The Impact Of Land-atmosphere Processes Over The “sensitivity Area” Of The Tibetan Plateau And Moisture Transport On Meiyu Precipitation Over The Middle And Lower Yangtze River

The Tibetan Plateau?TP?is known as the?roof of the world?,the world's?third pole?and is characterized by complex terrain in different regions.It has strong dynamic and thermodynamic impacts on the atmosph eric circulation over East Asia and global and has become the critical?sensitive area?of global weather and climate change.Located downstream of the TP,the middle and lower Yangtze River in China frequently experiences torrential rainfall and severe rainstorms within the zonally elongate Meiyu belt.This paper dissects the synergy effect of multiple factors between apparent heat source over main body of TP,surface latent heat flux?SLHF?over the?sensitive area?,East Asian monsoon moisture transport and crest line of the subtropical high through comprehensive diagnostic analysis and numerical simulation test by analyzing the relevant characteristics of climatic scale.From the synoptic scale perspective,multi-factor comprehensive diagnosis analysis and simulation test of precipitation process in meiyu period in the middle and lower reaches of the Yangtze river were carried out.The climatic correlation factors influencing the abnormal precipitation process in the meiyu period in the middle and lower reaches of the Yangtze river were further verified.This paper reveals the possible mechanism of regional distribution of precipitation in the middle and lower Yangtze River in Meiyu period.The main conclusions are summarized as follows:?1?The precipitation reaches its peak in July with high rainstorm occurrence in the middle and lower Yangtze River.The the middle and lower Yangtze River is located at a core zone with the highest rainstorm frequency within the east-west stretched Meiyu belt.It reveals that there is a significant correlation between the inter-annual variation of drought or flood in the middle and lower Yangtze River and the land-atmosphere processes over southeast margin of the TP?SEMTP?by tracking and analyzing the upstream?strong signal?.Results show that the SEMTP is the most?sensitivity area?of the?strong signal?.Such a signal over SEMTP is more pronounced in SLHF than surface sensible heat of the land-air processes.?2?From the perspective of climatic correlation?interannual variation?,the distribution characteristics of water vapor transport in the plateau and monsoon regions are different in the drought and flood years of the the middle and lower Yangtze River.The SLHF in the?sensitive area?of the plateau is not only a?strong signal?of the precipitation circulation pattern in the meiyu period of middle and lower Yangtze River,but also closely related to the apparent heat source Q₁ of the main body of the plateau.The increase of apparent heat source Q ₁in the main body of the plateau modulates the SLHF and ridge position of subtropical high pressure in the southeastern edge of the plateau.In July,cyclonic?anticyclonic?circulation in the TP and adjacent regions at the surface is enhanced when the apparent heat source?Q1?over the TP is stronger?weak?.In the meantime,low?high?SLHF over the SEMTP corresponds to high?low?humidity at upper levels,strong?weak?zonal moisture transport and a southerly?northerly?pattern of East Asian summer monsoon?EASM?moisture transport circulation in July.Low?high?SLHF over the SEMTP also corresponds to high?low?cloud,precipitation and rainstorms in t he the middle and lower Yangtze River under the background of a southward?northward?shifted subtropical high system.The anomalous moisture flow originating from the mid to low latitude oceans,converges in the east of the SEMTP and turns eastward?northward?.Northward?eastward?movement of the moisture transport flux corresponds to high?low?SLHF.The two different EASM water vapor transport circulation patterns described the response characteristics of the?strong signal?of SLHF over the SEMTP and the apparent heat source Q₁ over the mainbody TP and its water vapor transport structure on the plateau.?3?In this paper,a new index of the ridge line of the western Pacific subtropical high pressure relative to the structure of water vapor transport in the meiyu period of the middle and lower reaches of the Yangtze river is proposed,which more reasonably characterizes the ridge line position of the subtropical high pressure and the north-south movement characteristics of the meiyu period in the middle and lower reaches of the Yangtze river.Under the background of plateau thermal drive,the SLHF over the SEMTP in July showed a significant climate characteristic?strong signal?,which had a more reasonable response relationship with the new ridge index of the western Pacific subtropical high pressure.In other words,the subtropical high pressure ridge line related to water vapor transport is south?north?,the SLHF over the?sensitive area?of the plateau is weak?strong?,so the precipitation in the middle and lower Yangtze River is high?low?and the precipitation in the eastern and northern China is low?high?.?4?On synoptic scales,the?sensitive area?over plateau is derived from the upstream?crossing point?where the middle and low latitude ocean water vapor flows converge and the water vapor?transfer station?over plateau based on the comprehensive diagnosis analysis and numerical simulation test of abnormal precipitation process in the middle and lower re aches of the Yangtze river.The change of SLHF in this area is the precursor of the change of the circulation pattern of abnormal precipitation in the meiyu period in the middle and lower reaches of the Yangtze River.Samples of high and low values of SLHF in the?sensitive area?over SEMTP were selected day by day in July of the typical year.It was found that the SLHF in the?sensitive area?over the SEMTP was abnormally low?high?,the precipitation system with a 3-day lag had moved out from the plateau,and the location of the high-correlation precipitation rain belt with a 4-5 day lag and the direction of water vapor transport were significantly different.The characteristics of low?high?SLHF migration in the?sensitive area?over SEMTP correspond to the easterly?northern?precipitation system,that is,the rain belt is located in the middle and lower Yangtze River?northern China?.The results showed that the apparent heat source Q1 in the main body of the TP and SLHF over the?sensitive area?was significantly correlated with the distribution of water vapor and precipitation in the the middle and lower Yangtze River 3-5days later,and had the precursory"strong signal"characteristics.?5?Numerical simulation and sensitivity analyses using the mesoscale Weather Research and Forecasting?WRF?model further reveal that SLHF over the SEMTP shows pronounced?strong signal?features.The variation of intensity of SLHF over the SEMTP corresponds to the moisture transport structure of the Meiyu rain belt and the location of the ridge of the subtropical high in East China.An increase in the apparent heat source Q₁ over the main body of the TP might explain the?strong signal?over the SEMTP.