Statistical Analysis And Numerical Simulation Of Local Soil Moisture-precipitation Coupling In Summer In China And Surrounding Areas

Land-atmosphere interaction is the main driving force for the Earth's surface water and energy budget,and it has an important impact on the climate system.Soil moisture-precipitation coupling is one of the cores of land-atmosphere interactions,Previously,much attention has been paid to the effect of soil moisture on Precipitation over large-scale spatial and temporal scales.In recent years,the effect of soil moisture change on local precipitation through boundary layer process has gradually attracted people's attention.however,the direct attribution research has been limited due to the complexity of the process and the lack of observational data.In recent years,there have been some researches on the methods and indices of local land-atmosphere coupling,But most of these methods have been applied to North American research.and there are few studies on East Asia.Therefore,by referring to the existing methods and indexes,and based on statistical analysis and numerical simulation,this paper studies the characteristics of local soil moisture and precipitation coupling in summer in China and surrounding areas.In this paper,we firstly calculate the local soil moisture-precipitation coupling characteristics from the point of climatology view,and select the potential convection day and convective precipitation day to calculate the sensitivity of convective precipitation to morning evaporation and soil moisture in the afternoon;Secondly,numerical simulations of two local strong convection cases in the afternoon are carried out to investigate the sensitivity of the afternoon strong convection simulation to different boundary layer parameterization schemes under the same surface conditions,and the sensitivity of the afternoon strong convection simulation to different soil moisture condition under the same boundary layer scheme.What is more,the land-atmosphere coupling diagnosis was performed for different simulation schemes.The main conclusions of this paper are as follows:(1)Soil moisture(SM),latent heat flux(LH)and lifting condensation level(LCL)data of four times a day in the summer of 32 years were used to calculate the land-atmosphere coupling strength of the land surface segment and the atmospheric segment.The results show that the strongest coupling moments of SM-LH and LH-LCL are at around 14 o'clock local time.Thecommon strong coupling areas are the arid and semi-arid areas such as the Mongolian plateau,the great desert of northwest India and parts of central Asia,which are considered as the hot spots of land-air coupling in summer.The north of India appeared as a hotspot of local land-atmosphere coupling only in early and middle June.In southern China,northeastern China,and southeastern Russia,there are significant negative correlations both in the land segment and atmospheric segment,indicating that the evaporation in this area is not limited by soil moisture.In addition,due to the influence of summer monsoon,the coupling of land surface segment in north China has seasonal signal conversion,showing positive coupling in June and negative coupling in July and August.Due to the influence of the Indian monsoon,there is also a seasonal coupling signal conversion in northern India,which shows positive coupling in the first half of June and negative coupling after the second half of June.(2)Based on the statistics of potential convection days and convection precipitation days in32 years,it is found that there are more potential convection days and fewer convection precipitation days in the Mongolian plateau region and the Indian desert,so the probability of convection precipitation is low there.Relatively speaking,there are fewer potential convection days and more convective precipitation days in the northern Tibetan Plateau and Myanmar,so the probability of convective precipitation is high there.Relatively speaking,there are fewer potential convection days and more convective precipitation days in the northern Tibetan Plateau and Myanmar,so the probability of convective precipitation is high there.Judging from the interannual variations of the two types of events,the trends of them are basically the same,and there is a significant reduction after 2000.From the perspective of changes in the summer,the trends of them are opposite.The potential convective days in June(convective precipitation days)are the most(least),and the potential convective days in July(convective precipitation days)are the least(most)months,so the probability of convective precipitation in June is the smallest,and the probability of convective precipitation in July is the largest.From the triggering feedback strength index(TFS and TFS_SM)of the land surface and the atmospheric segment,the Mongolian Plateau,Northwest India and Myanmar are the common high-value areas of the two indices,that is,there is a significant positive correlation among soil moisture,evaporative fraction and convective precipitation probability in this area.In Myanmar,because of the long-term surface humidity,the range of soil moisture change is small,and the changes of sensible and latent heat fluxes are mainly affected by atmospheric conditions.Therefore,the region is not a land-atmosphere coupled hotspot.However,in Mongolia plateau and northwest India,,the evaporation is mainly limited by the soil moisture,and the variation range of soil moisture is large enough,so the variation of soil moisture in the morning can significantly affect the occurrenceprobability of convective precipitation in the afternoon.In addition,the magnitude of the trigger feedback intensity is significantly larger than the amplified feedback strength.Therefore,the soil moisture and evaporative fraction mainly affect the probability of precipitation.Once the precipitation is triggered,the precipitation intensity will not be affected by the soil moisture and evaporative fraction.(3)The 11 boundary layer parameterization schemes of WRF model were used to simulate the afternoon local thermal convection in Nanjing and Shanghai respectively,and the radar data assimilation was added to conduct the soil moisture sensitivity test.The results show that the UW,GBM and Shin-Hong schemes have better simulation effect on local thermal convection,while the non-local ACM2 and MRF schemes have poorer simulation effect on local thermal convection.Under dry(wet)soil conditions,the simulated convective echo intensity is weak(strong)and scattered(concentrated),and the simulated convective echo location is also biased under different surface conditions.From the mechanism of soil moisture affecting precipitation,soil moisture significantly affects the sensible heat and latent heat flux of the underlying surface of the farmland.When the soil moisture decreases,the sensible heat flux increases and the latent heat flux decreases,and vice versa.Therefore,the change of soil moisture will not only affect the near-surface water vapor content through evaporation,but also affect the near-surface air temperature,thus affecting the urban heat island effect.The horizontal inhomogeneity of thermal conditions will generate local sub-circulation.Thus affect the location of local strong convection prominently.(4)Using the energy mixing diagram method to diagnose the local land-atmosphere coupling of different simulation schemes,and evaluate the contribution of surface,entrainment and advection flux to the development of the planetary boundary layer(PBL).Because of using the same land surface model,the simulated surface water and heat fluxes between different boundary layer schemes are very similar;The boundary layer simulated by QNSE,ACM2,BouLac and MRF schemes has higher height,more intense entrainment and smaller average specific humidity in the boundary layer;In UW and GBM schemes,the boundary layer height is lower,the entrainment is weaker,and the average specific humidity in the boundary layer is larger.In the case of Nanjing,the near surface environment is relatively dry,the boundary layer height is generally high,and the water vapor mainly comes from surface evaporation,so the wetter boundary layer is beneficial to trigger local convection.In the case of Shanghai,The near-surface environment is relatively humid,and the water vapor in the boundary layer mainly comes from surface evaporation and horizontal advection.However,the advection transport of water vapor simulated by ACM2 and MRF schemes is very weak,so the simulated convective precipitationintensity is weak.In addition,the land-air coupling varies greatly under different soil moisture conditions.When the soil changes from dry to wet,the sensible heat flux decreases,the evaporation of the surface strengthens,the turbulence intensity of the boundary layer weakens,the entrainment of dry air weakens,and the height of the boundary layer decreases.