Comparative Study On Predecessor Rain Events Associated With The Bay Of Bengal Tropical Cyclones In Pre-and Post-monsoon Seasons

The tropical cyclone in the Bay of Bengal is one of the weather systems that cause heavy precipitation in China's low-latitude highlands.Before the main circulation reaches the land and reaches the low-latitude plateau area,it can cause the predecessor rain event in the low-latitude plateau area.Based on NCEP reanalysis data,era-interim reanalysis data,China meteorological administration station precipitation data,and JTWC best track data,this paper identified 21 cases of predecessor rain events caused by tropical cyclones in the low-latitude plateau region in the Bay of Bengal from 1981 to 2012.They show a typical bimodal distribution,mainly occurring in April—may in the pre-monsoon season and October—November in the post-monsoon season,and less during the monsoon outbreak.However,the probability of PRE events caused by tropical cyclones in the Bay of Bengal in the low-latitude plateau area before and after the monsoon onset was significantly different.Compared with the late monsoon,the pre monsoon on the low-latitude plateau caused by the cyclone in the Bay of Bengal in the early monsoon outbreak were more likely to occur,and the precipitation on the low-latitude plateau caused by the PRE event was stronger,with a larger impact range.The PRE events in the Bay of Bengal were caused by the anomalies of the tropical cyclone in the Bay of Bengal,the subtropical high in the south China sea,the east Asian trough,and the high altitudejet stream.However,there were significant differences in the anomalous patterns of the circulation before and after the onset of the monsoon.In the early stage of the monsoon onset,tropical cyclones in the Bay of Bengal to match the subtropical jet stream anomaly can only cause low latitude plateau PRE event,during the monsoon onset,the PRE event tend to need more match the abnormal circulation factor,high and low level circulation anomaly threshold requirements higher,relative to a small probability event,low probability of occurrence.The PRE events caused by the Bay of Bengal tropical cyclone on the low latitude plateau before and after the onset of the monsoon are the result of the interaction of the Bay of Bengal tropical cyclone with the subtropical high pressure,high altitude jet stream and other systems.PRE rainfall are in thousands of miles away from the Bay of Bengal tropical cyclone body circulation of the right front,north of low pressure circulation over the Bay of Bengal on the east side with the south China sea area between the subtropical high pressure gradient has resulted in a lower troposphere have strong south airflow to the abundant water vapor from the Bay of Bengal tropical cyclone near to the low latitude plateau,causing the lower humidity increase in the low latitude plateau area,the formation of convection instability.At the same time,PRE precipitation areas were located on the equatorial side of the entrance area of the high-level subtropical jet,and the coupling of the secondary circulation of the high-level jet and the low-level circulation jointly caused the occurrence of PRE events.However,in the early and late monsoons,the occurrence mechanism of PRE events was also significantly different,which was mainly reflected in the different degrees of the coupling between the tropical cyclone in the Bay of Bengal and the upper and lower strata system and the contribution of topographic forcing.