Abstract: The thermal-moisture dynamics of permafrost and cold region engineering caused by the trend of warming and humidification of the Qinghai-Tibet Plateau (QTP) are the focus of research on permafrost ecological and geological evolution in the third pole. Currently, the model of permafrost ground surface energy budget under the influence of rainfall does not consider the influence of rainwater temperature and neglects the effect of rainfall energy pulse. On the basis of the previous frozen soil hydro-thermal coupling theory, a permafrost hydrothermal coupling model considering the rainfall energy was constructed by introducing the surface energy equilibrium theory considering the rainwater sensible heat. Based on the on-site monitoring data of the Beiluhe in the QTP, the validity of the model was verified, and the influence mechanism of the summertime rainfall on the ground surface energy equilibrium and the thermal-moisture dynamics of the active layer was analyzed. The results show that the average deviation errors of soil volumetric water content, temperature, and heat flux simulated by the modified model considering rainwater sensible heat are within ±1.198%，±0.704℃, and ±1.66 W/m2, respectively, and the consistency indexes are greater than 0.877, 0.929, and 0.937, respectively. The optimized model improves the assessment of the surface heat absorption or heat release state and can better predict the thermal-moisture dynamics of the active layer after rainfall. Summertime rainfall increases the surface evaporation latent heat and rainwater sensible heat but reduces the surface net radiation, sensible heat, and soil surface heat flux to cool the surface. The cooling efficiency is positively correlated with rainfall intensity. In addition, it is affected by the time of occurrence of the rainfall. The daytime rainfall has a significant cooling effect, and the rainwater sensible heat promotes the cooling of the surface. The rainwater at night temporarily heats the surface, but the significant effect of evaporation latent heat makes the ground surface continuously cool. Due to the reduced surface temperature gradients and the infiltration of rainwater, the decrease in the thermal vapor flux densities, the increase in the liquid water flux densities promote the downward transportation and accumulation of liquid water during heavy rainfall events and continuous rainfall events. However, compared with the increased convective heat exchange of liquid water, the reductions of heat conduction, latent heat of water vapor diffusion, and water vapor convective heat are more significant, which reduces the total soil heat flux, decreases soil temperature, and mitigates the warming rate for the active layer.

Key words: permafrost, rainwater sensible heat, coupling theory, ground surface energy equilibrium, thermal-moisture dynamics, summertime rainfall