Tropical cyclones in the Atlantic Ocean typically develop in association with westward moving middle and upper tropospheric tropical waves at subtropical latitudes and are characterized, in their mature stage, by a tropospheric-deep tower of potential vorticity occupying their warm cores. Hurricane Karen (October 2001) acquired this characteristic structure but initially developed as an extratropical cyclonic disturbance. In this paper we employ a high resolution, MM5 simulation to investigate the extratropical genesis phase and tropical conversion of Atlantic Hurricane Karen.

Karen developed as an extratropical disturbance on the cold side of a baroclinic zone associated with a midlatitude cyclone to its northeast. A burst of convection in its incipient stages of growth provided sufficient latent heating to establish a PV tower at the center. This heating attempted to drive the development of a warm core, but strong cold advection on the cold side of the baroclinic zone impeded this development until the system achieved seclusion from the large scale cold environment, a characteristic of subtropical cyclones. The subsequent interaction between the low-level circulation and the intrusion of high PV, high qe air from the upper troposphere quickly established a tropical cyclone structure to Karen. Details of this unusual tropical cyclone formation will be discussed in the light of diagnosis of the model output.