Analysis of Miniature Supercells Observed in the Outer Rainbands of Hurricane Gustav (2008)

Observations made by airborne Doppler radar on 1 September 2008 of the outer rainbands of Hurricane Gustav are used to document the structure of ten convective cells. Each cell was located over 50 km offshore and beyond the range of coastal Doppler radars. The cells were distributed through rainbands approximately 300 km away from the storm center in the right-forward and right-rear quadrants of the cyclone. Each cell developed in minimal environmental CAPE (<1500 Jkg^-1) and wind shear (< 10 ms^-1). As a result, the observed cells showed variation in structure and mesocyclogenesis. Of the four rainbands, nine cells were identified as distinct single-cell and compared to linear rainband convection. The cells were between 2 to 5 km in diameter and 6-8.5 km deep. Cells in the right-rear rainbands did not develop a mesocyclone. The front-forward quadrant cells developed updraft maxima of 7 ms^-1 and one with a vorticity maxima of 0.007 s^-1. Mesocyclones developed in these six cells and at the time of observation, each cell displayed unique structure. Mesocyclogenesis occurred primarily as a result of tilting and stretching of low-level horizontal streamwise vorticity into the vertical by the updrafts. Mesocyclones did not develop in the right-rear cells. These observations add to the spectrum of offshore miniature supercells in the outer rainbands of tropical cyclones.