Two hurricane eye features that have yet to be adequately explained
are the clear-air moat that forms at the outer edge of the eye and the
hub cloud that forms near the circulation center. To investigate whether
these features can be explained by the spatial distribution of the
subsidence field, we have derived an analytical solution of the
Sawyer-Eliassen transverse circulation equation for a three-region
approximation with an unforced central eye region of intermediate or
high inertial stability, a diabatically-forced eyewall region
of high inertial stability, and an unforced far-field of low inertial
stability. This analytical solution isolates the conditions under which
the subsidence is concentrated near the edge of the eye. The crucial
parameter is the dimensionless dynamical radius of the eye, defined as
the physical radius of the eye divided by the characteristic Rossby
length in the eye. When this dimensionless dynamical radius is less than
0.6, there is less than 10% horizontal variation in the subsidence rate
across the eye, while if it is greater than 1.8, the subsidence rate at
the edge of the eye is more than twice as strong as at the
center of the eye. When subsidence is concentrated at the edge of the
eye, the largest temperature anomalies occur near there rather than at
the vortex center. This warm-ring structure, as opposed to a warm-core
structure, is often observed in intense hurricanes.
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