The Numerical Simulation Of Land-atmosphere Interaction Process Over The Tibetan Plateau And Its Impact On Plateau Summer Precipitation

The Tibetan Plateau(TP)is the most complex and highest plateau in the world,with an average altitude of over 4,000 m,and is known as the "Roof of the World" and the "Third Pole of the Earth".The thermal and dynamical effects of the TP not only regulate atmospheric circulation,energy and moisture cycles in Asia,but also have a profound impact on global climate and the environment.The thermal effects of the TP on the free-layer atmosphere are mainly through the land-atmosphere interaction processes.Due to the complex topography and underlying surface condition over the TP,there are significant differences in the moisture,heat and momentum flux exchange in different area,which in turn affect the development of the boundary layer structure of the upper atmosphere and the regional circulation of the temperature and water vapor fields.Therefore,in order to reveal the regional dynamical and thermal effects of the TP and their impact on global-scale climate effects,land-atmosphere interaction processes is urgently needed to be analyzed.Due to the harsh environment and complex topography of the TP,observations in the western part of the plateau and complex mountainous areas are still limited.Moreover,The observation data are insufficient to represent the regional scale conditions,which hardly meet the analysis of surface heat source conditions and land-atmosphere interaction processes over the TP.Because of the complete physical process and the high spatiotemporal resolution,the regional climate model(RCM)has became a powerful tool to study the multi-temporal scale land-atmosphere interaction processes over the TP.The mechanism of the impact of surface heating field on regional energy and water budget was initially revealed.However,the parameterization scheme in current RCM is insufficient to actually characterize the heterogeneity surface conditions over the TP,leading to large uncertainties in the simulations of land-atmosphere interaction processes.Therefore,in order to reveal the mechanisms of how land-atmosphere interaction processes affecting convection and precipitation activities,there is a need to effectively improve the accuracy of precipitation simulations over the TP,and to improve the key parameterization schemes for surface-atmosphere coupling for the actual surface conditions on the plateau.Accordingly,this paper conducts related research from the following four aspects:Firstly,in order to reduce the influence of the size of the nested domain of the model and prevent the accumulation of bias between the model field and the driving field,the influence of spectral nudging(SN)simulating precipitation over the TP was evaluated based on WRF(Weather and Research and Forecasting).The results showed that using the default spectral approximation scheme did not reduce the bias of the overestimation of precipitation simulations well.Eliminating the nudging of potential temperature and water vapor mixing ratio in the model field,and only nudging the horizontal wind field and geopotential height could effectively improve the simulation of precipitation over the TP.Compared with the traditional continuous integration simulation and the default SN,this scheme reduced the overestimation of the water vapor transport on the southern slope of the TP and the upward movement intensity over the southern slope.Therefore,the water vapor arrived the interior of the TP was weakened,which reduced the overestimation and improved the accuracy in simulating precipitation intensity,daily precipitation peaks and its spatial distribution over the TP.Secondly,differences of three land surface schemes CLM4,Noah-MP and PX in the WRF model in summer precipitation simulation over the TP were revealed.Based on the analysis of the precipitation area,Noah-MP effectively reduced the error of the precipitation overestimation over the central and western TP.Beside,Noah-MP also reduced the false alarm rate and more accurately simulated the occurrence of nonprecipitation events.The study further analyzed the feedback mechanism between soil moisture-energy flux-precipitation.Result showed that Noah-MP simulated relatively drier soil moisture,which in turn reduced the latent heat flux and atmospheric water vapor content,thereby reducing the moist static energy,which was beneficial to decrease the overestimation of the frequency and intensity of precipitation over the TP.The advantages of Noah-MP also provide a basis for studying the interaction between the land surface and atmosphere.Thirdly,the topographic turbulent orographic form drag(TOFD)scheme was applied to reveal the impact of the complex terrain on regional climate simulations over the TP.Results showed that BBW04 TOFD scheme effectively improved the simulation of wind speed and its spatial distribution over the TP.By more clearly resolving the turbulent drag of the complex terrain,the water vapor transport was weakened over the southern slope of the TP,and reduced the overestimation of precipitation over the southern TP.At the same time,the TOFD scheme can reduce the underestimation of the sensible heat flux in the Mt.Everest region.In addition,the application of the TOFD scheme improved the simulation of the spatial heterogeneity of the surface roughness at the complex underlying surface over the eastern TP,and the effect of the turbulent drag on wind speed and precipitation simulation.Finally,by incorporating different thermal roughness length(Zoh)parameterization scheme in Noah-MP,the simulation of the land-atmosphere water and heat exchange process and precipitation over the TP was improved.Results showed that the overestimation of the land-atmosphere water and heat exchange strength and sensible heat flux,while the cold bias of surface temperatures was also decreased.The study further revealed the relationship between the variation of surface heating field and precipitation over the TP.Due to the enhancement in surface and near-surface temperature,the latent heat flux and surface evaporation over the eastern TP was increased,leading to a stronger CAPE and the increase of moist static energy within the boundary layer,which in turn increased of convective precipitation.Over the southern slope of the TP,when the sensible heat flux and latent heat flux decreased,the intensity of the surface heating field was weakened,which reduced the divergence of water vapor flux in the whole layer of atmosphere,and thus resulting in an increase in precipitation.