Ensemble numerical simulation of permafrost thermal regimes over the Tibetan Plateau using the Flexible Permafrost Model: 1950–2023

Abstract. Permafrost is a subsurface phenomenon that is difficult to be measured directly, and understanding its dynamics as well as influences under a warming climate depends critically on numerical simulations. However, this task presents significant challenges as the state-of-the-art land surface models are weak in their ability to represent permafrost processes. In this study, we introduce a new land surface scheme specifically designed for permafrost applications, the Flexible Permafrost Model (FPM). This model serves as an adaptable framework for implementing innovative parameterizations of permafrost-related physics. The FPM accounts for heat flow at and below the soil surface, while simultaneously resolving the land-atmosphere energy exchanges through comprehensive treatment of radiative balance and turbulent flux dynamics. We simulate the ground thermal regime and test the model with a network of permafrost measurements across the Tibetan Plateau. Our result yields root mean square error values of 1.0 m for active layer thickness and 1.0 °C for the mean annual ground temperature at 15 m depth of permafrost. We estimate that the current extent of permafrost (2010–2023) on the Tibetan Plateau is approximately 1.07±0.02×106 km2. Long-term simulations indicate that the permafrost temperature increased at a rate of 0.11 °C per decade since 1980 with a decreased area of 14.6×104 km2 (∼12.4 %). These ensemble simulations provide valuable information on the dynamics of permafrost over the Tibetan Plateau. Furthermore, our findings suggest that current land surface models, which utilize shallow soil columns (typically ∼3 m), are insufficient for permafrost simulations over the Tibetan Plateau due to the typically deep active layer (that is, 2.68±0.82 m by mean) and may not be suitable for future projections.