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Relict permafrost presents an ideal opportunity to understand the impacts of climatic warming on the ground thermal regime since it is characterized by mean annual ground temperature close to 0 °C and relatively thin permafrost. The long-term and continuous observations of permafrost thermal state and climate background are of great importance to reveal the links between the energy balance on hourly to annual timescales, to evaluate the variations of permafrost thermal state over multi-annual periods and to validate the remote sensing dataset. Until now there are few data available in relict permafrost regions although those data are important to understand the impacts of climate changes on permafrost especially in the boundary regions between permafrost and seasonally frozen ground regions. In this study, we present 11 years of meteorological and soil data in a relict permafrost site of the Mahan Mountain on the northeast of the Qinghai-Tibet Plateau. The meteorological data are comprised of air and ground surface temperature, relative humidity, wind speed and direction, shortwave and longwave downward and upward radiation, water vapor pressure, and precipitation on half-an-hour timescale. The active layer data include daily soil temperature and soil moisture at five different depths. The permafrost data consist of ground temperature at twenty different depths up to 28.4 m. The high-quality and long-term datasets are expected to serve as accurate forcing data in land surface models and evaluate remote-sensing products for a broader geoscientific community. The datasets are available from the National Tibetan Plateau/Third Pole Environment Data Center (https://doi.org/10.11888/Cryos.tpdc.271838, Wu and Xie, 2021). [ABSTRACT FROM AUTHOR]

Precipitation plays an important role in permafrost hydrology; it can alter the hydrothermal condition of the active layer and even influence the permafrost aggradation or degradation. Moisture recycling from evaporation and transpiration can greatly contribute to local precipitation in some regions. This study selected four monitoring sites and used an isotope mixing model to investigate local moisture recycling in permafrost regions of the central Qinghai‐Tibet Plateau (QTP). The results showed that the local water vapour flux in the summer and autumn were dominantly influenced by westerlies and the Indian monsoon. Moistures for precipitation in Wudaoliang (WDL) and Fenghuoshan (FHS) mainly came from the western QTP, eastern Tianshan Mountains, western Qilian Mountains, and the surrounding regions. In comparsion, more than half of precipitation at Tanggula (TGL) was mostly sourced from the Indian monsoon. Local moisture recycling ratios at the four sites ranged from 14% ± 3.8% to 31.6% ± 4.8%, and depended on the soil moisture and relative humidity. In particular, the higher soil moisture and relative humidity promoted local moisture recycling, but frozen ground might be a potential influencing factor as well. The moisture recycling ratios of the study area were consistent with the results from both the Qinghai Lake Basin and the Nam Co Basin, but differed from those of the northwestern QTP. This difference may indirectly confirm the great spatial variability in precipitation on the QTP. Moreover, the rising air temperature and ground temperature, increasing precipitation, higher soil moisture, higher vegetation cover, and expanding lakes in the study area may be conductive to enhancing future local moisture recycling by altering ground surface conditions and facilitating the land surface evaporation and plant transpiration. [ABSTRACT FROM AUTHOR]

The ground surface soil heat flux (G0) is very important to simulate the changes of frozen ground and the active layer thickness; in addition, the freeze-thaw cycle will also affect G0 on the Tibetan Plateau (TP). As G0 could not be measured directly and soil heat flux is difficult to be observed on the TP in situ due to its high altitude and cold environment, most of previous studies have directly applied existing remote sensing-based models to estimate G0 without assessing whether the selected model is the best one of those models for those study regions. We use in-situ observation data collected at 12 sites combined with Moderate Resolution Imaging Spectroradiometer (MODIS) data (MOD13Q1, MODLT1D, MOD09CMG, and MCD15A2H) and the China meteorological forcing dataset (CMFD-SRad and CMFD-LRad) to validate the main models during the freeze-thaw process. The results show that during the three stages (complete freezing (CF), daily freeze-thaw cycle (DFT), and complete thawing (CT)) of the freeze-thaw cycle, the root mean square error (RMSE) between the models' G0 simulated value and the corresponding G0 "measured value" is the largest in the CT phase and smallest in the CF phase. The simulated results of the second group schemes (SEBAL, Ma, SEBALadj, and Maadj) were slightly underestimated, more stable, and closer to the measured values than the first group schemes (Choudhury, Clawson, SEBS, Choudhuryadj, Clawsonadj, and SEBSadj). The Maadj scheme is the one with the smallest RMSE among all the schemes and could be directly applied across the entire TP. Then, four possible reasons leading to the errors of the main schemes were analyzed. The soil moisture affecting the ratio G0/Rn and the phase shift between G0 and net radiation Rn are not considered in the schemes directly; the scheme cannot completely and correctly capture the direction of G0; and the input data of the schemes to estimate the regional G0 maybe bring some errors into the simulated results. The results are expected to provide a basis for selecting remote sensing-based models to simulate G0 in frozen ground dynamics and to calculate evapotranspiration on the TP during the freeze-thaw process. The scheme Maadj suitable for the TP was also offered in the study. We proposed several improvement directions of remote sensing-based models in order to enhance understanding of the energy exchange between the ground surface and the atmosphere. [ABSTRACT FROM AUTHOR]