Principal Investigator: H.E. Willoughby
Collaborating scientist(s):
M.L. Black
J.F. Gamache
C.E. Samsury
G.A. Soukup
Objective: Physical understanding of hurricane intensification.
Rationale: The low pressure and strong winds in hurricanes result from tightly coupled dynamic and thermodynamic processes. The energy derives from the warm underlying sea, but interactions with the surrounding atmosphere and gradient balance adjustment dominate the process of intensification. Why is it that extreme hurricanes are rare even though the sea is warm enough to sustain them over large areas of the tropical oceans throughout late summer?
Method: Analysis of thermodynamic soundings, flight--level observations, and Doppler radar data near the centers of hurricanes. Diagnostic modeling of secondary flows and the evolution of the primary balanced vortex.
Accomplishment: Central sea--level pressures of the most intense tropical cyclones are about 10% lower than ambient. 3 km of adiabatic descent in the eye is enough to produce the vertically integrated 30C warming necessary for the low hydrostatic pressure. In balanced models, convective entrainment of air from the eye into the eyewall causes the subsidence. Balanced models also predict, and observations confirm, that the eye contracts during intensification. Eye soundings show warm and dry air aloft, separated by an inversion from cloudy air below that follows a moist adiabat almost to the surface.

I hypothesize that the air above the inversion has remained in the eye since it was enclosed when the eye wall formed. Loss of mass through entrainment into the eyewall is balanced by shrinking of the eye's volume with little detrainment aloft from the eyewall into the eye. The air sinks as mass below it is drawn outward from the bottom of the eye. Dewpoint depressions at the inversion are 10--30C rather than the 100C that would occur if the air originated at the tropopause.

The moist air below the inversion derives primarily from frictional inflow under the eyewall and secondarily from descent induced by evaporation of condensate mixed across the inside edge of the eyewall. 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 frictional inflow and loss into the eyewall (Figure).

The convective bubbles in the eyewall are buoyant with respect to the air around the eye, but not with respect to the eye itself. Collectively, they act as a ``heat pump'' that does work on the eye by forcing thermally indirect descent. When the convection is intense, net entrainment lowers the inversion, warming and drying the eye. When the convection weakens, net frictional inflow raises the inversion, cooling the eye and filling it with cloud. This interpretation suggests that convection intensifies tropical cyclones through induced adiabatic warming of the eye sounding, not through vertical mixing that acts to adjust the eyewall sounding toward thermodynamic equilibrium with the sea.

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