Comparison of Two Heavy Rainfall Events in Shali River Basin
Based on the conventional and automatic observation data, NCEP(1°×1°) reanalysis data, the causes of two heavy rainfall events occurred on 5 and 14 J uly, 2007 in Shali river area are diagnosed and analyzed. The diagnostic results show that the dynamic conditions of two heavy rainfall events are different und er different influencing systems. The results show that the movement direction a nd intensity transformation of the positive center of the vertical helicity at 7 00hPa matches the heavy rainfall regions quite well in two heavy rainfall events . The heavy rainfall center lies on the front of the movement of the vertical he licity center. The vertical helicity value is positive at lower level while negative at upper level on July 5, whose dynamic c ondition is more beneficial for the occurrence of the heavy rainfall. The moist potential vorticity analysis indicates that the atmospheric instability at lower level of JUL-5 heavy rainfall event is not only convectively unstable but also symmetrically instable, which is favorable for the development of both vertical convection and slantwise convection. For the JUL-14 event, the lower level atmos phere is convectively stable and the boundary layer is convectively unstable. At the same time, the middle-level atmosphere is convectively unstable and has obv iously symmetric instability, so the slantwise vorticity gets a bigger growth, w hich leads to the strong ascending movement and the moist air upward transport a nd makes the precipitation intensity increase.