Global Journal of Researches in Engineering Civil And Structural Engineering Volume 13 Issue 7 Version 1.0 Year 2013 Type: Double Blind Peer Reviewed International Research Journal Publisher: Global Journals Inc. (USA) Online ISSN: 2249-4596 & Print ISSN: 0975-5861 Engineering Applications of the Newly Available Roughness- Length Measurements by AOML at 213 ASOS Stations By S. A. Hsu Louisiana State University, United States Abstract - Most recently, the Hurricane Research Division of the U. S. Atlantic Oceanographic and Meteorological Laboratory (AOML) has made extensive surveys of the roughness length (Zo) in each of the 213 Automated Surface Observation Stations (ASOS) located in tropical-cyclone prone regions. The original 8 values of Zo for each of the 45 degree segments within the 360 degree compass in each ASOS station are averaged geometrically to obtain one typical value for each of these 213 ASOS stations. Six ASOS stations are verified independently by the gust factor method during 5 hurricanes. Since the difference is within the 10 % composite accuracy for field measurements in wind speed, the computed geometric mean Zo values for each of the 213 ASOS stations are recommended for practical use. Applications of the proposed geometric mean Zo value to estimate the 3-second gust, peak gust, and peak factor during Hurricane Katrina are also provided for engineers as an example. Keywords: roughness length; hurricanes; asos stations, turbulence intensity; power-law exponent; gust factor; peak factor. GJRE-E Classification : FOR Code: 0 05 EngineeringApplicationsoftheNewlyAvailableRoughnessLengthMeasurementsbyAOMLat213ASOSStations Strictly as per the compliance and regulations of : © 2013. S. A. Hsu. This is a research/review paper, distributed under the terms of the Creative Commons Attribution- Noncommercial 3.0 Unported License http://creativecommons.org/ licenses/by-nc/3.0/), permitting all non commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Engineering Applications of the Newly Available Roughness-Length Measurements by AOML at 213 ASOS Stations S. A. Hsu 3 Abstract - Most recently, the Hurricane Research Division of aviation safety reasons. However, since the wind 01 the U. S. Atlantic Oceanographic and Meteorological direction in a tropical cyclone is rotational in nature and 2 Laboratory (AOML) has made extensive surveys of the since the strongest wind may come from any direction, it roughness length (Zo) in each of the 213 Automated Surface is not necessary for practical operation to have 8 Zo for Year Observation Stations (ASOS) located in tropical-cyclone prone each ASOS. Instead, a typical Zo value or the geometric regions. The original 8 values of Zo for each of the 45 degree 13 segments within the 360 degree compass in each ASOS mean for each ASOS is needed for most engineering station are averaged geometrically to obtain one typical value applications. Therefore, the original list which consists of for each of these 213 ASOS stations. Six ASOS stations are 8 Zo values for each ASOS is geometric averaged. Our verified independently by the gust factor method during 5 results are provided in the Appendix with one geometric hurricanes. Since the difference is within the 10 % composite mean Zo for each of the 213 ASOS. accuracy for field measurements in wind speed, the computed VII Version I geometric mean Zo values for each of the 213 ASOS stations III. Validating the Rela tion between are recommended for practical use. Applications of the zo, Gust Factor and Turbulence proposed geometric mean Zo value to estimate the 3-second gust, peak gust, and peak factor during Hurricane Katrina are Intensity also provided for engineers as an example. Keywords: roughness length; hurricanes; asos stations, According to Panofsky and Dutton (1984, XIII Issue turbulence intensity; power-law exponent; gust factor; pp.130-131), it is common in engineering practice to peak factor. describe the variation of the wind speed with height, i.e. Volume the wind profile with a power law such that ) D D DD I. Introduction E ( U 2 /U 1 = (Z2/Z1)^ p (1) ost recently, in its “Tropical Cyclone Wind Exposure Documentation Project”, the According to Hsu (2008), Hurricane Research Division (HRD) at the M P = 0.2996 Zo ^ 0.168 (2) Atlantic Oceanographic and Meteorological Laboratory (AOML), U. S. National Oceanic and Atmospheric Where U2 and U1 are the wind speed at height Administration (NOAA), has made an extensive survey Z2 and Z1, respectively, p is the power-law exponent, and monitoring of the roughness length (Zo) at 213 and Zo is the roughness length. Automated Surface Observation Stations (ASOS). Most Now, for each ASOS Station the appropriate ASOS are located at hurricane-prone airports (see value of p based on Eq. (2) is also provided in the www.aoml.noaa.gov/hrd/asos/index.html). Because Zo Appendix. is a parameter needed for wind and turbulence According to Hsu (2013), for 5 second gust over estimates for civil, structural and environmental the 2 minute duration, which is available routinely from engineers (see, e.g., Hsu, 2013), the purpose of this the wind speed measurements by ASOS, we have study is to utilize these newly available Zo measure- G = 1 + 2.04 P (3) ments by AOML for engineering applications. Global Journal of Researches in Engineering = 1 + 2.04 TI (4) II. Geometric Mean zo for each ASOS Where G is the gust factor (the ratio of 5-s gust Environment to 2-min sustained wind speed) and TI represents the According to AOML the 360 degree compass for the longitudinal turbulence intensity. wind direction measurement is divided into 8 segments A forementioned equations are validated as follows : so that there is one Zo value for each 45 degrees at each ASOS. These 8 Zo values may be needed for a) Composite Field Accuracy for Wind Speed Measurements Author: Professor Emerit us and Certified Consulting Meteorologist Coastal Studies Institute, Louisiana State University. In order to validate Equations (1) thru (4), one e-mail: [email protected] must first determine the composite field accuracy for the © 2013 Global Journals Inc. (US) Engineering Applications of the Newly Available Roughness-Length Measurements by AOML at 213ASOS Stations wind speed measurements. According to U. S. National b) Validation during Hurricane Bonnie in 1998 Data Buoy Office (see http://www.ndbc.noaa.gov/rsa. On the basis of Tables 2 and 3 and Fig.1, we shtml), The composite accuracy of field measurements can say that the geometric mean Zo for KILM as listed in for the wind speed and wind gust is +/- 10 %. In other the Appendix is valid for engineering applications. words, if the difference between measurements and Furthermore, it is shown that p =TI. estimates related to wind and gust characteristics is Table 2 : Comparisons of wind speed measurements by within 10 %, one may accept those estimates as ASOS and Texas Tech University at Wilmington Airport reasonable. Note that this 10 % margin of error can also (KILM), North Carolina, USA, during Hurricane Bonnie in be related to the different anemometers used in the 1998 field. An example is shown in Table 1. Table 1 : Comparison of generated longitudinal ASOS Texas Tech turbulence intensity (TI) from different anemometers Station Station 2013 0.2-Second Gust (m/s) NA 38.2 during Hurricane Bonnie in 1998 3-Second Gust (m/s) NA 33.6 Year (1). UVW (2). Propeller- Difference Mean TI 5-Second Gust (m/s) 32.9 33.5 1-minute Sustained (m/s) NA 25.0 14 anemometer Vane between between anemometer (1) and (2) (1) and (2) 2-minute Sustained (m/s) 25.2 24.4 0.175 0.195 10 % 0.185 (Data Source : Schroeder, 1999). (Data source: Schroeder, 1999) rsion I 35.00 30.00 y = 1.420x VII Ve 25.00 R² = 0.920 20.00 15.00 XIII Issue v 10.00 5.00 Volume 0.00 ) E DDDD second gust, m/s 0.00 5.00 10.00 15.00 20.00 25.00 ( - 5 2 - minute sustained wind speed, m/s Figure 1 : Win d speed and gust measurements at ASOS Station at Wilmington Airport, N.C., USA, during Hurricane Bonnie in 1998 Table 3 : A comparison of measurements against 3 coming ashore near Pass Christian, MS, the aircraft estimates of p using Eq. (3) and the geometric mean of measure- ments of maximum wind speed was 68.4 m/s Zo from Appendix at Wilmington Airport during at 350 m and at the near-surface (77m) it dropped down Hurricane Bonnie in 1998 to 47 m/s. Therefore, according to Eq. (1), we have Source (1). P based (2). P from Difference (68.4/47) = (350/77) ^ p on either Appendix between So that p = Ln (68.4/47)/Ln (350/77) = 0.248 (5) measured or for KILM (1) and (2) estimated Since this value is identical to that at KASD for Slidell Airport, LA (which is not very far from Pass UVW, Table 1 0.175 0.185 0.054 Global Journal of Researches in Engineering Propeller-vane, 0.195 0.185 0.051 Christian), as provided in the Appendix, we can say that Table 1 the geometric mean Zo for KASD is verified for practical ASOS, Table 2 0.150 0.185 0.189 use. Note that, during Katrina, nearly all surface wind TTU, Table 2 0.183 0.185 0.011 measurements were not available because of massive Fig. 1 0.206 0.185 0.102 power failure. Therefore, these aircraft measurements by Mean 0.182 0.185 0.016 U.S. Air Force Hurricane-Hunters are greatly appreciated. c) Validation during Hurricane Katrina in 2005 d) Validation during Hurricane Rita in 2005 According to Henning (see http://ams.confex.