Three-Dimensional Structure of a Mesoscale Convective System in a Baiu-Frontal Depression Generated in the Downstream Region of the Yangtze River

摘要

A Baiu-frontal meso-a-scale depression was generated on 18 June 2001 in the mesoscale observational region, which was located in the downstream region of the Yangtze River. Based mainly on the data collected in this observation, we analyze the three-dimensional structure of a meso-a-scale convective system. We describe this structure and discuss the evolution of Baiu-frontal convective systems near the east coast of the continent. This convective system was composed of several meso-β-scale convective groups, which were bandshaped echoes located near the depression center, cellular echoes formed one after another ahead of the center, and a band-shaped echo extending more than 500 km in the rearward of the depression. There were three flows toward the convective system in the lower troposphere: moist southwesterly jet, dry west-to-northwesterly wind, and cold shallow easterly wind, which appeared near the coast. These convective groups were formed where the southwesterly wind, with convectively-unstable air, was lifted up to the free-convection level. This lifting resulted from low-level convergence, which was formed both by easterly and west-to-northwesterly wind in the central portion of the depression, by easterly wind at the forward portion, and by west-to-northwesterly wind at the rear portion. The system-relative speed of the southwesterly wind in the forward portion was larger than that in the rear. It is suggested that this strong inflow caused the formation of a deep convective structure in the forward portion, which reached the tropopause. The easterly wind played an important role in lifting such a strong inflow, and forming the deep convective groups in the meso-a-scale convective system, and then contributed to evolving the depression through the extension of the convective-updraft area. The mechanism that the easterly wind was formed locally near the coastline is discussed. The enhancement of low-level convergence, due to the formation of such a local wind system, is suggested to be one of the causes for the development of a mesoscale convective system, near the east coast of the continent during Baiu seasons.