| The mesoscale convective systems (MCS) over Yangtze-Huaihe basin were classified into MCC, PECS, MβCCS and MβECS based on the FY-2D geostationary satellite blackbody temperature data during June to August from 2007-2013. The mesoscale convective vortex (MCV) during rainstorm and heavy rainstorm days were divided into two kinds:developing MCV (D-MCV) and non-developing MCV (ND-MCV), from which each case was simulated with mesosale model WRFV3.5. Combined with the NCEP/NCAR global objective analysis data, the background field, vapor condition, vorticity budget and vertical structure were analyzed.The results indicate that PECS and MβECS, with larger average size, lower blackbody temperature and longer lifetime, occur more frequently than MCC and MβCCS. The peak time for MCS initiation is around 14:00-17:0OBJT while most MCS cases mature in the period of 17:00-19:00BJT and dissipate during 18:00-23:00BJT, however, it differs for diverse types. Most MCS cases appear in the middle and eastern part of Yangtze-Huaihe basin while rarely on the sea. Eastward, northeastward and southeastward are the three main moving directions for MCSs, and the moving distance of MCS case is closely related to its lifetime. The percentage of MCS cases which may lead to MCV is about 33.8% and the average level MCV cases occur over Yangtze-Huaihe basin is lower than that of North America. MCV cases are more likely to be associated with PECS and MβECS, most of which appear in the northern and western part of their original MCSs. The distinction between MCV thickness caused by different kinds of MCSs is quite modest, and it seems to has little relationship with its origin MCS type and the grade of rainstorm days. The composite analysises show that before the formation of D-MCV, high-level jet stream is stronger than that of ND-MCV. In addition, Yangtze-Huaihe area is in the former of trough-line, which may produce strong southwesterly stream and contribute to the formation of new convection.Both of the two MCV cases chosen occurred on the shear line on low layer with the surroundings of circulation situation for Meiyu period. The amount of water vapor transportation leads to the different accumulative rainfall of the two processes, which is closely related with low-level jet and K index can indicate the generation and activity of MCS cases. The vertical vorticity budget of the two MCV cases are analyzed by using the model output data. The results show that the contribution from vertical advection and tilting counteract on different levels during each stage of both MCV lifetimes, and the contribution from horizontal advection and convergence is opposite. The major vorticity on low layer during the development stage of MCV results from vertical advection and convergence as a whole. Both of the two MCV circulation emerge on the low layer firstly, and then develop upward. The closed vortex of ND-MCV develops stronger than that of D-MCV and the absolute value of each item of the vorticity equation decreasing after ND-MCV matured, which leads to a longer lifetime compared with D-MCV. D-MCV can trigger new convection during its developing stage, and the secondary convection is located on its southern and down shear direction. On the contrary, after the formation of ND-MCV, its original convection weakens quickly.During the forming and developing stage of D-MCV, there are two low level jets with different height in south of the vorticity. After the formation of D-MCV, the southwesterly airstream in its southern part enhances the jet in the lower level, which may contribute to the change of horizontal vorticity result from vertical wind shear and then lead to updrafts. The two updrafts caused by different low level jets together provide dynamic conditions for the secondary convection. After D-MCV gets mature, the low level jet gradually weakens, and the unstable stratification at the convection area converts to the neutral stratification, which weakens the vertical movement and convection. For ND-MCV, the downward airflow caused by heavy rainfall is one of the factors decreasing unstable condition and weakening convection. Comparing to D-MCV, the vertical wind shear in southern area is quite weak when ND-MCV occurs, which is disadvantageous to form the vertical circulation. After the formation of ND-MCV, the onset of the northerly winds in the west weakens low level jet, which could also be one of the reasons why ND-MCV failes to strengthen its original MCS and motivate secondary convection. |
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