A DIAGNOSTIC STUDY OF CYCLOGENESIS IN THE WESTERN NORTH PACIFIC-OCEAN

The phenomenon of explosive cyclogenesis is studied from the perspective of the synoptic-scale framework within which various intensities of maximum 24-h pressure falls are occurring. This study is accomplished with a construction of composite groups of cyclones that have experienced similar maximum intensification rates within a specified 5-degrees latitude-longitude geographical domain over the Kuroshio Current in the western North Pacific Ocean. An examination of diagnostics computed from the composite fields of geopotential height and temperature reveals several trends. As the degree of intensification increases, the downstream surface ridge and attendant warm, moist inflow become more prominent, the cyclonic vorticity of the initial surface circulation is greater, the downstream frontogenesis is stronger and occurs through a deeper layer of the troposphere, and the location and strength of the vertical-motion forcing become more favorable for development. As a consequence of these results, it is concluded that synoptic-scale forcing mechanisms extending over a large domain, in a composite sense, play a role in determining the amount of intensification experienced by a cyclone. These mechanisms supporting cyclogenesis include not only dynamic support in the form of midtropospheric thermal and vorticity advection but also by deep tropospheric frontogenetic processes occurring both upstream and downstream of the surface low. Since these mechanisms are well resolved by contemporary numerical models and routinely available data, the aforementioned trends might be used operationally to evaluate the potential for cyclone intensification.