EFFECT OF TYPHOON KROSA(0716) ON AN AUTUMN HEAVY RAINFALL IN BEIJING

In early October 2007,a series of rainfall events successively occurred in Beijing during the normally rainless autumn days,among which was a heavy rainfall event on October 5th and 6th.In this paper,by means of numerical simulation and sensitivity experiments,we found that this unusual heavy rainfall event in Beijing was highly affected by the activities of typhoon Krosa(0716) in the southeast coastal area of China.Results show that the peripheral circulation of typhoon Krosa had a remote influence on this heavy rainfall event.The waves,generated in the peripheral circulation of Krosa,propagated northward at the tropopause and interacted with the westerly trough waves,resulting in strong divergence in the upper troposphere over Beijing.The water vapor from the northern East China Sea and the Yellow Sea was transported to the middle and lower reaches of the Yangtze River by the easterly winds on the north side of Krosa,and then was advected northward to 40 °N around Beijing by the flows of the remnant subtropical high.The moist convergence zone at the middle-and lower-troposphere moved 1.5° northward under the influence of Krosa so that part of the zone was over Beijing.In addition,dynamical analyses indicated that during the event,in the troposphere over Beijing,there were strong convergence below 500 hPa,strong divergence above 500 hPa(especially between 300 and 200 hPa),ascending motions in the whole troposphere,and moist convergence below 600 hPa.All these conditions are very favorable to heavy rainfall.Moreover,the long-lived strong cold advection at the tropopause and the warm advection at the middle-and lower-troposphere helped to maintain the rainfall to a certain extent.Sensitivity experiments showed that these dynamical conditions can hardly establish without the influence of the typhoon.