Miniature supercells in Hurricanes Gustav (2008) and Ivan (2004): Environments and cell structure

Tornadoes spawned within the outer rainbands of landfalling tropical cyclones regularly pose a threat to coastal regions. These tornadoes, while less intense than their Great Plains counterparts, can still cause considerable loss of live and property damage. Much of the challenge faced by local forecasters during a TC landfall is to identify which convective cells will become tornadic. If common distinguishing features of the tornadic cells can be identified while the cells are still offshore, then forecasters may be able to increase their lead time and better forewarn the public. The overall objective of this study is to document the structure of offshore convective cells, in particular miniature supercells, before they move onshore and spawn tornadoes.

Hurricane Ivan made landfall along the Gulf coast early in the morning on 16 September 2004 as a Category 3 hurricane. Over 115 tornadoes were reported as Ivan crossed the southeast United States. The most significant tornadic activity occurred in the afternoon and evening of 15 September when an intense outer rainband developed ~350km east of the storm center and produced 14 tornadoes over the Florida panhandle. Hurricane Gustav made landfall along the Louisiana coast on the morning of 1 September 2008 as a Category 2 hurricane. Over 40 tornadoes were reported as Gustav crossed the lower Mississippi Valley region. The most significant tornadic activity occurred in the afternoon and evening of 1 September when multiple intense outer rainbands developed ~350 km east of the storm center and produced 22 tornadoes over southern Alabama, Mississippi, and Louisiana.

This study documents the local environments and three-dimensional structure of convective cells embedded within each hurricane's outer rainbands when the cells were more than 100 km offshore. Extensive observational datasets composed primarily of airborne NOAA WP-3D Doppler radar data and then supplemented with flight-level, dropsonde, rawinsonde, buoy, and WSR-88D radar data are used. The three-dimensional winds and convective structure within nine deep convective cells are documented via pseudo-dual-Doppler analyses. The majority of cells exhibited supercell characteristics (i.e., hook echoes and rotating updrafts though the lower and mid-levels), but Ivan's cells contained stronger low-level updrafts and mesocyclones. Both environments were characterized by moderate CAPE with relatively dry mid-levels, but the 0-6 km vertical shear was much stronger in Ivan, suggesting that the local vertical shear may be a critical factor in offshore miniature supercell development. A synopsis of our preliminary results will be presented at the conference.