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The freeze–thaw (F-T) cycle of the active layer (AL) causes the "frost heave and thaw settlement" deformation of the terrain surface. Accurately identifying its amplitude and time characteristics is important for climate, hydrology, and ecology research in permafrost regions. We used Sentinel-1 SAR data and small baseline subset-interferometric synthetic aperture radar (SBAS-InSAR) technology to obtain the characteristics of F-T cycles in the Zonag Lake-Yanhu Lake permafrost-affected endorheic basin on the Qinghai-Tibet Plateau from 2017 to 2019. The results show that the seasonal deformation amplitude (SDA) in the study area mainly ranges from 0 to 60 mm, with an average value of 19 mm. The date of maximum frost heave (MFH) occurred between November 27th and March 21st of the following year, averaged in date of the year (DOY) 37. The maximum thaw settlement (MTS) occurred between July 25th and September 21st, averaged in DOY 225. The thawing duration is the thawing process lasting about 193 days. The spatial distribution differences in SDA, the date of MFH, and the date of MTS are relatively significant, but there is no apparent spatial difference in thawing duration. Although the SDA in the study area is mainly affected by the thermal state of permafrost, it still has the most apparent relationship with vegetation cover, the soil water content in AL, and active layer thickness. SDA has an apparent negative and positive correlation with the date of MFH and the date of MTS. In addition, due to the influence of soil texture and seasonal rivers, the seasonal deformation characteristics of the alluvial-diluvial area are different from those of the surrounding areas. This study provides a method for analyzing the F-T cycle of the AL using multi-temporal InSAR technology. [ABSTRACT FROM AUTHOR]

Relict permafrost presents an ideal opportunity to understand the impacts of climatic warming on the ground thermal regime since it is characterized by a 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 in permafrost thermal state over multiannual periods and to validate the remote sensing dataset. We present 11 years of meteorological and soil data from the Mahan Mountain relict permafrost site of northeastern Qinghai–Tibet Plateau. The meteorological data comprise air and land surface temperature, relative humidity, wind speed and direction, shortwave and longwave downwards and upwards radiation, water vapor pressure, and precipitation on a half-hour timescale. The active layer data include daily soil temperature and soil volumetric water content at five different depths. The permafrost data consist of the ground temperature at 20 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 (10.11888/Cryos.tpdc.271838, Wu and Xie, 2021). [ABSTRACT FROM AUTHOR]

Permafrost has great influences on the climatic, hydrological, and ecological systems on the Qinghai–Tibet Plateau (QTP). The changing permafrost and its impact have been attracting great attention worldwide like never before. More observational and modeling approaches are needed to promote an understanding of permafrost thermal state and climatic conditions on the QTP. However, limited data on the permafrost thermal state and climate background have been sporadically reported in different pieces of literature due to the difficulties of accessing and working in this region where the weather is severe, environmental conditions are harsh, and the topographic and morphological features are complex. From the 1990s, we began to establish a permafrost monitoring network on the QTP. Meteorological variables were measured by automatic meteorological systems. The soil temperature and moisture data were collected from an integrated observation system in the active layer. Deep ground temperature (GT) was observed from boreholes. In this study, a comprehensive dataset consisting of long-term meteorological, GT, soil moisture, and soil temperature data was compiled after quality control from an integrated, distributed, and multiscale observation network in the permafrost regions of QTP. The dataset is helpful for scientists with multiple study fields (i.e., climate, cryospheric, ecology and hydrology, meteorology science), which will significantly promote the verification, development, and improvement of hydrological models, land surface process models, and climate models on the QTP. The datasets are available from the National Tibetan Plateau/Third Pole Environment Data Center (https://data.tpdc.ac.cn/en/disallow/789e838e-16ac-4539-bb7e-906217305a1d/ , last access: 24 August 2021, 10.11888/Geocry.tpdc.271107, Lin et al., 2021). [ABSTRACT FROM AUTHOR]

Warming permafrost on a global scale is projected to have significant impacts on engineering, hydrology and environmental quality. Greater warming trends are predicted on the Qinghai–Tibetan Plateau (QTP), but most models for mountain permafrost have not considered the effects of water phase change and the state of deep permafrost due to a lack of detailed information. To better understand historical and future permafrost change based on in situ monitoring and field investigations, a numerical heat conduction permafrost model was introduced which differentiated the frozen and thawed state of soil, and considered unfrozen water content in frozen soil, distribution of ground ice and geothermal heat flow. Simulations were conducted at two sites with validation by long‐term monitoring of ground temperature data. After forcing with reconstructed historical ground surface temperature series starting from 1966, the model predicted permafrost changes until 2100 under different RCP scenarios. The results indicate a slow thermal response of permafrost to climate warming at the two investigated sites. Even under the most radical warming scenario (RCP8.5), deepening of the permafrost table is not obvious before 2040. At both sites, the model indicates that shallow permafrost may disappear but deep permafrost may persist by 2100. Moreover, the simulation shows that the degradation modes may differ between zones of discontinuous and continuous permafrost. The main degradation mode of the site in the discontinuous zone appears to be upward thawing from the permafrost base, while that of the site in the continuous zone is downward thawing at the permafrost table with little change at the permafrost base. [ABSTRACT FROM AUTHOR]

The long-term and large-scale soil moisture (SM) record is important for understanding land atmosphere interactions and their impacts on the weather, climate, and regional ecosystem. SM products are one of the parameters used in some Earth system models, but these records require evaluation before use. The water resources on the Qinghai-Tibet Plateau (QTP) are important to the water security of billions of people in Asia. Therefore, it is necessary to know the SM conditions on the QTP. In this study, the evaluation metrics of multilayer (0-10, 10-40, and 40-100 cm) SM in different reanalysis datasets of the European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim [ERA]), National Centers for Environmental Prediction Climate Forecast System and the Climate Forecast System version 2 (CFSv2), and China Meteorological Administration Land Data Assimilation System (CLDAS) are compared with in situ observations at 5 observation sites, which represent alpine meadow, alpine swamp meadow, alpine grassy meadow, alpine desert steppe, and alpine steppe environments during the thawing season from January 1, 2011, to December 31, 2013, on the QTP. The ERA SM remains constant at approximately 0.2 m3⋅m−3 at all observation sites during the entire thawing season. The CLDAS and CFSv2 SM products show similar patterns with those of the in situ SM observations during the thawing season. The CLDAS SM product performs better than the CFSv2 and ERA for all vegetation types except the alpine swamp meadow. The results indicate that the soil texture and land cover types play a more important role than the precipitation to increase the biases of the CLDAS SM product on the QTP. [ABSTRACT FROM AUTHOR]

The characteristics of long-term variation for extreme precipitation events were analyzed at the Tanggula site in the continuous permafrost regions of the Qinghai-Tibetan Plateau (QTP). In addition, the impacts of extreme precipitation events in summer on soil thermal-moisture dynamics were also investigated. The results showed that local extreme precipitation indices fluctuated significantly and that the trend magnitudes of local very wet days (R95p), annual total wet-day precipitation (PRCPTOT), number of heavy precipitation days (R10mm), maximum length of dry spell (CDD), and simple daily intensity index (SDII) were larger than those of the western QTP, other regions of China, and even the global average. The freeze-thaw cycling in the local active layer occurred from October to the next September during 2006 to 2014. The influence of extreme precipitation event in summer on local soil hydrothermal conditions could reach soil depths up to 105 cm or so, and these were more pronounced than with light or moderate precipitation events. Soil temperature reacted more promptly to local extreme precipitation events than did soil moisture. The rate at which local soil temperature fell after an extreme precipitation event was greater than the rate of increasing temperature on nonprecipitation days. Moreover, the amount of precipitation received during extreme precipitation events had a greater effect on local soil moisture and temperature than duration time for these events. Consecutive extreme precipitation events with a longer duration time did not necessarily to have a greater effect than a single precipitation event with a shorter duration. Finally, the thawing process of active layer and local water migration modes could also affect the response of soil hydrothermal conditions to an extreme precipitation event to a large extent. [ABSTRACT FROM AUTHOR]

• Lakes in the endorheic basin exhibited a rapid expansion since 2000. • The significant lake expansion was observed in the Hoh Xil region. • Lake expansion was closely related to precipitation and permafrost degradation. • Meltwater from permafrost thawing contributes to 21% of lake volume change. • Glacier retreat contributed to expansion of several large lakes. The QTP (Qinghai-Tibet Plateau) holds the greatest concentration of high-elevation inland lakes in the world, and most of these lakes are located in the QTP endorheic basin. Accelerated lake expansion in the endorheic basin since 2000 has been confirmed by many studies, but there is disagreement as to which components of the water balance explain most of the lake expansion. In this paper, based on Landsat images, meteorological data, glacier data and permafrost monitoring data, we analyzed the spatial–temporal changes in lake area and further explored the driving factors behind rapid lake expansion in different zones in the endorheic basin. The results suggested that the spatial pattern of trends in lake area in the endorheic basin from 2000 to 2017 showed a southwest-northeast transition from contracting to slightly expanding to rapidly expanding. More importantly, the dramatic lake expansion observed in most regions was statistically significant (0.01 ~ 26.29 km2/yr). Although retreating glaciers may have contributed to lake growth, this can not fully explain the recent lake expansion. We find that increasing precipitation was the primary driver behind lake expansion, and that permafrost degradation accelerated lake expansion. Lakes on the northern slope of the Gangdise Mountains, fed by the most rapidly retreating glaciers, were relatively stable or even contracted. In contrast, most of the non-glacier-fed lakes in the Hoh Xil region and the central endorheic basin nevertheless experienced significant expansion and slight expansion, respectively, which were mainly attributed to the greatest increase in AP (annual precipitation). Remarkable lake expansion was found in the continuous permafrost zone. The increasing ALT (active layer thickness) and soil temperature of permafrost resulted in an increase in meltwater from ground ice, the partial release of soil moisture and an amount of unfrozen water content, which has contributed to supply lakes and accelerated lake expansion. [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]

Ground ice is a distinctive feature of permafrost terrain. The vertical distribution and factors controlling the hydrochemistry of ground ice are important for studying soil moisture and salt migration during the freeze–thaw process in soil. These factors are also important components of hydrological cycles in cold regions. The hydrochemical characteristics of ground ice on the Qinghai-Tibetan Plateau (QTP) are not well known. We examined the characteristics of ground ice hydrochemistry using data from 9 soil profiles in permafrost regions of the central QTP. The isotopes and anion concentrations of subsurface water on the QTP were higher than those in Arctic polygonal ground regions. The spatial distribution of anions was complex. Well-developed hydrochemical depth gradients occurred within the soil profile. Isotopes decreased and anions increased with depth, suggesting general vertical patterns of soil hydrochemistry across different permafrost regions. Cl − and SO 4 2− concentrations in soil water increased with depth, while NO 3 − concentration did not change with depth. Freeze-out fractionation, self-purification, and desalination greatly impact soil hydrochemistry. The major factors controlling variation of soil water chemistry were soil moisture, air temperature, and active layer thickness. The results could provide a framework for understanding ground ice origins and the moisture and salt migration pathways in the context of permafrost changes. This information could be useful in developing process-based permafrost hydrologic models. [ABSTRACT FROM AUTHOR]

Air and soil temperatures are important factors that contribute to hydro-thermal processes and ecosystem dynamics in permafrost regions. However, there is little research regarding soil thermal dynamics during freeze-thaw processes in permafrost regions with thermal orbits on the Tibetan Plateau. Thermal orbits can provide simplified illustrations of the relationships between air and ground temperatures. This paper presents a new quantitative analysis for thermal orbits by combining the characteristics of ellipse and linear regression theories. A sensibility analysis of thermal orbits was conducted with different air and ground temperatures and vegetation types on the Tibetan Plateau. Results indicated that the thermal orbit regression slopes and intercepts had variations in characteristics between air and ground temperatures at different depths. More specifically, both air and ground temperatures showed homologous variation with increasing depth. This type of analysis is important for a better understanding of permafrost thermal properties as they relate to soil moisture, climate change, and vegetation effects in permafrost regions on the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

• Algorithms and parameterizations to determine unfrozen water content are reviewed. • Selected eighteen unfrozen water content parameterizations for the unfrozen water content are evaluated. • Influential factors on unfrozen water content parameterizations are summarized and discussed. • Several future research questions are highlighted and addressed. Unfrozen water plays an important role in a number of processes, including water and heat transfer, frost heave, thaw settlement and simulations for the hydro-thermo-mechanical interactions in frozen soil. Past studies have demonstrated that considering the unfrozen water content in cold regions can significantly improve accuracy in coupling heat and water transfer modeling in frozen soil. However, differences between experimental data and theoretical understanding have resulted in discrepancies between parameterizations. To address this, we presented the first study to synthesize the algorithms and parameterizations used for unfrozen water content; we also discussed influential factors on unfrozen water content in frozen soil. We then provided a comprehensive discussion of the progress in algorithms and parameterizations regarding unfrozen water content and summarized them into four categories, which were calculated using soil temperature, specific surface area of soil particles, soil water curve, and different types of water. Selected unfrozen water content parameterizations were then evaluated based on those previous results as well as the data collected from our field observation station in permafrost region on the Qinghai-Tibet Plateau (QTP). These results revealed that empirical parameterizations were useful for calculating unfrozen water content. In addition, the physical parameterizations had higher accuracy for calculating unfrozen water content, but they were more complicated and difficult to use in practical applications. Unfrozen water content parameterizations were influenced by many factors, and the warming and cooling processes were especially important when calculating unfrozen water content. Finally, future research should aim to improve our theoretical understanding and to develop simple parameterizations that couple land surface processes models in cold regions. It is expected that this review will provide a sound theoretical basis for the further study of the unfrozen water content in frozen soil and its subsequent effects on hydrothermal transfer processes in cold regions. [ABSTRACT FROM AUTHOR]