SAR (synthetic aperture radar) observations in Hurricanes Bonnie, Danielle, Georges, and Mitch in 1998 clearly demonstrate secondary circulations in the atmospheric boundary layer (roll vortices) in the region between the main rainbands of a hurricane. These roll vortices contribute significantly to the vertical exchange of momentum, heat, and moisture in the atmosphere. The mechanisms of their formation and effects on the structure and dynamics of the planetary boundary layer in hurricanes are investigated here.

A high-resolution 2-dimensional (cross-large scale flow) boundary layer model is set up (5 km wide and 3 km height) and a series of experiments are conducted aiming to simulate horizontal rolls in high wind conditions. Initial vertical velocity, temperature, etc. profiles are used from the GFDL hurricane model. The model simulations indicate the development of roll vortices in the boundary layer on a time scale of a few hours and a spatial scale of ~2 km that are superimposed on the mean wind filed. They are generated mainly by thermal and dynamic instability in the boundary layer. The roll layer is coincided with the depth of the boundary layer. The associated convective fluxes affect greatly the vertical profiles in the boundary layer and air-sea fluxes.