A Numerical Study of a Mesoscale Convective System during TOGA COARE. Part Ⅱ: Organization

摘要

In Part Ⅰ, the authors presented a successful numerical simulation of the life cycle of a warm-pool mesoscale convective system (MCS) that occurred on 15 December 1992 during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. In this study, the simulation results of Part Ⅰ are diagnosed to investigate the organization of the MCS and the convective onsets that occurred during the growing and mature stages of the MCS. During the life cycle of the MCS, four convective onsets occur in the presence of large-scale ascent, convective available potential energy (CAPE), and surface potential temperature drop-off (SPTD). It is found that the first convective onset is caused by the existence of upward motion, CAPE, and SPTD in the model initial conditions. The second convective onset is regulated by the favorable occurrence of SPTD. The third and fourth convective onsets arise from the development of upward motion associated with the westward propagation of the quasi-2-day wave. The four mesoscale precipitation features clustered together to form the MCS in response to the evolution of the vertical motion field. The organization of the MCS is characterized by the presence of a midtropospheric mesovortex situated near the position of the first convective onset. Analysis of the relative vorticity (RV) budget indicates that the mesovortex originates and intensifies largely from vortex stretching induced by deep convective heating. A decrease in RV above (below) the mesovortex arises because of the combined effects of the tilting and horizontal advection terms (the tilting, stretching, and solenoidal terms). Our results suggest that the mesovortex played little role in the subsequent onsets (i.e., second, third, and fourth) and that other warm-pool MCSs occurring near the transequatorial flow are likely to be associated with mesovortices.