Changes of timing and duration of the ground surface freeze on the Tibetan Plateau in the highly wetting period from 1998 to 2021

A dual cooling and warming effect of precipitation over the Tibetan Plateau (TP) on permafrost has been reported recently through numerical methods. However, predictive relationships among snow cover depth in the cold season, rainfall in the summer season, associated vegetation dynamics, and the thermal regime of frozen ground still have not been well demonstrated, although it will become even more important as precipitation has been projected to increase continuously. Previous studies have confirmed the pivotal wetting period since 1998 over the TP. In this study, the changes in the first date, last date, duration, and number of days in the ground surface freeze from 1998-2021 are investigated based on the records of 51 meteorological stations. The results indicate that greening and the absence of spring snow depth are favorable to the thermal response of surface seasonally frozen ground to atmospheric warming and wetting. However, enhanced summer rainfall warms the permafrost but cools the seasonally frozen ground remarkably after 1998. The thermal response of frozen ground to wetting depends on the type of precipitation, which varies seasonally. The combined effects of thinning snow cover depth, increasing rainfall, expanding vegetation cover, and air warming have dominated the thermal degradation of frozen ground, especially in wetter zones.