Observations from 1,047 meteorological stations from September 1, 2006 to August 31, 2015 revealed regional differences in the freezing and thawing processes of seasonally frozen ground (SFG) across China. SFG generally undergoes a one-way freezing process (i.e., top-down), and the stations with a large freeze depth generally experienced long freeze durations. During the thawing process, soil is generally characterized by two-way thawing (i.e., top-down and bottom-up) in the region north of 35 ' N, ' N, especially north of 30 ' N ' N (except in northeastern China). The onset of thawing from the bottom occurs earlier than that from the top at most stations in the two-way thawing region. The stations exhibiting one-way thawing (i.e., bottom-up) were mainly located on the southern edge of eastern China (east of 110 degrees E) degrees E) and in southern part of Xinjiang and southeast part of the Qinghai-Tibet Plateau. The freezing process lasts several days to more than four months longer than the soil thawing process, and this difference tends to be larger in high-latitude and high-altitude regions. All of the sites experienced a discontinuous freeze-thaw process, the station-average duration of which was less than a quarter of that of the continuous freeze-thaw process. Strong associations of soil freeze depth with air temperature (as characterized by the air freezing index and air thawing index) implied a dominant influence of air temperature on the soil freeze-thaw process. During the freezing process, this relationship was partially modulated by snow cover in snowy regions, such as northeast China, northwest China, and the eastern Tibetan Plateau. This paper provides the first overview of regional differences in the freezing and thawing processes of SFG over China, and the findings improve our understanding of the soil freeze-thaw process and provide important information to support research into regional landscapes, ecosystems, and hydrological processes.

Atmospheric conditions, topsoil properties and land cover conditions play essential roles in ground surface temperature (GST), surface air temperature (SAT) and their differences (GST-SAT). They determine the strength of the thermal forcing of the lower atmospheric boundary and the distributions of frozen ground in cold regions. However, the relative importance of these factors at various time scales and the underlying physical mechanisms remain less well understood. Here, we investigate the spatiotemporal patterns of GST-SAT and examine 11 potential factors in three categories in influencing the GST-SAT variations from 1983 to 2019 over the Tibetan Plateau (TP) using boosted regression tree models. The results show that the TP has experienced asynchronous warming in GST and SAT since 2001: a warming hiatus in SAT but continued warming in GST, resulting in a significantly increasing trend in GST-SAT. The relative importance of the three categories that influence the GSTSAT spatial variation was: atmospheric variables (56.1 %) > shallow soil properties (24.4 %) > interfacial land cover features (19.5 %). The importance of the factors also varied with the combinations of annual, seasonal, daily, day-time and night-time time scales, manifested by positive or negative effects. The interdecadal changes of net radiation, precipitation, wind speed and soil moisture amplified the asynchronous warming between air and shallow ground over the TP since the 2000s. These findings provide an in-depth understanding of the spatiotemporal variations of GST-SAT and the underlying mechanisms. This study will benefit the development of the Earth system models on the TP.

Snowmelt water is an essential runoff source of some alpine rivers in China. This study selected the Upper Burgin River (UBR), a typical snow-fed river, to quantitatively assess the runoff contributions of different components, as well as the causes of runoff variations under the background of cryosphere change and global warming. Based on the spatial-temporal distributions of snow and glaciers during a year, as well as the altitudinal variations of 0 degrees C isotherm, the high flow hydro graphs in UBR was separated into two parts: seasonal snowmelt flood of lower altitudes (<3,000 in) and glacier-snow melt flow in high altitudes (3,000-4,296 m). The daily baseflow hydrograph of-URR. was separated by the digital filtering technique. It is concluded that the contributions of snowmelt flow, glacier melt flow, and baseflow (includes rainfall runoff component) to total annual flow volumes are 27.2% (12.7%), 8.5% (+1.7%), and 64.3% (3.0%), respectively. The speed of air temperature rise in spring may be the controlling factor for monthly snowmelt flow distributions in the snow -fed river. The volume of snowmelt was determined by spring precipitation (SP) and previous winter's precipitation (PWP). The PWP changes can explain 43.7% of snowmelt changes during 1981-2010 in UBR, while snowmelt change in 1957-1980 is more impacted by SP. The determining factor of snowmelt variation was changed from SP to PAP during the recent decades. Precipitation in current year, excluding previous year's rainfall and snowfall, can only explain 32%-70% of the variability in total nmoff.

Surface air temperatures are significant indicators of environmental and climatic change that affect a diverse set of physical systems including permafrost. Most temperature products, such as gridded or reanalysis data, are still at a relatively low spatial resolution, limiting the ability to simulate heterogeneous permafrost changes and leading to large uncertainties. Here we apply a downscaling method based on elevation to obtain high-resolution surface air temperatures from the sixth Coupled Model Intercomparison Project in Northern Hemisphere permafrost regions. Root-mean-square errors and mean absolute errors after downscaling are reduced by 34 and 37%, respectively, relative to meteorological site data and gridded observations from the Climatic Research Unit. Compared to the downscaled surface air temperature data, non-downscaled model projections overestimate by 0.12-0.39 degrees C in the discontinuous, isolated, and sporadic permafrost regions and underestimate up to 0.18 degrees C in the continuous permafrost region under different emission scenarios. The warming rates in Northern Hemisphere permafrost regions were 0.093 degrees C/10 year during the historical (1850-2014) period and are projected to be 0.22 degrees C/10 year for SSP1-2.6, 0.48 degrees C/10 year for SSP2-4.5, 0.75 degrees C/10 year for SSP3-7.0, and 0.95 degrees C/10 year for SSP5-8.5 during 2015-2100, which is 1.4-1.6 times the warming of non-permafrost regions. Warming rates in high latitudes are 1.2-1.7 times higher than those in high-elevation regions. Continuous permafrost regions' warming will be 1.2-1.4 times higher than in other permafrost regions. For permafrost with high ground ice content, warming will be 1.1 times greater than in permafrost regions with medium or low ground ice content.

This paper presents the results of 30 years of permafrost thermal monitoring in the Tiksi area in the eastern Russian Arctic. At a stone ridge site, the mean annual temperatures in the upper 30 m of the ground have increased by 1-2.4 degrees C compared to the first years of observations, with trends of degrees C/yr. At the same time, its change was uneven. In the last 20 years, the rate of increase has increased compared with the first decade of observations. At wet tundra sites in the foothill plain, the mean annual temperatures at the top of permafrost have increased by 2.4-2.6 degrees C between 2005 and 2022 at rates of 0.11-0.15 degrees C/yr, and the active layer thicknesses have increased at rates of 0.05-0.41 cm/yr.

Knowledge of the difference between soil and air temperatures (Delta T) is helpful to improve our understanding on the land-atmosphere thermal interactions and temperature-dependent soil processes. Based on 272 stations across China, this study investigated the spatiotemporal variations of the annual and seasonal Delta T (difference between soil temperature at a depth of 0.4 m and air temperature) from 1981 to 2014, and quantified the relative contributions of multiple environmental variables (snow cover, precipitation, vegetation, soil moisture, and solar radiation) to Delta T variation for the first time. Air temperature primarily controls soil temperature dynamics, but the asynchronous trends of soil and air temperatures may lead to the complexity of the land-atmosphere relationship. Almost no apparent trends in Delta T were detected for the entire China (except in summer), but the spatial heterogeneity of trends was evident. Snow cover conditions greatly dominated the Delta T dynamics both annually and seasonally (except in summer). The relative contribution of snow cover duration to Delta T variation was significantly greater than that of mean snow depth for the entire China, but the regional differences in the contributions of the two variables were noticeable at different seasons. The greening of vegetation closely associated with the Delta T variation in annual, autumn and winter, and soil moisture exerted a great influence on summer Delta T, associated with sunshine duration (a proxy for surface solar radiation). The amount of precipitation made a slight impact on Delta T at either seasonal or annual scales.

Using 1980-2014 observation data of 378 meteorological stations, this study quantified the correlations between snow cover and the annual maximum seasonally freeze depth (MSFD) across China and the contributions of snow cover to MSFD. Snow cover exhibited a weak effect on MSFD across China, which may be related to the thinness and short duration of the snow cover. Regional differences in the effects of snow cover on MSFD were relatively large, and the effects of snow cover were more significant than those of air temperature (characterized by the airfreezing index) at stations in Northeast China, northeastern Inner Mongolia, and the north of the Tianshan Mountains. In regions with a thin snow cover or short snow cover duration (SCD), the cooling effect of snow cover can dominate, but it only slightly influences MSFD owing to the short SCD. With increasing average snow depth (ASD) and SCD, the relative contributions of snow cover to MSFD gradually increased, peaking at SCDs of 120-140 days or ASDs of 8-10 cm, which is attributable to the increasing insulating effect. However, with further increase in ASD and SCD, the insulating effect decreases because of high albedo and the latent heat effect of snow melting. This decrease resulted in declining MSFD-snow cover correlations and snow cover contributions to MSFD. Compared with SCD, ASD has a greater influence on MSFD. By adding an ASD variable to the Stefan formula, this modified formula outperformed the conventional formula in MSFD estimation at SCDs of 60-140 days. This study demonstrated the importance of snow cover variables for soil freeze depth analysis and simulation in areas with large snow cover and further elucidated the effects of snow cover on soil freeze depth.

Soil freeze depth variations greatly affect energy exchange, carbon exchange, ecosystem diversity, and the water cycle. Given the importance of these processes, obtaining freeze depth data over large scales is an important focus of research. This paper presents a simple empirical algorithm to estimate the maximum seasonally frozen depth (MSFD) of seasonally frozen ground (SFG) in snowy regions. First, the potential influences of driving factors on the MSFD variations were quantified in the baseline period (1981-2010) based on the 26 meteorological stations within and around the SFG region of Heilongjiang province. The three variables that contributed more than 10% to MSFD variations (i.e., air freezing index, annual mean snow depth, and snow cover days) were considered in the analysis. A simple multiple linear regression to estimate soil freeze depth was fitted (1981-2010) and verified (1975-1980 and 2011-2014) using ground station observations. Compared with the commonly used simplified Stefan solution, this multiple linear regression produced superior freeze depth estimations, with the mean absolute error and root mean square error of the station average reduced by over 20%. By utilizing this empirical algorithm and the ERA5-Land reanalysis dataset, the multi-year average MSFD (1981-2010) was 132 cm, ranging from 52 cm to 186 cm, and MSFD anomaly exhibited a significant decreasing trend, at a rate of -0.38 cm/decade or a net change of -28.14 cm from 1950-2021. This study provided a practical approach to model the soil freeze depth of SFG over a large scale in snowy regions and emphasized the importance of considering snow cover variables in analyzing and estimating soil freeze depth.

Boreal forest and wetland have important influences on the development and protection of the ecosystem-dominated Xing'an permafrost. However, the responses of different ecosystems to climate change and the impacts on the underlying permafrost are still unclear. Here, based on the multi-period land use/land cover (LULC) data and long-time series of air temperature, combined with the ordinary least squares (OLS) and ordinary kriging (OK) methods, the effects of land use and cover change (LUCC) on the distribution of mean annual air temperature (MAAT) and permafrost in Northeast China were analyzed. From 1980s to 2010s, MAAT showed an upward trend (0.025 degrees C per yr) and extents of permafrost showed a decreasing trend (-3668 km(2)yr(-1)) in Northeast China. Permafrost degradation mainly occurred in forested land and grassland, with areal reductions of 4.0106 x 10(4) and 3.8754 x 10(4) km(2), respectively. The transformation of LULC aggravates the degradation of permafrost. The conversions of forested land and grassland to cultivated land and forested land to grassland resulted in the shrinkage of permafrost extent by 6233 km(2) from 1980s to 2010s . Our results confirm the significant impacts of LUCC on the Xing'an permafrost resulting in its degradation. Additionally, they can provide a scientific basis for ecological environment protection and restoration and sustainable development of boreal forest and wetland ecosystems in permafrost regions of Northeast China.

The water and energy in the land surface and lower atmosphere have a strong coupling relationship. Apart from the land surface temperature (Ts) and air temperature (Ta), the land surface-air temperature difference (Ts-Ta) is also an essential parameter reflecting the coupling process. However, the global spatiotemporal variations and influencing factors of Ts-Ta remain not well explored. Here, ERA5-land reanalysis data, GIMMS NDVI data, and elevation data were used to analyze the global spatiotemporal heterogeneity and influencing factors of Ts-Ta. It was found that annual mean Ts-Ta exhibited a decreasing trend from the equator to polar areas. And the annual Ts-Ta increased at 0.009 degrees C/ 10a from 1981 to 2020. The variations of global net radiation mainly determined the spatiotemporal heterogeneity of global Ts-Ta. The different properties of the land surface and near-surface atmosphere were the main factors affecting the Ts-Ta, including soil moisture, vegetation, snow cover, and the water vapor content in the atmosphere. In addition, Ts and Ta also affected each other. These findings are conducive to a better understanding of the land-atmosphere coupling, and it is of great significance to take better measures to adapt the global climate change.