Study of the Vorticity and Moisture Budget of a Northeast Vortex Producing Heavy Rainfall

Detailed analysis are conducted on the structure,the vorticity and moisture budgets of a northeast vortex which caused sustained heavy rainfall in Northeast China on 25-29 July 2005.The results show: 1) The northeast vortex was a deeper depression related with cold air.In the early formation stage,the cyclonic vorticity appeared in the middle troposphere and then extended to the lower and upper level,respectively;While in the developing stage of the vortex,the cyclonic vorticity increased most rapidly in the upper troposphere,then extended downwards to the middle and the lower troposphere,which produced the development of surface cyclone with front.Under the interaction of frontogenesis between upper and lower level,there formed a nearly vertical deep Bow-Shaped front zone with a little tilted forwards in the low level.2) The calculation of vorticity budget indicates that both the horizontal advection term and horizontal divergence term contributed most significantly to the strengthening of vortex,but their values are different during the different stages of the vortex.3) The maxima of the potential vorticity anomaly were located in the upper troposphere,and then extended downwards in the southeastern part of the vortex which is near the area of heavy rainfall.In addition,the analysis of satellite data TBB shows that there existed the typical spiral cloud band during the period of the heavy rainfall and the area of intense convective activities coincided with that of heavy precipitation.4) In the early stage of heavy rainfall of the northeast vortex,the moisture budget was mainly from the north boundary;With the reinforcement of southwest monsoon in the low latitude,a moisture transportation from the low to the middle or higher latitude was formed along the west edge of subtropical high and the moisture supply is mainly caused by the convergence of north and south boundary.5) Finally,comparison has been conducted between the heavy rainfall cases of northeast vortexes during the summer of 2005 and 1998,and it shows that the sunstained heavy rainfall in larger area was closely related to the Asian blocking highs in middle or higher latitude,the maintenance of vortex and the position of subtropical high in West Pacific,as well as the moisture supply of summer monsoon coming from low latitude area.