High-resolution air temperature distribution data are of crucial significance for studying climate change and agriculture in the Yellow River Basin. Obtaining accurate and high-resolution air temperature data has been a persistent challenge in research. This study selected the Yellow River Basin as its research area and assessed multiple variables, including the land surface temperature (LST), Normalized Difference Vegetation Index (NDVI), Digital Elevation Model (DEM), slope, aspect, longitude, and latitude. We constructed three downscaling models, namely, ET, XGBoost, and LightGBM, and applied a stacking ensemble learning algorithm to integrate these three models. Through this approach, ERA5-Land reanalysis air temperature data were successfully downscaled from a spatial resolution of 0.1° to 1 km, and the downscaled results were validated using observed data from meteorological stations. The results indicate that the stacking ensemble model significantly outperforms the three independent machine learning models. The integrated model, combined with the selected set of multiple variables, provides a feasible approach for downsizing ERA5 air temperature data. The stacking ensemble model not only effectively enhances the spatial resolution of ERA5 reanalysis air temperature data but also improves downscaled results to a certain extent. The downscaled air temperature data exhibit richer spatial texture information, better revealing spatial variations in air temperature within the same land class. This research outcome provides robust technical support for obtaining high-resolution air temperature data in meteorologically sparse or topographically complex regions, contributing significantly to climate, ecosystem, and sustainable development research.

Frost jacking of electricity transmission tower (ETT) pile foundations in permafrost regions of the Qinghai–Tibet Plateau is becoming increasingly serious. This study investigated the effects of thermosyphons on the frost jacking characteristics of such pile foundations. Field tests of pile foundations with and without thermosyphons were carried out in the Wonkhu–Budongquan section of the Qinghai–Tibet Plateau. A three-dimensional finite-element model was established to analyze the effectiveness of thermosyphons, ground temperature distributions, and jacking displacements and force characteristics of ETT pile foundations. The results showed that the power of thermosyphons decreases yearly within 30 years. The thermosyphons can effectively increase the cold reserves of the permafrost and reduce the soil temperature; the soil temperature around the pile and the depth of thaw is the lowest in the fifth year, and this will increase the top of permafrost by 48 cm in the thirtieth year. Thermosyphons can effectively increase the adfreezing force of the pile side below the active layer and reduce the frost jacking displacement, thus alleviating frost jacking damage to ETT pile foundations.

This study focused on the coupling heat transfer mechanism and the cooling efficiency of L-shaped two-phase closed thermosyphons (L-shaped TPCTs) in the wide subgrade of permafrost regions. Considering the fact that time–space dynamics change the effects of the air temperature, wind speed, and geotemperature, a coupled air temperature–L-shaped TPCT–subgrade soil heat transfer model was established using the ANSYS 15.0 software platform, and the rationality of the model was verified through measured data. The heat-transfer characteristics of the L-shaped TPCTs and the long-term thermal stability of the subgrade were studied under different inclination angles of the evaporator (α = 15°, 30°, 50°, 70°, and 90°). Then, the cooling effectiveness of a composite subgrade with TPCTs and an XPS insulation board was numerically calculated. The results show that the heat flux of the L-shaped TPCT was the greatest when α = 50°, and the heat flux reached the maximum value of 165.7 W·m⁻² in January. The L-shaped TPCT had a relatively good cooling effect on the subgrade as a whole when α = 50° and 70°, but the thawing depth at the center of the subgrade with L-shaped TPCTs reached 9.0 m below the ground surface in the 30th year. The composite subgrade with L-shaped TPCTs/vertical TPCT/XPS insulation board is an effective method to protect the permafrost foundation and improve the long-term thermal stability of the wide subgrade. The maximum heat flux of evaporation section of the L-shaped TPCT is 18.8% higher than that of the vertical TPCT during the working period of the TPCTs of the composite subgrade.

The fractal characteristics of soil microstructures, including the size, distribution, and dynamic evolution process of soil particles, cracks, and intergranular pores, are important factors influencing the macroscopic physical properties of soils. Quantitative characterization, qualitative analysis, and the impact of fractal characteristics on macroscopic properties have been important research directions in recent years. This paper summarizes the research status on soil microstructure fractal characteristics, elaborating on the kinds of soil fractal characteristics, the calculation methods of fractal characteristic parameters, and the influence of fractal characteristics on macroscopic properties. Based on existing research results, this paper proposes that future research on soil microstructure fractal characteristics should focus on the following aspects: (1) advancing fractal characteristic research towards higher precision and multi-dimensionality to reveal the internal relations between soil fractal characteristics and macroscopic physical properties; (2) strengthening interdisciplinary collaboration to promote theoretical innovation in fractal analysis and build a more comprehensive system for studying the evolution of soil fractal characteristics; and (3) a close integration with engineering tests to promote the application and transformation of research results, providing valuable references for optimizing construction schemes and improving the service performance of engineering structures.

In permafrost regions, long distance buried pipelines are widely used to transport oil and natural gas resources. However, pipeline problems occur frequently due to the complicated surrounding environment and transportation requirement of positive temperature. In this study, a thermal insulation layer was applied to mitigate permafrost degeneration around the buried oil-gas pipelines. Based on engineering background of the Sebei-Xining-Lanzhou natural gas pipeline in China, an indoor model test was designed and carried out in which many key indices, such as the temperature regime, vertical displacement, pipeline wall stress, and water content, were closely monitored. The test results indicate that the large heat loss of the buried pipeline produces a rapid increase in ground temperatures which seriously reduces the bearing capacity of the permafrost foundation. The buried oil-gas pipelines with a thermal insulation layer can effectively reduce the thawing range and vertical displacement of the permafrost foundation around the buried pipelines, so as to control the stress of the pipeline wall in the normal range and protect the safe and stable operation of the buried oil-gas pipelines. The experimental results can serve as a reference for the construction, operation, and maintenance of buried oil-gas pipelines in permafrost regions.