Process-based modelling of active layer dynamics under changing Arctic precipitation: insights from Zackenberg, Greenland

Arctic regions are undergoing rapid climatic change, including an increasing proportion of rain in annual precipitation. This change is expected to alter ground thermal regimes, permafrost stability, and soil hydrological processes. While previous studies have primarily focused on the effects of rising air temperatures on permafrost, the impact of changes in precipitation remains insufficiently quantified, given the variability and uncertainty of future precipitation projections.

This study evaluates the effects of altered precipitation patterns on active layer dynamics at Zackenberg, northeast Greenland. We combined long-term field observations with pedon-scale modelling using the CryoGrid community model, a coupled soil thermal-hydrological model that explicitly represents water and ice dynamics. We derived model parameters from site-specific measurements and calibration, and then validated their performance against independent observations. We obtained future precipitation scenarios from bias-corrected HIRHAM5 RCP4.5 and RCP8.5 projections.

The model reproduces the main hydrothermal dynamics of the active layer well. Simulation uncertainties remain due to simplified representations of percolation, snow insulation, and limited soil moisture data. Parameter evaluation reveals equifinality among evaporation depth, the evapotranspiration ratio, and saturated hydraulic conductivity. Uncorrected HIRHAM5 forcing exhibits a pronounced cold bias, resulting in underestimated active layer thickness (ALT) and emphasising the need for bias correction. High interannual variability in precipitation amount and rain–snow partitioning strongly influences both ALT development and freeze-back dynamics, largely independent of mean air temperature trends. Differences between RCP8.5 and a wetter, modified scenario suggest that wetter conditions can constrain active layer deepening under otherwise identical forcing, indicating that increased soil moisture may partially buffer warming effects. Our results show that uncertainties in precipitation projections have a major impact on active layer dynamics, providing process-based evidence that precipitation is a key driver in warming Arctic permafrost regions.