Diversity Characteristics And Mechanism Of Intraseasonal Variation Of Summer Precipitation In Southeastern China

The middle and lower reaches of the Yangtze River Valley and the southern China are two regions with the most summer-mean precipitation in China,and there are significant intraseasonal oscillation modes in the two regions.Therefore,the study of the variation characteristics and related mechanisms of intraseasonal precipitation in the two regions will help to understand the causes of precipitation deeply and provide a theoretical reference for the further development of the extension forecast of precipitation.Previous studies often focus the general characteristics of intraseasonal precipitation and circulation anomalies through the composition results of all rainfall events,while this paper focuses on the diversity characteristics of intraseasonal precipitation through objective classification methods such as cluster analysis.Further,this research will clarify the physical mechanisms of different types of precipitation events.The main conclusions are as follows:(1)Two typical types of 10-30-day rainfall events over the middle and lower reaches of the Yangtze River Valley were identified.Through analyzing the western Pacific subtropical high(WPSH)index and South Asian high index in the peak phase of the 10-30-day persistent heavy rainfall events,it's found that the WPSH shifts westward in most rainfall events(80%),while the South Asian high extends eastward in some events(64%)and retreats westward in some events(36%).Thus,we propose that there're two typical types 10-30-day rainfall events in the middle and lower reaches of the Yangtze River Valley,namely “eastward South Asian high event” and “westward South Asian high event”.The former corresponds to the eastward extension of the South Asian high and the westward extension of the WPSH,while the latter corresponds to the westward retreat of the South Asian high and the westward extension of the WPSH.(2)Different precipitation triggering mechanisms in the “eastward South Asian high event” and “westward South Asian high event” were clarified.For the “eastward Southeast Asian high event”,there's a strong anticyclonic circulation anomaly over the western North Pacific prior to rainfall occurrence and the related southwesterly wind anomaly prevails through southeastern China,which could advect moisture into the middle and lower reaches of the Yangtze River Valley,moistens the boundary layer,and then induces atmospheric convective instability,finally induces local convective precipitation.For the “westward South Asian high event”,the anticyclonic circulation anomaly over western North Pacific is weak while a strong northerly wind appears north of the middle and lower reaches of the Yangtze River Valley.It could bring the cold air mass southward,initiating frontal rainfall.The zonal changes of two types of South Asian high are affected by disturbances in mid-high latitudes.In the eastward South Asian high event,precipitation around the middle and lower reaches of Yangtze River Valley can promote the eastward extension of the South Asian high through vertical potential vorticity advection.In the westward South Asian high event,large-scale cyclonic circulation anomalies with an equivalent-barotropic structure in mid-high latitudes causes the westward retreat of the South Asian high and induces precipitation over the middle and lower reaches of Yangtze River Valley.(3)Three typical propagation routes corresponding to 10-30-day rainfall events over the southern China were identified.Through the cluster analysis of the time-latitude diagrams of 10-30-day rainfall events over the southern China in summer from 1979 to 2008,it's found that about 80% of the rainfall events correspond to three typical propagation routes,namely,the “southward propagation”events from northern China to South China,the "northwestward propagation" events propagating northwestward from the tropical western Pacific Ocean to the South China,and the "northward propagation" events from the southern South China Sea to South China.The three types of rainfall events are similar in quantity,each accounting for about one third.(4)The physical reasons for the influence of rainfall events with different propagation routes on the interdecadal variation of intraseasonal precipitation in the southern China are expounded.The intensity of 10-30-day rainfall variability over the southern China in summer increased significantly after the early 1990 s.By comparing the differences between the previous stage(P1: 1979-1993)and the latter stage(P2: 1994-2008),we found that the significant increase in the frequency of “northward propagation” events in P2 stage was the main reason for this interdecadal change,while the frequency of “southward propagation” and “northwestward propagation” events and the precipitation intensity of the three types of rainfall events changed little.