The global climate is becoming warmer and wetter, and the physical properties of saline soil are easily affected by the external climate changes, which can lead to complex water-heat-salt-mechanics (WHSM) coupling effect within the soil. However, in the context of climate change, the current research on the surface energy balance process and laws of water and salt migration in saline soil are not well understood. Moreover, testing systems for studying the impact of external meteorological factors on the properties of saline soil are lacking. Therefore, this study developed a testing system that can simulate the environmental coupling effect of the WHSM in saline soil against a background of climate change. Based on meteorological data from the Hexi District in the seasonal permafrost region of China, the testing system was used to clarify the characteristics of surface energy and WHSM coupling changes in sulfate saline soil in Hexi District during the transition of the four seasons throughout the year. In addition, the reliability of the testing system was also verified using testing data. The results showed that the surface albedo of sulfate saline soil in the Hexi region was the highest in winter, with the highest exceeding 0.4. Owing to changes in the external environment, the heat flux in the sulfate saline soil in spring, summer, and early autumn was positive, while the heat flux in late autumn and winter was mainly negative. During the transition of the four seasons throughout the year in the Hexi region, the trends of soil temperature, volumetric water content, and conductivity were similar, first increasing and then decreasing. As the soil depth increased, the influence of external environmental factors on soil temperature, volumetric water content, and conductivity gradually weakened, and the hysteresis effect became more pronounced. Moreover, owing to the influence of external environmental temperature, salt expansion in the positive temperature stage accounts for approximately five times the salt-frost heave deformation in the negative temperature stage, indicating that the deformation of sulfate saline soil in the Hexi region is mainly caused by salt expansion. Therefore, to reduce the impact of external climate change on engineering buildings and agriculture in salted seasonal permafrost regions, appropriate measures and management methods should be adopted to minimize salt expansion and soil salinization.

Exploring the complex relationship between the freeze-thaw cycle and the surface energy budget (SEB) is crucial for deepening our comprehension of climate change. Drawing upon extensive field monitoring data of the Qinghai-Tibet Plateau, this study examines how surface energy accumulation influences the thawing depth. Combined with Community Land Model 5.0 (CLM5.0), a sensitivity test was designed to explore the interplay between the freeze-thaw cycle and the SEB. It is found that the freeze-thaw cycle process significantly alters the distribution of surface energy fluxes, intensifying energy exchange between the surface and atmosphere during phase transitions. In particular, an increase of 65.6% is observed in the ground heat flux during the freezing phase, which subsequently influences the sensible and latent heat fluxes. However, it should be noted that CLM5.0 has limitations in capturing the minor changes in soil moisture content and thermal conductivity during localized freezing events, resulting in an imprecise representation of the complex freeze-thaw dynamics in cold regions. Nevertheless, these results offer valuable insights and suggestions for improving the parameterization schemes of land surface models, enhancing the accuracy and applicability of remote sensing applications and climate research.

Driven by human activities and global climate change, the climate on the Qinghai-Xizang Plateau is experiencing a warming and humidifying trend. It significantly impacts the thermal-moisture dynamics in the active layer of the permafrost, which in turn affects the ecological environment of cold regions and the stability of cold region engineering. While the effect of air temperature on permafrost thaw has been well quantified, the processes and mechanisms behind the thermal-moisture response of the permafrost under the combined influence of increased rainfall and rising air temperature remain contentious and largely unknown. A coupled model was applied to quantify the impacts of increased rainfall, rising air temperature, and their compound effects on the thermal-moisture dynamics in the active layer, considering the sensible heat of rainwater in the ground surface energy balance and water balance process. The results indicate that the compound effect of warming and humidifying resulted in a significant increase in surface net radiation and evaporation latent heat, a more significant decrease in surface sensible heat, and a smaller impact of rainfall sensible heat, leading to an increase in surface soil heat flux. The compound effect of warming and humidifying leads to a significant increase in the liquid water flux with temperature gradient. The increase in liquid water flux due to the temperature gradient is larger than that of warming alone but smaller than the effect of humidifying alone. Warming and humidifying result in a smaller increase in soil moisture content during the warm season compared to rainfall increases alone. The thermal conductivity heat flux in the active layer increases significantly during the cold season but less than the effect of warming alone. The convective heat flux of liquid water flux increases noticeably during the warm season but less than the effect of rainfall increases alone. Increased rainfall significantly cools the soil during the warm season, while both warming and humidifying lead to a more pronounced warming effect on the soil during the cold season than during the warm season. An increase in the average annual temperature by 1.0 degrees C leads to a downward shift of the permafrost table by 10 cm, while an increase in rainfall by 100 mm causes an upward shift of the permafrost table by 8 cm. The combined effect of warming and humidifying results in a downward shift of the permafrost table by 6 cm. Under the influence of climate warming and humidifying, the cooling effect of increased rainfall on permafrost is relatively small, and the warming effect of increased temperature still dominates.

Accurate initial soil conditions play a crucial role in simulating soil hydrothermal and surface energy fluxes in land surface process modeling. This study emphasized the influence of the initial soil temperature (ST) and soil moisture (SM) conditions on a land surface energy and water simulation in the permafrost region in the Tibetan Plateau (TP) using the Community Land Model version 5.0 (CLM5.0). The results indicate that the default initial schemes for ST and SM in CLM5.0 were simplistic, and inaccurately represented the soil characteristics of permafrost in the TP which led to underestimating ST during the freezing period while overestimating ST and underestimating SLW during the thawing period at the XDT site. Applying the long-term spin-up method to obtain initial soil conditions has only led to limited improvement in simulating soil hydrothermal and surface energy fluxes. The modified initial soil schemes proposed in this study comprehensively incorporate the characteristics of permafrost, which coexists with soil liquid water (SLW), and soil ice (SI) when the ST is below freezing temperature, effectively enhancing the accuracy of the simulated soil hydrothermal and surface energy fluxes. Consequently, the modified initial soil schemes greatly improved upon the results achieved through the long-term spin-up method. Three modified initial soil schemes experiments resulted in a 64%, 88%, and 77% reduction in the average mean bias error (MBE) of ST, and a 13%, 21%, and 19% reduction in the average root-mean-square error (RMSE) of SLW compared to the default simulation results. Also, the average MBE of net radiation was reduced by 7%, 22%, and 21%.

The presence of taliks (perennially unfrozen zones in permafrost areas) adversely affects the thermal stability of infrastructure in cold regions, including roads. The role of heat advection on talik development and feedback on permafrost degradation has not been quantified methodically in this context. We incorporate a surface energy balance model into a coupled groundwater flow and energy transport numerical model (SUTRA-ice). The model, calibrated with long-term observations (1997-2018 on the Alaska Highway), is used to investigate and quantify the role of heat advection on talik initiation and development under a road embankment. Over the 25-year simulation period, the new model is driven by reconstructed meteorological data and has a good agreement with near surface soil temperatures. The model successfully reproduces the increasing depth to the permafrost table (mean absolute error <0.2 m), and talik development. The results demonstrate that heat advection provides an additional energy source that expedites the rate of permafrost thaw and roughly doubles the rate of permafrost table deepening, compared to purely conductive thawing. Talik initially formed and grew over time under the combined effect of water flow, snow insulation, road construction and climate warming. Talik formation creates a new thermal state under the road embankment, resulting in acceleration of underlying permafrost degradation, due to the positive feedback of heat accumulation created by trapped unfrozen water. In a changing climate, mobile water flow will play a more important role in permafrost thaw and talik development under road embankments, and is likely to significantly increase maintenance costs and reduce the long-term stability of the infrastructure.

The surface energy budget over the Qinghai-Tibet Plateau (QTP) and the Arctic significantly influences the climate system with global consequences. The performances of 30 selected Coupled Model Intercomparison Project Phase 6 (CMIP6) models were evaluated based on six sites in the QTP and Arctic. The simulation results for latent heat flux (LE) were more accurate in the QTP, where the correlation coefficient and root mean square error (RMSE) were 0.9 and 30 W m(-2), respectively. The results for sensible heat flux (H) were more accurate in the Arctic, the correlation coefficient and RMSE were 0.8 and 24 W m(-2), respectively. Furthermore, the multiple models mean results revealed that the surface energy flux had seasonal variation and regional differences over the QTP and the Arctic. In the QTP, H was the lowest in winter, increased in spring, and reached the maximum in summer. However, the transitional changes in spring and autumn were not apparent in the Arctic, mainly due to seasonal net radiation difference between the two places. LE was affected by precipitation and surface soil moisture content. This work is important for understanding land-atmosphere interactions and useful for improving the accuracy of land surface models simulations.

The surface energy balance is a key issue in land surface process research and important for studies of climate and hydrology. In this paper, the surface energy fluxes (net radiation, ground heat flux, sensible heat flux and latent heat flux) at the Tanggula (TGL) and Xidatan (XDT) sites were measured and the distributions of the regional surface energy fluxes on the Tibetan Plateau were obtained using a revised surface energy balance system (SEBS) model. The results show that the surface energy fluxes have obvious seasonal variations. At both sites, the sensible heat flux is highest in spring and lowest in summer, and the latent heat flux is highest in summer and lowest in winter. The high elevation, snow cover, freeze-thaw process, precipitation, vegetation and soil texture are important influencing factors for land surface energy fluxes. The time-phase difference between the net radiation and ground heat flux for bare soils is estimated to be 2-3 hr. The ratio of ground heat flux and net radiation ranged from approximately 0.18 to 0.33, and a parameterization scheme for the remote sensing of ground heat flux over the Tibetan Plateau bare soil in summer is developed. The simulation results of the regional surface energy fluxes show that the distributions of surface parameters, such as vegetation, soil texture and soil moisture content, are important for understanding regional changes in the surface energy fluxes.

Surface energy budget and soil hydrothermal regime are crucial for understanding the interactions between the atmosphere and land surface. However, large uncertainties in current land surface process models exist, espe-cially for the permafrost regions in the Qinghai-Tibet Plateau. In this study, observed soil temperature, moisture, and surface energy fluxes at four sites in permafrost regions are chosen to evaluate the performance of CLM5.0. Furthermore, the soil property data, different thermal roughness length schemes, and dry surface layer (DSL) scheme are investigated. The results show that the soil property data is important for CLM5.0. The default scheme in CLM5.0 yields large errors for surface energy fluxes. The combination of the thermal roughness length and DSL scheme significantly improved the simulation of surface energy fluxes, especially for latent heat flux. The optimization of DSL scheme significantly improved soil temperature simulation and decreased the RMSE from 1.95 degrees C, 2.07 degrees C, 2.02 degrees C, and 2.95 degrees C to 1.34 degrees C, 1.35 degrees C, 1.35 degrees C and 2.29 degrees C in TGL site, respectively. The combination of the thermal roughness length and DSL scheme performed the best in shallow soil moisture, decreasing the RMSE from 0.136 m3 m- 3 to 0.049 m3 m- 3 in the XDT site but slightly enhancing the errors in middle soil. The interactions between surface energy and soil hydrothermal regime also discussed. However, the thermal roughness length and the DSL schemes are highly dependent on the condition of the underlying surface. Different schemes should be selected for different regions.

Glaciers, as massive freshwater reservoirs, support the planet's living systems and have an impact on our daily lives, even for communities living far away. Ongoing and future climate change is predicted to have strong impacts on the mass balance of alpine glacier around the world. To understand the relationship between climate and glacier dynamics, a range of mass balance models are currently used. Most of these models however, ignore subsurface heat fluxes as a component of glacier mass balance. Here, we set out to investigate the importance of subsurface heat flux for the mass balance of an alpine glacier using a surface energy mass balance model (SEM) coupled with a multilayer subsurface heat conduction model (MSHCM) that resolves the subsurface glacier temperature. As a case study, we investigate the Urumqi Glacier No.1 in the Tianshan Mountains (NW China), which has a long and continuous time series of surface and subsurface glacier temperature measurements. We evaluate the results of both glacier temperature models (SEM and MSHCM) using these in situ observations and investigate the sensitivity of mass balance to five meteorological factors: air temperature, precipitation, incoming shortwave radiation, relative humidity, and wind speed. The mass balance of the glacier was simulated first by including the influence of subsurface heat flux, and second, the subsurface heat flux was neglected. Observed and simulated mass balance and the englacial temperature were found to be reasonably close in both cases. Furthermore, the mass balance was simulated with a zero surface temperature assumption, which resulted in a 6% overestimation of the summer ablation. We concluded that the mass balance of Urumqi Glacier No.1 was most sensitive to variations in temperature, followed by precipitation. Furthermore, our results show that subsurface heat flux in the ablation area can generally be neglected in estimating the mass balance of alpine glaciers during ablation season.

Rainfall can potentially change upper thermal-moisture boundary conditions and influence the hydrological and thermal state of the active layer in permafrost regions. Studying the relationship between rainfall and ground temperature represents an emerging issue in permafrost engineering and environment but the interactive mechanisms of rainfall and the active layer are not well understood. This study aims to analyze the effects and mechanisms of summertime rainfall on the thermal-moisture dynamics of the active layer by field observations and simulation. The observation data demonstrated that frequent light rainfall events had a minor impact on the active layer, whereas consecutive rainfall events and heavy rainfall events had significant effects on soil temperature and water content. Moreover, the soil temperatures were more susceptible to summertime rainfall events. These rapidly cooled the shallow ground and delayed the temperature rise. Summertime rainfall significantly increased the surface latent heat flux, but decreased the net radiation, sensible heat flux, and soil surface heat flux. Rainfall also enhanced the amount of downward liquid water and water vapor, but the impact of rainfall on the increase in the convective heat transfer of the liquid water was lower than the decreases in the heat conduction flux, latent heat flux by vapor diffusion, and heat flux by convection of vapor. Thus, the reduction in the total soil heat flux caused by rainfall directly leads to a cooling effect on the soil temperature and delays the increase in soil temperature. The cooling effect of rainfall events may mitigate the warming rate and maintain the active layer at a relatively low temperature. The results provide new insights into understanding the inner mechanisms of the effect of rainfall on the active layer and on the possible long-term change trends of permafrost under increasing precipitation in the central Qinghai-Tibet Plateau. (c) 2021 Elsevier B.V. All rights reserved.