Eye soundings show warm and dry air aloft, separated by an inversion from cloudy air below. 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 between it and 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. 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 entrained 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 entrainment 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.
Willoughby, H. E., 1997: Tropical cyclone eye thermodynamics. Mon. Wea. Rev., 125 , (draft).
Last modified: 8/9/96