HURRICANE EYE DYNAMICS AND THERMODYNAMICS
Physical understanding of hurricane intensification.
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?
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.
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
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.
Willoughby, H. E., 1995: Eye Thermodynamics. Preprints, 21st Conference
on Hurricanes and Tropical
Meteorology, American Meteorological Society, 357-358.
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