Effects of seasonal rainfall variations on the hydrothermal state and thermal stability of the permafrost active layer in the central Qinghai-Tibet Plateau of China

Abstract

Rainfall significantly impacts the hydrothermal state and thermal stability of the permafrost active layer by modifying surface energy boundary conditions. The quantitative characterization of hydrothermal changes in the Qinghai-Tibet Plateau has long been a crucial issue, yet the mechanisms underlying the effects of seasonal rainfall on the hydrothermal state within the permafrost active layer remain poorly understood. Therefore, this study investigated the influence of seasonal rainfall variations on the hydrothermal state within the permafrost active layer based on a water-vapour-heat coupled model that optimises the surface energy balance boundary conditions. The results show that regardless of whether the spring rainfall changes or not, the Bowen ratio is >1, indicating that the main energy exchange mode between the atmosphere and surface is sensible heat flux. But following the increase in rainfall in summer and autumn, the Bowen ratio is <1, indicating that the main energy exchange mode is latent heat flux. Meanwhile, increased seasonal rainfall leads to an increase in liquid water flux and a decrease in water vapour flux, whereas decreased seasonal rainfall leads to the opposite trend. Heat conduction, water vapour thermal convection, and water vapour diffusion latent heat decrease with increasing rainfall in various seasons. However, liquid water thermal convection increases with increasing rainfall during various seasons. Seasonal rainfall variations ultimately affect permafrost thermal stability by influencing water-vapour migration and energy changes within the permafrost active layer. Among them, the active layer thickness remains basically unchanged after the spring rainfall increases by 2.0 times, whereas it decreases by 4 and 6 cm after the summer and autumn rainfall increase by 1.5 and 2.0 times, respectively. Additionally, when the rainfall during spring, summer, and autumn decreases by 2.0, 1.5, and 2.0 times, respectively, the active layer thickness increases by 7, 6, and 3 cm, respectively. Therefore, increased seasonal rainfall can alleviate permafrost degradation. Otherwise, it is unfavourable for the preservation of permafrost thermal stability.