Antarctica is the coldest continent on the earth. The temperature of the highest point in Antarctica， Dome A， has reached -93.0 ℃. Three extreme low temperature events are simulated through Polar Version 3.8.1 of the Weather Research and Forecasting Model （WRF）. Compared with the observation of automatic weather station at Dome A， the simulated results are satisfactory. Diagnoses are made based on the simulation outputs. The following results were obtained： the enhancement of the high-pressure center between the Indian Ocean and the Atlantic Ocean and its high-pressure ridge stretching into Antarctic plateau led the pressure rising at Dome A and made this region fine and cloudless， which were the foundation for the extreme low temperature events；the enhancement of the Antarctic cold vortex， longstanding cold advection and stable thermal inversion layer provided enough cooling factors and strengthen the radiation cooling effect at night； stable vertical field and extremely low downward longwave radiation made the extreme cold events continue. However， more in-situ observations are required， especially in the vast interior of Antarctica.
Arctic sea ice plays a very important role in global climate change and sea-ice concentration （SIC） is a crucial parameter for sea ice monitoring. The accuracy of SIC data is an essential basis for Arctic research. Therefore， comparing and assessing the products of sea ice data retrieved from different satellite observations are necessary. The results of existing research indicate that the AMSR2/ASI is a best dataset for SIC quantity studies and real-time shipping guide. However， the temporal coverage of these datasets is relatively short. There are some limits in the study of long-time series. In this paper， the SICCI dataset released by ESA SICCI and the NSIDC dataset released by NSIDC were compared and assessed using the BLM dataset released by Bremen University. The results showed that sea ice area obtained by the three datasets all had significant trends of reduction during 2003 - 2010 and 2013 - 2015. Sea ice concentration and sea ice area of SICCI are lower than BLM and those of NSIDC are higher than BLM. Compared with the NSIDC dataset， the bias of annual and monthly mean and daily sea ice area of SICCI are lower about 81.88%， 80.90%， 81.44%， respectively. The bias of SIC between SICCI and BLM is -3.28%， which is lower than 4.36% of NSIDC. Hence， the SICCI dataset is the best dataset for SIC quantity studies and Arctic sea ice area study. Latitude mean comparisons demonstrate that the SICCI dataset is able to successfully detect the small ice floe area near the continent. The bias of SIC between SICCI and BLM are 0.05% and 0.44% in the region of low latitude and low SIC， respectively， so the SICCI dataset is the best dataset to study sea ice in the regions of low-mid latitude or low SIC. In contrast， NSIDC has the lowest bias of 1.08% and root-mean-squared error （RMSE） of 7.76% in the regions near North Polar. Therefore， it is the most suitable for study in Arctic Polar and should be used for further study in the regions of middle and high latitudes. In addition， quantitative evaluation via BLM indicates that the SICCI dataset is suitable for study sea ice in the shipping route of low latitude or low SIC areas， and the NSIDC dataset performs well in the shipping route of high latitude or high SIC area.
The meteorological data from five meteorological stations in Altay Prefecture were analyzed by means of liner regression method， 9-year moving average analysis method to get spatial and temporal variations of air temperature in the prefecture， from 1962 to 2013. The results show that： （1） Annual mean air temperature， mean maximum and mean minimum air temperature had increased significantly with the rate of 0.40， 0.29， 0.58 ℃·（10a）-1， and the maximum ones happened in winter and autumn. （2） The extreme maximum air temperature， the yearly maximum value of daily minimum air temperature， warm day and warm night had risen with the rate of 0.19 ℃·（10a）-1， 0.58 ℃·（10a）-1， 1.45 d·（10a）-1， 3.37 d·（10a）-1； diurnal temperature range had decreased with a rate of -0.29 ℃·（10a）-1； the growing season length had increased with a rate of 3.31 d·（10a）-1. Warm day and warm night had increased in every season at more than 50% stations； in the rest stations， all indices had increased， except for the maximum air temperature and the growing season length. （3） The extreme minimum air temperature and the yearly minimum value of daily maximum air temperature had increased with the rate of 0.68， 0.48 ℃·（10a）-1. Cold days， cold nights， freezing days， frost days had decreased with the rate of -1.57， -3.69， -1.79 and -4.40 d·（10a）-1； cold night and frost day at all stations had significantly decreased. （4） The decrease of cold indices was larger than the increase of warm indices， and the decrease of night indices was larger than the increase of day indices. These results would provide theoretical basis for the sustainable development of local agriculture and prevent meteorological disasters in Altay Prefecture.
Based on the daily precipitation data of 45 meteorological stations in northern Xinjiang from 1961 to 2018， the temporal and spatial distribution and circulation characteristics of different types of snowstorms in northern Xinjiang were studied. The results show that among the three types of snowstorms in northern Xinjiang， the number of partly snowstorms was the most， followed by regional snowstorms， and the number of large regional snowstorms is the least. Partly snowstorm days accounted for 73.1% of total snowstorm days， regional snowstorm days accounted for 20.9% and large regional snowstorm days only accounted for 6.0%. Except for partly snowstorms， the days of the other two types of snowstorms increased significantly. The snowstorms in northern Xinjiang mainly occurred in November and March， and the snowstorms in January were the least. Before the 1990s， local snowstorms dominated the northern part of Xinjiang， and the snowstorm centers were mainly located in the northern part of the Ili Valley and Tacheng area. Since the 1990s， the number of days of regional and large regional snowstorms had increased significantly. The number of days of snowstorms has increased significantly in the Altay and the middle part of the northern slopes of the Tianshan Mountains. Urumqi has become a new center of snowstorms on the northern slopes of the Tianshan Mountains. The circulation situation of snowstorm in northern Xinjiang can be divided into three types and six models. Among them， the front wave type is the most， the low trough type is the second one， and the low vortex type is the last one. Before the 1990s， the front wave type was the most， and into the 21st century， the low trough type has significantly increased.
Based on daily precipitation record and record of weather phenomena from 162 meteorological stations in Northeast China， climatic characteristics and temporal and spatial variation of snowfall from 1961 to 2017 were analyzed using statistical analysis methods. The results show that the snowfall and snowfall days were the maximum in December， the light snow and moderate snow were in November or December， the heavy snow and snowstorm had the highest probability of occurrence in late winter and early spring. The snowfall was larger in mountainous areas than in plain， and in plain snowfall had been decreased from north to south and from east to west， the high snowfall area had mainly located in the northern part of the Great Hinggan Mountains， the Xiao Hinggan Mountains and the Changbai Mountains， and the snowfall intensity center had been located in the Changbai Mountains and the plain areas of central Liaoning Province. The annual， autumn， winter and spring snowfalls had been accounted for 4.7%， 7.0%， 84.4% and 7.6% of the precipitation in the same period， respectively； the daily maximum snowfall had been accounted for the highest proportion of total annual snowfall in western mountainous areas and southern Dalian areas of the province； the longest continuous snowfall days were less than 2 days. In the 57 years， the snowfall and the snowfall intensity had increased significantly with the rates of 1.93 mm?（10a）-1 and 0.11 mm?d-1?（10a）-1， and the snowfall days had decreased significantly at the rate of 2.08 d?（10a）-1； the increase of snowfall had been mainly due to the increase of snowfall at all levels； the decrease of snowfall days had been mainly due to the decrease of traces and light snowfall days； the increase of snowfall intensity had been mainly due to the heavy snow and snowstorm intensity. The annual， autumn and winter snowfall as a percentage of precipitation in the same period had increased by 0.36%， 0.48% and 0.45% per decade， while spring snowfall had decreased by 0.11% per decade. The contribution rate of medium snow， heavy snow and snowstorm to snowfall had increased， and the contribution rate of light snowfall and traces snowfall days had decreased. After a mutation of snowfall and snowfall days in 1987， the contribution rate of traces snowfall days， snowstorm days and light snowfall had changed significantly. On the regional average， the amount of snowfall in 2001 - 2017 had increased by 27.8% as compared with that in 1961 - 1980， and the snowfall days had decreased by 22.4%.
Based on the data of temperature， precipitation and snow cover days in the Qinling Mountains from 1962 to 2016 from 32 meteorological stations， the spatial and temporal changes and influence factors in the cold season， snow cover days were analyzed by REOF， M-K test and wavelet analysis. It is found that the average snow cover days in the cold season of the Qinling Mountains is higher than that in its southern slopes. There are more snow cover days in the higher altitude. Under global climate change， the snow cover days decrease rapidly. The snow cover days are significantly decreasing in the cold season. There are five districts where the decadal variation of snow cover days are significant. From the end of the 20th century to the beginning of the 21st century， there were sudden changes in snow cover days. The periodic changes of snow cover days in the cold season mainly concentrated in 10 ~ 20 a， and there were also have obvious 4 a period in the southern slopes of the Qinling Mountains. The seasonal increase of SST in the northwest Pacific Ocean was the external forcing factor for the decrease of snow days in cold season in the Qinling Mountains. The significant increase of mean temperature in cold season and the significant decrease of precipitation in cold season in the mountains directly resulted in the decrease of snow cover days. In the Qinling Mountains， the abrupt change of decreasing number of snow days in cold season lagged behind the abrupt change of temperature increase by about 4 ~ 7 a.
The basic variation characteristics of snow cover in the Mount Hua and its relationship to temperature and precipitation were analyzed by using linear tendency estimate， M-K inspection， correlation analysis and path analysis， based on the snow cover observation data in the Mount Hua weather station from 1953 to 2016 and satellite remote sensing data from 1989 to 2016. The results demonstrated that： （1） Average days of snow cover were 78.5 d. Snow cover mainly occurred from October to the following May. The beginning date of snow cover had delayed； the ending date of snow cover had advanced； the days from the beginning to the ending had decreased； the days of snow cover had decreased significantly with the rates of winter half year and winter from 1953 to 2016； the annual， winter half year and winter snow days had significantly reduced by 8.3 d?（10a）-1， 7.6 d?（10a）-1， 4.7 d?（10a）-1， respectively. There was no significant decreasing trend of annual maximum snow depth and annual accumulated snow depth had decreased significantly with the rate of 88.2 cm?（10a）-1 from 1981 to 2016. Monthly decreasing trend of snow cover days， maximum snow depth and accumulated snow depth were the most significant in March during the 36 years. There was no obvious decrease of snow cover area， light snow cover area and deep snow cover area in the Mount Hua from 1989 to 2016. （2） From 1953 to 2016 average temperature had risen， precipitation had decreased， the days of snow cover had a significant negative correlation with average temperature and a significant positive correlation with precipitation in the Mount Hua. Temperature was the key factor impacting snow cover days in year， winter half year and winter. The annual， winter half year and winter days of snow cover all had changed abruptly in the years close to the year that average temperature changed abruptly. （3） Average temperature and precipitation had significantly correlated with atmospheric circulation index in winter half year and in winter from 1953 to 2016. The snow cover days of winter and winter half year had negatively correlated with atmospheric circulation indexes， such as Tibet Plateau Region index， India-Burma Trough intensity index， Antarctic Oscillation and Western Pacific Sub Tropical High Western Ridge Point index， and had positively correlated with East Pacific 850 hPa Trade Wind index and Asia Polar Vortex area index significantly.
The cold wave is one of the common meteorological disasters in winter， autumn and spring in northern China， and has severely impacted on socioeconomic development and people’s lives. The Hexi Corridor is a fragile ecological environment and affected by the cold wave， revealing the characteristics of the spatial and temporal of the cold waves’ frequency can provide a reference for disaster prevention and mitigation in agriculture and animal husbandry. In our study， we collected the daily minimum temperature data of 12 meteorological stations in Hexi Corridor from 1961 to 2018， data statistics and spatial visualization methods were used to analyze the spatial-temporal variation characteristics of the cold wave frequency in the Hexi Corridor in recent 60 years， and the response relation between the Arctic Oscillation （AO） anomaly and the cold wave variation was discussed. The results showed that： （1） From the perspective of time series analysis， the cold wave mainly occurred from October to April， of which November， December and April were high frequency of cold wave. The cold wave frequency showed gradual decrease from 1961 to 2018， and a lowest value appeared in the 1980s. From the perspective of seasonal variation， the cold wave frequency is decreasing， the trend was high in autumn， middle in spring， and low in winter. （2） The frequency of cold waves in the Hexi Corridor has obviously spatial differences. The cold wave frequency was high in the western region， while the frequency was low in the central region， and the eastern region was middle level. （3） The conditions of AO has a spatial-temporal response relation with the frequency of the cold wave in the Hexi Corridor. Cold wave was high frequency in all meteorological stations of Hexi Corridor when the AO was negative phase， especially in the eastern and western.
Freezing index is a comprehensive representation of the length and cold severity of winter， and the thawing index is the length and >0 ℃ accumulated temperature of the thawing season in a certain region. The freezing/thawing index are key parameters to calculate the thickness of active layer and the seasonal soil freezing depth， and it can be used in prediction of the permafrost distribution. In this study， the air and ground freezing/thawing indices and their spatial and temporal variation trends from 1977 to 2017 in the middle and lower reaches of the Yarlung Zangbo River Basin （YZRB） were calculated and analyzed on the basis of the daily air and ground surface temperatures obtained from 11 national meteorological stations. Results showed dramatic changes of freezing/thawing indices in the middle and lower reaches of the YZRB over the past four decades. The air freezing index， ground surface freezing index， air thawing index， and ground surface thawing index ranged from 208.4 ℃·d to 508.0 ℃·d， and from 136.9 ℃·d to 371.0 ℃·d， and from 2 171.8 ℃·d to 2 499.8 ℃·d， and from 3 350.2 ℃·d to 4 315.2 ℃·d respectively， with a climate tendency rate of -36.6， -48.7， 90.7 and 115.8 ℃·d·（10a）-1， respectively. Both the air and ground surface freezing indices of this study area were highest at Jiali Station with an altitude of 4 488.8 m， and lowest at Linzhi Station with an altitude of 2 991.9 m. The air and ground surface thawing indices were highest at Zedang Station with an altitude of 3 560 m， and lowest at Jiali Station with an altitude of 4 488.8 m. The days with negative air temperature within a year decreased at a rate of -6.28 d·（10a）-1， and the changes of the days with negative ground surface temperature within a year showed similar trend ［-5.57 d·（10a）-1］. The periglacial landforms， which is formed under the presence of the freezing-thawing cycles could provide a reference for the monitoring and prevention of geological disasters， such as thawing slumps and solifluctions， are worthy of great attentions in future studies.
In the southern margin of permafrost zone of the Eurasian continent， permafrost in the Greater Khingan Mountains is greatly sensitive to local factors such as vegetation and moisture for its formation， development and preservation. In order to reveal the specific features of permafrost in these areas， drilling， ground penetrating radar （GPR） and long-term monitoring of permafrost temperature were conducted in Xinlin forest area on eastern slope of the Greater Khingan Mountains. In the Greater Khingan Mountains， the active layer thickness （ALT） is significantly affected by vegetation type and grazing intensity. The ALT reaches 2.5 m at severe grazing plots， but shrinks to 1.5 m at light and non-grazing grazing plots. The ground temperature at the depth of 2.0 m was 0.04 ℃ at Carax tato zone in November 2013， and began to recover gradually after grazing termination and the Gammu Highway diversion， from -0.12 ℃ to -0.69 ℃. Meanwhile， the ground temperature at the depth of 1.5 m was reduced from 0.17 ℃ to -0.42 ℃. Thus the ALT was less than 1.5 m at Carax tato zone. Changing with vegetation type， the annual ALT was 0.8 m， 1.3 m and 0.7 m， respectively， at larch forest zone， Carax tato zone and shrub zone. The annual average temperature at 11m depth were -1.34， -0.98 and -2.19 ℃ for the three ground surfaces. According to the profiles of ground temperature， the geothermal profiles belonged to the positive gradient type， indicating a relatively stable permafrost at shrub zone. However， it is more complicated at larch forest plot with negative - zero - positive gradient type geothermal curve and Carax tato zone with negative - twist type geothermal curve. Permafrost thickness in larch forest， Carax tato zone and shrub zone is 63.8 m， 60.3 m and 84.6 m， respectively. Because of the combined effect of vegetation and human activities， permafrost in the study area experienced a slight degradation， degradation of termination due to disturbance removal and gradual recovery.
In order to investigate the creep deformation behavior of warm frozen soil and its relationship with various influencing factors， uniaxial creep tests were carried out in laboratory on the specimens with moisture contents of 15%， 25% and 35%， load levels of 100 kPa， 200 kPa and 300 kPa， and temperatures of -1.5 ℃， -0.7 ℃ and -0.3 ℃. Then， the creep deformation behaviors of warm frozen soil under different temperatures， ice contents and load conditions were analyzed. The results revealed that the creep deformations of the warm frozen soils could reach a considerable level， and they showed an attenuated tendency under current conditions. Soil temperature was the most important external factor for influencing frozen soil creep while ice content was the key inherent one. When contained a lot of ice， the warm frozen soil might develop a remarkable variation in creep rate when soil temperature or load changed， even on the order of magnitude. Under current test conditions， the Burgers viscoelastic model could be applied to express the creep deformation process of warm frozen soil with a satisfactory fitting degree.
In order to reflect the influence of freezing-thawing cycles on the permeability coefficient anisotropy of undisturbed loess and the permeability differences between undisturbed and remolded loess， Q3 loess samples with obvious anisotropy were collected from Xi’an. Triaxial permeability tests were conducted to analyze the change law of permeability coefficient of horizontal and vertical undisturbed loess and remolded loess varying with initial water content， freezing-thawing cycles and cell pressure before and after freezing-thawing cycles. It can be obtained that before the freezing-thawing cycle， the vertical permeability coefficient of undisturbed loess is 2×10-6 ~ 18×10-6 cm?s-1， while the horizontal permeability coefficient of undisturbed loess and the permeability coefficient of remolded loess is 0 ~ 4×10-6 cm?s-1. After the freezing-thawing cycles， the relationship curves between the permeability coefficient and the initial water content of the horizontal and the vertical undisturbed loess and remolded loess present different characteristics of gradual rise， parabola form and gentle change. However， with the increase of freezing-thawing cycles， the permeability coefficients of the horizontal and the vertical undisturbed loess and remolded loess shows an increasing trend of order of magnitude. Moreover， the ratio of vertical to horizontal permeability coefficient （kv /kh ） of undisturbed loess decreases from 4.38 to 0.90 gradually， which can be seen that the freezing-thawing cycles can not only improve the permeability of loess significantly， but also strongly weaken the anisotropy of loess. By comparing the correlation between cell pressure， permeability coefficient and porosity， the results show that there is a negative linear relationship between the porosity and cell pressure of the undisturbed and remolded loess. It can be explained that the change of porosity is the main reason for the change of loess permeabilityby an exponential rule of permeability coefficient and the cell pressure， as well as the similar variation curves of the permeability coefficient and porosity.
Taking saline soil in the Qarhan area as the research object， using Scanning Electron Microscope （SEM） equipment and particle pore and crack image recognition and analysis system （PCAS）， microscopic quantitative identification and structural analysis are carried out. Applying image segmentation principle and fractal theory to study the microstructure of saline soil under traffic cyclic loading， the characteristics of pore distribution and its microscopic mechanism under different cyclic stress ratios and different frequencies are discussed. The results show that the cyclic stress ratio R increases from 0.375 to 0.75， the critical failure dynamic strain increases from 2.32% to 3.17%， and the number of dynamic load cycles decreases accordingly. The vibration frequency f increases from 0.5 Hz to 2.0 Hz， the critical failure dynamic strain reduces from 2.82% to 2.48%， and the number of dynamic load cycles increases correspondingly. The microscopic parameters of saline soil increase with the cyclic stress ratio， fractal dimension value of pore scale Dv increases by 0.12， probability entropy Hm decreases by 0.017， fractal of the morphology of pores D decreases by 0.27， average shape coefficient F increases by 0.14； the microscopic parameters increase with the vibration frequency， Dv increases by 0.014， Hm decreases by 0.002， D changes by 0.06， average shape coefficient F changes by 0.003. Under dynamic loading， the pore space arrangement of sample is consistent with the fold and drum deformation of the macroscopic soil. Based on the comparison of microstructure parameters about the unloaded sample and loaded sample， the interaction mechanism between cyclic stress ratio and vibration frequency on the saline soil microscopic pore characteristics and macroscopic deformation has systematically analyzed.
In cold regions， the construction and service life of foundation engineering are restricted by tangential frost heaving force， one of the key parameters in design of foundations. Therefore， it is significant to study the tangential frost heaving force in depth. In this paper， the current research status of tangential frost heaving force was summarized by reviewing relevant literature at home and abroad. The development mechanism of tangential frost heaving force was described in details in terms of the conditions and process of tangential frost stress； the influence factors and the distribution laws of the tangential frost heaving force were also represented briefly. Starting with the experiment and theory， the test methods and theoretical calculation methods of tangential frost heaving force were expounded， and how to determine the value of tangential frost heaving force was also stated. This review also put forward prospects for the future research of tangential frost heaving force.
To prevent the fracture of freezing pipe is an urgent problem in the formation of frozen wall in deep expansive clay layer. According to the soil properties of the deep thick expansive clay layer of a mine auxiliary shaft in Huainan mining area， distribution of the freezing temperature field and formation law of the frost heaving force are studied through the thermo-mechanical coupling calculation. When frozen for 152 days and 236 days， the average temperature of clay layer is -14.42 ℃， -16.58 ℃ and fine sand layer is -15.86 ℃， -17.32 ℃ respectively. The average temperature of clay layer is 1.44 ℃ and 0.74 ℃ higher than that of fine sand layer in the same period. The average thickness of the frozen wall in clay layer is 8.92 m and 10.25 m respectively， and that in fine sand layer is 9.54 m and 10.77 m respectively. The average thickness of the frozen wall in clay layer is 0.62 m and 0.56 m smaller than that in fine sand layer in the same period. Sand is easier to freeze and faster than clay. When frozen for 90 days， the average frost heaving force of the outer， middle and inner circles of the deep clay layer is about 1.1 times of that of fine sand layer in the same period. When the clay layer was frozen for 151 days， the average frost heaving force in the frozen wall surrounded by three circles of freezing pipes reached 81.1% of the initial ground stress， which was 1.16 times of the fine sand layer in the same period. When frozen for 236 days， the frost heaving force of inner circle of fine sand layer is 3.91 MPa， which is about 5.11% bigger compared with that of middle circle of 3.72 MPa， while that of inner circle of clay layer is 4.81 MPa， which is about 1.48% bigger compared with that of middle circle of 4.74 MPa. The average frost heaving force in the frozen wall surrounded by three circles of freezing pipes in clay layer is 88.6% of the initial ground stress， which is 1.28 times of that of fine sand layer in the same period. There is a significant non-uniformity in the frost heaving force at the deep expansive clay layer and the interface with the fine sand layer. The main position of the maximum frost heaving force is basically corresponding to the pipe break in the actual project excavation. It highlights that the non-uniform frost heaving force is the important reason for fracture of freezing pipes， which provides a reference for the design of similar frozen projects in the future.
In order to study anisotropic properties of sandstone in Tianshan Highway， Xinjiang， freeze-thaw cycle test， uniaxial/triaxial compressive tests were carried out. Strength and deformation parameters under different confining pressure and numbers of freeze-thaw cycles were discussed， the impact of bedding angle and failure modes was analyzed. The results show that elastic modulus， inner friction angle and cohesion of samples increase at first and then decrease with the increase of angle， and reach the minimum when the angle is 60°， reach the maximum at the angle of 0°or 90°； strength and deformation parameter decreases with numbers of freeze-thaw cycles， however， these parameters increase with the increase of confining pressure； with the increase of confining pressure， anisotropy of samples is decreasing； with numbers of freeze-thaw cycles， anisotropy of samples is increasing； the failure modes of sandstone can be summarized as splitting failure cross weak bedding face， shear failure cross weak bedding face， tension-shear failure， shearing slip damage along the weak bedding face， and splitting failure along weak bedding face. These results could be useful for engineering construction in cold regions.
In seasonal frozen areas， subgrade fill soil is often in the state of unstable state when spring thawing. In this paper， four influencing factors including freezing temperature， thawing temperature， confining pressure and moisture content were selected to conduct a triaxial test on the silty clay specimen from Zhangjiakou. The test results were analyzed by grey relational analysis method， and a sensitivity order of each factor to the strength was given. The results show that the sensitivity of moisture content， thawing temperature and frozen temperature is more than 60%， which should be considered importantly. When the moisture content is 9%， the strength of the specimen is high and brittle failure will occur. With the moisture content increase， the failure will change to ductile failure. Thawing temperature mainly affects the thawing rate and the rate of drainage consolidation in the shear process. The lower the temperature， the stronger the soil specimen； freezing temperature affects the strength by changing the degree of cementation between soil particles and ice crystals； the lower the freezing temperature， the stronger the cementation. However， below -10 ℃， the strength increases slowly. The strength of soil increases with the increase of confining pressure. Under the influence of different confining pressures， the shape and trend of stress-strain curves are almost the same. This analysis results will be useful for the practical projects in seasonal frozen areas.
The deserts and sandy lands in China totally or partly located in seasonally frozen soil regions. The aim of this study on freeze-thaw processes of dunes was to further elucidate wind erosion and morphological evolution of dunes during seasonal freezing. The freeze-thaw processes and its controlling factors of mobile dunes and fixed dunes by barrier located in the East Desert of the Yellow River in Ningxia were analyzed through field observation and laboratory experiment. The results show： The freezing period of dunes was from mid-November to early March next year， there were distinct differences in freezing duration and thickness of frozen layers among geomorphic sites of mobile dunes （leeward slope > windward slope > top of dune）. During the seasonal freezing period， there was a surface unfrozen layer on dune which protected the frozen layer， and the thickness of it was about 10 cm. The frozen layer thickness of mobile dune was much smaller than that of fixed sand dune by barriers， which caused by the cyclic process of thawing and wind erosion on frozen layer surface in the windward slope of mobile dune. The thickness of frozen layer of dune top and leeward slope on mobile dune affected by short-term （32 h） and long-term （15 d） average temperature， respectively. The results from field observation and laboratory experiment indicated that the dune could not be frozen with sand moisture content under 1.6%； there was a power function relation between the hardness of frozen soil and moisture content （P<0.001）， and the hardness increased slowly with temperature decreasing.
In the winter of 2017 and 2018， aerial photography of ice surface on Lake Wuliangsuhai was used to determine the density and fractal dimension of the cracks on ice surface， by using improved adaptive-thresholding and image segmentation. The results show that the fractal dimension of ice cracks at a fixed location changes within the range of 1.35 ~ 1.50， and both the amount and fractal dimension of the cracks increase with time. An obvious linear relationship is found between the fractal dimension and the ice thickness during the period of ice growth， with a correlation coefficient of R2=0.75. For ice cracks at different locations of ice surface， the density varies within 0.017 ~ 0.079， and fractal dimension within 1.38 ~ 1.64. A significant logarithmic relationship is also found between the two parameters， with a correlation coefficient of R2>0.96. Aerial photographs in different days give very similar results of curve fitting， revealing that a larger crack density attributes to a larger fractal dimension of the cracks. As a physical index of ice surface morphology， the relationship between fractal dimension of ice cracks and ice thickness， crack density is developed in this study， which provide a possibility to monitor the ice growth and decay through the evolution of cracks on ice surface.
Icicles hanging from the eaves are a common phenomenon in the mid and high latitudes， and their abrupt breaking and falling may have detrimental effects on people’s work and life. Particularly for bridge construction， tunneling and transmission projects， it will cause significant damage to facilities and human. Due to lack of knowledge in icicles’ natural growth and development， current methods for icicle prevention and management are nothing but mechanical destruction and heating， which cost is a lot. In this paper， a outdoor experiment was used to investigate the initial growth and development of eaves icicle， which will guide to simulate the initial growth of icicle in low-temperature room using high-precision titration from metal carriers. By observing the velocity of water droplet and the formation rate of ice column （length， diameter and weight）， the relationship between the properties of ice column and the time and water supply was obtained. The results showed that the formation of ice column and the flow distribution of the eaves are influenced by the development of ice column. There is a synergetic relationship between surface water flow velocity and the development process of ice column； the ice column shape resistance and the flow rate increment produced by the runoff concentration affects the formation rate. These research results will be useful for preventing and managing eaves icicles.
In order to deepen the understanding of the process of water circulation in the upper reaches of Heihe River， based on the measured data of precipitation isotopes and meteorological data from 5 sites in the study area from August 2015 to August 2016， in addition to the analysis of precipitation isotope characteristics， TrajStat software is mainly used. The backward trajectory calculation module and the potential source contribution function analysis （PSCF） method focus on the analysis of the water vapor source of the precipitation in the study area， and supplement the analysis with the method of water vapor flux. The results show that： （1） The precipitation isotope is obviously low in summer and high in winter， and the slope （8.02） and intercept （11.02） of the meteoric water line in the study area are higher than the slope （8） and intercept （10） of the global atmospheric waterline. There is a temperature effect （δ18O=0.43x-10.82， r=0.54， P<0.0001）， there is no precipitation effect （δ18O=-0.05x-7.81， r=0.03， P<0.0001）； （2） Precipitation in the study area is affected by many kinds of water vapor， and the west wind water vapor has the greatest influence. In summer， except the west wind water vapor， the southeast water vapor has obvious influence and the source of water vapor is complicated； （3） The potential evaporation sources in the study area are mainly concentrated in some areas with large evaporation and relatively high humidity， such as： Qilian Mountains， Hexi area， northern part of the Qaidam Basin， the southeastern part of the plateau and the southwestern part of the Jiuquan area are the main potential evaporation sources for summer precipitation in the study area； （4） When precipitation is small and temperature is high， the slope and intercept of atmospheric water line equation of continuous precipitation are small. The stable isotope value of rainstorm is relatively depleted， which is most affected by water vapor of southeast monsoon， followed by water vapor of north and west wind， a variety of water vapor convergence is a necessary condition for the occurrence of heavy rain events.
Under the background of climate warming， the annual and seasonal temperatures of the Yangtze River since the 1960s have shown a significant warming trend； evaporation and runoff have generally increased. Since the beginning of the 21st century， precipitation in the source regions of the Yangtze River has increased. Tuotuo River is the main river in the source areas of the Yangtze River， which is of great practical significance to study the impact of climate warming on the runoff in the source areas of the Yangtze River.By using the hydrological and meteorological observation data from 1981 to 2015， the influence of climate factors on runoff are analyzed in this paper. The results show that the annual and seasonal mean temperature， the mean maximum and minimum temperatures had increased in the 31 years； the annual and seasonal （except winter） precipitations had increased in the 31 years in the Tuotuo River basin； evaporation had shown a decreasing trend in summer， autumn and the year round， together with an increasing trend in spring and winter. Among the climate factors， precipitation was the most important factor to effect runoff， which had increased in summer and had a very close relationship with runoff in summer. The influence of glacier and snow melt water on runoff was followed by the increase of annual mean minimum temperature， and the influence of evaporation was obviously lower than the former two.
Based on the measured hydrological data and the statistical data of water conservancy and water conservation measures in the upper reaches of the Weihe River， the variations of hydrological factors and water conservancy and water conservation measures in the basin are analyzed. According to the double cumulative curve， ordered clustering， Lee-Heghinan， sliding T test and other methods， the study period was divided into base period （1956 - 1981） and measure period （1982 - 2016） according to the changes of water conservancy and water conservation measures in the basin. By using the water balance method， the runoff and sediment runoff of rivers are analyzed and restored， and water and sediment reducing effects of taking water on water conservancy project are evaluated. The correlation formulas of annual runoff， annual sediment runoff and annual precipitation in the base period and the measure period are established， respectively， and the effect of water and sediment reducing in the measure period is analyzed and calculated. The basic period is mainly water and sediment reducing for water conservancy projects， water and sediment are reduced by 6.8% and 3.9%， respectively； for the measure period the comprehensive water reducing effects are 42.9%， and the comprehensive sediment reducing effects are 76.5%， the water-reducing and sediment-reducing effects of water conservation measures is larger than that of water conservancy projects， and the comprehensive sediment-reducing effect is much larger than the comprehensive water-reducing effect. Analyzing and studying the influence of human activities on runoff and sediment in the river basins can provide important basis for future regional water resources management， comprehensive river basin management and ecological environment protection.
The spatial distribution of stable hydrogen and oxygen isotopes in precipitation is the basic data in isotope hydrology and isotope ecology. An increasing attention has been paid to the products of stable hydrogen and oxygen isotopes at high spatial resolutions in recent years. In this study， based on the precipitation samples collected at 23 stations across the Tianshan Mountains during 2012 - 2013， applicability of two widely-applied global products is evaluated， including OIPC （the Online Isotopes in Precipitation Calculator） and RCWIP （the Regionalized Cluster-based Water Isotope Prediction）. According to the physical geography regionalization of arid land in China， the study area is divided into： （Ⅰ） the Junggar Basin desert area； （Ⅱ） the Ili-Bayanbulak mountain area； （Ⅲ） the Tarim Basin desert area； and （Ⅳ） the Turpan Basin-Hami Gobi desert area. In this paper， the values of root-mean-square error （RMSE）， determination coefficient （R2）， mean bias error （MBE） and mean absolute error （MAE） were used in comparison between the observed and simulated isotopic composition in precipitation. Results show as follows： （1） On a temporal basis， the simulation of OIPC and RCWIP in the summer half year （from April to October） is obviously better than that in the winter half year （from November to March）. Among the natural subregions， the best simulation is seen in the Junggar Basin desert area and the worst is in the Turpan Basin-Hami Gobi desert area. （2） By comparing the evaluation parameters of RMSE， R2， MBE and MAE， RCWIP is generally better than OIPC in this study area. （3） By analyzing the multi-year data of stable hydrogen and oxygen isotopes in precipitation in Urumqi， the inter-annual variability of precipitation isotopes does not greatly affect the long-term representativeness， and the products are still useful especially when in-situ observations are unavailable.
Northeast China is one of the most important grain producing area in China， which is also the most typical area affected by seasonal freezing and thawing. The freeze-thaw process affects the migration and transformation of water and nutrients in spring farmland by changing the soil evaporation capacity， infiltration capacity and the hydraulic connection between surface water and groundwater. Therefore， studying the variation and influencing factors of soil temperature and moisture during freezing and thawing period is of great significance for guiding water and fertilizer management in seasonal freeze-thaw agriculture areas. However， most of the studies on the effects of freeze-thaw process on soil water migration have just used soil column freezing test， which was carried out using homogeneous soil at the specified freezing and thawing temperature and frequency， and the water migration in the farmland soil could not be well reflected. In order to better understand the effect of freezing-thawing process on soil temperature and moisture variation in a seasonal freezing agricultural areas， six sampling were carried out using soil-drilling method in each sampling location before and during the freezing and thawing period to monitor soil moisture， and temperature probes were pre-embedded to monitor soil temperature. Results show that the surface soil temperature during freezing and thawing period was mainly affected by the thickness of snow， while the deep soil temperature was mainly affected by the initial soil moisture content. During the freezing period， the moisture content of the frozen soil layer almost increased， among which the shallow soil increased the most； and the slower the freezing rate， the lower the initial water content， and the higher the water content of the adjacent soil layer， the more the water content increased of the soil layer. During the thawing period， the soil moisture content of each underlying surface and soil layer was basically decreasing， and this phenomenon mainly occurred in the soil layer of 0 - 30 cm. The water was mainly loss was mainly lost by evaporation， and the frozen soil layer had an inhibitory effect on soil evaporation. The melting of the frozen soil layer was the main reason for the difference in soil moisture content between the underlying surfaces and that between different melting stages.
The spatial and temporal variations of daytime precipitation and nighttime precipitation were analyzed based on observed data from 47 meteorological stations from 1961 to 2018. The resulted indicated that： （1） The spatial distribution of daytime and nighttime precipitation were basically consistent， in the 58 years， with a decreasing trend from southeast to northwest， precipitation in daytime less than that in nighttime； （2） Spatially， there was an obvious decreasing tendency of precipitation in daytime and nighttime in the Qinghai Plateau， but in the eastern agriculture area， the western Qaidam and the Qingnan pastoral there was an increasing trend， and others areas showed an obviously increasing tendency； （3） There was a few increasing days of daytime precipitation but a decreasing days of nighttime precipitation， it had an decreasing tendency of daytime and nighttime precipitation in the eastern agriculture， but an increasing trend in Qaidam region； （4） There were quasi-periodic variations of 2 years in daytime rainfall and 3 years in nighttime rainfall in the plateau； （5） There was no clear abrupt variation of nighttime rainfall in the Qinghai Plateau， the eastern agriculture， the Qinghai Lake area and Qaidam basin， except for a jumping in 2003， but nighttime rainfall had an abrupt in Qinghai lake area in 1979 and 2003， respectively.
Lanzhou New Area is located in the west of the Loess Plateau， which is a typical arid area. Many engineering slopes with over 30° are formed during road construction. Revegetation on slopes plays an important role in improving local landscape and preventing soil erosion， and soil moisture status on slopes has a great impact on revegetation. Three types of soil preparation （the strip-shaped pit， the round-shaped pit and the original slope） were selected to study on the soil moisture of slopes of loess in Lanzhou New Area formed by engineering excavation in the early stage of revegetation. The results show that： （1） In the three types of soil preparation， the soil moisture condition of the strip-shaped pit is the best， and there is a statistically significant difference between the strip-shaped pit， the the round-shaped pit and the original slope （P<0.05）. （2） On the undisturbed slope plots under different irrigation frequency in 0 - 20 cm soil layer soil moisture content is low and in 20 to 50 cm soil layer soil moisture content is higher. With the increase of soil depth， the variation coefficient of soil water content decreases. With the decrease of irrigation frequency， the variation coefficient of soil water content increases， and the active soil layer increases from 0 - 10 cm to 0 - 40 cm， while the variation coefficient of 40 - 50 cm soil layer remains below 10% in different irrigation stages. （3） In the early stage of vegetation reconstruction of the slope， the soil moisture should be maintained at 8.4% ~ 10.8%， that is， the field water capacity 38% - 49%， in order to ensure the normal growth of plants； when the root length of the planted vegetation is more than 10 cm， it can be considered to change the sprinkling irrigation frequency from daily to alternate days， otherwise vegetation will be in danger of death；the results can provide reference for vegetation restoration and ecological construction of similar slopes of loess.
Based on monthly mean air temperature， maximum temperature， minimum temperature， precipitation and annual maximum frozen soil depth datafrom five meteorological stations in Chang Tang Nature Reserve （CTNR） from 1971 to 2017， in combination with the satellite remote sensing data during 1973 - 2017， the changes in major ecological environment variables such as climate（e.g.， temperature and precipitation）， water（lakes， glaciers） and grassland vegetation were analyzed using the linear trend analysis and correlation analysis methods. The results shows that： （1） In the past 47 years （1971 - 2017）， the annual mean temperature had been significantly increasing with a rate of 0.46 ℃·（10a）-1 in CTNR， a much higher rate than that in the global and Asia. The seasonal mean temperature inceased in CTNR at a rate of 0.37 - 0.55 ℃·（10a）-1 with the maximunm rate occurring in winter， and the minimun rate in summer. Besides， the annual precipitation amount exhibited a significantin creasing trend with a rate of 11.0 mm·（10a）-1， especially in spring and summer. （2） During the 1975 - 2017， the area of Selinco showed a significant expansion with a rate of 38.48 km2·a-1. The coverage area of Puruogangri Glacier presented a shrinking tendency with a total retreated area of 2.11 km2 from 1973 to 2017， and the maximum annual depth of permafrost in nature reserves was 35.7 cm per 10 years on average. （3） The NDVI in the reserve increased by 25.3% from 1999 to 2013， with an average increasing rate of 0.0184 per decade， indicating the vegetation coverage increased significantly. （4） Overall， this study area is featured by warm and humid climate， melting glaciers， lake expansion， permafrost degradation， and increasing vegetation cover in CTNR. More importantly， the glacier ablation is one of the main factors leading to the changes in the spatial and temporal distribution of water resources and the water cycle process change， which will undoubtedly exert a profound impact on social and economic development in the Qinghai-Tibet Plateau.
Under the influence of climate warming and human activities， the alpine meadow in Qinghai-Tibet Plateau has significantly degenerated， making scholars attach great importance to it. However， their influence on microorganism is still uncertain. This study regarded original ecological alpine meadow in upper reaches of Shule River in the northeastern Qinghai-Tibet Plateau， natural degeneration alpine meadow and artificial plowing and supplementing alpine meadow soil as the targets， and systematically researches on the influence of degeneration and plowing and supplementing on the quantity of microorganism in this area. The results showed that the quantity of bacteria which can be cultured in the study area varied between 4.3×106 ~ 4.5×107 CFU·g-1， and bacteria， which could be cultured， have clear seasonal variations under different types of alpine meadow ecosystems： bacterial quantity reached the highest level in June and lowest in April under original ecological alpine meadow ecosystem， but it has no clear seasonal variations in natural degeneration and plowing and supplementing alpine meadow； What’s more， the quantity of bacteria which could be cultured in natural degeneration and plowing and supplementing alpine meadow were evidently less than original ecological alpine meadow. Related analyses demonstrated that what resulted in the phenomena mentioned above lies in the difference of soil physical and chemical properties and vegetation characteristics. Correlation analysis showed that soil bacterial quantity was significantly positively correlated with soil total nitrogen， vegetation cover and soil moisture. So it can be seen that measures of plowing and natural supplementing in this area do not recover the quantity of microorganism， and even less than degeneration meadow soil. The results provide data base for using microorganism to overview different types of alpine meadow ecological environment in Qinghai-Tibet Plateau.
Soil organic carbon stocks （SOCS） in the Qinghai-Tibet Plateau have important implications for ecology succession， while the spatial distribution of SOC data are scarce， especially in seasonally frozen ground regions. Here， based on the multiple environmental variables and soil profile data （n=378）， we evaluated the soil organic carbon （SOC） pool and spatial distribution of seasonally frozen ground which down to 2 m depth on the Qinghai-Tibet Plateau using the geographically weighted regression （GWR） approach. Our results showed that SOCS exhibited a decreasing tread from south-eastern to the north-western， and the SOC pools were estimated to be 15.37 Pg in the top 2 m. The average SOC under different vegetation types showed a decreasing trend as follows： forest>shrub> alpine meadow> alpine grassland> alpine desert. Among all soil types， brown earths， chernozems， and peat soils had the largest SOC， while brown pedocals， brown desert soils， grey-brown desert soils， Aeolian soils， chisley soils， saline， frigid desert soils， cold desert soil， and cold desert soils had the smallest SOC. Our results provided the storage， pattern， and spatial distribution of SOC in the seasonally frozen ground area， providing a scientific basis for future studies pertain to the Earth System Models.
Using the observation temperature （OT） in Shiquanhe （west）， Shenzha （middle part） and Anduo （east） in the edge of the Chang Tang Nature Reserve from 1979 to 2018， together with the ERA-Interim （ET）， NCEP/NCAR （NT1）， NCEP/DOE（NT2） and JRA-55（JT） reanalyzed temperature data， an applicability study was carried out from two aspects of annual and monthly mean variations using the parameters， such as perennial temperature variation， mean square deviation and correlation. The results show that from the perspective of annual and monthly variations， the four kinds of reanalysis data have poor applicability in Shiquanhe （west）， and JRA-55 data have good applicability in Anduo （east） and Shenzha （middle）； the correlation between the reanalysis temperature data and the observed temperature in winter is better than that in summer. In terms of monthly mean temperature， one can see a minimum in winter （January or December） and a maximum in summer （July or August）. On the whole， the JRA-55 data has a good applicability in Chang Tang Nature Reserve. According to the studies on the annual temperature change and monthly temperature change， JRA-55 has the best correlation with the observed temperature， while ERA-Interim temperature trend is not accurate and NCEP/NCAR and NCEP/DOE are significantly lower.
In inland river basin in arid regions， water resource is the dominant factor for care of the environment and economic developing. Carrying capacity is a sound basis for judging the coordinated development of water resources and economic， social and ecological environment. The evaluation indicators are constructed from two aspects， i.e.， overloading status and carrying capacity. The water resources carrying capacity in the counties of Zhangye Prefecture in Heihe River basin is evaluated. The results show that： （1） In terms of overloading status， the overloading status of total water consumption in Zhangye Prefecture overall had been improved form 2015 to 2017， but the total water consumption of Minle County was overloaded， and the overloading status of groundwater use of all counties was significantly improved， and groundwater over-exploitation had been effectively controlled. Overall， from 2015 to 2017， the overloading status of water resources in Zhangye Prefecture had been improved overall， from the overloading of four counties （Ganzhou， Linze， Gaotai， Minle） in 2015 to critical overloading of the other four counties （Linze， Gaotai， Shandan， Sunan） in 2017， which reflected that the management of water resources utilization had been effective； （2） In terms of water resources carrying capacity， the number of population with overload of water resources of all counties in the prefecture were all less than in the 2017 level， and the economic scale that can be carried by water resources in 2020 and 2030 will be about 90% of 2017； （3） according to the regional development plan， two scenarios of future water resources carrying capacity improvement， i.e.， scenario of agricultural water-saving efficiency improvement and scenario of reduced arable irrigation area， respectively， were proposed. Based on the above analyses， countermeasure for improving the water resources carrying capacity in Zhangye Prefecture is suggested.
Based on the daily average temperature， the average maximum temperature and the average minimum temperature data from 1961 to 2017 and DMSP/OLS satellite nighttime light data， the influence of urbanization on the trend of average temperature and extreme temperature indices of Liaoning Province was quantitatively analyzed. The results show that the temperature in Liaoning Province exhibits a significant warming trend， and the rate of warming trend at urban stations is obviously faster than at rural stations. The growth rate of the average minimum temperature is the fastest， followed by the average temperature， and the average maximum temperature is relatively slow. The warming rates of four seasons are as follows： winter > spring > autumn > summer； the contributions of urbanization from large to small are in order： the average minimum temperature > the average temperature > the average maximum temperature. The significant increase of the minimum temperature and the slight growth of the maximum temperature at the city stations should lead to the decrease of the daily range of temperature， and enhance the contribution of urbanization. Urbanization aggravates the significant decrease of the extreme low temperature events and the significant increase of extreme high temperature events in Liaoning Province. Urbanization has a significant impact on extreme temperature events. The urban effect of the extreme temperature index related to the cold events is negative， while that related to the warm events is positive. In terms of the contribution rate of urbanization effect， the relative index is larger， followed by the duration index， and the absolute index except the daily temperature range is relatively small. The extreme temperature index based on the lowest temperature is more affected by urbanization than that based on the highest temperature. The spatial asymmetry of urban heat island intensity and the difference of aerosol concentration between urban station and rural station results in the increase of maximum temperature not as significant as that of minimum temperature， which may explain the urban effect on the extreme temperature index.
Due to the complex terrain and snow distribution of the Tibetan Plateau （TP）， many snow depth retrieval algorithms have not reached the ideal accuracy. Based on the new generation of AMSR2 passive microwave data， the TP was used as the study area in this study， the Brightness Temperature （BT） and the Brightness Temperature Difference （BTD） were input as parameters， and the elevation and latitude parameters were also introduced into the SD retrieval model. The simulated annealing algorithm （SA） was used to feature selection， and the SD retrieval model of the TP that based on the RF algorithm was constructed. The results showed that the RF algorithm had higher accuracy over the TP compared with the AMSR2 global SD products； the goodness of fit （R2） were increased from 0.41 to 0.60， the root mean square error （RMSE） were decreased from 7.36 cm to 4.88 cm， and the BIAS were decreased from 3.24 cm to -0.16 cm， respectively. The BTD of low frequency are more suitable for SD retrieval with RF algorithm over the TP. When the elevation approached or exceeded the average elevation of the TP by 4 000 m， the retrieval result was not well， but the RMSE was only 3.78 cm， and the BIAS was only -0.09 cm. The southern part of the plateau （25° ~ 30° N） had poor retrieval results due to its complex topography and relatively few meteorological stations， with the RMSE of 5.94 cm and the BIAS of -0.39 cm. The grassland， bare land and farmland with the low vegetation cover area had a maximum RMSE of 3.19 cm， and BIAS of -0.49 cm. The main land cover type on the TP was grassland， and RMSE and BIAS of the random forest algorithm on the grassland underlay were about 3 cm and 0 cm， respectively
The ground surface deformation seriously affects the stability of ecological environment and engineering facilities in permafrost regions. The differential interferometric synthetic aperture radar （D-InSAR） technology， as a new space observation method， provides a new way for monitoring the ground surface deformation over permafrost. In the past 20 years， the ground surface deformation monitored by D-InSAR technology has obtained good results in permafrost regions. In this paper， the basic principles of D-InSAR technology are introduced firstly. Then the development and application of D-InSAR technology in permafrost regions are described. At last， it is analyzed that the advantages and the existing problems of D-InSAR technology in monitoring the ground surface deformation over permafrost， and proposed the possible solutions. It is expected to provide a reference for the further study of D-InSAR technology to monitor the ground surface deformation in permafrost regions.
