Ice sheet inception: subgrid hypsometric parameterization of mass balance in an ice sheet model

Studies of ice sheet inception and mountain glaciation are limited by model resolution in both atmospheric and glaciologic models. The accumulation and ablation of snow and ice are controlled by local climate, which is sensitive to topographic features that are subgrid in continent-scale investigations. Climate patterns are complex in areas of high relief, but the first-order influence of elevation on temperature and atmospheric humidity is quantifiable and is central to glacier mass balance. We make substantial improvements to the descriptions of accumulation and ablation in a large-scale ice sheet model by incorporating subgrid topographic information. The distribution of relief in the large-scale model is estimated through a hypsometric parameterization based on the minimum, maximum, and median topographic elevation. This permits accounting of high-elevation accumulation and melting in deep valleys. We combine this with a parameterization of downslope subgrid ice flux, simulating but not explicitly describing the role of alpine glaciers. The detailed budgeting of mass balance greatly improves our skill in modelling present-day ice distribution in North America. We anticipate similar improvements in our ability to model nucleation centres of the Quaternary ice sheets. This approach may also prove useful in projections of glacier and ice field response to climate change.