Hurricanes that do and do not spawn tornado outbreaks: Offshore traits and their evolution at landfall

Are there differences in the vortex-scale stability and vertical shear of the horizontal wind from one hurricane to the next that allow one to forecast when there will be an outbreak of tornadoes? Can the flight patterns used to improve hurricane track forecasts supply the insights to make such a forecast? To explore these issues we examine the GPS sondes deployed from USAF and NOAA aircraft and the rawinsondes launched by the NWS sites prior to and during landfall for two hurricanes: Ivan (2004) and Dennis (2005). Both hurricanes were a category 3 at landfall and their circulation centers struck the coast only 60 miles apart along the Gulf of Mexico. Dennis generated about 10 tornadoes with 9 that were rated F0 on the Fujita scale, while Ivan produced at least 52 tornadoes within a day of landfall. The favored region for tornadogenesis was either ahead or to the right of track, approximately 200 to 600 km from the circulation center.

Both hurricanes were sampled during landfall as well as the previous day. Within two days of landfall aircraft deployed about 136 GPS sondes in Dennis and 131 in Ivan. About 20 NWS rawinsonde sites within 1000 km of the hurricane track supply another 80 soundings for each hurricane. The minimum sea-level pressure varied considerably in Dennis as it crossed over Cuba but it was deepening and achieved 930 hPa prior to landfall. Ivan was steadier, varying between 940 and 930 hPa till landfall.

We intend to use these sondes to explore the following issues:

(1) Are there differences between Dennis and Ivan in convective available potential energy, lower tropospheric shear, lifted index and the bulk Richardson number?

(2) How do these variables differ across the track of each hurricane?

(3) How do these key variables evolve as the hurricane makes landfall?

(4) What kind of synoptic pattern is the hurricane interacting with as it makes landfall?

(5) What is the depth of the vertical shear of the horizontal winds during landfall?

We believe that this is one of the first investigations that uses the GPS sondes jettisoned from the NOAA G-IV high altitude aircraft, deployed primarily to improve track forecasts, to explore the vortex-scale environment that impacts tornadogenesis. We will assess if the typical patterns flown by the G-IV can pay dividends regarding the forecasting of tornado outbreaks associated with a hurricane. This is also the first study of its kind to compare two hurricanes using data over the ocean and land. The findings will be discussed in light of prior work including Novlan and Gray (1974), Gentry (1983), McCaul (1991), McCaul and Weisman (1995), Spratt et al. (1997), Bogner et al. (2000) and McCaul et al. (2004).