ABSTRACT

Small-scale vortices associated with damaging wind gusts in hurricanes have been documented in several observational studies.  These vortices have a horizontal extent of roughly 100 m to 1 km and have been likened to tornadoes because of their size and extreme wind speeds.  In this study a hierarchy of large-eddy simulations is used to document the dynamics of these vortices.  Composite analysis at the time of peak near-surface wind speed shows how they are associated with a coherent vortical structure that resembles one-half of the famed “hairpin vortex” that is a characteristic of turbulence in sheared boundary layers.  In this case, the peak wind speeds are located on the radially outward part of the misovortex where the circulation of the misovortex adds to the mean flow of the hurricane.  A conceptual model for the formation of these misovortices involves three main components: 1) downward transport of high-momentum air from the level of maximum winds; 2) upward tilting of high-vorticity air from the highly sheared surface layer; and 3) amplification via a misovortex-scale secondary circulation.  Simulations also show that these vortices can form within the eyewall of hurricanes and then move towards the relatively calm eye, which explains observations of extraordinarily high “gust factors” and non-Gaussian wind-speed distributions in some hurricanes.