| The distribution and conversion of net radiation by the process of surface energy balance is an important part of surface radiation budget and a critical factor affecting the formation and change of the climate pattern.Great differences in the characteristics of surface energy balance exist at different underlying surfaces.It has great significance to explore the characteristics of surface energy balance of various underlying surfaces.The bulk aerodynamic method is widely applied in weather and climate models,and the dynamic,thermal roughness length and stability parameters are crucial in the bulk aerodynamic method.The land surface processes of the Tibetan Plateau(TP)exerts significant effects on regional and global weather and climate through its thermodynamic and mechanical forcing.Compared to the central and eastern TP,only a few stations exist in the western TP and there are lacks of long-term flux observations and systematic researches on the western part of the TP for a long time due to the harsh natural environment.Based on the long-term observation data at Shiquanhe at Shiquanhe(32.50°N,80.08°E,4279.3 m above sea level)in the western TP,the thesis analyzed the characteristics of surface energy balance,calculated the key parameters of land surface processes,and evaluated the performances of the stability parameterization schemes for simulating the land surface processes by using a numerical model in this area.It could supplement the deficiency of the observation and research on the land air interaction process in the western TP to a certain extent.The main conclusions are summarized as follows:1).The variation characteristics of conventional meteorological elements,the monthly averaged daily variation and seasonal variation characteristics of radiation and surface energy balance components,and the closure ratio of surface energy were analyzed.The seasonal wind speed varied slightly and was relatively low with annual value of 2.17 m s-1,and westerlies were dominant with the annual westerly wind frequency 59.2%at Shiquanhe.The seasonal variations of air temperature,solar radiation and specific humidity were obvious.The difference between the maximum temperature and the minimum temperature in 30-min interval could reach 47.1 K,and the maximum daily difference was 22.40 K.The average specific humidity was 2.6 g kg-1,lower than that in the eastern or central TP.The monsoon could affect Shiquanhe area approximately in late May,and then most of the annual precipitation occurred during June to September.The diurnal variations of the seven energy fluxes were different to some extended,and the seasonal variations were obvious with their maxima in summer and minima in winter.The diurnal and seasonal variations of soil respiration caused by the biological and chemical processes existed,and around0.1 mg m-2 s-1absorption(release)CO2 occurred in the summer afternoon(midnight of winter),but the absorption and release CO2generally were offset from a yearly perspective.The surface energy balance ratio(?)varied diurnally and seasonally.The?was above 0.6 in summer months,while in winter months?was around 0.5.Comparing the results of the Shiquanhe site with the central and eastern TP sites,it was found that(1)their seasonal and diurnal variations of the fluxes were similar,(2)latent heat flux at Shiquanhe(daily daytime mean<90 W m-2)was distinctively smaller than at the central and eastern TP sites during the wet season(generally>100 W m-2)due to less precipitation,and(3)affected by various factors,the residual energy was larger at Shiquanhe,leading to a smaller surface energy balance ratio.2).The land surface parameters,such as aerodynamic roughness length(z0m),zero-plane displacement(d),thermal roughness length(z0h),surface albedo,and soil heat capacity,thermal conductivity,thermal diffusivity and water flux density,were estimated.The aerodynamic roughness and zero-plane displacement were affected by the distribution of ground features in directions,and the averaged values were 5.58×10-2 m and 0.44 m,respectively.The thermodynamic roughness length or the excess resistance to heat transfer(k B-1)varied with the atmospheric stratification,and the atmospheric stratification of the boundary layer at Shiquanhe was mainly unstable.The performances of thermodynamic roughness parameterization schemes were different.Compared to other parameterization schemes the mean k B-1 calculated by the Z95 scheme agreed best with the value calculated with observation under unstable stratification.Surface albedo was affected by soil moisture and had weak seasonal changes,with an average of 0.20,which was equivalent to deserts and Gobi.The annual average soil heat capacity,thermal conductivity,thermal diffusivity,and water flux density were 0.95×106 J m-3 K-1,0.24 W m-1 K-1,2.73×10-7 m2s-1 and 0.12×10-5 m s-1,respectively,which were relatively consistent with the observations in the desert and Gobi.3).The performances of the widely used Noah LSM were evaluated with different stability parameters schemes for the surface layer.The Noah LSM performed well in simulating the surface fluxes of momentum,sensible heat,and upward longwave radiation,as well as soil temperature at Shiquanhe,and its performances were sensitive to the schemes used to estimate the surface layer stability parameter.The four non-iterative schemes employed in this study had an advantage in computational efficiency compared to the iterative scheme adopted by the Noah LSM.According to the Nash efficiency coefficient,the scheme proposed by Li performed best and was better than the iterative scheme,and its simulation efficiency of momentum flux,sensible heat flux,upward longwave radiation,and 10 and 40 cm soil temperatures were 1%,88%,8%,6%,and 13%higher than Noah's default scheme,respectively,but its simulation efficiency of the 5 cm soil temperature was lower than Noah's default scheme.Further,Li scheme is suitable for an unstable surface layer,which occurs frequently at Shiquanhe,and has potential to improve the land surface model performance at the TP. |
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