PDF | 2 MB - Australian Building Codes Board

abcb.gov.au

PDF | 2 MB - Australian Building Codes Board

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.

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