Potential Vorticity Streamers as Precursors to Tropical Cyclone Genesis in the Western Pacific.

摘要

The term tropical transition (TT) is used to describe the formation of a tropical cyclone from an extratropical precursor. The overarching goal of this thesis is to re-examine TT in the western North Pacific. This is accomplished via the synthesis of a subjective climatology of all tropical cyclones (TCs) from 2002-2008 and a case study analysis of ensemble prediction data for the particularly intriguing event of TS 16W in September of 2008. The climatological analysis indicated that TT events represented 14% of all TCs during the study period. The maximum frequency of TT events occurred in the late summer / early fall. The resulting storm systems tended to form farther to the north than non-TT events and were found to be relatively weak and short- lived. The results of the case study analysis lend credence to the earlier finding that the genesis pathway of TS 16W exhibited a two-stage evolution. The first stage involved the forcing of near-continuous deep convection by a tropopause-level potential vorticity (PV) anomaly. The second stage involved the rearrangement of the upper-level PV structure via diabatic processes, resulting in a necessary reduction of vertical wind shear.