Tropical transition: possible mechanisms and observational needs

Tropical transition refers to the formation of a warm core tropical cyclone from a cold-core disturbance, usually of extratropical origin. Some of the largest forecast errors of track and intensity occur with these events. Their proximity to the North American coast produces significant adverse consequences for poor forecasts, namely, minimal time for coastal warnings and inland flood preparations.

In this paper we will present a synthesis of recent studies of tropical transition, primarily modeling studies of individual cases such as Diana (1984), Michael (2000), Gabrielle (2001), Humberto (2001) and Alex (2004). The primary emphasis will be on the sequence of events leading to the formation of Humberto. A tropopause-based cyclonic potential vorticity feature migrated slowly southward during its nearly two-week life cycle and initiated convection over the western Atlantic Ocean. The convection occurred on the southwest flank of the trough and it is demonstrated through numerical simulations that vertical wind shear was crucial for organizing convection. The origin of mesoscale vorticity was not the ensuing mesoscale convective system itself, but rather, secondary convection that developed at the intersection of a rearward propagating bore-like disturbance with the remnant of a surface cold front produced in the preceding hurricane Grabrielle as it underwent extratropical transition. Once the initial vortex formed, there was a constant interplay between environmental shear, tilting the vortex, and new convection and associated vorticity anomalies on scales of about 40 km that counteracted the shear. These processes led to a quasi-steady, tilted, slowly intensifying mesoscale vortex that eventually grew to become a tropical cyclone.

We will conclude the talk with specific recommendations for observations that need to be made to conclusively support or refute the results of previous simulations of tropical transition.