cyclone ‘Joan’ in 1975, and ‘Leo’ in 1977.

57


ACKNOWLEDGEMENTS

The support of Dr. Lam Pham and Mr. Brian Ashe of the Australian Building Codes

Board during this project is gratefully acknowledged.

The interest and advice from the members of the Steering Committee are also

acknowledged:

Michael Syme (CSIRO)

Fabio Finocchiaro (Northern Territory Administration)

Nabil Yazdani (Western Australia Administration)

Lance Glare (Queensland Administration)

Cam Leitch (Cyclone Testing Station, James Cook University)

Jeff Kepert (Bureau of Meteorology)

Bob Cechet (Geoscience Australia)

Sergio Detoffi (Standards Australia)

The author also gratefully acknowledges the assistance and free sharing of

information given by the following persons in face-to-face meetings, telephone calls

or e-mails during the course of the project:

Debbie Abbs (CSIRO)

Bob Cechet (Geoscience Australia)

Jeff Kepert (BoM)

Yuri Kuleshov (BoM)

Mark Leplastrier (Insurance Australia Group)

Neville Nicholls (Monash University)

58

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