The Impacts Of Spring Soil Moisture Over The Qinghai–Xizang Plateau On Summer Precipitation In China

Soil moisture,a comprehensive indicator of land surface and hydrological processes,contains much information about land surface and hydrological processes.However,only very rare ground soil observation data are available over the Qinghai-Tibet Plateau.At present,the research about the impacts of soil moisture on summer precipitation is mainly concentrated in Eastern China.Based on a series of numerical simulations and statistical diagnosis,this paper studies the impacts of soil moisture over the Qinghai-Tibet Plateau on summer precipitation in China.Firstly,the Community Land Model version 4.5(CLM4.5)driven by atmospheric forcing data(CRUNCEP)is used to simulate spatial and temporal characteristics of soil moisture in the Qinghai-Xizang Plateau for the period 1981–2016.Compared with station data,two reanalysis products(i.e.,the ERA-Interim and GLDAS-CLM)and the microwave remote sensing FY-3B/MWRI data,it is found that CLM4.5 can well reproduce the spatial and temporal variations of soil moisture in the Qinghai–Xizang Plateau.And the relationship between soil moisture over the Qinghai-Tibet Plateau and the summer precipitation in China was analyzed using SVD method.Finally,the impacts of soil moisture over the Qinghai-Tibet Plateau on summer precipitation in China was analyzed using the synthetic analysis method.The main conclusions are as follows:(1)The results show that the spatial and temporal distributions of simulated soil moisture are consistent with station observations.The simulation is significantly correlated with observations in all soil layers but with high systematic deviations.And the consistency between simulation and observations gradually decreases from the surface downward.The spatial distribution of soil moisture in the Qinghai–Xizang Plateau is characterized by increasing distribution from the northwest to the southeast of the Qinghai-Tibet Plateau.The high-value areas are found in the Sanjiangyuan Wetland and the southeastern Plateau,while the dry regions are located in the Qaidam Basin and the Tarim Basin.Soil moisture generally increases from surface downward.Temporal variation of soil moisture presents an alternate dry and wet pattern.There is little difference in soil moisture variation in different layers.The simulation also reasonably reproduces monthly changes in soil moisture in the summer.Soil moisture in most of the southwestern region including the southern periphery of the plateau obviously increases in the summer.The dry area in the northern Qaidam Basin also markedly shrinks to the north.The simulated soil moisture provides more detailed description of spatial distribution and monthly changes of soil moisture in the summer compared with reanalysis data and microwave remote sensing data.(2)The temporal scale of soil moisture memory over the Tibetan Plateau in May varies from 20 days to 60 days,and the memory of soil moisture in May have an impact on the short-term climate in July.The surface soil moisture in the south-central and northern Tibetan Plateau is positively related to the summer precipitation in South China,while it is negatively related to that in the middle and lower reaches of the Yangtze River.The surface soil moisture in the western Tibetan Plateau has a positive correlation with summer precipitation in the middle and lower reaches of the Yangtze River.There is a negative correlation between the surface soil moisture in the west part of the plateau and the summer precipitation in South China.The middle and deep(i.e.,10~50cm,50~138cm)soil moisture in the key area of the Tibetan Plateau is positively related to the summer precipitation in South China,while it is negatively related to that in the middle and lower reaches of the Yangtze River.The wetter the surface soil moisture in the south-central and northern Tibetan Plateau in May,the lower surface temperature,less sensitive heat flux and more latent heat flux,leading to weaker surface heating.The surface soil moisture in the southeastern and western Tibetan Plateau is negatively related to the summer precipitation in Southwest China and eastern part of the Northwest China,while it is positively related to that in western part of the Northwest China.And the surface soil moisture in the northern Tibetan Plateau is positively related to the summer precipitation in Southwest China and eastern part of the Northwest China,while it is negatively related to that in western part of the Northwest China.(3)The possible mechanism of the influence of soil moisture in the key areas of the Qinghai-Tibet Plateau in late spring on precipitation of July in East China: the wetter the surface soil moisture in the southeastern and northern Tibetan Plateau in May,the lower surface temperature,less sensitive heat flux and more latent heat flux,leading to weaker surface heating.And the dryer the surface soil moisture in the western Tibetan Plateau,leading to stronger surface heating.Negative anomalies enable the energy exchange between the land and the atmosphere to reach equilibrium at lower temperatures.Lower surface temperatures reduce the amount of heat transferred from the surface to the atmosphere,which reduces the amount of heat obtained in the lower atmosphere.It is favorable for the stronger South Asia high over the Tibetan Plateau and the weaker Western Pacific Subtropical High.With the convergence of water vapor in South China and North China,corresponding to the updraft,the precipitation in South China and North China is more.Corresponding to the subsidence airflow,the precipitation in the middle and lower reaches of the Yangtze River and Southwest China regions is less,and vice versa.(4)The possible mechanism of the influence of soil moisture in the key areas of the Qinghai-Tibet Plateau in late spring on precipitation of July in West China: the wetter the surface soil moisture in the southeastern Tibetan Plateau in May,the lower surface temperature,less sensitive heat flux and more latent heat flux,leading to weaker surface heating.And the dryer the surface soil moisture in the northern Tibetan Plateau,leading to stronger surface heating.With the convergence of water vapor in Western Northwest China,corresponding to the updraft,the precipitation in Western Northwest China is more.Corresponding to the subsidence airflow,the precipitation in Eastern Northwest China and Southwest China regions is less,and vice versa.