Affected by the high latitude blocking and high-altitude cold vortex in the flood season of 1998, the northwest part of the northeast region began to rain continuously from the end of early June to mid August and the number of days of rainfall increased and the rainfall increased obviously. According to the weather system affecting rainfall, the continuity of rainfall and the time of flood occurrence, the rainfall from June to August 1998 was divided into five main rainfall processes. Namely June 14th ~ 24th, July 5th ~ 10th, July 17th ~ 21st, July 22nd ~ 30th and April 2nd ~ 14th. In view of the five main rainfall processes, the meteorological causes of each rainfall process were analyzed from two aspects of circulation situation and physical field. For the physical quantity field, the two basic aspects of the dynamic factor and the water vapor factor were mainly considered. Therefore, the basic factors such as vorticity, divergence, vertical velocity and water vapor flux were selected to analyse.
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