Development Of Land Surface Schemes On The Tibet Plateau: A New Fractional Vegetation Coverage Scheme And A Big Root Model

The Tibetan Plateau?TP?plays key roles in the formation of Asian monsoon.Improving land surface parameterization schemes is crucial to enhance scientific understanding of complex land-atmosphere interactions on the TP.In this study,Common Land Model?CoLM?was set up on the TP and forced by CMFD dataset,and its simulations were evaluated by observations from three soil moisture and temperature monitoring networks on the TP.In general,temporal dynamics of soil moisture and temperature was well capture by the simulations,but,estimation of soil temperature in regions covered by grassland showed obvious overestimations.Considering the unique characteristics of the grassland on the TP,three improvement strategies were proposed to improve the land surface model:The first improvement is to replace the default soil texture data with the newly The Global Soil Dataset for Earth System Modeling?GSDE?and to include the effect of organic matter in calculating soil thermal and hydraulic parameters.Among them,the change of soil texture mainly affected the soil water simulation results of arid areas and deep soils in the TP,and soil organic matter would improve the soil moisture simulation of surface soils in grassland.However,the improvement of soil parameters has little effect on the simulation results of soil temperature.Secondly,A dynamic FVC parameterization based on clumping index?CI?and leaf area index?LAI?is developed?SMFVC?.The results show that soil temperature simulation by CoLM is significantly improved by using SMFVC,and the highest reduction of root mean square error?RMSE?reaches to 10.6?.SMFVC also shows ability in improving albedo simulation in summer.In addition,compared to the FVC parameterization scheme based on remote sensing data?RSFVC?,SMFVC shows more significant improvement,while RSFVC overestimates soil temperature in summer.Thirdly,we developed a“big root”model to further improve soil moisture simulation in CoLM.The results show that the big root model simulates soil moisture in the vegetation cover region of the TP better than the original soil moisture module,and reduces the underestimation of the surface soil moisture simulation during the non-freezing period.When radius of rhizosphere in big root model takes the optimal value,decrements of RMSE of 5 cm soil moisture simulations in Naqu and Maqu networks are 0.03 m³ m^-3 and 0.04 m³ m^-3,respectively.In addition,we identified a significant linear relationship between the optimal radius of rhizosphere and soil organic carbon.In this study,by introducing a new FVC parameterization scheme?SMFVC?and a big root model in the CoLM,the simulations of surface energy budget and moisture budget are improved,respectively.These two schemes can be easily coupled into other LSMs and/or earth system models?ESMs?,which are highly expected to further improve the performance of models,especially in arid/semi-arid grassland regions.