Analysis of offshore deep convection within landfalling hurricanes just prior to tornadogenesis

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 come ashore. 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 defining characteristics of tornadic convection prior to tornadogenesis within the rainbands of landfalling tropical cyclones.

This study utilizes an extensive observational dataset composed primarily of land-based NEXRAD WSR-88D and airborne NOAA WP-3D Doppler radar data and then supplemented with surface (both onshore and offshore), rawindsondes, GPS dropsondes, and aircraft flight-level data from Hurricanes Ivan and Jeanne of 2004. The three-dimensional wind fields of deep convective cells within offshore hurricane rainbands are documented via dual-Doppler analyses. The thermodynamic structure is inferred from flight-level, GPS dropwindsonde, and surface (buoy) observations. Several cells exhibit supercell characteristics (i.e., rotating updrafts and hook echoes) offshore and two produced tornadoes soon after moving onshore. The same cells, viewed from the land-based WSR-88D radars, exhibit higher echo tops (ETs), greater vertically integrated liquid (VIL), greater rotational shears, and higher spectral widths than nontornadic cells while still offshore. A synopsis of these preliminary (and additional) results will be presented at the conference.