Thermodynamic Evolution of the Hurricane Inner-Core during Eyewall Replacement Cycles and Ramifications of the Relict Wind Maximum

Flight-level aircraft data are used to examine inner-core thermodynamic changes associated with eyewall replacement cycles (ERCs) and the role of the relict inner eyewall circulation on the evolution of the post ERC eye. At the start of an ERC, a warm dry eye typically undergoes a dramatic moistening and cooling process. Relative humidity values can quickly approach those observed in the moat. Near the end of an ERC the mean relative humidity difference between the eye and moat can be < 5%, but the mean equivalent potential temperature can be 10 K greater in the eye than in the moat. These two air masses are separated by the relict wind maximum. After and ERC, this feature is generally not evident in microwave imagery and can even be difficult to identify in the tangential wind profile, but it can have substantial ramifications on the subsequent evolution of the storm. The relict wind maximum remains within the post-ERC eye as a well-defined and persistent region of high inertial stability that is resistant to horizontal mixing. It acts as a “containment vessel” that confines the old-eye air mass within the post-ERC eye. It may also restrict subsidence over the storm center. This focuses subsidence induced warming in an annulus along the inner periphery of the active outer eyewall convection. This disproportionate warming modifies the surface pressure and tangential profile and transforms them into a more “U-shape”. This makes the eyewall more barotropically unstable. This has implications on Rossby wave theory, polygonal eyewalls, eye-eyewall mixing, and the maximum intensity of a hurricane. In addition, this steepens the pressure profile and increases the pressure gradient which contributes to the intensification of the primary eyewall. The lack of active convection and subsidence over the storm center allows the minimum central pressure to rise while this intensification occurs, creating an anomalous pressure-wind relationship. When the relict wind maximum dissipates, the air masses in the eye are free to mix and subsidence can resume over the storm center.