The Surface Heat Source Estimation Over The Tibetan Plateau And Its Variability

The Tibetan Plateau?TP?,known as“the roof of the world”,is located in the central and eastern part of the subtropical Asia continent.The thermal effect of the TP can influence the atmospheric circulation over the TP and its adjacent areas.It also stimulates the remote correlation of the Asian-Pacific Oscillation and regulate the development of tropical ENSO.This indicates that the thermal effect of the TP also has the influence on the atmospheric circulation over the northern hemisphere.Driven by the thermal effect over the TP,the large-scale atmospheric water circulation forms a continuous“Asian water tower”on the TP,which further regulates the regional and global water circulation.The seasonal and annual variations of the TP thermal effect play an important role in the precipitation of China and Asia.The reasonable estimation of the surface heat fluxes will help to enhance our understanding of the role for TP in climate change.However,previous studies use the bulk transfer method with the gradient of air temperature and humidity,surface roughness length,and turbulent transfer coefficient to estimate the surface heat flux over the TP.Due to the great uncertainties of the variables in the near surface,the estimated surface heat flux using bulk transfer method may be with some error.The maximum entropy production?MEP?model avoids using the above variables in the near surface,only needs the net radiation,surface temperature,surface humidity or soil moisture.Therefore,the observations of the Third Tibetan Plateau Atmospheric Scientific Experiment?TIPEX-III?,the data of ERA5,ERA-Interim,JRA-55,MERRA-2,NCEP-I,NCEP-II reanalysis datasets,the data from satellite datasets and the maximum entropy production?MEP?model are used to estimate the summer surface heat source over the Tibetan Plateau?TP?during 1980-2018.The daily observed precipitation,land surface temperature,air temperature and wind speed of the Chinese Meteorological Administration with multiple statistical methods are used to analyze the variations of the surface heat source over the TP and the main factors associated with these variations.The main conclusions are summarized as follows:?1?Surface heat fluxes at Anduo,Bange,Biru,Jiali,and Naqu sites over the central TP have been estimated using the MEP model with the surface energy balance and the observation data from TIPEX?III from August 2014 to September 2015.The MEP sensible heat flux(SH_{MEP OBS})and latent heat flux(LE_{MEP OBS})are highly correlated with those of the TIPEX-III observations with the values of determination coefficient I are greater than 0.88 and 0.73 at the five sites over the central TP.The root-mean-square error?RMSE?values of SH_{MEP OBS} and LE_{MEP OBS} are 34.3 and 55.5 W m^-2,respectively.The agreement between the MEP and TIPEX?III heat fluxes is higher when the observed surface energy balance closure is better.The values of SH_{MEP OBS} and LE_{MEP OBS} tend to be less than those of the heat fluxes derived from bulk transfer method in previous studies.The summer sensible?SH?and latent heat flux?LE?at Shiquanhe site over the western TP is estimated by the MEP model driven by observations from TIPEX?III from August 2014 to June2016.The r values of MEP SH and MEP LE are 0.59 and0.82,respectively.The RMSE values of SH_{MEP OBS} and LE_{MEP OBS} are 11.1 and 9.2 W m^-2,respectively.Compared with the errors reported in previous studies,the errors of the estimated LE are acceptable.Thus,the MEP model can reasonably estimate surface heat fluxes over the central and western TP.?2?The summer SH(SH_merged)at Anduo,Bange,Biru,Naqu,and Shiquanhe sites over the TP are estimated by the SH of ERA5,ERA-Interim,and MERRA-2 from August 2014 to August 2016.The summer LE(LE_merged)at Anduo,Bange,Biru,Naqu,and Shiquanhe sites over the TP are calculated based on the LE estimated by the MEP model driven by the summer net radiation,surface temperature,and soil moisture of ERA5 and ERA-Interim and MERRA-2 LE from August 2014 to August 2016.Compared with the SH_EC and LE_EC from August 2014to August 2016.The r values of SH_mergedand LE_mergedare 0.81 and 0.69,respectively.The RMSE values of SH_mergedand LE_mergedare 13.71 and 24.40 W m^-2,respectively.Compared with the results reported in previous studies,the r and RMSE values of SH_mergedand LE_merged are acceptable.Therefore,the summer SH,LE,net radiation,surface temperature,and soil moisture of ERA5,ERA-Interim,JRA-55,and MERRA-2 and MEP model are used to estimate the summer SH_mergedand LE_merged over the whole TP during 1980-2018.The results show that the regional average values of SH_merged(LE_merged)are 37.4,45.0,45.7,41.8?65.9,43.1,28.6,49.4?W m^-2 over the eastern,central,western,and whole TP,respectively.The climatology of SH_merged and LE_merged increases?decreases?from southeast to northwest.?3?The summer SH_merged(LE_merged)presents downward?upward?trend with the value of-0.94?1.0?W m^-2 decade^-1 during 1980-2018 over the whole TP.Meanwhile,the SH_merged and LE_merged also have an obviously interdecadal variation.SH_merged presents the increasing?decreasing?trend of 3.4?-2.2?W m^-2 decade^-1 during 1980-1994?1995-2018?.LE_mergedpresents the decreasing?increasing?trend of-3.4?1.5?W m^-2 decade^-1 during 1980-1995?1996-2018?.The summer surface heat source(Q_merged=SH_merged+LE_merged)presents an upward trend with the value of 0.1 W m^-2 decade^-1 during 1980-2018 over the whole TP.The r value between Q_mergedand LE_merged is 0.48,which is significant.Therefore,LE_merged plays an important role in Qmerged.?4?The correlation between SH_mergedand the difference between surface temperature and surface air temperature?Ts-Ta?is significant positive with r value of 0.51 over the whole TP.This indicates that Ts-Ta can have a great effect on the spatial distribution characteristics and variability of SH_merged,especially in the central and eastern TP.The correlation between LE_mergedand soil moisture is significant positive with r value of 0.44 over the whole TP.This indicates that soil moisture and precipitation can have a great effect on the spatial distribution characteristics and variability of LE_merged,especially in the central and eastern,central.And western TP.