A Wind Gust Factor Database and the Development of Associated GFE Tools Jonathan Blaes 16 November 2012

The Problem with Forecasting TC Wind Gusts Lack of science Varying forecast strategies and tools Limited external consistency Limited collaboration Limited shift to shift forecast consistency Time/efficiency constraints TC Wind forecasts are a mess too!

Examining Gust Factors We examined the sustained winds, wind gusts, peak winds, and gust factors for ten tropical cyclones. A table of the tropical cyclones examined along with a map of their tracks is shown. Storm NameDateLandfallWinds at LandfallGF Obs Irene27-Aug-11NC85 MPH1055 Hanna6-Sep-08NC/SC70 MPH917 Ernesto30-Aug-06South FL70 MPH1107 Gaston29-Aug-04SC75 MPH1164 Charley13-Aug-04FL75 MPH290 Isabel18-Sep-03NC105 MPH3095 Floyd16-Sep-99NC105 MPH1283 Dennis4-Sep-99NC105 MPH5045 Fran5-Sep-96NC115 MPH551 Bertha12-Jul-96NC105 MPH431

Methods Observations of winds and wind gusts from between 21 and 45 ASOS or AWOS METARs were collected for each storm Locations varied for each storm, tried to capture the variations in the wind field. Only used hourly METARs, no special observations The hourly wind gust factor (GF) was computed as the ratio of the wind gust to the sustained wind speed. Total of 14,938 gust factors were computed The various station locations and TCs result in a data set that contains varied sensor types, roughness length, exposure, stability, wind trajectory, etc.

Sustained Wind vs. Gust Factors

Gust Factor Frequency

Wind Speed vs. Gust Factor Occurrences

Mean Gust Factor The mean gust factor value of the 10 storms examined was 1.47 which compares closely to several previous studies.

Gust Factor Regression Equations

Notes from the Gust Factor Database Minimum GF observed was ~ 1.1 with only 17.9% or 2,678/14,938 observations with a gust factor of less than 1.2. The GF is varied at low sustained wind speeds but generally converges and decreases with increasing sustained wind speed. A one size fits all gust factor does not apply Large variation in GF with changes in sustained winds. Sustained winds MPH – GF typically range from 1.2 to 1.6 Sustained winds MPH – GF typically range from 1.1 to 1.4 Sustained winds MPH – GF typically range from 1.1 to 1.3 Large fraction (41.5%) of GF occur in the 1.2 to 1.6 range The average GF among the 14,938 observations was A regression curve was developed: y = ln(x)

TCWindGust and WindGust_from_WindGustFactor tools This dataset has spawned the development of several GFE tools Two pathways 1)directly create the WindGust grid based on GF x Wind 2)create semi persistent WindGustFactor grids are used to create the WindGust grid based on GF x Wind (next talk)

TCWindGust and WindGust_from_WindGustFactor tools Both methods provide the user with multiple GF options. 1) “CSTAR Regression" is based on a regression equation developed at RAH from 14,938 gust factors 2) "CSTAR Mean of 1.47" based on the same RAH data set but uses a simple average of all GFs. 3) The remaining options are based on research by Harper et al. (2008) which provided examined the peak gust for various exposures.

Hurricane Sandy Examined 12 ASOS/AWOS in NC/SC Total of 1,369 gust factors Sustained winds rather weak Mean GF of 1.58 y = ln(x)

Hurricane Sandy Sandy shown in bold red

Next Steps Continue refinements to the tools How to handle winds less than 10 MPH? GF of 1.55 How to handle higher end winds >70 MPH as GF asymptotes out toward 1.0. GF of 1.2 is reached at 76.6 MPH while a GF of 1.15 is reached at MPH Examine over water GF and incorporate into tools Need to add quantitative data for over water environments. AKQ has a dataset or we can build upon prior research which suggest over water GF ranges from 1.1 to 1.2 Examine GF associated with Sandy in NJ/NY Develop training materials Explore some sort of verification Test the methodology at WFOs ILM, RAH, MHX, and MFL during the 2013 TC season

Acknowledgements Thanks to student volunteer Dan Brown for examining nearly all of the ~15K gust factors and generating most of the charts as well. NC State student volunteers Rebecca Duell and Lindsey Anderson helped develop the overall methodology and completed the analysis of Hurricane Irene. Bryce Tyner and Dr Aiyyer Collaborative Investigator Reid Hawkins Rest of the CSTAR TC Winds team

References Durst, C. S., 1960: Wind speeds over short periods of time. Meteor. Mag., 89, 181–186. Conder, M. R., and R. E. Peterson, 2000: Comparison of Gust Factor Data from Hurricanes. Preprints, 24th AMS Conf. on Hurricanes and Tropical Meteor. Fort Lauderdale, FL. J53-J54. [Available online at Krayer, William R., Richard D. Marshall, 1992: Gust factors applied to hurricane winds. Bull. Amer. Meteor. Soc., 73, 613– 618. [Available online at CO;2] 2.0.CO;2 Paulsen, B. M., J. L. Schroeder, 2005: An Examination of Tropical and Extratropical Gust Factors and the Associated Wind Speed Histograms. J. Appl. Meteor., 44, 270–280. [Available online at Powell, Mark D., Samuel H. Houston, Timothy A. Reinhold, 1996: Hurricane Andrew's Landfall in South Florida. Part I: Standardizing Measurements for Documentation of Surface Wind Fields. Wea. Forecasting, 11, 304–328. [Available online at CO;2http://dx.doi.org/ / (1996) CO;2] Schroeder, J. L., M. R. Conder, and J. R. Howard, 2002: "Additional Insights into Hurricane Gust Factors,” Preprints, Twenty- Fifth Conference on Hurricanes and Tropical Meteorology, San Diego, California, Vickery, P.J., and P.F. Skerlj, 2005,: Hurricane gust factors revisited, J. Struct. Eng., 131, [Available online at NEW/Hurricane/Hurricane_Gust_Factors_Revisited.pdf] NEW/Hurricane/Hurricane_Gust_Factors_Revisited.pdf Yu, Bo, Arindam Gan Chowdhury, 2009: Gust Factors and Turbulence Intensities for the Tropical Cyclone Environment. J. Appl. Meteor. Climatol., 48, 534–552. [Available online at

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