Projecting Circum-Arctic Excess Ground Ice Melt with a sub-grid representation in the Community Land Model

Abstract. We implement a sub-grid representation of excess ground ice within permafrost soils and its melting using the latest version of Community Land Model (CLM5). Based on the original CLM5 tiling hierarchy, we duplicate the natural vegetated landunit by building extra tiles for up to three different excess ice conditions in each grid point. Single-grid simulation cases initialize the same amount of excess ice in same soil layers, while prescribing different volumetric ice contents and sub-grid distributions. For the same total amount of excess ice initialized at the same soil layers, different sub-grid variability of excess ice leads to different excess ice melting rates on the grid level, as well as to different impacts to permafrost thermal properties and local hydrology. We prescribed a tiling scheme based on the dataset “Circum-Arctic Map of Permafrost and Ground-Ice Conditions” (Brown et al., 2002) within horizontal and vertical distributions of three different types of excess ice in the CLM grid to test applicability of sub-grid representation. Compared to the excess ice initialized homogeneously in each grid, the sub-grid scale excess ice and the tiling scheme amend the overly early timing of initial melting and the overly high melting rate of excess ice in warming climate. Initializing the excess ice depths according to local active layer thickness reduces underestimation of excess ice melt for the Arctic coastal regions. Further developments rely on additional global observational datasets on both the spatial and vertical distributions of excess ground ice, where our development of sub-grid representation demonstrated the potential for more realistically projecting excess ice melt in the circum-Arctic domain.