Abstract: In the past 50 years, the rainfall in the Qinghai-Tibet Plateau has shown a fluctuating increase trend, and summer rainfall is the main source of annual rainfall. The changes in rainfall will inevitably lead to change of active layer thermal-moisture dynamics. However, there are few literature reports on the effects of summer rainfall changes on the hydrothermal state of permafrost. Therefore, based on the water-vapor-heat coupling model that optimizes the surface energy balance boundary conditions, the measured meteorological data in the Beiluhe area of the Qinghai-Tibet Plateau in 2013 was used as the model-driven data to study the response mechanism of the active layer after the summer rainfall increased to 2 times. The results show that after the summer rainfall increases to 2 times, the net radiation and latent heat of evaporation increase, while the sensible heat and surface heat flux decrease. The energy exchange between the surface and the atmosphere is converted from sensible heat to latent heat of evaporation, and the overall heat used to heat the active layer is reduced by 1.4% in summer. The increase in summer rainfall leads to an increase in the transport flux of liquid water, while the transport flux of water vapor decreases. As a result, the decrease of the heat transport by conduction, the convection by water vapor, and the latent heat of water vapor diffusion is about 2.6 times of the increase of the convection by liquid water. Therefore, increased summer rainfall leads to the cooling of the soil, and the thickness of the active layer raised by 0.14 m as a whole, which has a cooling effect on the frozen soil.

Key words: permafrost, active layer, thermal-moisture dynamics, summer rainfall