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

Abstract. Permafrost remains a largely subsurface phenomenon, and understanding its dynamics as well as influences under a warming climate heavily relies 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 both vertical and lateral 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 1.1 m for the thickness of the active layer and 1.5 °C for the mean annual ground temperature of permafrost. We estimate that the current extent of permafrost (2010–2023) on the Tibetan Plateau is approximately 1.15 ± 0.02 × 10⁶ km², which aligns closely with published estimates. Long-term simulations indicate that the permafrost temperature has increased by 0.26 °C since 1980 with a decreased area of 13.2 × 10⁴ km² (∼10.5 %). 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, are insufficient for permafrost simulations over the Tibetan Plateau due to the typically deep active layer (that is, 2.88 ± 0.95 m by mean) and may not be suitable for future projections.