Modeling depth hoar snow and its impact on permafrost and greenhouse gas fluxes

Recent permafrost temperature observations show warming, likely due to the combined impacts of more snow insulation and increased air temperatures. Depth hoar refers to coarse, faceted snow crystals that form near the bottom of the snowpack due to a strong temperature gradient that induces a vapor gradient. The thin and sparse connection between depth hoar crystals results in lower snow density. The depth hoar formed in a snowpack likely enhances permafrost warming during the winter season, and the impact could be sequentially fed back to CO2 fluxes from the permafrost soil during the next growing season. However, little quantitative assessments have been made on the impact of depth hoar on permafrost temperature and the associated feedback to CO2 fluxes. To address this deficiency, we coupled the depth hoar process to the land surface model CHANGE. The model assessed the impact of the depth hoar on permafrost and the associated greenhouse gases, based on two experiments that included or excluded the depth hoar process, over the pan-Arctic scale for the period 1979–2019. The differences between the two experiments illustrated that the depth hoar induced lower snow density and the resultant warmer permafrost temperature was linked to both larger vegetation photosynthesis and decomposition of soil organic carbon. These results strongly suggest that these snow processes improvement should be included in land surface models for better simulations and future projections on the Arctic environmental changes.