Mesoscale Development and Along-Frontal Variation of a Meiyu/Baiu Front and Precipitation Observed in the Downstream Region of the Yangtze River

摘要

This paper has investigated the mesoscale structure and evolution of a Meiyu/Baiu front and precipitation along the front observed in the downstream region of the Yangtze River on 21 June 2002 by using data from intensive observations of upper-air, Surface, and five Doppler radars, as well as GMS IR and GANAL re-analysis data. It is found that the front collocated with a large-scale wind shear line. The frontal zone was characterized by a subsynoptic-scale low-level jet (LLJ) to the south and a thermally direct circulation in the middle troposphere South of the surface front. The front evolved from an inactive front with little convection along it to an intensive one triggering a strong meso-alpha-scale rainband. The front initially intensified mainly in association with the divergence related to the evaporative cooling of precipitation systems north of the front and further developed when strong convection evolved along the front. The meso-alpha-scale rainband triggered by the front was composed of several meso-beta-scale convective systems. Meso-beta-scale convective systems were narrow and consolidated in the western part, but wide and weak in the eastern part of the downstream region of the Yangtze River. Three-dimensional kinematic and reflectivity structures of two meso-beta-scale convective systems, where one was in the western part and the other was in the eastern part, have been examined comprehensively. In the western part, the convective system evolved in a quasi-steady state and was characterized by a deep and strong convective cell just north of the front and limited stratiform precipitation further north of the front. The orientation of the LLJ to the front was at a sharp angle. The primary updraft triggered by the front sloped largely northward in the lower troposphere and became nearly upright and strong from the middle troposphere. In the eastern part, on the other hand, the convective system changed remarkably with time and was featured by Multiple shallow and weak convective cells across the front and extended stratiform precipitation both south and north of convective cells. The LLJ that oriented nearly normal to the front overran the front at lower levels and penetrated far to the north of the front. It appears that the variable structure and evolution of the LLJ would have a great impact on the development of distinct convective systems in the downstream region of the Yangtze River. The mesoscale along-frontal variability of the Meiyu/Baiu frontal zone appears to be responsible for distinct modes of convective organization along the front in a limited distance.