The role of the occlusion process in the extratropical-to-tropical transition of Atlantic Hurricane Karen

A fine-scale numerical simulation is employed to examine the extratropical-to-tropical conversion (ETC) of Atlantic Hurricane Karen in October 2001. The extratropical precursor to Karen developed along the trailing cold front of a major cyclone in the central north Atlantic in response to cyclonic vorticity advection by the thermal wind associated with an upper-level short wave that had penetrated into the subtropics. The study focuses on the extratropical occlusion process and the occluded structure of the pre-Karen storm to advance a new theory on the nature of ETC. Recent ETC research has suggested that conversion proceeds via diabatically induced rearrangement of tropopause-level PV that serves to reduce the vertical shear above the sea-level pressure (SLP) minimum. The analysis shows that as the extratropical precursor to Karen entered its post-mature phase, an upper tropospheric PV minimum developed above the surface cyclone center as a natural consequence of the dynamic and diabatic processes characteristic of the extratropical occlusion process. This PV minimum acted to reduce the vertical shear above the SLP minimum. Since the shear reduction occurred while the cyclone was situated over SSTs greater than 26?C, rapid development of deep convection at the center of the storm, vorticity production via column stretching in a vorticity-rich environment, and development of a tropospheric deep warm core via latent heat release ensued. The evolution suggests that the process of ETC can, at least in some instances, be viewed as a boundary-dependent consequence of the general process of extratropical occlusion.