MODELING THE RESPONSE OF GLACIERS TO CLIMATE CHANGE USING A 2D GLACIER MODEL THAT REPRESENTS THE EFFECTS OF TOPOGRAPHY ON SURFACE MASS BALANCE: UPDATES AND VALIDATION

As glaciers contract into cirques across the western US and beyond, it is imperative that we have tools to accurately predict their response to climate change. One of the most important, typically overlooked, factors in mountain glacier response to climate change is the complex effect of topography on glaciers. In this study, we apply, update, and validate the MATLAB version of the Plummer and Philips (2003) model to quantify how glacier sensitivity varies with topography. To represent snow and ice ablation, the model calculates the surface energy balance based on shortwave, longwave, turbulent and ground fluxes at the Earth surface. The energy balance approach to calculating ablation has the advantage of being more physically based than degree-day models which assume a linear relationship between air temperature and melt. But the surface energy balance approach is complex and requires estimation of many parameters and relies on many inferences about the snow/ice surface. Validating such a model is thus time consuming, complex, and necessarily incomplete. Here we present results from an effort to update and validate the glacier model of Plummer and Phillips (2003) originally developed to inversely estimate past climate based on paleoglacier geometry. Specifically, we explore the importance of the surface mass balance timestep, how snow surface temperature is calculated, and how surface temperature dependent energy fluxes vary with that method. We also explore the importance of model resolution in calculating the distribution of surface mass balance by comparing 25-m and 200-m grids. While the surface mass balance can easily be calculated on a finer grid, the ice flow calculation, based on the 2-D from of the shallow-ice approximation is inaccurate at resolutions finer than about 100 m due to underlying assumptions in the derivation. To validate the model, we compare simulated mass balance against known constraints of surface mass balance gradients from modern glaciers. We also implement a new package to evaluate the plausibility of estimated paleoclimate with recent compilations of data bounding what conditions are possible based on modern glaciology. The goal of this effort is to produce a model that can simulate glacier response to climate change from the Last Ice Age into the future.