Improved permafrost modelling in mountain environments by including air convection in a hydrological model

Abstract. Permafrost occurrence in mountainous regions is influenced by complex topography and surficial geology, leading to high spatial heterogeneity. Coarse sediments create a unique thermal regime that allows permafrost to persist even under positive mean annual air temperatures due to natural convection lowering ground temperatures. Although this process has been recognized as a key factor in explaining the persistence of permafrost formation within coarse sediments, studies assessing the impact of natural convection on ground temperatures and permafrost are limited, partly due to the absence of hydrological models that take this process into account. This article expands on a well-established hydrological model to incorporate the effects of natural air convection on heat transfer. The modified model includes airflow through Darcy's equation and the Oberbeck–Boussinesq approximation to account for density-driven buoyancy effects, as well as a heat advection–conduction equation for the air phase without assuming local thermal equilibrium between the air and the other phases. The model was tested on a talus slope in the Canadian Rockies, where conventional models failed to represent field-based evidence of permafrost. The results revealed that coarse-sized sediments can lower ground temperatures by several degrees when natural convection is considered. Additionally, the study demonstrated that local thermal equilibrium approaches underestimate the impact of natural convection especially on short timescales. This enhanced model improves our understanding of permafrost dynamics in alpine landforms and enables a more accurate analysis of permafrost extent and its influence on groundwater discharges.