Analysis of Dynamic and Thermal Field Structure Characteristics on Rainstorm Area in Interaction Process Between Westerly Trough and Subtropical High
Based on the conventional observations, AWS, NCEP/NCAR (1°×1°, by 6 h) reanalysis and FY2C satellite data, this paper investigated the dynamic and thermal field structure characteristics throughout the three interaction stages between westerly trough and subtropical high (SH) in the rainstorm which occurred in Shandong from 15 to 18 August 2007. The results indicated that there are three stages sequentially throughout the whole process. Firstly, SH advanced westward accompanied with the shear line shifting westward at a snail’s pace; secondly, the transversal trough moved southward with the SH weakening; thirdly, the trough transformed from transversal into upright with SH southward retreating. The three stages share some features in common: There was a shear line and a θse frontal zone in the mid and low levels, and a low level jet (LLJ) at 700 hPa. Besides, the convective cloud cluster had the feature of backpropagating, causing the rainstorm to merge several times in its life. The difference of the three stages lies in that: (1) In the first stage, the slope of θse frontal zone was gentle and north tilted with shear line and frontal zone backward tilted, so it was typical warm front precipitation. With the saturation region extending broad and upward to 300 hPa, the rainstorm area got extensive and uniform distributed, wavering around the water vapor convergence center in the junction of θse frontal zone and the warm ridge at 850 hPa. So, the low cyclonic convergence, shear line convergence and frontal uplift are the dynamic mechanism of the rainstorm, while the LLJ is the enhancement mechanism. (2) In the second stage, the θse frontal zone was sharpsloped and intersectedgradient by leaning northward and southward in turn with height, and the colddry air invaded into the warm ridge from the low and mid layers respectively, thus creating typical severe convection precipitation and furious upward movement. Also, with the saturation region extending upward to 200 hPa and getting narrow, the distribution of the rainstorm area became nonuniform, the intensity was high and range got small, located around the θse warm ridge vertical axis. Hence, the main dynamic mechanism of the Xintai strong convection precipitation is frontal uplift, and the enhancement mechanism is the invasion of colddry air in the middle troposphere. (3) In the third stage, the θse frontal zone was sharpsloped and south tilted, having a forerake characteristics, which resulted in the typical upper trough precipitation. With the saturation region extending narrow and upward to 500 hPa, the rainstorm area became long, narrow, dispersive and in a low intensity, located at the 850 hPa shear line which also was an position near the frontal zone in the θse warm tongue. Thus, dynamic mechanism of the rainstorm is the shear line convergence, and the enhancement mechanism is the invasion of dry air in the middle troposphere.