Frozen soil was simulated at six seasonally frozen and seven permafrost stations over the northern Tibetan Plateau using the Variable Infiltration Capacity (VIC) model for the period of 1962–2009. The VIC model resolved the seasonal cycle and temporal evolution of the observed soil temperatures and liquid soil moisture well. The simulated long‐term changes during 1962–2009 indicated mostly positive trends for both soil temperature and soil moisture, and negative trends for soil ice content at annual and monthly time scales, although differences existed among the stations, soil layers, and seasons. Increases in soil temperature were due mainly to increases in daily air temperature maxima and internal soil heat conduction, while decreases in soil ice content were related to the warming of frozen soil. For liquid soil moisture, increases in the cold months can be attributed to increases in soil temperature and enhanced soil ice melt while changes in the warm months were the results of competition between positive precipitation and negative soil temperature effects. Precipitation and liquid soil moisture were strongly correlated with evapotranspiration and runoff but had various degrees of correlations with base flow during May–September. Seasonally frozen stations displayed longer and more active hydrological processes than permafrost stations. Slight enhancement of the surface hydrological processes at the study stations was indicated, due to the combined effects of precipitation changes, which were dominant, and frozen soil degradation.

Gridded daily precipitation, temperature minima and maxima, and wind speed are generated for the northern Tibetan Plateau (NTP) for 1957–2009 using observations from 81 surface stations. Evaluation reveals reasonable quality and suitability of the gridded data for climate and hydrology analysis. The Mann–Kendall trends of various climate elements of the gridded data show that NTP has in general experienced annually increasing temperature and decreasing wind speed but spatially varied precipitation changes. The northwest (northeast) NTP became dryer (wetter), while there were insignificant changes in precipitation in the south. Snowfall has decreased along high mountain ranges during the wet and warm season. Averaged over the entire NTP, snowfall, temperature minima and maxima, and wind speed experienced statistically significant linear trends at rates of −0.52 mm yr−1 (water equivalent), +0.04°C yr−1, +0.03°C yr−1, and −0.01 m s−1 yr−1, respectively. Correlation between precipitation/wind speed and climate indices characterizing large-scale weather systems for four subregions in NTP reveals that changes in precipitation and wind speed in winter can be attributed to changes in the North Atlantic Oscillation (NAO), the Arctic Oscillation (AO), the East Asian westerly jet (WJ), and the El Niño–Southern Oscillation (ENSO) (wind speed only). In summer, the changes in precipitation and wind are only weakly related to these indices. It is speculated that in addition to the NAO, AO, ENSO, WJ, and the East and South Asian summer monsoons, local weather systems also play important roles.

The Tibetan Plateau and the surrounding (TPS) with its vast land mass and high elevation affects regional climate and weather. The TPS is also the headwater of 9 major Asian rivers that provide fresh water for 1.65 billion people and many ecosystems, with wet season (May–September) precipitation being the critical component of the fresh water. Using station observations, ERA-Interim and MERRA2 reanalysis, we find that wet season precipitation displays vertical gradients (i.e., changes with elevation) that vary within the region on the TPS. The decrease of precipitation with elevation occurs in the interior TPS with elevation larger than 4000 m, little or no change over the southeastern TPS, and increase elsewhere. The increase of precipitation with elevation is caused by increasing convective available potential energy (CAPE) and decreasing lifting condensation level (LCL) with elevation overwhelming the effects of decreasing total column water vapor (TCWV) with elevation. The decreasing precipitation with elevation is due to the combined effects of increasing LCL and decreasing TCWV. LCL and CAPE play a more important role than TCWV in determining the spatial patterns. These findings are important for hydrology study in observation scarce mountainous areas, water resources and ecosystem managements in the region.

In this study, the spatiotemporal distributions of monthly and annual temperature minima (T_min) and maxima (T_max), extreme T_min and T_max, the highest (lowest) T_min (T_max), frost day (FD), icing day (ID), summer day (SD) and tropical night (TR) at 112 stations and over the ten large river basins on the Tibetan Plateau and its surroundings (TPS) during 1963–2015 are examined. Mann-Kendall test is applied for the trends. The analyses show: the northwest experiences the hottest summer while the central TPS has the coldest winter and most frequent frost and icing days. The northwest (southeast) features the highest (lowest) monthly extreme temperature ranges. The northwest has the most frequent hot summer days, whereas the southeast has the least frequent frost and icing days. The entire TPS displays few tropical nights. Most stations show positive trends for all monthly and annual T_min and T_max variables. February displays the most positive trends for both monthly T_min and T_max variables while April shows the highest number of stations with decreasing trends in monthly T_max. The trends of FD and ID are negative, whereas the trends of SU and TR are positive. Over river basins, the trends of monthly T_min are all positive and statistically significant and the trends of monthly T_max are all positive except for one negative trend and around 1/3 of the positive trends are statistically significant. Relatively larger increases in monthly T_min and T_max are noted for the cold season than the warm. The monthly and annual T_min variables increase more than T_max variables.

Investigating the changes in the extreme temperature indices is crucial for the assessment of climate change and to recommend practical adaptation strategies particularly in countries like Nepal, which is highly prone to the impacts of extreme climatic events. This study analyses the mean temperatures and the changes of extreme temperature indices at 25 stations located in different physiographic regions in Nepal at monthly and annual time scales for 1986–2015. Ten extreme temperature indices recommended by ETCCDI are selected and examined. Quality control is first conducted for original daily maximum and minimum temperature data. In Nepal, it is warming in general with maximum temperature (0.04°C year⁻¹) increasing faster than minimum temperature (0.02°C year⁻¹) on countrywide average which is contrary to the changes in the Tibetan Plateau and its surroundings and the global average. The changes in extreme temperatures are spatiotemporally heterogeneous and unique. Annually, cold extremes increase but the percentage of days below 10th percentile becomes less frequent. Terai and Siwalik plain regions show negative annual trends in cold indices (TNn and TXn), but mixed changes in other extreme indices. There are strong correlations between the trends of maximum temperature‐related variables and elevation. This analysis also adds the evidence of pronounced warming in higher elevation only for day‐time extreme temperatures but not for temperature minima in Nepal which differs from the other studies around the world. This study also reveals that December and January tend to have colder days in the Terai. However, summer months are getting hotter in each region across the country. Kathmandu, the most urbanized city in Nepal shows a significant increase in minimum temperature (T_min) and minimum T_min (TNn) indicating the influence of urbanization in temperature change ‐ an urban heat island effect.