- Email: [email protected]
Implications of Typhoon York on the design wind speeds in Hong Kong
Journal of Wind Engineering and Industrial Aerodynamics 90 (2002) 1569–1583
Implications of Typhoon York on the design wind speeds in Hong Kong M. Pande*, T.C.E. Ho, M. Mikitiuk, G.A. Kopp, D. Surry A.G. Davenport Wind Engineering Group, Boundary Layer Wind Tunnel Laboratory, Faculty of Engineering Science, The University of Western Ontario, London, Ont., Canada N6A 5B9
Abstract There are significant differences between the design wind speeds recommended by the Hong Kong Code-1983 (Code of Practice on Wind Effects, Hong Kong, Building Development Department, Hong Kong, 1983) and those predicted by the Monte-Carlo simulation model (Design wind speeds in tropical cyclone-prone regions, Ph.D. Thesis, The University of Western Ontario, London, Ontario, 1985). The differences are due to the difficulty in determining the ratio of surface to upper-level wind speeds in typhoons, particularly for regions of complex topography. Typhoon York, which struck the Hong Kong area in September 1999 and was the strongest typhoon in that region since 1983, provides an opportunity to revisit the Hong Kong code design wind speeds and the wind field simulation models (e.g., Design wind speeds in tropical cyclone-prone regions, Ph.D. Thesis, The University of Western Ontario, London, Ontario, 1985). In this paper, Boundary Layer Wind Tunnel Laboratory’s typhoon model is used to recreate Typhoon York and an alternative approach is developed to model the hurricane boundary layer. The wind speeds and wind direction recorded at different anemometer locations around the Hong Kong area during the passage of Typhoon York are compared with the simulated time histories at these stations. The results show good agreement between the observed and the simulated wind speeds which support lower ratios of upper-level to surface wind speeds than are used in the current Hong Kong code. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Typhoon; Wind field model; Hong Kong code; Design wind speed
*Corresponding author. Tel.: +1-519-661-3338; fax: +1-519-661-3339. E-mail address: [email protected] (M. Pande). 0167-6105/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 7 - 6 1 0 5 ( 0 2 ) 0 0 2 7 1 - 4
1570
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1. Introduction The specification of design wind speeds for Hong Kong has long been debated in the Wind Engineering fraternity due to the uncertainties involved in the ratio of surface to upper-level wind speeds in typhoons. The complex topography in Hong Kong creates additional difficulty in the determination of design wind speed at building heights. To compound the problem, the surrounding terrain significantly affects the wind flow characteristics at the various anemometer sites and makes it difficult to reference the wind speed to a standard height and surface roughness condition. Various researchers [1,2] have made measurements on topographical models of the anemometer sites, in boundary layer wind tunnels, to determine the effect of the surrounding terrain and correct the readings taken at these locations. The implied upper-level mean wind speed in the Hong Kong code-1983 [3] and Building Department Practice Notes for Authorized Persons and Registered Structural Engineers, No. 150 (PNAP) [4] is 64 m/s for a return period of 50 years. Monte Carlo computer simulation methods used at the Boundary Layer Wind Tunnel Laboratory (BLWTL) at the University of Western Ontario together with experimental and analytical estimates of the way wind speed varies with height suggest a value of 48 m/s for the same return period. The passage of Typhoon York (the strongest typhoon since 1983) directly over the southwest part of Hong Kong has provided a unique set of wind speed measurements that demand comparison with wind field simulation models (e.g. [5]). In addition, it provides an opportunity to review the advances, which have recently taken place in the area of meteorology and to incorporate our current knowledge of the behaviour of typhoons in predicting the wind structure during extreme tropical cyclone events over the sea. In this paper, the data recorded at a number of meteorological stations are analysed. Of particular interest are those at Waglan Island, Central Plaza and Di Wang in Shenzhen. The uniqueness of the data recorded at Waglan Island is that the wind here flows over open water for a long fetch and, if corrected for the topographic error, the data can provide information on the characteristics of typhoons over sea. The data sets at Central Plaza and Di Wang are unique because they are both about 380 m above sea level and hence should provide the strongest evidence to date of the upper-level design wind speed in Hong Kong, including the effect of complex topography. Typhoon York has been recreated for this paper using a wind field simulation model [5]. The time histories are reconstructed at a number of meteorological stations in Hong Kong including Central Plaza, Cheung Chau and Waglan Island and an asymptotic wind velocity profile is used to reduce the upper-level wind speeds in the typhoon to the anemometer level. The simulated time histories are compared with the actual time histories recorded at these stations during the passage of the storm.
2. Typhoon York (Hong Kong observatory, 1999) ‘‘York developed as a tropical depression about 430 km northeast of Manila on 12 September 1999. Entering the South China Sea, York intensified into a tropical
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1571
storm on 13 September. Its movement then became erratic. Heading north at first, York strengthened into a severe tropical storm on 14 September. It then turned northwestwards before becoming almost stationary and attaining typhoon strength the following night. York picked up speed to about 20 km/h and then began to head towards Hong Kong on the early morning of 16 September. As York approached Hong Kong, local winds reached gale force offshore. Hurricane signal no. 10 was hoisted for the first time since 1983 and was in force for 11 h, the longest on record. It made landfall near Zhuhai and weakened into a severe tropical storm that evening. Moving further inland, York rapidly became an area of low pressure the next day’’.
3. Typhoon York’s wind field Information regarding wind speeds measured at various anemometer locations around Hong Kong was obtained from Hong Kong Observatory. The 6-hourly track information provided by the Hong Kong Observatory was used in the recreation of Typhoon York. The location of different anemometer stations along with the track of Typhoon York is shown in Fig. 1. Winds of hurricane intensity struck Hong Kong on 16 September, first from the northeast direction and later from the southwest direction as shown in Fig. 2. Local winds experienced a lull during the passage of the eye over Hong Kong on 16 September. Several wind speed traces spanning the period from 15 to 17 September are available for Waglan Island, Central Plaza, Green Island,
Fig. 1. Anemometer stations around Hong Kong area and track of Typhoon York (15–17 September 1999).
1572
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
Fig. 2. Hourly wind speed and wind direction recorded at various anemometer locations around Hong Kong during the passage of Typhoon York.
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1573
Table 1 Anemometer wind speeds (m/s) (wind direction)—Typhoon York Station
1-min
10-min
Hourly
1-min peak gust
Waglan Island Central Plaza Cheung Chau Lau Fau Chan Kai Tak Sai Kung Green Island Chek Lap Kok
51.8 45.0 38.5 34.6 23.1 40.0 35.9 25.8
45.7 39.5 32.8 30.8 17.8 32.2 30.5 21.7
42.3 37.2 31.1 28.9 16.2 29.9 30.1 21.1
64.9 54.8 44.4 39.5 58.3 42.8 42.8 33.1
(151) () (461) (61) (211) (261) (161) (3551)
(241) () (2021) (71) (221) (231) (161) (21)
(271) () (2001) (71) (1321) (241) (201) (3561)
() () () () () () () ()
Cheung Chau, Lau Fau Shan, Sai Kung, Kai Tak and Chek Lap Kok. The actual traces of hourly mean wind speeds for these measuring stations have been reproduced in Fig. 2. The traces at Cheung Chau, Waglan Island and Central Plaza indicate that these stations were briefly within the region of maximum winds of the typhoon as shown by the drop in wind speeds at about the time when the eye was closest to each station. The maxima (Fig. 2) reveal that in the former case winds were coming from the northeasterly direction while for the latter maximum, the winds came from the southeasterly direction. The trace at Central Plaza shows a decrease in the second maximum wind speed as the storm passed. The trace at Cheung Chau indicates two maxima of almost the same value. The eye of the storm was over Cheung Chau at around 3 p.m. on 16 September. Table 1 shows values of maximum 1-min peak gust, 1- and 10-min averaged and hourly averaged wind speed recorded at different stations around Hong Kong. Also shown are the directions of these wind speeds. The 10-min and hourly mean wind speeds and wind directions were derived using a moving average applied on the 1-min wind speed data. The highest wind speeds were recorded at Waglan Island because of its exposed location on an isolated island in the southeast waters of Hong Kong. Its data have been regarded as the most reliable, and most of the analysis of the Hong Kong wind speed have been based on data from this station. In the next section, Waglan Island data will be analysed in more detail.
4. Analysis of Waglan Island data Lun [6] and Chen [7] were among the first researchers to carry out an extremevalue analysis on the Waglan Island data (Lun used data between 1953 and 1969, while Chen used data between 1953 and 1974) and in the Hong Kong Wind Code Commentary [8] are quoted as obtaining 50-year return period mean-hourly wind speeds of 51.5 and 44.3 m/s and gust wind speeds of 77.2 and 70.5 m/s, respectively. The Hong Kong Code Commentary also states that the reason for the discrepancy is probably due to the difference in the data period and differences in data interpretation. But both Lun and Chen did not take into account the aerodynamics
1574
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
of the island that cause speed-up. Typhoon York recorded a maximum mean-hourly wind speed of 42.3 m/s and a 3 s peak gust of 64.9 m/s at Waglan Island. Using the extreme-value distribution of Chen, Typhoon York was apparently a 30-year return period event. Melbourne [2] conducted wind tunnel studies on a topographical model of Waglan Island to determine the topographic correction factors, and applied them on the annual-extreme wind speed data for Hong Kong to obtain the 50-year return period mean-hourly wind speed and gust speed, at a reference height of 50 m above mean sea level of 37 and 63.5 m/s, respectively. Using Melbourne’s topographic correction factors, Typhoon York’s maximum recorded mean-hourly wind speed and peak gust at Waglan Island can be reduced to a reference height of 50 m above mean sea level, producing a maximum mean-hourly wind speed and peak gust of 38 and 61.8 m/s, respectively, which would make Typhoon York a 64-year return period event. Similar wind tunnel studies were also carried out at the BLWTL [1] and the extreme wind speed estimates were derived from the Waglan Island data. These predicted a 50-year return period surface mean-hourly wind speed of 39.0 m/s, which compared very well with the results of the Monte-Carlo simulation by Georgiou [5] at the same site. BLWTL’s analysis of extreme wind speed at Waglan Island together with the Monte-Carlo simulation suggest a 50-year return period upper-level meanhourly wind speed of 48 m/s. Using BLWTL’s topographic correction factors, Typhoon York’s upper-level maximum mean-hourly wind speed was 47 m/s and suggest that it was a once-in-50-year storm. The Hong Kong Wind Code commentary and PNAP [4] derive the 50-year return period mean-hourly upper-level wind speed based on the 50-year return period gust wind speed of 70.5 m/s at Waglan Island as suggested by Chen [6]. The gust velocity profile given in the Hong Kong Code is used to translate the 50-year return period Waglan Island gust wind speed to upper level, assuming a gradient height of 200 m and power law exponents for mean wind speed and turbulence intensity of 0.22 and –0.26, respectively. A gust factor of 1.26 is applied to the gust wind speed at gradient height to derive the mean wind speed. This gives a mean wind speed at upper level of 64.0 m/s. If the mean velocity profile is used to derive the upper-level mean wind speed based on 50-year return period Waglan Island mean wind speed, then it would give a value of 55.0 m/s. It is important to note here that these estimates are based on the Waglan Island data without accounting for the aerodynamics of the island and hence tend to be conservative.
5. Typhoon turbulence characteristics It is worthwhile to compare the typhoon turbulence characteristics that were observed during Typhoon York with those described in the Hong Kong Code Commentary [8], PNAP [4], Melbourne [2] and Sharma et al. [9]. Melbourne, during the study of the Waglan Island anemometer data, found that the turbulent boundary layer had characteristics, which were more appropriate to flow over rough terrain rather than wind flow over oceans. Choi [10] also reported that the average
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1575
Table 2 Comparison of turbulence intensities and gust factors over sea Reference
Height (m)
Turbulence intensity (%)
Gust factor
Melbourne
50
>20
>1.74
HK wind code
10–50
27–11
2.0–1.4
PNAP 150 General terrain Built up terrain
75 75
10 18
1.37 1.67
Sharma et al. Sea Coast
10 10
14 21
1.53 1.78
Typoon York
50
24
1.8
turbulence intensity of the tropical cyclone wind (17.5%) at 10 m height at the coastal site was approximately 59% higher than that in a monsoon wind (11%) at the same location. The reason for this behaviour of typhoon winds over sea exposure was not very clear at that time. Recent studies [9] have revealed that the turbulence over the sea in a typhoon is not only generated by the mechanical shearing of the wind due to surface friction but also by the buoyancy forces due to the heat transfer in the convectively unstable tropical atmosphere. The correction factors for the peak gust given in Melbourne [2] were applied to the peak gust data recorded at Waglan Island during Typhoon York. The Deaves and Harris wind field model for extreme winds was used to estimate the turbulence intensities. The peak gust factor for Typhoon York was observed to be 1.8 and the maximum turbulence intensity calculated from the Deaves and Harris model was 24%, at 50 m above mean sea level. Table 2 compares these data with different turbulence characteristics from several references.
6. BLWTL’s simulated Typhoon wind speeds Typhoon York was recreated using the typhoon model developed by BLWTL and described in detail in [5]. This model predicts the gradient height wind speed from a balance of forces in equilibrium with the storm pressure field, which has a minimum timescale of variation of the order of an hour. The input parameters required for this wind speed simulation model are track position and direction, central pressure difference, radius to maximum wind and translation speed. In the recreation of Typhoon York, parameters were obtained from the 6-hourly track information provided by the Hong Kong Observatory. To simulate the wind field at an hourly interval, all the track parameters were linearly interpolated from the available 6-hourly track information. The central pressure difference was calculated using a
1576
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
periphery pressure of 1010 mb, which is a typical value for the Northwest Pacific region [5]. The maximum central pressure difference during the life of Typhoon York was 45 mb. At the time of maximum central pressure difference, the storm was translating at a speed of 4.1 m/s. 6.1. Estimation of Rmax and Holland’s Pressure profile parameter, B In the absence of any direct determinations of radius to maximum winds, Rmax ; from the center of the storm, it was estimated using the distance of the eye to the anemometer station, at the time when the station was on the eyewall of the typhoon. The calculations at Cheung Chau, Waglan Island and Central Plaza anemometer locations that passed briefly through the eye, showed a consistent value of 30 km for Rmax : The Holland’s pressure profile parameter, B; defines the shape of the pressure profile. Increasing it alters the shape of the profile to concentrate more of the pressure drop near Rmax and thus leads to higher wind speeds at the radius to maximum winds. It is ideally determined using the pressure profiles of the typhoon; however the pressure profiles within the typhoon are rarely available. This and the lack of the upper-level wind speed record during the life of the typhoon made the estimation of Holland’s pressure profile parameter, B; very difficult. For the recreation of Typhoon York, B was determined using a non-linear regression model. The model assumed B and Rmax to be the only unknown parameters in the equation of surface wind speed. The observed wind speed data were fitted in the modeled equation using a non-linear least-squares fit. The statistical model estimated a B value of 1.3 and a Rmax value of 30 km. These values were adopted in the simulation of Typhoon York. An example of the gradient height wind field produced by BLWTL’s typhoon model is presented in Fig. 3. The parameters used to simulate the wind field are also indicated in the same figure. For this choice of parameters, the maximum upper-level wind speed in the storm was 43.5 m/s, which according to Georgiou would suggest it was a once-in-20-year storm. (Note: if B were taken as 1.5, the maximum wind speed would be 46.7 m/s associated with a 40-year storm.) 6.2. Asymptotic profile—typhoon boundary layer profile An earlier version of Georgiou’s 1985 model uses a simple relationship for the hurricane boundary layer over water, based on empirical relationships which indicated that the sustained over-water wind speeds at 10 m are approximately 0.8 of the upper-level wind speeds. More recent works [11,12] are based on the parameterization of the fluxes of momentum, heat and moisture in the planetary boundary layer (PBL) as described by Blackadar and Tennekes [13] and Zilitinkevich [14]. Parametric relations result from the matching of mean profiles of wind, temperature and moisture predicted by the surface and outer-layer similarity theories for a PBL in which flow is assumed to be horizontally homogeneous and quasistationary. Cardone and Vickery use a convenient form of parameterization first proposed by Deardorff [15] and later updated by Arya [16]. This parameterization
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1577
Fig. 3. Gradient wind field (m/s) calculated using BLWTL’s Typhoon model.
expresses the PBL fluxes in terms of layer-averaged mean PBL properties which are less affected by sampling errors than their local values. Cardone and Vickery solved the Navier–Stokes equations for the storm parameters to obtain the vertically integrated mean wind speed over the thickness of the boundary layer and hence this scheme of parameterization of surface fluxes worked well for them. Vickery showed that the ratio of the surface-level wind speeds to the vertically integrated mean wind speeds vary as a function of the air-sea temperature difference and the vertically integrated mean wind speed. In this paper, an alternative approach, suggested originally by Davenport and described by Chen [17], is used to determine the variation of wind speed with height in the tropical cyclone boundary layer. It is described below. The surface-layer and outer-layer similarity theories for a PBL [18] also suggest that the surface stress, t; can be expressed as t ¼ rðu * Þ2 ¼ rðCg Vg Þ2 ; where Cg is the geostrophic drag coefficient, u * is the friction velocity and Vg is the gradient wind speed. From similarity arguments it follows that the wind speed near the surface approaches that of an aerodynamic boundary layer over a rough surface with the
1578
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
well-known logarithmic profile Vz z ¼ 2:5 ln ; z0 u* where Vz is the surface wind speed at a height z and z0 is the roughness length of the surrounding terrain. An additional result of this theory is that the geostrophic drag coefficient, Cg ; and the inflow angle, a0 are functions of the Rossby number, R0 [18]. Field observations at a number of different sites suggest that the following approximations are suitable [19]: Cg E0:114ðR0 Þ0:07 ; sin a0 E10:7Cg : These results have the effect of accounting for the buoyant heat transfer within the PBL and can be adapted for typhoons by using an equivalent Rossby number, R00 ; which takes into consideration the effects of both the Coriolis forces as well as the strong centrifugal forces acting inside a typhoon. Thus, it can be written as fz0 VT sin a z0 0 R0 ¼ ; þ 1 Vg Vg r where VT is the translational velocity of the storm, a is the angle between the storm track and any point at a radial distance r; measured positive clockwise from the direction of translation. The ratio of the wind speed at any height z with the gradient wind speed is represented as Vz z ¼ 0:285ðR00 Þ0:07 ln z0 Vg and sin a0 E1:22ðR00 Þ0:07 : It is interesting to note that the asymptotic velocity profile and Vickery’s model of the hurricane boundary layer are based on the same surface and outer-layer similarity theories. The difference is only in the treatment of universal constants in the parametric relations for the surface fluxes of momentum, heat and moisture. Vickery uses Arya’s [16] parameterization, initially proposed by Deardorff [15], whereas the asymptotic velocity profile uses the relationship between the geostrophic drag coefficient and surface Rossby number for estimating the surface flux in a convective PBL. The asymptotic velocity profile cannot be used in the eye of the storm (roRmax ) as the flow regime changes here and does not remain cyclostrophic. The asymptotic velocity profile, unlike Georgiou’s [5] over-water profile, varies both temporally as well as spatially. It depends upon the radial distance to the storm center, as well as
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1579
600 Asymptotic Profile, z0 = 0.01, r = 30 km Power Law - Rough sea Georgiou (1985) Cardone (1996), Air-sea temp. difference = -1,Vg = 47m/s Vickery (2000), Air-sea temp. difference = -1,Vg = 47m/s
500 z (m)
400 300 200 100 0 0.0
0.1
0.2
0.3
0.4
0.5 Vz/Vg
0.6
0.7
0.8
0.9
1.0
Fig. 4. Comparison of boundary layer profiles.
the storm direction, gradient wind speed, translational speed, Coriolis parameter and aerodynamic roughness length. A comparison of the asymptotic velocity profile, at a radial distance of 30 km (Rmax ) over-water with Georgiou’s [5] over-water profile is shown in Fig. 4. Also, shown in the same figure is the power-law profile for the rough-sea case. As mentioned earlier, Cardone [11] and Vickery [12] estimate the ratio of surface to vertically integrated mean speed over the height of the PBL based on air–sea temperature difference and the vertically integrated mean wind speed itself. The air– sea temperature difference for Typhoon York was –11C and the maximum gradient mean hourly wind speed was 47.0 m/s (Rmax ) based on Waglan Island data. For these data, Cardone’s model estimates the ratio of surface to vertically integrated mean speed of 0.69. Vickery’s model also suggests the same value but after performing some preliminary comparisons between simulated and observed wind speeds, Vickery recommended empirical adjustments in the eyewall region by increasing the winds by 10%. The good agreement between asymptotic velocity profile and Cardone and Vickery is not so surprising since all these approaches are based on the same similarity theories. The comparisons of different profiles shown in Fig. 4 indicate that the asymptotic velocity profile suggests a shallower depth of the boundary layer for the typhoon compared to the other two profiles in the region of maximum winds. At distances away from the eyewall, the effect of the centrifugal forces diminishes and the Coriolis effects become dominant. The asymptotic velocity profile also suggests a successive variation in the gradient height over the domain of the storm, which agrees well with the wind observations by Doppler sodar carried out for three typhoons by Amano et al. [20]. For comparison of wind speeds at anemometer height, the determination of z0 is required. For the complex terrain in Hong Kong, the different terrain types were identified by visual examination of the Hong Kong maps. Representative roughness lengths were estimated at each anemometer location, for different azimuths, by weighted averaging of the terrain roughness, based on the fetch and the area occupied by the terrain. Inverse weights were used for the fetch and direct weighting
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1580
was applied for the area occupied by the terrain. Mathematically, the roughness model used in this study can be represented as xi n z0i Ai 1 X x ; z0 ¼ A i i¼1 where z0 is the local roughness length for a given direction; z0i ; Ai and xi are the roughness length, area and fetch of the terrain, respectively, which affect the flow of the wind for this direction. The roughness classification given in ESDU [21] was used for determining the roughness length for the different types of terrain.
7. Discussion The simulated time histories of upper-level winds and the derived anemometerlevel wind speeds at different anemometer stations are compared with the recorded 10-min mean wind speeds and wind directions in Figs. 5 and 6, respectively. A visual comparison of the simulated and the observed wind speeds shows good agreement. At Waglan Island, where the topographic corrections were available from model tests, the simulated wind speeds agree well with the observed wind speeds except on the front side of the storm where the observed values at the radius to maximum winds are higher than the simulated values. The difference can be attributed to the chosen value of pressure profile parameter, B; for the typhoon. The non-linear regression model used to determine the B value does not have a bias toward higher wind speeds and calculates the parameter based on the overall fit. The variation of B
Fig. 5. Comparison of observed and simulated wind speeds at different anemometer locations.
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1581
Fig. 6. Comparison of observed and simulated wind directions at different anemometer locations.
affects the wind speeds at the radius to maximum winds the most, but does not have much effect further away from the eyewall. Thus, adopting a higher value of B would increase the wind speeds at the radius to maximum winds without having much effect further away from it. This would result in higher gradient and surface wind speeds and better agreement with the recorded wind speeds. If matching Waglan Island speeds had been used to determine the best value of B; then B would equal 1.5. The Central Plaza anemometer is located 380 m above sea level, in a region of complex topography with a line of hills immediately to the south of the building and hills towards the north and northeast at a farther distance. Velocity profiles were taken in this area using a 1:2500 topographic model which included all of Hong Kong Island (and the hills to the south of building) and most of the urban area and the hills in the north [1]. These profiles were used to reduce the upper-level wind speeds to anemometer height. The comparison between the observed and the simulated wind speeds at certain stages of Typhoon York is poor, which is possibly due to a gap in the hills not captured in the 451 spatial resolution topographical profiles. Considering the effects of the surrounding topography, the modeled wind speeds seem to be in good agreement with the recorded wind speeds. The gradient wind field (Fig. 6) on the front and on the rear side of the typhoon looks identical, as the eye of the storm passed briefly over the station. In the former case, winds approached the anemometer location from the northeast, which has lesser effect of topography as compared to the east, southeast, south, and southwest, for later winds. The effect on the wind speeds due to the change in the topography, as the winds changed direction over time, is reflected well in the BLWTL profiles taken at this site. The simulated time history at Cheung Chau shows good agreement with the observed values, both on the front as well as on the rear side of the storm. Comparison of wind data measured at the Di Wang Tower in Shenzhen at the 350 m level during Typhoon York [22] is also shown in Figs. 5 and 6. The tower was instrumented with the wind and structural monitoring system jointly developed by the Hong Kong Polytechnic University and the China Academy of Building Research. Good agreement is found between the measured wind data and the results from Georgiou’s 1985 model in conjunction with asymptotic profiles. It should be
1582
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
noted that the Central Plaza data originate from a reliable meteorological record maintained by the Hong Kong Observatory, the Di Wang data have been extracted from Xu and Zhan’s paper, without verifying the reliability of the instrumentation installation and monitoring system used.
8. Conclusions Waglan Island mean-hourly wind speed data recorded during Typhoon York was analysed after applying the topographic correction factors. The return period of this event was estimated, using extreme-value analysis based on the work of various research available in the literature. It was observed that the return period of Typhoon York was in the range of 30–64 years. According to Georgiou’s 1985 model, this range of return period corresponds to wind speeds between 45 and 48 m/s. Hence Typhoon York was roughly a 50-year return period event and thus provides an opportunity to validate the various design wind speed practices prevalent in the Hong Kong region. The peak gust data for Typhoon York, measured at Waglan Island, was also analysed to study the characteristics of typhoons over sea and to determine the gust factor and turbulence intensity during this event. The peak gust and turbulence intensity estimated for Typhoon York was compared to the work of other researchers and the Hong Kong Wind Code. The comparison showed that during an extreme event the sea has characteristics similar to flow over quite rough terrain. The results are in good agreement with Melbourne’s recommendation of turbulence intensity in excess of 20%, at a reference height of 50 m, during a typhoon event. The wind field for Typhoon York was simulated using BLWTL’s typhoon wind field model. Asymptotic profiles were used to model the boundary layer in a tropical cyclone. The comparison of asymptotic velocity profiles to Cardone and Vickery’s hurricane boundary layer model showed good agreement. The results show a good comparison between the observed and the simulated wind speeds. Asymptotic profiles provide a theoretical model for the boundary layer in a typhoon, especially over water. The comparisons also show a reasonable match between the simulated and observed wind speeds over land, though the validity of asymptotic profiles over land still needs further investigation. It could prove very useful for areas that do not have complex topography. The results generally support the BLWTL simulation model. This is particularly significant for the two high-level anemometers examined on the Central Plaza and Di Wang buildings, and hence suggest the current Hong Kong specified upper-level mean wind speed of 64 m/s is unduly conservative.
References [1] G. Lythe, D. Surry, A.G. Davenport, Wind profiles over Hong Kong: results of experiments with a 1:2500 and 1:500 scale model of the area, Engineering Science Research Report, The University of Western Ontario, BLWT-SS15-1981.
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1583
[2] W.H. Melbourne, Design wind data for Hong Kong and surrounding coastline, Proceedings of the Third International Conference on Tall Buildings, Hong Kong and Guangzhou, 1984. [3] Code of Practice on Wind Effects, Hong Kong, Building Development Department, Hong Kong, 1983. [4] Building Department Practice Note for Authorized Persons and Registered Structural Engineers, No. 150, Hong Kong, June, 1994. [5] P.N. Georgiou, Design wind speeds in tropical cyclone-prone regions, Ph.D. Thesis, The University of Western Ontario, London, Ontario, 1985. [6] C.L.K. Lun, The application of the theory of extreme to the estimation of design wind speeds, M.Sc. Thesis, 1970. [7] T.Y. Chen, Comparison of surface wind speeds in Hong Kong, ROHK, Hong Kong, Tech. 41, 1975. [8] E.C.C. Choi, Commentary on the code of practice on wind effects, Hong Kong, 1983. [9] R.N. Sharma, P.J. Richards, A re-examination of the characteristics of tropical cyclone winds, J. Wind Eng. Ind. Aerodyn. 83 (1999) 21–33. [10] E.C.C. Choi, Characteristics of typhoons over the South China Sea, J. Ind. Aerodyn. 3 (1978) 353–365. [11] E.F. Thompson, V.J. Cardone, Practical modelling of hurricane surface wind fields, J. Waterway, Port, Coastal Ocean Eng. 22 (4) (1996) 195–205. [12] P.J. Vickery, P.F. Skerlj, L.A. Twisdale, Hurricane wind field model for use in hurricane simulations, J. Struct. Eng., ASCE (2000) 1203–1221. [13] A.K. Blackadar, H. Tennekes, Asymptotic similarity in neutral barotropic planetary boundary layers, J. Atmos. Sci. 25 (1968) 1015–1022. [14] S.S. Zilitinkevich, Dynamics of the atmospheric boundary layer, Leningrad (in Russian; English translation published 1973), 1970. [15] J.W. Deardorff, Numerical investigation of neutral and unstable planetary boundary layers, J. Atmos. Sci. 29 (1972) 91–115. [16] S.P. Arya, Suggested revisions to certain boundary layer parameterization schemes used in atmospheric circulation models, Mon. Weather Rev. 105 (2) (1977) 215–227. [17] D. Chen, Vulnerability of tall buildings, M.E.Sc. Thesis, The University of Western Ontario, London, Ontario, 1999. [18] Tennekes, H., Similarity Laws and Scale Relations in Planetary Boundary Layers, Workshop on Micrometeorology, American Meteorological Society, 1972, pp. 177–213. [19] A.G. Davenport, The Structure of wind and climate, Proceedings of the Conference on Safety of Structures Under Dynamic Loads, Vol. 1, Trondheim, Norway, 1978, pp. 209–256. [20] T. Amano, H. Fukushima, T. Ohkuma, S. Kawaguchi, S. Goto, The observation of typhoon winds in Okinawa by Doppler sodar, J. Wind Eng. Ind. Aerodyn. 83 (1999) 11–20. [21] ESDU Strong Winds in the Atmospheric Boundary Layer, Part 1: Mean Hourly Wind Speed, Engineering Sciences Data Unit Item No. 82026, London, England, 1982. [22] Y.L. Xu, S. Zhan, Field measurements of Di Wang Tower during Typhoon York, J. Ind. Aerodyn. 89 (2001) 73–93.
Implications of Typhoon York on the design wind speeds in Hong Kong M. Pande*, T.C.E. Ho, M. Mikitiuk, G.A. Kopp, D. Surry A.G. Davenport Wind Engineering Group, Boundary Layer Wind Tunnel Laboratory, Faculty of Engineering Science, The University of Western Ontario, London, Ont., Canada N6A 5B9
Abstract There are significant differences between the design wind speeds recommended by the Hong Kong Code-1983 (Code of Practice on Wind Effects, Hong Kong, Building Development Department, Hong Kong, 1983) and those predicted by the Monte-Carlo simulation model (Design wind speeds in tropical cyclone-prone regions, Ph.D. Thesis, The University of Western Ontario, London, Ontario, 1985). The differences are due to the difficulty in determining the ratio of surface to upper-level wind speeds in typhoons, particularly for regions of complex topography. Typhoon York, which struck the Hong Kong area in September 1999 and was the strongest typhoon in that region since 1983, provides an opportunity to revisit the Hong Kong code design wind speeds and the wind field simulation models (e.g., Design wind speeds in tropical cyclone-prone regions, Ph.D. Thesis, The University of Western Ontario, London, Ontario, 1985). In this paper, Boundary Layer Wind Tunnel Laboratory’s typhoon model is used to recreate Typhoon York and an alternative approach is developed to model the hurricane boundary layer. The wind speeds and wind direction recorded at different anemometer locations around the Hong Kong area during the passage of Typhoon York are compared with the simulated time histories at these stations. The results show good agreement between the observed and the simulated wind speeds which support lower ratios of upper-level to surface wind speeds than are used in the current Hong Kong code. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Typhoon; Wind field model; Hong Kong code; Design wind speed
*Corresponding author. Tel.: +1-519-661-3338; fax: +1-519-661-3339. E-mail address: [email protected] (M. Pande). 0167-6105/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 7 - 6 1 0 5 ( 0 2 ) 0 0 2 7 1 - 4
1570
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1. Introduction The specification of design wind speeds for Hong Kong has long been debated in the Wind Engineering fraternity due to the uncertainties involved in the ratio of surface to upper-level wind speeds in typhoons. The complex topography in Hong Kong creates additional difficulty in the determination of design wind speed at building heights. To compound the problem, the surrounding terrain significantly affects the wind flow characteristics at the various anemometer sites and makes it difficult to reference the wind speed to a standard height and surface roughness condition. Various researchers [1,2] have made measurements on topographical models of the anemometer sites, in boundary layer wind tunnels, to determine the effect of the surrounding terrain and correct the readings taken at these locations. The implied upper-level mean wind speed in the Hong Kong code-1983 [3] and Building Department Practice Notes for Authorized Persons and Registered Structural Engineers, No. 150 (PNAP) [4] is 64 m/s for a return period of 50 years. Monte Carlo computer simulation methods used at the Boundary Layer Wind Tunnel Laboratory (BLWTL) at the University of Western Ontario together with experimental and analytical estimates of the way wind speed varies with height suggest a value of 48 m/s for the same return period. The passage of Typhoon York (the strongest typhoon since 1983) directly over the southwest part of Hong Kong has provided a unique set of wind speed measurements that demand comparison with wind field simulation models (e.g. [5]). In addition, it provides an opportunity to review the advances, which have recently taken place in the area of meteorology and to incorporate our current knowledge of the behaviour of typhoons in predicting the wind structure during extreme tropical cyclone events over the sea. In this paper, the data recorded at a number of meteorological stations are analysed. Of particular interest are those at Waglan Island, Central Plaza and Di Wang in Shenzhen. The uniqueness of the data recorded at Waglan Island is that the wind here flows over open water for a long fetch and, if corrected for the topographic error, the data can provide information on the characteristics of typhoons over sea. The data sets at Central Plaza and Di Wang are unique because they are both about 380 m above sea level and hence should provide the strongest evidence to date of the upper-level design wind speed in Hong Kong, including the effect of complex topography. Typhoon York has been recreated for this paper using a wind field simulation model [5]. The time histories are reconstructed at a number of meteorological stations in Hong Kong including Central Plaza, Cheung Chau and Waglan Island and an asymptotic wind velocity profile is used to reduce the upper-level wind speeds in the typhoon to the anemometer level. The simulated time histories are compared with the actual time histories recorded at these stations during the passage of the storm.
2. Typhoon York (Hong Kong observatory, 1999) ‘‘York developed as a tropical depression about 430 km northeast of Manila on 12 September 1999. Entering the South China Sea, York intensified into a tropical
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1571
storm on 13 September. Its movement then became erratic. Heading north at first, York strengthened into a severe tropical storm on 14 September. It then turned northwestwards before becoming almost stationary and attaining typhoon strength the following night. York picked up speed to about 20 km/h and then began to head towards Hong Kong on the early morning of 16 September. As York approached Hong Kong, local winds reached gale force offshore. Hurricane signal no. 10 was hoisted for the first time since 1983 and was in force for 11 h, the longest on record. It made landfall near Zhuhai and weakened into a severe tropical storm that evening. Moving further inland, York rapidly became an area of low pressure the next day’’.
3. Typhoon York’s wind field Information regarding wind speeds measured at various anemometer locations around Hong Kong was obtained from Hong Kong Observatory. The 6-hourly track information provided by the Hong Kong Observatory was used in the recreation of Typhoon York. The location of different anemometer stations along with the track of Typhoon York is shown in Fig. 1. Winds of hurricane intensity struck Hong Kong on 16 September, first from the northeast direction and later from the southwest direction as shown in Fig. 2. Local winds experienced a lull during the passage of the eye over Hong Kong on 16 September. Several wind speed traces spanning the period from 15 to 17 September are available for Waglan Island, Central Plaza, Green Island,
Fig. 1. Anemometer stations around Hong Kong area and track of Typhoon York (15–17 September 1999).
1572
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
Fig. 2. Hourly wind speed and wind direction recorded at various anemometer locations around Hong Kong during the passage of Typhoon York.
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1573
Table 1 Anemometer wind speeds (m/s) (wind direction)—Typhoon York Station
1-min
10-min
Hourly
1-min peak gust
Waglan Island Central Plaza Cheung Chau Lau Fau Chan Kai Tak Sai Kung Green Island Chek Lap Kok
51.8 45.0 38.5 34.6 23.1 40.0 35.9 25.8
45.7 39.5 32.8 30.8 17.8 32.2 30.5 21.7
42.3 37.2 31.1 28.9 16.2 29.9 30.1 21.1
64.9 54.8 44.4 39.5 58.3 42.8 42.8 33.1
(151) () (461) (61) (211) (261) (161) (3551)
(241) () (2021) (71) (221) (231) (161) (21)
(271) () (2001) (71) (1321) (241) (201) (3561)
() () () () () () () ()
Cheung Chau, Lau Fau Shan, Sai Kung, Kai Tak and Chek Lap Kok. The actual traces of hourly mean wind speeds for these measuring stations have been reproduced in Fig. 2. The traces at Cheung Chau, Waglan Island and Central Plaza indicate that these stations were briefly within the region of maximum winds of the typhoon as shown by the drop in wind speeds at about the time when the eye was closest to each station. The maxima (Fig. 2) reveal that in the former case winds were coming from the northeasterly direction while for the latter maximum, the winds came from the southeasterly direction. The trace at Central Plaza shows a decrease in the second maximum wind speed as the storm passed. The trace at Cheung Chau indicates two maxima of almost the same value. The eye of the storm was over Cheung Chau at around 3 p.m. on 16 September. Table 1 shows values of maximum 1-min peak gust, 1- and 10-min averaged and hourly averaged wind speed recorded at different stations around Hong Kong. Also shown are the directions of these wind speeds. The 10-min and hourly mean wind speeds and wind directions were derived using a moving average applied on the 1-min wind speed data. The highest wind speeds were recorded at Waglan Island because of its exposed location on an isolated island in the southeast waters of Hong Kong. Its data have been regarded as the most reliable, and most of the analysis of the Hong Kong wind speed have been based on data from this station. In the next section, Waglan Island data will be analysed in more detail.
4. Analysis of Waglan Island data Lun [6] and Chen [7] were among the first researchers to carry out an extremevalue analysis on the Waglan Island data (Lun used data between 1953 and 1969, while Chen used data between 1953 and 1974) and in the Hong Kong Wind Code Commentary [8] are quoted as obtaining 50-year return period mean-hourly wind speeds of 51.5 and 44.3 m/s and gust wind speeds of 77.2 and 70.5 m/s, respectively. The Hong Kong Code Commentary also states that the reason for the discrepancy is probably due to the difference in the data period and differences in data interpretation. But both Lun and Chen did not take into account the aerodynamics
1574
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
of the island that cause speed-up. Typhoon York recorded a maximum mean-hourly wind speed of 42.3 m/s and a 3 s peak gust of 64.9 m/s at Waglan Island. Using the extreme-value distribution of Chen, Typhoon York was apparently a 30-year return period event. Melbourne [2] conducted wind tunnel studies on a topographical model of Waglan Island to determine the topographic correction factors, and applied them on the annual-extreme wind speed data for Hong Kong to obtain the 50-year return period mean-hourly wind speed and gust speed, at a reference height of 50 m above mean sea level of 37 and 63.5 m/s, respectively. Using Melbourne’s topographic correction factors, Typhoon York’s maximum recorded mean-hourly wind speed and peak gust at Waglan Island can be reduced to a reference height of 50 m above mean sea level, producing a maximum mean-hourly wind speed and peak gust of 38 and 61.8 m/s, respectively, which would make Typhoon York a 64-year return period event. Similar wind tunnel studies were also carried out at the BLWTL [1] and the extreme wind speed estimates were derived from the Waglan Island data. These predicted a 50-year return period surface mean-hourly wind speed of 39.0 m/s, which compared very well with the results of the Monte-Carlo simulation by Georgiou [5] at the same site. BLWTL’s analysis of extreme wind speed at Waglan Island together with the Monte-Carlo simulation suggest a 50-year return period upper-level meanhourly wind speed of 48 m/s. Using BLWTL’s topographic correction factors, Typhoon York’s upper-level maximum mean-hourly wind speed was 47 m/s and suggest that it was a once-in-50-year storm. The Hong Kong Wind Code commentary and PNAP [4] derive the 50-year return period mean-hourly upper-level wind speed based on the 50-year return period gust wind speed of 70.5 m/s at Waglan Island as suggested by Chen [6]. The gust velocity profile given in the Hong Kong Code is used to translate the 50-year return period Waglan Island gust wind speed to upper level, assuming a gradient height of 200 m and power law exponents for mean wind speed and turbulence intensity of 0.22 and –0.26, respectively. A gust factor of 1.26 is applied to the gust wind speed at gradient height to derive the mean wind speed. This gives a mean wind speed at upper level of 64.0 m/s. If the mean velocity profile is used to derive the upper-level mean wind speed based on 50-year return period Waglan Island mean wind speed, then it would give a value of 55.0 m/s. It is important to note here that these estimates are based on the Waglan Island data without accounting for the aerodynamics of the island and hence tend to be conservative.
5. Typhoon turbulence characteristics It is worthwhile to compare the typhoon turbulence characteristics that were observed during Typhoon York with those described in the Hong Kong Code Commentary [8], PNAP [4], Melbourne [2] and Sharma et al. [9]. Melbourne, during the study of the Waglan Island anemometer data, found that the turbulent boundary layer had characteristics, which were more appropriate to flow over rough terrain rather than wind flow over oceans. Choi [10] also reported that the average
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1575
Table 2 Comparison of turbulence intensities and gust factors over sea Reference
Height (m)
Turbulence intensity (%)
Gust factor
Melbourne
50
>20
>1.74
HK wind code
10–50
27–11
2.0–1.4
PNAP 150 General terrain Built up terrain
75 75
10 18
1.37 1.67
Sharma et al. Sea Coast
10 10
14 21
1.53 1.78
Typoon York
50
24
1.8
turbulence intensity of the tropical cyclone wind (17.5%) at 10 m height at the coastal site was approximately 59% higher than that in a monsoon wind (11%) at the same location. The reason for this behaviour of typhoon winds over sea exposure was not very clear at that time. Recent studies [9] have revealed that the turbulence over the sea in a typhoon is not only generated by the mechanical shearing of the wind due to surface friction but also by the buoyancy forces due to the heat transfer in the convectively unstable tropical atmosphere. The correction factors for the peak gust given in Melbourne [2] were applied to the peak gust data recorded at Waglan Island during Typhoon York. The Deaves and Harris wind field model for extreme winds was used to estimate the turbulence intensities. The peak gust factor for Typhoon York was observed to be 1.8 and the maximum turbulence intensity calculated from the Deaves and Harris model was 24%, at 50 m above mean sea level. Table 2 compares these data with different turbulence characteristics from several references.
6. BLWTL’s simulated Typhoon wind speeds Typhoon York was recreated using the typhoon model developed by BLWTL and described in detail in [5]. This model predicts the gradient height wind speed from a balance of forces in equilibrium with the storm pressure field, which has a minimum timescale of variation of the order of an hour. The input parameters required for this wind speed simulation model are track position and direction, central pressure difference, radius to maximum wind and translation speed. In the recreation of Typhoon York, parameters were obtained from the 6-hourly track information provided by the Hong Kong Observatory. To simulate the wind field at an hourly interval, all the track parameters were linearly interpolated from the available 6-hourly track information. The central pressure difference was calculated using a
1576
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
periphery pressure of 1010 mb, which is a typical value for the Northwest Pacific region [5]. The maximum central pressure difference during the life of Typhoon York was 45 mb. At the time of maximum central pressure difference, the storm was translating at a speed of 4.1 m/s. 6.1. Estimation of Rmax and Holland’s Pressure profile parameter, B In the absence of any direct determinations of radius to maximum winds, Rmax ; from the center of the storm, it was estimated using the distance of the eye to the anemometer station, at the time when the station was on the eyewall of the typhoon. The calculations at Cheung Chau, Waglan Island and Central Plaza anemometer locations that passed briefly through the eye, showed a consistent value of 30 km for Rmax : The Holland’s pressure profile parameter, B; defines the shape of the pressure profile. Increasing it alters the shape of the profile to concentrate more of the pressure drop near Rmax and thus leads to higher wind speeds at the radius to maximum winds. It is ideally determined using the pressure profiles of the typhoon; however the pressure profiles within the typhoon are rarely available. This and the lack of the upper-level wind speed record during the life of the typhoon made the estimation of Holland’s pressure profile parameter, B; very difficult. For the recreation of Typhoon York, B was determined using a non-linear regression model. The model assumed B and Rmax to be the only unknown parameters in the equation of surface wind speed. The observed wind speed data were fitted in the modeled equation using a non-linear least-squares fit. The statistical model estimated a B value of 1.3 and a Rmax value of 30 km. These values were adopted in the simulation of Typhoon York. An example of the gradient height wind field produced by BLWTL’s typhoon model is presented in Fig. 3. The parameters used to simulate the wind field are also indicated in the same figure. For this choice of parameters, the maximum upper-level wind speed in the storm was 43.5 m/s, which according to Georgiou would suggest it was a once-in-20-year storm. (Note: if B were taken as 1.5, the maximum wind speed would be 46.7 m/s associated with a 40-year storm.) 6.2. Asymptotic profile—typhoon boundary layer profile An earlier version of Georgiou’s 1985 model uses a simple relationship for the hurricane boundary layer over water, based on empirical relationships which indicated that the sustained over-water wind speeds at 10 m are approximately 0.8 of the upper-level wind speeds. More recent works [11,12] are based on the parameterization of the fluxes of momentum, heat and moisture in the planetary boundary layer (PBL) as described by Blackadar and Tennekes [13] and Zilitinkevich [14]. Parametric relations result from the matching of mean profiles of wind, temperature and moisture predicted by the surface and outer-layer similarity theories for a PBL in which flow is assumed to be horizontally homogeneous and quasistationary. Cardone and Vickery use a convenient form of parameterization first proposed by Deardorff [15] and later updated by Arya [16]. This parameterization
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1577
Fig. 3. Gradient wind field (m/s) calculated using BLWTL’s Typhoon model.
expresses the PBL fluxes in terms of layer-averaged mean PBL properties which are less affected by sampling errors than their local values. Cardone and Vickery solved the Navier–Stokes equations for the storm parameters to obtain the vertically integrated mean wind speed over the thickness of the boundary layer and hence this scheme of parameterization of surface fluxes worked well for them. Vickery showed that the ratio of the surface-level wind speeds to the vertically integrated mean wind speeds vary as a function of the air-sea temperature difference and the vertically integrated mean wind speed. In this paper, an alternative approach, suggested originally by Davenport and described by Chen [17], is used to determine the variation of wind speed with height in the tropical cyclone boundary layer. It is described below. The surface-layer and outer-layer similarity theories for a PBL [18] also suggest that the surface stress, t; can be expressed as t ¼ rðu * Þ2 ¼ rðCg Vg Þ2 ; where Cg is the geostrophic drag coefficient, u * is the friction velocity and Vg is the gradient wind speed. From similarity arguments it follows that the wind speed near the surface approaches that of an aerodynamic boundary layer over a rough surface with the
1578
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
well-known logarithmic profile Vz z ¼ 2:5 ln ; z0 u* where Vz is the surface wind speed at a height z and z0 is the roughness length of the surrounding terrain. An additional result of this theory is that the geostrophic drag coefficient, Cg ; and the inflow angle, a0 are functions of the Rossby number, R0 [18]. Field observations at a number of different sites suggest that the following approximations are suitable [19]: Cg E0:114ðR0 Þ0:07 ; sin a0 E10:7Cg : These results have the effect of accounting for the buoyant heat transfer within the PBL and can be adapted for typhoons by using an equivalent Rossby number, R00 ; which takes into consideration the effects of both the Coriolis forces as well as the strong centrifugal forces acting inside a typhoon. Thus, it can be written as fz0 VT sin a z0 0 R0 ¼ ; þ 1 Vg Vg r where VT is the translational velocity of the storm, a is the angle between the storm track and any point at a radial distance r; measured positive clockwise from the direction of translation. The ratio of the wind speed at any height z with the gradient wind speed is represented as Vz z ¼ 0:285ðR00 Þ0:07 ln z0 Vg and sin a0 E1:22ðR00 Þ0:07 : It is interesting to note that the asymptotic velocity profile and Vickery’s model of the hurricane boundary layer are based on the same surface and outer-layer similarity theories. The difference is only in the treatment of universal constants in the parametric relations for the surface fluxes of momentum, heat and moisture. Vickery uses Arya’s [16] parameterization, initially proposed by Deardorff [15], whereas the asymptotic velocity profile uses the relationship between the geostrophic drag coefficient and surface Rossby number for estimating the surface flux in a convective PBL. The asymptotic velocity profile cannot be used in the eye of the storm (roRmax ) as the flow regime changes here and does not remain cyclostrophic. The asymptotic velocity profile, unlike Georgiou’s [5] over-water profile, varies both temporally as well as spatially. It depends upon the radial distance to the storm center, as well as
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1579
600 Asymptotic Profile, z0 = 0.01, r = 30 km Power Law - Rough sea Georgiou (1985) Cardone (1996), Air-sea temp. difference = -1,Vg = 47m/s Vickery (2000), Air-sea temp. difference = -1,Vg = 47m/s
500 z (m)
400 300 200 100 0 0.0
0.1
0.2
0.3
0.4
0.5 Vz/Vg
0.6
0.7
0.8
0.9
1.0
Fig. 4. Comparison of boundary layer profiles.
the storm direction, gradient wind speed, translational speed, Coriolis parameter and aerodynamic roughness length. A comparison of the asymptotic velocity profile, at a radial distance of 30 km (Rmax ) over-water with Georgiou’s [5] over-water profile is shown in Fig. 4. Also, shown in the same figure is the power-law profile for the rough-sea case. As mentioned earlier, Cardone [11] and Vickery [12] estimate the ratio of surface to vertically integrated mean speed over the height of the PBL based on air–sea temperature difference and the vertically integrated mean wind speed itself. The air– sea temperature difference for Typhoon York was –11C and the maximum gradient mean hourly wind speed was 47.0 m/s (Rmax ) based on Waglan Island data. For these data, Cardone’s model estimates the ratio of surface to vertically integrated mean speed of 0.69. Vickery’s model also suggests the same value but after performing some preliminary comparisons between simulated and observed wind speeds, Vickery recommended empirical adjustments in the eyewall region by increasing the winds by 10%. The good agreement between asymptotic velocity profile and Cardone and Vickery is not so surprising since all these approaches are based on the same similarity theories. The comparisons of different profiles shown in Fig. 4 indicate that the asymptotic velocity profile suggests a shallower depth of the boundary layer for the typhoon compared to the other two profiles in the region of maximum winds. At distances away from the eyewall, the effect of the centrifugal forces diminishes and the Coriolis effects become dominant. The asymptotic velocity profile also suggests a successive variation in the gradient height over the domain of the storm, which agrees well with the wind observations by Doppler sodar carried out for three typhoons by Amano et al. [20]. For comparison of wind speeds at anemometer height, the determination of z0 is required. For the complex terrain in Hong Kong, the different terrain types were identified by visual examination of the Hong Kong maps. Representative roughness lengths were estimated at each anemometer location, for different azimuths, by weighted averaging of the terrain roughness, based on the fetch and the area occupied by the terrain. Inverse weights were used for the fetch and direct weighting
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1580
was applied for the area occupied by the terrain. Mathematically, the roughness model used in this study can be represented as xi n z0i Ai 1 X x ; z0 ¼ A i i¼1 where z0 is the local roughness length for a given direction; z0i ; Ai and xi are the roughness length, area and fetch of the terrain, respectively, which affect the flow of the wind for this direction. The roughness classification given in ESDU [21] was used for determining the roughness length for the different types of terrain.
7. Discussion The simulated time histories of upper-level winds and the derived anemometerlevel wind speeds at different anemometer stations are compared with the recorded 10-min mean wind speeds and wind directions in Figs. 5 and 6, respectively. A visual comparison of the simulated and the observed wind speeds shows good agreement. At Waglan Island, where the topographic corrections were available from model tests, the simulated wind speeds agree well with the observed wind speeds except on the front side of the storm where the observed values at the radius to maximum winds are higher than the simulated values. The difference can be attributed to the chosen value of pressure profile parameter, B; for the typhoon. The non-linear regression model used to determine the B value does not have a bias toward higher wind speeds and calculates the parameter based on the overall fit. The variation of B
Fig. 5. Comparison of observed and simulated wind speeds at different anemometer locations.
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1581
Fig. 6. Comparison of observed and simulated wind directions at different anemometer locations.
affects the wind speeds at the radius to maximum winds the most, but does not have much effect further away from the eyewall. Thus, adopting a higher value of B would increase the wind speeds at the radius to maximum winds without having much effect further away from it. This would result in higher gradient and surface wind speeds and better agreement with the recorded wind speeds. If matching Waglan Island speeds had been used to determine the best value of B; then B would equal 1.5. The Central Plaza anemometer is located 380 m above sea level, in a region of complex topography with a line of hills immediately to the south of the building and hills towards the north and northeast at a farther distance. Velocity profiles were taken in this area using a 1:2500 topographic model which included all of Hong Kong Island (and the hills to the south of building) and most of the urban area and the hills in the north [1]. These profiles were used to reduce the upper-level wind speeds to anemometer height. The comparison between the observed and the simulated wind speeds at certain stages of Typhoon York is poor, which is possibly due to a gap in the hills not captured in the 451 spatial resolution topographical profiles. Considering the effects of the surrounding topography, the modeled wind speeds seem to be in good agreement with the recorded wind speeds. The gradient wind field (Fig. 6) on the front and on the rear side of the typhoon looks identical, as the eye of the storm passed briefly over the station. In the former case, winds approached the anemometer location from the northeast, which has lesser effect of topography as compared to the east, southeast, south, and southwest, for later winds. The effect on the wind speeds due to the change in the topography, as the winds changed direction over time, is reflected well in the BLWTL profiles taken at this site. The simulated time history at Cheung Chau shows good agreement with the observed values, both on the front as well as on the rear side of the storm. Comparison of wind data measured at the Di Wang Tower in Shenzhen at the 350 m level during Typhoon York [22] is also shown in Figs. 5 and 6. The tower was instrumented with the wind and structural monitoring system jointly developed by the Hong Kong Polytechnic University and the China Academy of Building Research. Good agreement is found between the measured wind data and the results from Georgiou’s 1985 model in conjunction with asymptotic profiles. It should be
1582
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
noted that the Central Plaza data originate from a reliable meteorological record maintained by the Hong Kong Observatory, the Di Wang data have been extracted from Xu and Zhan’s paper, without verifying the reliability of the instrumentation installation and monitoring system used.
8. Conclusions Waglan Island mean-hourly wind speed data recorded during Typhoon York was analysed after applying the topographic correction factors. The return period of this event was estimated, using extreme-value analysis based on the work of various research available in the literature. It was observed that the return period of Typhoon York was in the range of 30–64 years. According to Georgiou’s 1985 model, this range of return period corresponds to wind speeds between 45 and 48 m/s. Hence Typhoon York was roughly a 50-year return period event and thus provides an opportunity to validate the various design wind speed practices prevalent in the Hong Kong region. The peak gust data for Typhoon York, measured at Waglan Island, was also analysed to study the characteristics of typhoons over sea and to determine the gust factor and turbulence intensity during this event. The peak gust and turbulence intensity estimated for Typhoon York was compared to the work of other researchers and the Hong Kong Wind Code. The comparison showed that during an extreme event the sea has characteristics similar to flow over quite rough terrain. The results are in good agreement with Melbourne’s recommendation of turbulence intensity in excess of 20%, at a reference height of 50 m, during a typhoon event. The wind field for Typhoon York was simulated using BLWTL’s typhoon wind field model. Asymptotic profiles were used to model the boundary layer in a tropical cyclone. The comparison of asymptotic velocity profiles to Cardone and Vickery’s hurricane boundary layer model showed good agreement. The results show a good comparison between the observed and the simulated wind speeds. Asymptotic profiles provide a theoretical model for the boundary layer in a typhoon, especially over water. The comparisons also show a reasonable match between the simulated and observed wind speeds over land, though the validity of asymptotic profiles over land still needs further investigation. It could prove very useful for areas that do not have complex topography. The results generally support the BLWTL simulation model. This is particularly significant for the two high-level anemometers examined on the Central Plaza and Di Wang buildings, and hence suggest the current Hong Kong specified upper-level mean wind speed of 64 m/s is unduly conservative.
References [1] G. Lythe, D. Surry, A.G. Davenport, Wind profiles over Hong Kong: results of experiments with a 1:2500 and 1:500 scale model of the area, Engineering Science Research Report, The University of Western Ontario, BLWT-SS15-1981.
M. Pande et al. / J. Wind Eng. Ind. Aerodyn. 90 (2002) 1569–1583
1583
[2] W.H. Melbourne, Design wind data for Hong Kong and surrounding coastline, Proceedings of the Third International Conference on Tall Buildings, Hong Kong and Guangzhou, 1984. [3] Code of Practice on Wind Effects, Hong Kong, Building Development Department, Hong Kong, 1983. [4] Building Department Practice Note for Authorized Persons and Registered Structural Engineers, No. 150, Hong Kong, June, 1994. [5] P.N. Georgiou, Design wind speeds in tropical cyclone-prone regions, Ph.D. Thesis, The University of Western Ontario, London, Ontario, 1985. [6] C.L.K. Lun, The application of the theory of extreme to the estimation of design wind speeds, M.Sc. Thesis, 1970. [7] T.Y. Chen, Comparison of surface wind speeds in Hong Kong, ROHK, Hong Kong, Tech. 41, 1975. [8] E.C.C. Choi, Commentary on the code of practice on wind effects, Hong Kong, 1983. [9] R.N. Sharma, P.J. Richards, A re-examination of the characteristics of tropical cyclone winds, J. Wind Eng. Ind. Aerodyn. 83 (1999) 21–33. [10] E.C.C. Choi, Characteristics of typhoons over the South China Sea, J. Ind. Aerodyn. 3 (1978) 353–365. [11] E.F. Thompson, V.J. Cardone, Practical modelling of hurricane surface wind fields, J. Waterway, Port, Coastal Ocean Eng. 22 (4) (1996) 195–205. [12] P.J. Vickery, P.F. Skerlj, L.A. Twisdale, Hurricane wind field model for use in hurricane simulations, J. Struct. Eng., ASCE (2000) 1203–1221. [13] A.K. Blackadar, H. Tennekes, Asymptotic similarity in neutral barotropic planetary boundary layers, J. Atmos. Sci. 25 (1968) 1015–1022. [14] S.S. Zilitinkevich, Dynamics of the atmospheric boundary layer, Leningrad (in Russian; English translation published 1973), 1970. [15] J.W. Deardorff, Numerical investigation of neutral and unstable planetary boundary layers, J. Atmos. Sci. 29 (1972) 91–115. [16] S.P. Arya, Suggested revisions to certain boundary layer parameterization schemes used in atmospheric circulation models, Mon. Weather Rev. 105 (2) (1977) 215–227. [17] D. Chen, Vulnerability of tall buildings, M.E.Sc. Thesis, The University of Western Ontario, London, Ontario, 1999. [18] Tennekes, H., Similarity Laws and Scale Relations in Planetary Boundary Layers, Workshop on Micrometeorology, American Meteorological Society, 1972, pp. 177–213. [19] A.G. Davenport, The Structure of wind and climate, Proceedings of the Conference on Safety of Structures Under Dynamic Loads, Vol. 1, Trondheim, Norway, 1978, pp. 209–256. [20] T. Amano, H. Fukushima, T. Ohkuma, S. Kawaguchi, S. Goto, The observation of typhoon winds in Okinawa by Doppler sodar, J. Wind Eng. Ind. Aerodyn. 83 (1999) 11–20. [21] ESDU Strong Winds in the Atmospheric Boundary Layer, Part 1: Mean Hourly Wind Speed, Engineering Sciences Data Unit Item No. 82026, London, England, 1982. [22] Y.L. Xu, S. Zhan, Field measurements of Di Wang Tower during Typhoon York, J. Ind. Aerodyn. 89 (2001) 73–93.