OCEANIC CYCLOGENESIS AS INDUCED BY A MESOSCALE CONVECTIVE SYSTEM MOVING OFFSHORE .2. GENESIS AND THERMODYNAMIC TRANSFORMATION

The genesis of intense cyclonic vorticity in the boundary layer and th e transformation of a low-level cold pool to a warm-core anomaly assoc iated with the long-lived mesoscale convective systems (MCSs), which p roduced 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 s ystem. It is shown that the midlevel vortex/trough at the end of the c ontinental MCS's life cycle is characterized by a warm anomaly above a nd a cold anomaly below. The mesovortex, as it drifts toward the warm Gulf Stream water, plays an important role in initiating and organizin g a new MCS and a cyclonic (shear) vorticity band at the southern peri phery of the previously dissipated MCS. It is found from the vorticity budget that the vorticity band is amplified through stretching of abs olute vorticity as it is wrapped around in a slantwise manner toward t he cyclone center. Then. the associated shear vorticity is converted t o 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 vertic al transfer of surface fluxes from the warm ocean and the convectively induced grid-scale transport are responsible for the development of a high-theta(e) tongue, which is wrapped around in a fashion similar to the vorticity band, causing conditional instability and further organ ization of the convective storm. Because the genesis occurs at the sou thern periphery of the vortex/trough, the intensifying cyclonic circul ation tends to advect the pertinent cold air in the north-to-northwest erly flow into the convective storm and the ambient warmer air into th e cyclone center, thereby transforming the low-level cold anomaly to a warm-cored structure near the cyclone core. It is shown that the tran sformation and the evolution of the surface cyclone are mainly driven by the low-level vorticity concentrations. It is found that many of th e cyclogenesis scenarios in the present case are similar to those note d in previous tropical cyclogenesis studies and observed at the early stages of tropical cyclogenesis from MCSs during the Tropical Experime nt in Mexico. Therefore, the results have significant implications wit h regard to tropical cyclogenesis from MCSs.