Thermodynamic structure of a hurricane's lower cloud and subcloud layers

The Global Positioning System dropwindsonde (GPS sonde) provides unprecedented 2 Hz vertical resolution of the subcloud and lower cloud layers in the extreme conditions associated with a hurricane. The GPS sondes deployed in Bonnie (1998), Mitch (1998), and Humberto (2001) have been examined to determine the performance of the temperature and moisture sensors with the ultimate desire to establish the most likely equivalent potential temperature structure from 2000 m altitude to the sea surface. The specific goals are: (1) identify and correct what portions of the soundings that have been compromised by the presence of cloud droplets or spray, and (2) determine what processes are affecting the energy content of the inflow layer. The inflow layer has been argued to control hurricane intensity (e.g., Malkus and Riehl 1960, Emanuel 1986).

Temperature profiles usually contain a dry adiabatic layer in the lowest 200 m. As winds increase above hurricane force there often appears a prominent super-adiabatic layer that can extend up to 80 m above the sea. The two possible causes of this strong lapse rate are the rapid loss of the sensible heat from spray and viscous dissipation. Water loading from spray can easily reduce this apparent unstable layer to neutral conditions.

Moisture profiles are more problematic. Sondes that fall through thick cloud often remain saturated for hundreds of m, even through a dry adiabatic layer. This structure is considered erroneous and a recommendation is offered to mitigate the problem. As the sonde nears the sea, moisture content increases for approximately 50% of the vertical profiles. The source of the moisture in these profiles must be the sea rather than cloud or rain. There are three mechanisms potentially responsible for this structure which includes spray adversely affecting the moisture sensor, evaporation of spray into the surface layer, and enhanced interfacial fluxes. It will be shown that the evaporation of spray, at least within 100 km of the circulation center, is not the cause since this process would not yield the observed dry adiabatic layer and strong vertical gradient of equivalent potential temperature. The relative humidity in this region is also already quite high (>90%), limiting the amount of spray evaporation. Surface fluxes are believed to be the most likely cause of the high equivalent potential temperature found near the sea surface.

Horizontal maps of temperature, moisture, and equivalent potential temperature are generated that reveal coherent mesoscale structures. Inflow is non-isothermal, relative humidity in the core region approaches 95%, and the equivalent potential temperature rapidly increases adjacent to the eyewall. The eye is a reservoir for high energy air below the hub cloud top. Near the eyewall the muted vertical gradients of equivalent potential temperature are conducive to the rapid energy increase of the inflow. Above the inflow a layer often exists that inhibits the loss of energy into the middle troposphere through either entrainment or the formation of small clouds.