The role of upshear convection in expediting the tropical transition of Atlantic Hurricane Karen (2001)

The process by which a baroclinic, vertically-sheared, extratropical cyclone is transformed into a warm-core, vertically-stacked tropical cyclone is known as tropical transition. In this study, a fine-scale simulation of the tropical transition of Atlantic Hurricane Karen in October 2001 is employed to examine the processes leading to the development of upshear convection and to explore the effects the upshear convection has on the transitioning cyclone. The analysis shows that, as in marine extratropical cyclones, the area upshear of the pre-transition cyclone is characterized by reduced slantwise stability. Eventually, an intense burst of slantwise convection occurs which instigates three important processes that conspire to produce a full-fledged tropical cyclone. First, the convection generates intense low-level vorticity on the western half of the cyclone which quickly dominates the cyclone's vorticity field eventually organizing the circulation into a small-scale, intense vortex. Second, upshear convection hastens the isolation of the cyclone's developing warm core by increasing the amount of cold air advection on the northern and western sides of the storm and cycling cooler, drier air into the boundary layer to the south of the cyclone. Third, upshear convection vertically redistributes potential vorticity from the tropopause to the surface and introduces a component to the upper-level winds which advects strong, shear-inducing PV gradients away from the column above the cyclone. These three processes render the initial extratropical cyclone into a frontless vortex with tropical-storm force winds and a warm core in a low-shear environment. These features are sufficient, given a warm enough ocean surface, to allow self-amplification as a tropical cyclone.