•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.
The deuterium excess is a second-order parameter linking water-stable oxygen and hydrogen isotopes and has been widely used in hydrological studies. The deuterium excess in precipitation is greatly ...influenced by below-cloud evaporation through unsaturated air, especially in an arid climate. Based on an observation network of isotopes in precipitation of arid central Asia, the difference in deuterium excess from cloud base to ground was calculated for each sampling site. The difference on the southern slope of the Tian Shan is generally larger than that on the northern slope, and the difference during the summer months is greater than that during the winter months. Generally, an increase of 1% in evaporation of raindrops causes deuterium excess to decrease by approximately 1‰. Under conditions of low air temperature, high relative humidity, heavy precipitation, and large raindrop diameter, a good linear correlation is exhibited between evaporation proportion and difference in deuterium excess, and a linear regression slope of <1‰ %–1 can be seen; in contrast, under conditions of high air temperature, low relative humidity, light precipitation, and small raindrop diameter, the linear relationship is relatively weak, and the slope is much larger than 1‰ %–1. A sensitivity analysis under different climate scenarios indicates that, if air temperature has increased by 5°C, deuterium excess difference decreases by 0.3‰–4.0‰ for each site; if relative humidity increases by 10%, deuterium excess difference increases by 1.1‰–10.3‰.
Soil temperature is an important physical variable of soil and plays a key role in controlling the underground hydro-thermal processes in permafrost regions on the Qinghai-Tibetan Plateau (QTP). ...Daily soil temperatures were observed at five different vegetation cover sites (alpine wet meadow, alpine meadow, alpine steppe, alpine desert steppe and alpine desert) from 2012 to 2015 in permafrost regions on the QTP. The performance of three reanalysis soil temperature products (National Centers for Environmental Prediction Climate Forecast System and Climate Forecast System Reanalysis (CFSR), European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim), and Global Land Data Assimilation System (GLDAS- NOAH)) at four depths (0–10, 10–40, 40–100 and 100–200 cm) was evaluated using the observation data. The results revealed that the CFSR soil temperature products had the best performance at most sites and that GLDAS-NOAH and Era-Interim had the poorest performance. However, the original CFSR soil temperature products underestimated the lowest temperatures. The calibration models for CFSR soil temperature products were established using the observed daily soil temperature from 2013 to 2015 and were validated with observed data from 2012. The results showed that the calibrated CFSv2 products were closer to the observations at different depths in the study sites. Moreover, we investigated the variations of seasonal and annual mean soil temperature from 1980 to 2015 at depths of 0–10, 10–40, 40–100 and 100–200 cm using the soil temperature calibration results. It was found that the soil temperatures at different depths all warmed fastest in spring, more slowly in winter and slowest in autumn at most sites. In addition, the average annual soil temperature exhibited significant warming trends in the permafrost regions on the QTP. The effect was largest with alpine desert steppe and smallest with alpine wet meadow, with statistically significant rates of 0.0599, 0.0468, 0.0438, 0.0282 and 0.0145 °C/year in alpine desert steppe, alpine desert, alpine steppe, alpine meadow and alpine wet meadow, respectively. This research provides a foundation for understanding the thermal properties of permafrost on the Qinghai-Tibetan Plateau under climate change.
•Evaluate the performance of the reanalysis soil temperature products at different depths.•Establish calibration models for the reanalysis soil temperature products.•Examine the warming trend in the seasonal and annual mean soil temperatures.
•Reveal the dynamics of soil temperature and soil moisture during the freezing and thawing process.•Demonstrate the characteristics thawing of freezing and thawing processes of active layer.•Analysis ...the characteristics of coupling of moisture and heat of active layer.
Hydro-thermal characteristics of the active layer are critical during freezing-thawing cycles, causing the moisture and heat exchanges between both permafrost and atmosphere in permafrost regions. There is better understanding of these characteristics in high-latitude permafrost areas, while comparatively little is known in the middle-low latitude areas. Here, we used field monitoring data along with statistical models to quantitatively analyze the hydro-thermal dynamics of the freezing-thawing processes at the Tanggula (TGL) site in permafrost regions of Qinghai-Tibetan Plateau (QTP). This combined approach was used to examine the hydro-thermal characteristics in high-altitude permafrost regions. Our results revealed that the duration of the freezing process was much shorter than that of the thawing process. During freezing-thawing processes, the amplitude variation in soil temperature had a significant logarithmic relationship with depth. There was a significant exponential relationship between soil water content at a depth of 5 cm and monthly precipitation. The averaged energy in the active layer consumed for phase change from water to ice was 145.53 MJ/m2. Finally, we analyzed the quantitative hydro-thermal characteristics and influential factors during the freezing and thawing processes; the different hydro-thermal processes occurring in high-altitude permafrost regions were compared with those in high latitude permafrost regions. Collectively, these results offer a perspective on the difference in permafrost across different region and also provide a reference for the parameterization of land surface models.
An arc array synthetic aperture radar (AA-SAR) is a new type of omnidirectional observation and imaging system. Based on linear array 3D imaging, this paper introduces a keystone algorithm combined ...with the arc array SAR 2D imaging method and proposes a modified 3D imaging algorithm based on keystone transformation. The first step is to discuss the target azimuth angle, retain the far-field approximation method of the first-order term, analyze the influence of the forward motion of the platform on the along-track position, and realize the two-dimensional focusing of the target slant range-azimuth direction. The second step is to redefine a new azimuth angle variable in the slant-range along-track imaging and use the keystone-based processing algorithm in the range frequency domain to eliminate the coupling term generated by the array angle and the slant-range time. The corrected data are used to perform along-track pulse compression to obtain the focused image of the target and realize the three-dimensional imaging of the target. Finally, in this article, the spatial resolution of the AA-SAR system in the forward-looking state is analyzed in detail, and the change in the spatial resolution of the system and the effectiveness of the algorithm are verified through simulation.
•The Qinghai-Tibet Plateau has assimilated 43.16 Tg C/a from 1981 to 2016.•The carbon sink on the Qinghai-Tibet Plateau has weakened during 1981-2016.•The carbon sink on the Qinghai-Tibet Plateau is ...projected to weaken by 2080–2100.
Cold regions contain a large amount of soil organic carbon, and the warming-accelerated loss of this carbon pool could cause important feedback to climatic change. The changes of carbon budgets in cold regions are poorly quantified especially for the Qinghai–Tibet Plateau (QTP) due to limited field observation data. By considering the soil freeze–thaw process and establishing new plant functional types with localized parameters, we used the Integrated Biosphere Simulator (IBIS) model to simulate the changes of carbon budget on the QTP during 1980–2016. The model was calibrated and validated using carbon flux data from eddy covariance observations at 16 sites. The results showed that the QTP has assimilated 43.16 Tg C/yr during 1980–2016, with permafrost and non-permafrost regions accounting for approximately 15% and 85% of the carbon sink, respectively. During the past four decades, the gross primary production and ecosystem respiration have increased by 1.74 and 2.04 Tg C/yr2, resulting in that the carbon sink on the QTP has weakened during 1980–2016. Moreover, the weakening of carbon sink is more pronounced in the non-permafrost regions. We project that the ecosystems will release 12.30 and 24.40 Tg C by 2080–2100 under the moderate and high shared socio-economic pathways (SSP 370 and SSP 585), respectively. This could largely offset the carbon sink and even shift the carbon sink to carbon source on the QTP.
Approximately one-third of the Earth's arid areas are distributed across central Asia. The stable isotope composition of precipitation in this region is affected by its aridity, therefore subject to ...high evaporation and low precipitation amount. To investigate the factors controlling stable water isotopes in precipitation in arid central Asia, an observation network was established around the Tianshan Mountains in 2012. Based on the 1052 event-based precipitation samples collected at 23 stations during 2012-2013, the spatial distribution and seasonal variation of δD and δ
18
O in precipitation were investigated. The values of δD and δ
18
O are relatively more enriched in the rainfall dominant summer months (from April to October) and depleted in the drier winter months (from November to March) with low D-excess due to subcloud evaporation observed at many of the driest low elevation stations. The local meteoric water line (LMWL) was calculated to be δD=7.36δ
18
O - 0.50 (r
2
=0.97, p<0.01) based on the event-based samples, and δD=7.60δ
18
O+2.66 (r
2
=0.98, p<0.01) based on the monthly precipitation-weighted values. In winter, the data indicate an isotopic rain shadow effect whereby rainout leads to depletion of precipitation in the most arid region to the south of the Tianshan Mountains. The values of δ
18
O significantly correlate with air temperature for each station, and the best-fit equation is established as δ
18
O=0.78T - 16.01 (r
2
=0.73, p<0.01). Using daily air temperature and precipitation derived from a 0.5° (latitude)×0.5° (longitude) gridded data set, an isoscape of δ
18
O in precipitation was produced based on this observed temperature effect.
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•Land cover types and soil properties were associated with metal elements.•Metal elements originated from geogenic processes or anthropogenic sources.•37.14% of soils showed moderate ...pollution of Hg.•84.44% of soils had moderate to high enrichment levels for Cd.
Permafrost-affected soils can serve as a major reservoir of metal elements (MEs) from anthropogenic sources by atmospheric transport. Understandings of the contents, sources, and ecological risks of MEs in high-altitudinal permafrost regions are helpful to mitigate environmental and human health hazards under climate change. Thus, we investigated the concentrations of 21 MEs of topsoil (0–50 cm) and evaluated the environmental quality using the ecological risk assessment methods in permafrost regions on the central Qinghai-Tibet Plateau (QTP). The results showed that (1) Ca, Al, Fe, K, Mg, Ti, and Mn (max values in mg/kg d.w.: 7.61 × 104, 5.93 × 104, 3.12 × 104, 2.33 × 104, 1.49 × 104, 0.52 × 104, and 0.06 × 104, respectively) were abundant in all sampling sites. (2) The concentrations of most MEs in the alpine wet meadow were the highest, followed by the alpine meadow and alpine desert steppe. (3) Land cover types and soil properties (soil organic carbon, pH, and soil texture) were associated with MEs. (4) Ca, Al, Fe, K, Mg, Mn, Rb, Sr, Th, Zn, V, Ni, As, Pb, Cu, and Co likely originated from geogenic/pedogenic processes, and Ti, Cr, Cd, and Hg were enriched by both natural and anthropogenic sources. (5) The modified contamination degree indicated that sampling sites in permafrost regions of the QTP were in a low pollution state, while the geoaccumulation index and enrichment factor have revealed that 37.14 % of soils showed moderate pollution of Hg, and 84.44 % of soils had moderate to high enrichment levels for Cd. This study reveals accumulation patterns of MEs in permafrost regions and provides a scientific basis for the research on the ecological security of MEs in permafrost regions influenced by climate change.
Nowadays, it is still a challenge for commercial nitrate sensors to meet the requirement of high accuracy in a complex water. Based on deep-ultraviolet spectral analysis and a regression algorithm, a ...different measuring method for obtaining the concentration of nitrate in seawater is proposed in this paper. The system consists of a deuterium lamp, an optical fiber splitter module, a reflection probe, temperature and salinity sensors, and a deep-ultraviolet spectrometer. The regression model based on weighted average kernel partial least squares (WA-KPLS) algorithm together with corrections for temperature and salinity (TSC) is established. After that, the seawater samples from Western Pacific and Aoshan Bay in Qingdao, China with the addition of various nitrate concentrations are studied to verify the reliability and accuracy of the method. The results show that the TSC-WA-KPLS algorithm shows the best results when compared against the multiple linear regression (MLR) and ISUS (in situ ultraviolet spectrophotometer) algorithms in the temperatures range of 4-25 °C, with RMSEP of 0.67 µmol/L for Aoshan Bay seawater and 1.08 µmol/L for Western Pacific seawater. The method proposed in this paper is suitable for measuring the nitrate concentration in seawater with higher accuracy, which could find application in the development of in-situ and real-time nitrate sensors.
Surface air temperature is an important factor for the permafrost thermal state in the Northern Hemisphere. It is therefore necessary to understand the variations and regional differences in air ...temperature to determine the interactions between permafrost degradation and climate change. In this study, we used observational data from the National Centers for Environmental Information, the China Meteorological Administration, and the World Data Centre for Meteorology to quantitatively analyze the variations and regional differences in air temperature from 1980 to 2018. The results demonstrated that the annual mean air temperatures were low in continuous permafrost regions and high in sporadic and isolated permafrost regions, with a significant warming rate of 0.371 ± 0.086 °C/decade. Air temperatures warmed the slowest during the winter and fastest during the spring, and no “warming hiatus” was observed in the permafrost regions of the Northern Hemisphere. The spatial patterns of freezing degree-days (FDDs) and thawing degree-days (TDDs) had different spatial characteristics. The decreasing rate of FDDs was −6.97 °C·days/year, while the increasing rate of TDDs was 6.4 °C·days/year. The air temperatures and warming trends had largely regional differences with respect to high latitude, transitional, and high altitude permafrost regions. Air temperature and its warming trend was the highest in high altitude regions. In addition, air temperature warming trends gradually decreased from the continuous permafrost zone to the island permafrost zone. The FDDs had a significant decreasing trend from the continuous permafrost zone to the island permafrost zone, whereas TDDs exhibited the opposite trend. The results indicate that the air temperature warming rate in the permafrost regions was approximately 2.0 times that of the global warming rate, and 1.3 times the global land warming rate from 1980 to 2018. These findings offer a perspective on the differences in permafrost and its thermal state across different regions under climate change.
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•Air temperatures showed significant warming trend in different permafrost regions.•FDD and TDD have different variation trends in different permafrost regions.•Examined difference in air temperature in different permafrost regions and types•Discussed the possible reasons for these differences
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