Near-surface energy budget closure has been a trending topic in land surface processes research, especially on the underlying surfaces of heterogeneous wetlands. In this investigation, the horizontal thermal advection caused by thermal inhomogeneity over the alpine wetland is calculated based on the eddy covariance data observed at the Flower Lake observation field and WRF modelling data over the Zoige alpine wetland, China. The contribution of horizontal thermal advection to the near-surface energy closure is analysed. The results show that the mean horizontal heat advection of the Zoige wetland is 20.2 W·m−2, and the maximum value reached 55.0 W·m−2 in the summer of 2017. After introducing thermal advection into the near-surface energy balance equation, the near-surface energy closure ratio increased from 72.3% to 81.0%.

Precipitation has a significant influence on the topsoil moisture and further impacts the land-atmospheric water and heat exchange process over the Yarlung Zangbo Grand Canyon region (YGC) where exhibits one of the highest frequencies of convective activity in China. The simulated performance of the Community Land Model version 5.0 (CLM5.0) on turbulent fluxes under seven roughness heights for heat transfer (Z 0h ) schemes at Motuo and Pailong stations over the YGC was evaluated. The results indicate that the CLM5.0 significantly overestimates the surface sensible heat flux (H) while the simulation performance of surface latent heat flux (LE) is better than H. By comparing and analyzing the simulation results, the Z 0h schemes suitable for the YGC are selected optimally. The Zeng et al. (J. Hydrometeorol., 2012, 13, 1359–1370) scheme (Z12) is more suitable for the simulations of H, with the simulated RMSE of H at Motuo and Pailong stations on typical sunny days being only 21.63 and 15.13 W m−2, respectively, 81.51% and 76.96% lower than the original Z 0h scheme of CLM5.0. The Garratt, J., R and Francey, R., J (Boundary. Layer. Meteorol., 1978, 15, 399–421) scheme (G78) is more suitable for simulating LE in the YGC. The simulated BIAS and RMSE of LE at Motuo station were 9.80% and 21.90% lower than that under the default scheme of CLM5.0 on typical cloudy days. In addition, except for the G78 and CLM5.0 default scheme, the Z 0h under the other schemes showed obvious diurnal variation characteristics, and H was positively sensitive to Z 0h , while LE was the opposite. Consequently, the optimal Z 0h schemes are of great application value for further comparative analysis of the water and heat exchange process between the Grand Canyon land surface and the atmosphere, to better reveal the mechanism of land-atmosphere interactions in the YGC.

The soil freeze-thaw phenomenon is one of the most distinguishing peculiarities of the land surface processes over the Qinghai-Tibetan Plateau. By utilizing soil temperature/moisture and air temperature data from the GLDAS-NOAH product, 4 observing networks and 10 meteorological stations over the Qinghai-Tibetan Plateau, the spatio-temporal distribution of the start/end date and the duration of soil freezing and their relationships with air temperature and altitude were investigated with the analytical methods of linear regression model, correlation analysis, sliding t-test and Mann-Kendall test. As a result, the spatial distribution of top-layer soil freeze-thaw processes was characterized by a trend of delaying freezing and advancing thawing from the northwest to the southeast throughout the Qinghai-Tibetan Plateau. Moreover, in the period 1948-2019, the begin date of soil freezing was delayed at a rate of 1.2 d/10a (p-lev < 0.0001, R-2 = 0.40), and the end date of the soil freezing was advanced at a rate of 0.5 d/10a (p-lev = 0.105, R-2 = 0.04). The duration of the soil freeze was shortened with a rate of 2.1 d/10a (p-lev < 0.0001, R-2 = 0.23) in the southern Changtang but 0.4 d/10a (p-lev < 0.0001, R-2 = 0.47) in Qinghai and the southern Tibet Plateau. Additionally, the start date of the soil freeze is to be delayed by 2.8 days (p-lev < 0.0001, R-2 = 0.53), and the end date of the soil freeze is to be advanced by 3.6 (p -lev < 0.0001, R-2 = 0.61) days as the air temperature rises by 1.0 C. In the high cold climatological zone on the Qinghai-Tibetan Plateau, there is a trend toward earlier start date of soil freezing by 9.9 days (p-lev < 0.0001, R-2 = 0.36), and later end date of soil freezing by 14.4 days (p-lev < 0.0001, R-2 = 0.56) as the altitude increases by 1000.0 m. Furthermore, the seasonal variations of soil temperature and moisture were significantly different from typical strong and weak freezing years, which were defined according to the duration of soil freezing period. These results are of great significance for understanding the soil freeze-thaw process and its variation over the Qinghai-Tibetan Plateau.

The seasonal characteristics of water and heat exchange in the alpine grasslands are significant， and the freezing‐thawing process has an important impact on the land‐atmospheric water and heat exchange.Based on the observation data of the land surface process in the Tangchama small watershed in the source area of the Yellow River from May 2014 to May 2015， this research divides the soil freezing‐thawing process into thawed stage （TT）， frozen stage （FF）， thawing to freezing （T-F） and freezing to thawing （F-T）， and the changes in the different states and period of the net radiation， sensible heat flux， latent heat flux and surface heat flux of the underlying surface of the alpine grassland are analyzed to explore the characteristics of water and heat exchange between the land‐atmosphere in the soil freezing‐thawing process.The results are as follows： （1） The average value of the net radiation flux in the thawed stage is generally greater than that of the other three stages， and the maximum value reaches 203.7 W·m-2.The frozen soil melts in the freezing‐thawing stage， and the soil moisture content gradually increases.The radiation ratio increased significantly during the frozen stage， the net radiation diurnal variation was the largest in the thawed stage， reaching 717.6 W·m-2， and the frozen stage was the smallest， followed by the freezing‐thawing stage.（2） The proportion of sensible heat flux and latent heat flux is different in the thawed and frozen stages.When completely thawed， due to precipitation and soil moisture content， the net radiation is mainly converted into latent heat flux.The maximum diurnal variation of latent heat flux is 193.7 W·m-2， while the sensible heat flux is only about 80.0 W·m-2.The diurnal average of sensible heat and latent heat in the thawing‐freezing phase， the freezing‐thawing period and the frozen period is not much different.The mean latent heat in the three period is 21.9 W·m-2， and the sensible heat is 20.3 W·m-2； The diurnal variation is greater than the latent heat in the three period， the soil suffers a freezing‐thawing cycle， the soil temperature difference is small， and the water content changes， and the net radiation is mainly converted into sensible heat during this period； the diurnal variation of sensible heat was greater than that of latent heat in the three stages.The freezing-thawing cycle occurred in the soil， the difference between ground and air temperature was small， and the moisture content changed.During this period， the net radiation was mainly converted to sensible heat.（3） The soil heat flux is positive （negative） in thawed （frozen） state， indicating that the surface soil absorbs （releases） heat from the atmosphere， and its daily variation range is large （small）.The above results show that the state and process of soil freezing and thawing have different characteristics for the water and heat exchange process between the land and atmosphere.

土壤冻融过程对气候变化非常敏感,如何准确监测土壤冻融过程具有重要的科学意义.利用2017年6月至2018年6月中国科学院若尔盖高原湿地生态系统研究站玛曲观测场地基微波辐射计观测数据、浅层土壤温度和近地面气温数据,通过构建归一化极化比值冻结因子、极化差值冻结因子、组合水平极化差值冻结因子和组合垂直极化差值冻结因子等不同土壤冻结因子,评估了黄河源区草原下垫面土壤冻融过程.结果表明:L波段微波辐射计监测土壤冻融状态的结果与近地面气温和浅层土壤温度表征的土壤冻融过程基本一致.当入射角为50°时,归一化极化比值冻结因子和极化差值冻结因子与实测数据的一致性分别达到83.6％和82.8％.每种冻结因子具有明显的季节性变化,四种冻结因子在春季时的准确度低于夏、秋、冬三个季节.归一化后的相对冻结因子的标准差在秋季最大,可达0.3;在冬季和夏季最小,值小于0.2.在土壤发生冻结和融化转换时,垂直极化和水平极化下的亮温同时下降,其差值较完全冻结或者完全融化时的亮温差大.研究结果可为微波遥感监测土壤冻融过程提供技术参考.

The energy non-closure near the land surface has been a key topic in the land surface processes research. The energy closure rate is still not high even after considering heat storage and photosynthesis energy consumption, while the contribution of advective energy to the closure rate needs to be considered further under the non-uniform underlying surface. In this paper, the advective energy caused by thermal heterogeneity of underlying surface is calculated by using the energy budget data collected from the Flower-Lake observation site in the Zoige Alpine Wetland in 2017, and the contribution of thermal advection to energy closure near the ground is estimated. The result shows: In summer of 2017, the maximum value of the advective heat flux was 23.8w/m2 at the Zoige alpine wetland. When the contribution of advective heat flux is introduced into the energy balance equation, the energy closure rate increases from 72.0% to 79.4%. With considering the contribution of horizontal heat transfer, it has a certain effect on improving energy closure rate for the flat terrain and thermal inhomogeneous underlying surface. The near surface thermal inhomogeneity leads to the accumulation of heat, which is the basic reason for the heat advection to affect the energy closure rate, and also an important reason for the difference between the wetland characteristics of water and heat exchange of the wetland with the other regions. Key words：Alpine wetland; eddy correlation; advective heat flux; energy closure rate; inhomogeneous land surface