Oceanic Cyclogenesis as Induced by a Mesoscale Convective System Moving Offshore. Part II: Genesis and Thermodynamic Transformation

Abstract

The genesis of intense cyclonic vorticity in the boundary layer and the transformation of a low-level cold pool to a warm-core anomaly associated with the long-lived mesoscale convective systems (MCSs), which produced the July 1977 Johnstown flash flood and later developed into a tropical storm, are examined using a 90-h real-data simulation of the evolution from a continental MCS/vortex to an oceanic cyclone/storm system. It is shown that the midlevel vortex/trough at the end of the continental MCS's life cycle is characterized by a warm anomaly above and a cold anomaly below. The mesovortex, as it drifts toward the warm Gulf Stream water, plays an important role in initiating and organizing a new MCS and a cyclonic (shear) vorticity band at the southern periphery of the previously dissipated MCS. It is found from the vorticity budget that the vorticity band is amplified through stretching of absolute vorticity as it is wrapped around in a slantwise manner toward the cyclone center. Then, the associated shear vorticity is converted to curvature vorticity near the center, leading to the formation of a ?comma-shaped? vortex and the rapid spinup of the surface cyclone to tropical storm intensity. Thermodynamic budgets reveal that the vertical transfer of surface fluxes from the warm ocean and the convectively induced grid-scale transport are responsible for the development of a high-?e tongue, which is wrapped around in a fashion similar to the vorticity band, causing conditional instability and further organization of the convective storm. Because the genesis occurs at the southern periphery of the vortex/trough, the intensifying cyclonic circulation tends to advect the pertinent cold air in the north-to-northwesterly flow into the convective storm and the ambient warmer air into the cyclone center, thereby transforming the low-level cold anomaly to a warm-cored structure near the cyclone core. It is shown that the transformation and the evolution of the surface cyclone are mainly driven by the low-level vorticity concentrations. It is found that many of the cyclogenesis scenarios in the present case are similar to those noted in previous tropical cyclogenesis studies and observed at the early stages of tropical cyclogenesis from MCSs during the Tropical Experiment in Mexico. Therefore, the results have significant implications with regard to tropical cyclogenesis from MCSs.