OCEANIC CYCLOGENESIS AS INDUCED BY A MESOSCALE CONVECTIVE SYSTEM MOVING OFFSHORE .1. A 90-H REAL-DATA SIMULATION

Recent observations have revealed that some mesoscale convective syste ms (MCSs) could undergo multiple cycles of convective development and dissipation, and, under certain environments, they appeared to be resp onsible for (barotropic) oceanic or tropical cyclogenesis. In this stu dy, oceanic cyclogenesis, as induced by an MCS moving offshore and the n driven by deep convection in a near-barotropic environment, is inves tigated by extending to 90 h the previously documented 18-h simulation of the MCSs that were responsible for the July 1977 Johnstown hash fl ood. It is demonstrated that the mesoscale model can reproduce very we ll much of the meso-beta-scale structures and evolution of the long-li ved MCS out to 90 h. These include the development and dissipation of the continental MCSs as well as The associated surface and tropospheri c perturbations, the timing and location in the initiation of a new MC S after 36 h and in the genesis of a surface mesolow over the warm Gul f Stream water after 60-h integration, the track and the deepening of the surface cyclone into a ''tropical storm,'' the maintenance of a mi dlevel mesovortex/trough system, and the propagation of a large-scale cold front with respect to the surface cyclone. It is found that the n ew MCS is triggered after the vortex/trough moved offshore and interac ted with the land-ocean thermal contrasts during the afternoon hours. The oceanic cyclogenesis begins at 150-180 km to the south of the vort ex, as the associated surface trough advances into the Gulf Stream and weakens. Then, the cyclone overpowers quickly the low-level portion o f the vortex circulation and deepens 14 hPa in 24 h. A comparison with a dry sensitivity simulation shows that, the cyclogenesis occurs enti rely as a consequence of the convective forcing. Without it, an 84-h s imulation produces only a surface trough with the minimum pressure bei ng nearly the same as that left behind by the previous MCSs. It is sho wn that the vortex/trough provides persistent convergence at its south ern periphery for the continued convective development, whereas the co nvectively enhanced low-level flow tends to (i) ''pump'' up sensible a nd latent heat fluxes from the warm ocean surface and (ii) transport t he high-theta(e) air in a slantwise fashion into the region having low er theta(e) aloft, thereby causing further conditional instability, in creased convection, and more rapid deepening. The interactions of the continental MCS/vortex and the oceanic cyclone/storm systems with thei r larger-scale environments are also discussed.