On the distribution of subsidence in the hurricane eye

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 centre. 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; when it is greater than 1.8, the subsidence rate at the edge of the eye is more than twice as strong as at the centre 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 centre. This warm-ring structure, as opposed to a warm-core structure, is often observed in the lower troposphere of intense hurricanes.