ANALYSIS OF A LOCAL RAIN STORM IN BEIJING ASSOCIATED WITH TWO CONVECTIVE LINES

A local heavy rain storm which occurred on 13 July 2009 in Beijing and exhibited two peculiar convective cloud lines is studied.The mechanism is also investigated using a four-dimensional variational Doppler radar analysis system.It is shown that in the large scale background with upper-level trough and lower-level wind convergence,several convective cells occurred firstly,which was triggered by a topographic convergence line on the border between Hebei province and the southwest area of Beijing.Meanwhile a zone of horizontal temperature gradient with wind convergence built up the initiation and evolution needed by a convective storm,and a straight line of cumulus oriented approximately in an east-northeast to west-southeast direction was observed.The rainfall rate became the largest after the formation of the cloud line,and the convective outflow due to the heavy rain collided with the ambient wind field and produced two new convergence lines.Then the two convergence lines cut off the transportation of energy and moisture to the old cloud line,replacing it with two new convective cloud lines.On the two sides of the convective line,the two new convective cloud lines parallel to the old line and perpendicular to the ambient wind are produced.The boundary layer convergence lines with intense convective advection indicate the initiation of new convective storm 30～60 minutes ahead of the observation.When there is weaker convective advection and large cold pool in the boundary layer in Beijing,the local rain storm decays quickly.