Characteristics Of Dry-wet Coupling Between Soil Moisture And Precipitation Over The Tibetan Plateau During Late Spring

The Tibetan plateau（TP）is one of the global hotspots for land-air interactions.The coupling of soil moisture and precipitation is both one of the important links and a frontier hot issue in land-air interaction research.During the spring of the seasonal transition,the most prominent feature of the surface processes on the TP is the melting of the soil.Soil melting accompanied by changes in the magnitude of soil moisture and its spatial distribution on the TP can make effects on the distribution of surface sensible and latent heat by altering the surface energy balance,and in turn by influencing plateau thermodynamics.It plays an important role in the evolution of weather and climate on the TP,East Asia and even globally.Focusing on the spring of soil melting stage,the analysis and understanding of the coupling characteristics of soil moisture and precipitation on the TP can not only provide a deeper understanding of the land-air interaction on the TP,but also provide a theoretical basis and scientific basis to deepen the understanding of the role of the land-air interaction on the TP in the development and evolution of weather and climate.Based on the spatial and temporal characteristics of soil moisture and precipitation on the TP in spring,we analyze the type of dry and wet coupling of soil moisture and precipitation and their spatial distribution characteristics in May on the TP using a combination of station observation data,reanalysis data and numerical simulation.The structural characteristics of the PBL in different dry and wet coupling types on the TP and the role of the turbulent vortex diffusion process of the PBL in the coupling of soil moisture and precipitation on the TP were explored through numerical simulation experiments.（1）The spatial distribution of spring soil moisture and precipitation on the TP averaged from 1979 to 2019 is basically consistent,both showing a decreasing feature from the southeast to the northwest.The soil moisture in the Qaidam Basin is the smallest（<0.1 m~3/m~3）and corresponds to the least amount of precipitation.The interannual variability of soil moisture in the central and western regions of the TP（west of 90°E）is large.On the interannual scale,precipitation in the eastern part of the TP（east of 90°E）showed an increasing trend（about 4.74 mm/10a）from 1979 to 2019.On the seasonal scale,precipitation on the TP in the latitudinal direction shows a bimodal distribution with time,with the most precipitation in July and September,while precipitation on the TP in the longitudinal direction shows a unimodal pattern,mainly concentrated in May to October.On the monthly scale,soil moisture increases significantly from April to May.There is a significant lagged correlation between soil moisture and precipitation on the TP from 0 to 4 days in May.（2）The coupling of soil moisture and precipitation on the TP in May has different characteristics.The central and western part of the TP（31°～36°N,80°～96°E）is the wet coupling region,the dry coupling region is concentrated in the Qaidam Basin,while the transition region is mainly distributed in the southeastern part of the TP（27°～32°N,96°～105°E）.The key variables characterizing the dry-wet coupling（convective triggering potential/low-level humidity index,CTP/HI）have significant spatial and temporal differences on the TP.The CTP and HI range from-600 to 400 J/kg and 3 to34°C in the wet coupling region,and range from 100 to 1000 J/kg and 10 to 60°C in the dry coupling region,and range from 100 to 400 J/kg and 10 to 34°C in the transitional region.In the dry coupling region,both CTP and HI are about twice as high as in the wet coupling region,which indicates that the coupling differences between soil moisture and precipitation on the TP correspond to significant differences in atmospheric stability and water vapor content in the PBL.The dry coupling region corresponds to low soil moisture（0.07 m~3/m~3）,the transition region is corresponding to high soil moisture（0.29 m~3/m~3）,and the wet coupling region corresponds to an intermediate soil moisture content（0.26 m~3/m~3）.The dry and wet coupling between soil moisture and precipitation on the TP is related to the magnitude of soil moisture.（3）The PBL process is the key link connecting the soil moisture and precipitation coupling process.The analysis of the PBL thermodynamic structure and its characteristics in the soil moisture and precipitation coupling relationship on the TP using the model shows that the turbulent diffusion process in the dry coupling region on the TP is stronger,close to twice as strong as that in the wet coupling region and the transition region.The water vapor saturation in the lower atmosphere is much greater in the wet coupling region than in the dry coupling region.The boundary layer in the transition region has the highest water vapor content of the three regions,but the degree of water vapor saturation is less than in the wet coupling region.The role of the entrainment is to transport the heat and momentum from the free atmosphere into the PBL,while transporting the water vapor from the PBL into the free atmosphere.For the effect on the PBL structure,the entrainment effect is small compared to the turbulent diffusion effect.（4）Based on numerical sensitivity experiments,the role of momentum and heat turbulent eddy diffusion processes in the PBL in the dry-wet coupling of soil moisture and precipitation on the TP is verified.The results show that the enhanced turbulent diffusion in the PBL favors the development of wet coupling and transition region to dry coupling region.This implies that the dry-wet coupling of soil moisture and precipitation on the TP is not only related to soil moisture,but also to vertical turbulent eddy diffusion process in the PBL.Specifically,enhancing the turbulent diffusion of heat（K_h）and momentum（K_m）in the PBL leads to a decrease in water vapor in the atmosphere of the wet coupling and transition region in the southwestern part of the TP,a decrease in water vapor saturation,and an increase in temperature,which further leads to a decrease in precipitation.The opposite result is obtained after reducing the intensity of turbulent diffusion.The enhancement of K_m and K_h plays a decreasing and increasing role in the lower atmospheric water vapor,respectively,and has an opposite effect on the change of the coupling relationship between soil moisture and precipitation.The enhancement of K_h has a greater effect on the lower atmospheric temperature,making the lower atmospheric temperature decrease and the upper atmospheric temperature increase.