Mini-supercells observed in an offshore outer rainband of Hurricane Ivan (2004)

Tornadoes spawned within the outer rainbands of landfalling tropical cyclones regularly pose a great threat to coastal regions. These tornadoes, while less intense than there Great Plains counterparts, can still cause considerable loss of live and property damage. In fact, between 1970 and 2000, tornadoes were responsible for ~5% of all deaths and ~13% of the total insured damage caused by landfalling hurricanes along U.S. coasts. Much of the challenge faced by forecasters is to identify which convective cells will become tornadic out of the hundreds that often move onshore. If common distinguishing features of the tornadic cells can be identified while the cells are still offshore and nontornadic, then forecasters may be able to increase their lead time and better forewarn the public. The objective of this study is to document the structure of such offshore convective cells within outer TC rainbands.

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 ~400km east of the storm center and produced 14 tornadoes over the Florida panhandle.

This study documents the three-dimensional structure of convective cells embedded within Hurricane Ivan's outer rainband when the cells were more than 100 km offshore. An extensive observational dataset composed primarily of airborne NOAA WP-3D Doppler radar data and then supplemented with WSR-88D radar, rawindsondes, GPS dropsondes, and aircraft flight-level data is employed. The three-dimensional winds and convective structure within three deep convective cells are documented via dual-Doppler analyses. The thermodynamic structure is inferred from flight-level, GPS dropwindsonde, and surface (buoy) observations. Each cell exhibited supercell characteristics (i.e., rotating updrafts and hook echoes) well offshore. The local environment was characterized by moderate low-level helicity and CAPE with relatively dry mid-levels. Two of the three cells produced tornadoes soon after moving onshore over 2 hours later. A synopsis of our preliminary results and their comparison with previous studies will be presented at the conference.