MESOSCALE INTERACTIONS IN TROPICAL CYCLONE GENESIS

With the multitude of cloud clusters over tropical oceans, it has been perplexing that so few develop into tropical cyclones. The authors po stulate that a major obstacle has been the complexity of scale interac tions, particularly those on the mesoscale, which have only recently b een observable. While there are well-known climatological requirements , these are by no means sufficient. A major reason for this rarity is the essentially stochastic nature of the mesoscale interactions that p recede and contribute to cyclone development. Observations exist for o nly a few forming cases. In these, the moist convection in the preform ation environment is organized into mesoscale convective systems, each of which have associated mesoscale potential vortices in the midlevel s. Interactions between these systems may lead to merger, growth to th e surface, and development of both the nascent eye and inner rainbands of a tropical cyclone. The process is essentially stochastic, but the degree of stochasticity can be reduced by the continued interaction o f the mesoscale systems or by environmental influences. For example a monsoon trough provides a region of reduced deformation radius, which substantially improves the efficiency of mesoscale vortex interactions and the amplitude of the merged vortices. Further, a strong monsoon t rough provides a vertical wind shear that enables long-lived midlevel mesoscale vortices that are able to maintain, or even redevelop, the a ssociated convective system. The authors develop this hypothesis by us e of a detailed case study of the formation of Tropical Cyclone Oliver observed during TOGA COARE (1993). In this case, two dominant mesosca le vortices interacted with a monsoon trough to separately produce a n ascent eye and a major rainband. The eye developed on the edge of the major convective system, and the associated atmospheric warming was pr ovided almost entirely by moist processes in the upper atmosphere, and by a combination of latent heating and adiabatic subsidence in the lo wer and middle atmosphere. The importance of mesoscale interactions is illustrated further by brief reference to the development of two typh oons in the western North Pacific.