HURRICANE EYE THERMODYNAMICS

Principal Investigator: H.E. Willoughby

Objective: Improved physical understanding of the eye's role in hurricane intensification.
Rationale: The eyes of hurricanes lie at their geometric centers and also plays a central role in the dynamics. Although hurricanes' energy derives from the warm underlying sea, their low pressure and strong winds in result from tightly coupled dynamic and thermodynamic processes, not simple vertical mixing of warm air. On one hand, understanding of the physics of eye formation is essential to explain the rarity of extremely intense hurricanes over a summertime tropical ocean that is invariably warm enough to support very low sea-level pressures. On the other hand, this understanding is equally essential to explanation rapid deepening, the rare process by which some hurricanes approach thermodynamic equilibrium with the oceanic energy source.
Method: Analysis of thermodynamic soundings, flight--level observations, and photographic images in the eyes of hurricanes.
Accomplishment: In an intense tropical cyclone, the central sea-level pressure might be 50-100 hPa lower than that outside the vortex. Warming by dry subsidence inside the eye causes the 10-30 hPa of this total pressure fall that occurs between the eyewall and the center of the eye. In balanced models and in nature, thermally forced flow from the lower part of the eye into the eyewall forces the subsidence as the eye contracts during intensification.

Eye soundings (Figure 1) show warm and dry air aloft, separated by an inversion from cloudy air below. The equivalent potential temperature at the inversion is often > 10 degrees cooler than that in the eyewall. Dewpoint depressions at the inversion are 10-30C rather than the 100C that would occur if the air originated at the tropopause and sank with no mixing with the surrounding cloud. The temperatures and dewpoints above the inversion can, however, be derived by 100 hPa of undilute dry subsidence from an initial sounding that is just slightly more stable than a moist adiabat.

I hypothesize that the air above the inversion has remained in the eye since it was enclosed when the eyewall formed and that it has subsided only 1-2 km since that time ( Figure 2 ). The cause of the subsidence is a portion of the enclosed air's being squeezed downward toward the inversion level. The greater proportion of the dry air lost as the eye shrinks is either drawn into the eyewall or converted to moist air by mixing in a relatively thin boundary layer at the inner edge of the eyewall.

The moist air below the inversion is in thermodynamic contact with the sea surface. It derives both from frictional inflow under the eyewall and from moist downdrafts induced by evaporation of condensate mixed into the eye. The moist air's residence time in the eye is much shorter than that of the dry air above the inversion. The depth of the inversion is determined by the balance between the sources of moist air and loss by entrainment into the eyewall. Convection in the eyewall does work on the eye by forcing thermally indirect descent. When the convection is intense, net outflow from the moist layer lowers the inversion, warming and drying the eye. When the convection weakens, frictional inflow and mixing into the eye raise the inversion, cooling the eye and filling it with cloud.

Key references:
Willoughby, H. E., 1995: Eye Thermodynamics. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, American Meteorological Society, 357-358.

Willoughby, H. E., 1997: More about eye thermodynamics. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, American Meteorological Society, 96-97.

Willoughby, H. E., 1998: Tropical cyclone eye thermodynamics. Mon. Wea. Rev. , 126 , (in press).

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