Synoptic Classification And Precipitation Climate Characteristics In Summer Over Yangtze River Basin Based On Self-Organizing Map

This paper uses the ERA-Interim daily reanalysis data and the CN05.1 grib observation data by the National Climate Center to classify the sea level pressure in summer over the Yangtze river basin during 1979-2015 based on the self-organizing map(SOM).The spatial and frequency characteristics of the 25 SOM synoptic patterns as well as the relationship between the main transfer paths and the change of the rain area are analyzed.The K-means clustering method is used to cluster the SOM weather type again,and the circulation characteristics of the extreme precipitation events and the persistent extreme precipitation events in different types are compared and analyzed.The conclusions are as follows:(1)25 kinds summer surface synoptic patterns over the Yangtze river basin are obtained by SOM.We discovered the stability and shift of synoptic patterns are related to the strength of the weather systems.The more stable synoptic pattern is with the stronger high and low pressure system as well as the higher stagnation frequency of synoptic patterns.On the contrary,the synoptic pattern is more unstable.Three systematic evolution paths associated with local precipitation were found depend on the SOM synoptic patterns transition probability.Warm air force on the first path is strong and push the front northward with subtropical pacific high pressure northward,which produced Northern Yangtze River rainfall pattern(NYRP)and mostly occurred in July.The second Path reflects that the cold air forces spurs the southward of front and resulted in Yangtze River rainfall pattern(YRP),which were likely to occur in June and July.Path 3 represents the influence of the typhoon movement to the precipitation in the Jiangnan region of lower reaches of the Yangtze River,which formed Southern Yangtze River rainfall pattern(S YRP)and it was concentrated in August.(2)The K-means clustering method divides the regional extreme precipitation events related to SOM synoptic patterns into three categories.The first and second regional extreme precipitation events not only is the most frequent and strong persistent,but also occurs in June and July more frequently.The frequency of third category event is the lowest and the persistence is weak.The first category event accounts for 40.3%of the total samples,and precipitation center is located in Hubei and Anhui places.The second category event is 53%,the largest precipitation center is located in the north of Jiangxi,Hunan,and the third category event is 6.7%of the total sample.The center is mainly located in the Zhejiang over the lower reaches of the Yangtze River and Hubei.The frequency of extreme rainfall in northern Yangtze River is lower than the south region,the frequency of extreme rainfall in the central and eastern regions is higher than in the west.(3)In the first category precipitation events,the north region was controlled by the circulation of the anomalous cyclones on the lower layer and by the abnormal anticyclone circulation on the upper layer,which has a good baroclinic structure to promote the occurrence of extreme precipitation events in northern area.In the second and third types of events,the anomalous cyclones in the middle and lower layers circulate southward.The anomalous anticyclonic circulation in the upper reaches is more westward,which promotes the occurrence of extreme precipitation events in the south of the Yangtze River.Among them,the abnormal vertical motion of the third vertical circulations is the strongest and the rising center reaches 20 Pa/s.The common feature of the three types is that the southwestern warm and humid air stream from the Bay of Bengal is also the main source of water vapor for extreme precipitation events in the Yangtze River basin.(4)Among the first-type and second-type persistent extreme precipitation events,the West Pacific Subtropical High(WPSH)and the South Asian High-Pressure(SAH)have opposite directions before the occurrence and run backwards after the occurrence.The occurrence and maintenance of the incident are both closely related with the stability of WPSH and the location of the SAH.The difference is mainly reflected in the fact that WPSH of the second-type event is more easterly and southerly than the first-type when extreme precipitation occurs at the upper and middle-level,and therefore the extreme precipitation falls southward.When the second-type of extreme precipitation events occur before and after,the WPSH and the SAH Movements are slow and system is stable,therefore the extreme precipitation is more persistent.At the lower level,the disruption of the first-type of extreme precipitation event was related to the high pressure generated in the north China region at 700 hPa,and the second type of persistent extreme precipitation decreased as the area's cyclone weakened.