Vertical Profiles of Thermodynamic Variables in Hurricanes Bonnie (1998) and Mitch (1998): Implications for Energy Transport into the Inflow Layer

The Global Positioning System (GPS) dropwindsonde samples at 2 Hz, providing 6-7 m vertical resolution in the inflow layer to a hurricane. Eighty-five GPS sondes deployed throughout Hurricane Bonnie (964 hPa) and 40 sondes dropped in and near the eyewall of Hurricane Mitch (930 hPa) are used to examine the temperature structure adjacent to the sea surface. My goals are to: (1) determine which thermodynamic structures are real, and, (2) interpret these structures in light of the energy transfer into the inflow layer, which is vital to the intensity of the hurricane.

Horizontal fields of temperature developed for Bonnie are interpreted with the benefit of reflectivity data from the 10 cm WSR-88Ds at Morehead City and Wilmington. Three main points are apparent. First, there is cool air collocated with the upwelled water in the right rear quadrant of the storm. Here I have compared the 10 m T field with a SST map derived from 13 AXBTs. Second, there is a cool annulus collocated with the eyewall. The collocation with the intense echoes favors downdrafts as the leading cause. Third, the warm air streaming from the continent cools as it approaches the eyewall. Air in the offshore flow does not contain any rain, but does have relative humidity of 70 to 80% and wind speeds in excess of 18 m/s. This makes a strong case for the dry air being cooled by the evaporation of spray, and not downdrafts. In the eyewall region relative humidity often exceeds 95% which inhibits evaporation of spray and attendant cooling.

The GPS sondes deployed in Mitch are in near surface winds exceeding 65 m/s. Many of the drops in and near the eyewall contain a superadiabatic layer in the lowest 100 m. Several of the drops in the high wind region of Bonnie also have a similar structure. The unstable lapse rate is interpreted as supporting evidence that spray rapidly surrenders its heat but undergoes little evaporation.

The GPS observations will be used to place the work of Byers (1944), Korolev et al. (1990), Pudov (1992), Andreas (1995), Cione et al. (2000) and Barnes and Bogner (2001) in a unifying context. It will be shown that the isothermal assumption applied from a distant environmental location several degrees from the eyewall is not justified, but near the eyewall the inflow is receiving enough heat to counter adiabatic expansion. The possible roles of spray and dissipative heating will be discussed.