Abstract:Using the daily average data of NCEP 2.5°×2.5° and observed rainfall, on the base of comparative analysis of two heavy rainfall events in the Meiyu period in 2011, it is concluded: (1) The phases of the west jet over the Yangtze River for the two rainfall events were contrary, the former in the westerly airflow on the bottom of high pressure ridge, and the latter in the southwest flow before the trough. The circulation pattern (two troughs and one ridge) at 500 hPa were different，with the south trough of the former being shallow, and that of the latter being deep. The shear line at 700hPa of the former was formed by north and southwest winds and that of the latter was by east and southwest winds. (2) The water vapor of the former came from the west of the Bay of Bengal and the southeast of Subtropical; that of the latter came from the west of the Bay of Bengal mainly. (3) For both cases, the structure of the Meiyu front exhibited small humidity difference and temperature gradient, and the frontal zone moved northward first and southward, but the frontal zone position of the former was further south and that of the latter was further north; the duration of the former was long and that of the latter was short. The dry cold air of the former, which mainly came from the high level troposphere, was weak at lower levels and strong at upper levels, at the same time no clear cold air added to the north front. The dry cold air of the latter mainly came from upper levels, and there was a cold front frontal zone to the north of the front.
