Gust factors and surface-to-gradient wind-speed ratios in tropical cyclones

Digital wind-speed data from tropical cyclones that have impacted the United States during the last decade have been analysed. Using data from automatic weather stations, dropsondes, radiosondes, and Doppler radar, relationships between surface and gradient wind speeds have been determined. In addition, gust factors have been computed for offshore, coastal, and inland sites. It has been found that the wind-speed characteristics of tropical cyclones are essentially the same as extra-tropical cyclones. Unusually high gust factors have been found, but they appear to be rare, and are usually associated with spiral rain-bands feeding into the tropical cyclone, rather than the main body of the storm. It has also been found that, as a result of unjustified assumptions regarding the relationship between wind speeds over the sea and over the land, the design wind speeds in some coastal areas of the United States are probably overstated by approximately 25%.