The Spatio-temporal Features Of Tibetan Plateau Soil Moisture And Its Influence On Summer Precipitation Over The Yangtze River Valley

As an important factor of surface processes,soil moisture has a great influence on atmospheric circulation and weather climate on local and surrounding areas.The observation sites of soil moisture in the Tibetan Plateau(TP)are sparse and the observation record is limited,which make it difficult to study the climate effects of TP.Firstly we assess the applicability of eight datasets on TP,including reanalysis data,land model simulation and satellite retrivals data.After selecting the optimal soil moisture substitute data,we study the spatial-temporal characteristics of soil moisture on TP,we also focus on its climate effects to the surrounding areas,especially in eastern China.The results show as following:1)The soil moisture has highly consistency among the surface layer,the middle layer and the deep layer,the correlation coefficient between different layers are above 0.8.After comparing the eight substitute datasets with observation on indiduvdual stations and large area,we finally find that the satellite inversion data SSM/I RETRIEVALS has a consistent interannual variation with observed data at most stations.Meanwhile,the performance metrics calculated from daily data shows that the SSM/I RETRIEVALS exhibit a significant positive correlation with every site's observation data.Besides,the standard deviation(SD)is close to the observations in the southeast and central area on TP,it's also applicable to large areas,so the SSM/I RETRIVALS should be the best substitute data for studying the variation characteristics of soil moisture in TP.2)The spatial distribution of soil moisture is similar to precipitation and temperature on each season over TP,it is characterized by a feature with large southern margin and a decline from southeast to northwest.The average soil moisture in the whole TP region has a significant increasing trend on each season,corresponding precipitation and temperature also have an increasing trend.The soil moisture in spring is mainly affected by the temperature in the previous period,but the summer soil moisture is mainly affected by the precipitation in the previous period.In autumn the soil moisture is determined by the combination of precipitation and temperature in the previous period.After removing the linear trend,the interannual cycle of soil moisture in spring is 4 years and 2.5 years,and the interannual variation of soil moisture in summer and autumn is 5 years and 7 years,respectively.3)The soil moisture in spring has two areas with high value of interannual variability and are mainly located in the eastern and western TP,the interannual variability of soil moisture in those two regions are approximately independent.The soil moisture in these two areas are consistent from surface to deep,and can last from spring to summer.Both of them exhibit a closely relationship to the summer precipitation in the Yangtze River valley,but the relationships are exactly the opposite.The east-west "dipole" mode on TP has a significant impact on the summer precipitation in the Yangtze River valley,with a correlation coefficient up to 0.72.A soil moisture index(TPSMI)is defined with eastern key area minus the western key area,it is found that the correlation coefficient between spring and summer TPSMI is 0.56,that is,the anomalous signals of spring soil moisture in the spring can stably continue to summer.4)A larger TPSMI means that the soil moisture of eastern TP is bigger,while the soil moisture of western TP is smaller.In this condition,there is a latent heat source(sensible heat source)in eastern(northwestern)TP.Diagnostic analysis and numerical simulation both indicate that the collective effect of those two heat sources induce a cyclone-anticyclone-cyclone wave train from the west of TP through China mainland to northeast China,which presents a prominent quasi-barotropic structure through the middle and upper troposphere.This has great contribution to the enhancement of Northeast Cold Vortex,which leads to the intrusion of cold air southward.At the same time,the South Asian anticyclone gets enhanced and eastward,while the Sub-tropical anticyclone gets strong and westward,there is a converge of warm moist airflow from south and cold dry airflow from north in the Yangtze River valley,in addition to the strong rising movement,which contribute to the increase of summer precipitation over the Yangtze River valley.On the contrary,when the TPSMI is smaller,the precipitation of the Yangtze River valley is much less.5)The WRF model was used to analyze the physical process of the summer precipitation in the Yangtze River valley influenced by the soil moisture in the TP.The numerical simulation results further validate the conclusions of the above observational analysis.The initial plateau soil moisture anomaly can last until the entire summer period.When the soil in the eastern part of the plateau is wet,and the western part of the soil is dry,the precipitation in the eastern part of the Yangtze River valley is much higher.However,when the soil in the eastern part of TP is dry and the soil in the west is wet,the precipitation in the entire Yangtze River valley is much less.