Oceanic Cyclogenesis as Induced by a Mesoscale Convective System Moving Offshore. Part I: A 90-h Real-Data Simulation

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  • 1 Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada
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Abstract

Recent observations have revealed that some mesoscale convective systems (MCSs) could undergo multiple cycles of convective development and dissipation, and, under certain environments, they appeared to be responsible for (barotropic) oceanic or tropical cyclogenesis. In this study, oceanic cyclogenesis, as induced by an MCS moving offshore and then driven by deep convection in a near-barotropic environment, is investigated by extending to 90 h the previously documented 18-h simulation of the MCSs that were responsible for the July 1977 Johnstown flash flood. It is demonstrated that the mesoscale model can reproduce very well much of the meso-β-scale structures and evolution of the long-lived MCS out to 90 h. These include the development and dissipation of the continental MCSs as well as the associated surface and tropospheric perturbations, the timing and location in the initiation of a new MCS after 36 h and in the genesis of a surface mesolow over the warm Gulf Stream water after 60-h integration, the track and the deepening of the surface cyclone into a “tropical storm,” the maintenance of a midlevel mesovortex/trough system, and the propagation of a large-scale cold front with respect to the surface cyclone.

It is found that the new MCS is triggered after the vortex/trough moved offshore and interacted with the land-ocean thermal contrasts during the afternoon hours. The oceanic cyclogenesis begins at 150–180 km to the south of the vortex, as the associated surface trough advances into the Gulf Stream and weakens. Then, the cyclone overpowers quickly the low-level portion of the vortex circulation and deepens 14 hPa in 24 h. A comparison with a dry sensitivity simulation shows that the cyclogenesis occurs entirely as a consequence of the convective forcing. Without it, an 84-h simulation produces only a surface trough with the minimum pressure being nearly the same as that left behind by the previous MCSs. It is shown that the vortex/trough provides persistent convergence at its southern periphery for the continued convective development, whereas the convectively enhanced low-level flow tends to (i) “pump” up sensible and latent heat fluxes from the warm ocean surface and (ii) transport the high-θe air in a slantwise fashion into the region having lower θe aloft, thereby causing further conditional instability, increased convection, and more rapid deepening. The interactions of the continental MCS/vortex and the oceanic cyclone/storm systems with their larger-scale environments are also discussed.

Abstract

Recent observations have revealed that some mesoscale convective systems (MCSs) could undergo multiple cycles of convective development and dissipation, and, under certain environments, they appeared to be responsible for (barotropic) oceanic or tropical cyclogenesis. In this study, oceanic cyclogenesis, as induced by an MCS moving offshore and then driven by deep convection in a near-barotropic environment, is investigated by extending to 90 h the previously documented 18-h simulation of the MCSs that were responsible for the July 1977 Johnstown flash flood. It is demonstrated that the mesoscale model can reproduce very well much of the meso-β-scale structures and evolution of the long-lived MCS out to 90 h. These include the development and dissipation of the continental MCSs as well as the associated surface and tropospheric perturbations, the timing and location in the initiation of a new MCS after 36 h and in the genesis of a surface mesolow over the warm Gulf Stream water after 60-h integration, the track and the deepening of the surface cyclone into a “tropical storm,” the maintenance of a midlevel mesovortex/trough system, and the propagation of a large-scale cold front with respect to the surface cyclone.

It is found that the new MCS is triggered after the vortex/trough moved offshore and interacted with the land-ocean thermal contrasts during the afternoon hours. The oceanic cyclogenesis begins at 150–180 km to the south of the vortex, as the associated surface trough advances into the Gulf Stream and weakens. Then, the cyclone overpowers quickly the low-level portion of the vortex circulation and deepens 14 hPa in 24 h. A comparison with a dry sensitivity simulation shows that the cyclogenesis occurs entirely as a consequence of the convective forcing. Without it, an 84-h simulation produces only a surface trough with the minimum pressure being nearly the same as that left behind by the previous MCSs. It is shown that the vortex/trough provides persistent convergence at its southern periphery for the continued convective development, whereas the convectively enhanced low-level flow tends to (i) “pump” up sensible and latent heat fluxes from the warm ocean surface and (ii) transport the high-θe air in a slantwise fashion into the region having lower θe aloft, thereby causing further conditional instability, increased convection, and more rapid deepening. The interactions of the continental MCS/vortex and the oceanic cyclone/storm systems with their larger-scale environments are also discussed.

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