7.5 Comparision of Hurricane Wind Data During Hurricane Bonnie

7.5 COMPARISION OF HURRICANE WIND DATA DURING HURRICANE BONNIE: THE TEXAS TECH WIND ENGINEERING MOBILE INSTRUMENTED TOWER EXPERIMENT (WEMITE) AND THE NWS WILMINGTON ASOS

Mark R. Conder*1, Richard E. Peterson1 John L. Schroeder2 and Douglas A. Smith2, 1Department of Geosciences, Texas Tech University, Lubbock, Texas 2Department of Civil Engineering, Texas Tech University, Lubbock, Texas

1. INTRODUCTION

The determination of maximum wind gusts during a hurricane’s passage at a particular site is important when considering the survivability of structures in hurricane-prone regions. However, it is often difficult to obtain wind records owing to anemometer failure, or more often, power failure, which erases some or all of the record. Another difficulty arises in accounting for the exposure of the anemometer which can greatly influence the magnitude of wind gusts recorded. Recent studies, such as by Krayer and Marshall (1993) showed that gust factors from hurricanes might be greater than for other, conventional wind records. The deployment of Texas Tech’s WEMITE tower at Wilmington, NC, during the passage of Hurricane Bonnie has provided an excellent opportunity for the examination of the winds associated with hurricanes. The tower was deployed near the NWS ASOS site for Wilmington and thus a comparison of the two wind records can be performed. This paper will present some preliminary findings from the comparisons of the WEMITE and ASOS wind data Figure 1. A rough diagram of the WEMITE tower. and will use a non-standard technique to estimate the roughness length upwind from the anemometer sites. This technique employs a gust factor model developed 3. DEPLOYMENT FOR HURRICANE BONNIE by Jon Wieringa (1973, 1976) to calculate the roughness lengths from a wind record if the Hurricane Bonnie came ashore near Cape Fear, specifications of the recording instrument are known. NC, on the evening of 26 August 1998. The winds at landfall were approximately 52 m/s. The hurricane 2. THE WEMITE TOWER traveled NE along the NC coast putting Wilmington, NC about 10 km northwest of the eye center at it’s closest point. The track of the storm is pictured in figure 2. The WEMITE tower was designed by John Schroeder, a Ph.D. student in Texas Tech’s Civil Engineering Department, and built by several C. E. students. The tower measures horizontal wind speeds at heights of 10, 20 and 35 feet and measures vertical wind speeds at the 10 and 35 foot heights. Temperature, pressure and relative humidity sensors are located at the base of the tower. The tower is designed to operate in wind speeds in excess of 150 mph and is self-powered – running on current generated by the wind or by the battery backup. The tower is transported from Lubbock to the coast where a hurricane is forecast to make landfall and then setup in an area as open as possible near to the eye’s path. This hurricane season the tower has already been deployed for Tropical Storm Charley, and Hurricanes Bonnie, Earl and Georges. The basic tower design is shown in figure 1. ______* Mark R. Conder, Texas Tech University, Department of Geosciences, Lubbock, TX Figure 2. The path of Hurricane Bonnie (heavy black 79409-2101; email: [email protected] line), 26-27 August 1998. The WEMITE team arrived at Wilmington early on For this study, the measuring characteristics of each the morning of 26 August and set up the tower in a large anemometer need to be known. These are summarized grassy field just to the west of the runways at the New in Table 1. Hanover County Airport. The deployment site and the Wilmington ASOS location are shown in figure 3. TABLE 1

4. INSTRUMENTATION AND DATA ANEMOMETER height distance const. response z L tr The WEMITE tower recorded data for almost 35 WEMITE 10.7 m 2.7 m hours from 05:15 EDT to 16:05 EDT on 27 August. The 0.2 sec peak gust (0.2 sec) recorded at 35 feet (10 m) was 38.2 ASOS 10.0 m 7.9 m 2.0 sec m/s at 15:25 on 26 August. The peak gust is reduced to 33.2 m/s when a 5-second average is taken. In The ASOS data is automatically recorded as a 5-second contrast, the record from the Wilmington ASOS has a gust and a 2-minute mean. The data from both records peak 5-second gust of 33.3 m/s from at 14:27 EDT on were converted to 5-second gusts corresponding to 10- 26 August. The peak gust from both sites occurred minute means for direct comparison. Gust factors (G = when the wind direction was approximately 60 degrees. umax / average U) were then calculated for each record.

The gust factors for the WEMITE tower are graphed in Figure 3. Aerial photo of the deployment site (Castle Hayne, NC Quadrangle, U. S. Geological Survey, 1980) figure 4 and the ones from the NWS ASOS in figure 5. the two instruments are plotted together with wind direction in figure 6.

Figure 4. Gust factors from the WEMITE tower plotted with the peak and mean wind speeds. Figure 6. Gust factors from WEMITE and ASOS with wind direction in degrees.

Another consideration that might increase the gust factors for the WEMITE tower during the last 10 hours of the record is that the mean wind speed decreases to under 8 m/s which may allow thermal turbulence to affect the winds - although it was still overcast during those times.

5. THE GUST FACTOR MODEL

The magnitude of wind gusts are dependent upon the roughness of the terrain and varies with the duration of the gusts (Wieringa, 1996). Wieringa uses a statistical analysis of the gust factor, G (G = umax/ average U) to obtain the following equation for roughness length. Figure 5. Gust factors from the NWS ASOS plotted with the peak and mean wind speeds.  3  ft A[1.42  0.3ln(4 10 /Ut)] zo  z exp  , (1) From the graphs, it is seen that the gust factors   G  1  A  ft A  from the ASOS record average 1.42, staying fairly constant during the period with a slight decreasing where is the median of gust factors, z is the trend. For the WEMITE tower, however, the gust anemometer height, Ut is the average wavelength of factors begin around 1.6 but then jumps up considerably maximum gusts and ft is a factor of the averaging time. to values well over two giving an overall average gust The value for ft is given as 1.0 for 10-minute averages. factor of about 2.0. This is due to the backing of the The attenuation of the wind gusts by the anemometer, A wind as the hurricane passes which brings a much is given by: rougher upstream fetch to the WEMITE tower. The 1  largest “jump” occurs in the 22:00 to 02:00 EDT time A  [(1  [2L /Ut]2 (1  [2t / t]2 )] 2 , (2) frame and corresponds to a change in the wind direction r from NE to NW. The wind direction is about 300 degrees by 01:00 EDT. Looking at the aerial photo in where L is the distance constant of the figure 3, although there are scattered obstacles anemometer, tr is the response time of the data recorder (buildings) to the NE of the tower’s position, once the and t is the gust period. Using the values for the wind changes to the NW, there is a dense forest upwind anemometry given in table 1 an estimation of Ut can be at a distance of 300 meters. The photo shows that the made using graphs provided by Wieringa (1976, 1980). exposure for the ASOS does not change appreciably It should be noted that while the formula is intended for with a change from NE to NW. The gust factors from use with the median of gust factors from a particular set of directions, for this study the formula will be evaluated for each 10-minute gust factor. Roughness lengths are roughness length. The roughness lengths calculated in then calculated and the results are shown in figures 7 this study by the Wieringa gust model are summarized and 8. in the following table.

TABLE 3

Site avg. roughness length Stand. Dev. WEMITE 0.59 m 0.58 m ASOS 0.30 m 0.24 m

Although the conversion of roughness length to a terrain classification is still a developing “art”, an estimation of the WEMITE data would result in a “forest” classification while from the ASOS data a “suburban” or “bushland” classification would result (Wieringa, 1993). While in each case the anemometer was located in an open exposure for approximately 200 meters, the roughness lengths obtained imply a more turbulent nature of the wind. Thus, the roughness may be more dependent upon terrain further upwind than currently Figure 7. Roughness lengths from the WEMITE tower. accounted for. Another consideration is that the graphs given by Wieringa are for mean winds speeds under about 20 m/s which makes estimation of the value for Ut difficult for hurricane cases.

7. FUTURE WORK

The WEMITE tower has records from anemometers at 6.09 meters and 3.05 meters and a comparison with those should be done. An internal boundary layer might appear in such a comparison. A comprehensive characterization of the surrounding terrain of the anemometer must be done to determine to what extent the local topography will affect the estimated roughness. While the gust factor model is based upon a Gaussian distribution of the wind about the mean, this may not be an accurate assumption for hurricane winds and should be considered. Figure 8. Roughness lengths from the NWS ASOS. 8. ACKNOWLDGEMENTS 6. SUMMARY The authors wish to thank the personnel at the A comparison of gust factors is given in table 2. Wilmington NWS Forecast Offices for providing the ASOS wind data. TABLE 2 Funding for this research is provided by NSF GRANT CMS-9409869 through the CSU/TTU Cooperative Wind Site or study average GF(5-s to 10-min) Engineering Program and is greatly appreciated. Wilmington (ASOS) 1.42 Wilmington (WEMITE) 1.68 9. REFERENCES Durst (1960) 1.35 Krayer and Marshall (1992) 1.49 Marshall, R. D. and W. R. Krayer, 1992: Gust factors applied to hurricane winds. Bull. Amer. While the value for the Wilmington ASOS fits well Meteor. Soc., 73, 613-617. within the range of gust factors determined from the other studies, the value of 1.68 from the WEMITE tower Wieringa, J., 1973: Gust factors over open water and seems very high. This may be indicative of the built-up country. Boundary Layer Meteorology, 3, increasingly long route of heavily wooded terrain the 424-441. wind traverses before reaching the anemometer. This can be seen in figure 6 where the gust factor is between Wieringa, J., 1980: Representativeness of wind 1.4 and 1.5 for the period before the wind backs to the observations at airports. Bull. Amer. Meteor. Soc., NW. This fetch should be associated with a longer 61, 962-971. Wieringa, J., 1993: Representative Roughness Parameters for homogeneous terrain. Boundary Layer Meteorology, 63, 323-363.