Title: A mechanism for extraordinary wind gusts in tropical cyclones
Abstract: Some extreme values of gust factor have been measured within the eyewalls of tropical cyclones, where gust factor is defined as the ratio of peak wind speed to average wind speed. In the famous case of Cyclone Olivia in Australia in 1996, a peak gust factor of 2.75 was measured at 10 m AGL. Similarly, gust factors exceeding 2.0 have been measured where wind turbines have been damaged by landfalling tropical cyclones. Recent large-eddy simulations of idealized tropical cyclones show consistent results, with near-surface gust factor as high as 1.9, but only in a narrow zone between the eye and eyewall of the tropical cyclone. Further analysis of these simulations reveals that the gusts are associated with a coherent vortical structure that is roughly 200 m in scale, similar to the tornado-scale vortices that have been observed with mobile Doppler radars. The peak wind speeds are located on the radially outward part of the misovortex where the flow of the vortex adds to the mean flow of the tropical cyclone. A conceptual model for the formation of these misovortices is under development, and involves both downward transport of high-momentum air that originates near the radius of maximum winds together with tilting of high-vorticity air from the tropical cyclone surface layer. Strong convergence in the tropical-cyclone corner-flow region appears to play a role in amplifying these processes.
Bio: George Bryan’s primary research foci are severe convective weather and numerical model development. His recent studies have focused on turbulent processes in hurricanes and tornadoes, including several articles on the vulnerability of offshore wind turbines to hurricane wind gusts. Bryan has developed novel approaches to simulate convective phenomena using both large-eddy simulation and direct numerical simulation. He is the primary developer of Cloud Model 1 (CM1), a numerical model for idealized research that has been utilized by researchers at institutions around the world. Recently, Bryan has helped collect and analyze data collected by unmanned aircraft systems in the hurricane boundary layer.