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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
Page 2 and 3: IMPACT OF CLIMATE CHANGE ON DESIGN
Page 4 and 5: Executive Summary The objective of
Page 6 and 7: 1. Introduction 1.1 Project scope B
Page 8 and 9: Figure 1.1. Average number of days
Page 10 and 11: 2. History of cyclonic design wind
Page 12 and 13: The ratios of the current (2002) va
Page 14 and 15: Station Table 2.3. Comparison of de
Page 16 and 17: 2.3 Summary and Discussion The chan
Page 18 and 19: 3. Simulation methods for predictio
Page 20 and 21: with the hurricane. An imposed pres
Page 22 and 23: C.M.L. Dorman (1984), Tropical cycl
Page 24 and 25: In the absence of other comparable
Page 26 and 27: averaging of many individual normal
Page 28 and 29: T.F. Hock and J.L. Franklin (1999),
Page 30 and 31: A road-sign failure near the eye wa
Page 34 and 35: References G.N. Boughton and D. Fal
Page 36 and 37: 6.3 Interpretations by Emanuel (200
Page 38 and 39: IWTC (2006) ‘The scientific debat
Page 40 and 41: 7. Observed trends in tropical cycl
Page 42 and 43: cyclones occurred in both the South
Page 44 and 45: 8. Predicted effects of climate cha
Page 46 and 47: environmental factors (that are kno
Page 48 and 49: 9. Observations and projections for
Page 50 and 51: 9.3 Report by Cook G.D. Cook (2007)
Page 52 and 53: (the ‘TIM’ cyclones). The latte
Page 54 and 55: are slightly conservative but adequ
Page 56 and 57: • Improving the understanding of
Page 58 and 59: Cyclonic wind speeds - Darwin 1960-
Page 60: ACKNOWLEDGEMENTS The support of Dr

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