A mechanism for extraordinary wind gusts in tropical cyclones

George Bryan1, Richard Rotunno1, and Daniel Stern2

1National Center for Atmospheric Research, Boulder, Colorado, USA 2University Corporation for Atmospheric Research, Monterey, California, USA

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 to average wind speed. In the famous case of Cyclone Olivia in 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 and eyewall of the . 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 -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.

Correspondence to:

George H. Bryan, NCAR/MMM 3090 Center Green Drive Boulder, CO 80301, USA

Email: [email protected] Phone: 303-497-8989