The Wet Q-Vector Diagnostic Analysis Of "05.7.18" Torrential Rain In North Sichuan Basin

In this paper, based on the rainfall precipitation data oftorrential rain in Guangyuan city from 1971 to 2005, an investigation is conducted in some climatic characteristics of torrential rain in North Sichuan Basin on the east side of Qinghai-Xizang Plateau in recent 35 years. By using the FY-2 infrared imagery and the routine sounding and surface observation data on 15~19 July 2005, a synoptic analysis of the regional torrential rain process occurred in North Sichuan Basin on 18—19 July 2005 is made. Using the wet Q-vector, a dynamic diagnosis is made. Through examining the relations between the heavy rain area and the fields of divergence and frontogenetic function of the wet Q-vector, the temporal and spatial distribution characteristics of the divergence field during this heavy rain process are analyzed, and the relation between the generation and development of the rainstorm system and the frontogenetic function is probed into, and the probable physical mechanism producing this rainstorm process is revealed. The results are as follows: (1) The wet Q-vector has a good diagnostic property for the secondary circulation of the mesoscale systems of the heavy rain, especially near the heavy rain time. The convergence area of its divergence field can correspond to the heavy precipitation area, and the position of maximum rainfall is near the northwest of the convergence center, and the corresponding relation between the lower-level divergence of the wet Q-vector and the heavy rain area is better than that of the upper level divergence. (2) The center of frontogenesis can correspond to the torrential rain area of 12 hours in the future. After the instability energy is released largely, the air mass is inclined to the stability stratification and has the frontolysis, and the heavy rain decreases gradually. (3) The probable physicalmechanism of this event is: (a) The upper trough line and the lower shear line blocked by Northwest Pacific subtropical high are the main influencing systems of the torrential rain, (b) The southerly low-level jet streams on 700hPa and 850hPa transport the warm and moist air from the Bay of Bangle and the South China Sea to Sichuan Basin continuously, which enhances potential instability energy continually and locates North Sichuan Basin in the convergence area of the left side of the jet. (c) The mesoscale convective complexes(MCCs), which develop in North Sichuan Basin, are the direct influencing systems which cause this heavy rainfall, (d) The northerly surface cold front triggers the release of instability energy in the warm and moist air mass and the heavy rain occurs.