Surface Energy Flux Simulation At Typical Sites In Permafrost Regions Of The Qinghai-Tibet Platea

Surface energy flux in the permafrost region of the Qinghai-Tibet Plateau is an important variable that affects both the state of permafrost and the state of atmosphere,and also is a basic variable driving climate models.However,the current land surface model has insufficient simulation on surface energy flux in the permafrost region.In this study,new soil layers stratification scheme,new lower boundary depth,organic matter parameterization,vegetation root parameterization and freeze-thaw front parameterization were adopted to improve the Noah MP’s ability to simulate hydro-thermal characteristics and surface energy flux at a permafrost site.Then,the improved model was used to simulate the surface energy flux at Xidatan permafrost site from 2012 to 2016,and the variation characteristics of surface energy flux at multiple time scales were analyzed.The main conclusions of this study are as follows:（1）Increasing the spatial resolution of soil layers,deepening the lower boundary depth,introducing organic matter scheme,introducing vegetation root scheme and introducing freeze-thaw front scheme improved the ability of Noah-MP LSM to simulate soil temperature and soil liquid water content at Xidatan permafrost site,which reduced the mean RMSE of soil temperature from 2.54℃to 0.89℃,reduced the mean MBE of soil temperature from 0.89℃to-0.13℃,reduced the mean RMSE of soil liquid water from 0.1 m³·m^-3 to 0.06 m³·m^-3 and reduced the mean MBE of soil liquid water from 0.01 m³·m^-3 to-0.01 m³·m^-3.（2）Higher spatial resolution in shallow soil layers reduced the impact of Noah-MP surface temperature simulation bias on the simulation of underlying soil layers’temperature,but with the thinner first soil layer,surface temperature change can make larger soil temperature gradient change during the Noah-MP simulation process,which caused the soil heat flux to be more fluctuant and then reduced the stability of Noah-MP surface energy flux simulation.Improving the spatial resolution of soil layers of permafrost active layer can mitigate the stepped characteristics of Noah-MP modeled freeze-thaw front（0℃isotherm）,and then obtained more accurate freeze-thaw front.（3）Coupling organic matter parameterization and vegetation root parameterization to the improved Noah-MP model,which had increased soil layers spatial resolution and deepen lower boundary depth,significantly improved the simulation of soil temperature at Xidatan site:the mean RMSE of soil temperature for 0～3.2 m soil layers decreased from 2.18℃to 1.59℃,and the mean MBE of soil temperature for 0～3.2 m soil layers decreased from 1.18℃to0.83℃.（4）Coupling Stefan method based freeze-thaw front parameterization to the improved Noah-MP model,which had increased soil layers spatial resolution,deepen lower boundary depth,coupled organic matter parameterization and coupled vegetation root parameterization,improved the simulation of permafrost water and heat,thus decreased the mean RMSE of soil temperature for 0～3.2 m soil layers from 1.59℃to 0.89℃,decreased the mean MBE of soil temperature for 0～3.2 m soil layers from 0.83℃to-0.13℃,decreased the mean RMSE of soil liquid water for 0～3.2 m soil layers from 0.09 m³m^-3 to 0.06 m³m^-3,and decreased the mean MBE of soil liquid water for 0～3.2 m soil layers from 0.03 m³m^-3 to 0.01 m³m^-3.（5）From 2012 to 2016,the annual surface net radiation and surface sensible heat flux decreased,while the annual surface sensible heat flux and soil heat flux increased.The annual soil heat flux was always positive,indicating the degradation trend of permafrost at Xidatan site.Influenced by the monson climate and annual variation of solar radiation,the monthly net surface radiation,latent surface heat flux and soil heat flux at Xidatan site showed a"unimodal"trend,while the monthly surface sensible heat flux at Xidatan site showed a"bimodal"trend.The diurnal and interdiurnal variations of surface energy flux at Xidatan site were significantly affected by snow cover,which mainly appeared a decrease of the surface net radiation and surface sensible heat flux.