Mesoscale modeling of climate effects on mountain glaciers

Motivated by the observed global retreat of mountain glaciers, we have developed a coupled atmosphere/ glacier model to better understand how regional and global climate influence mountain glacier mass. This model is applied to one of the planet's largest mountain glacier systems- the Patagonian Icefields of South America. The atmospheric component of this model is a modified version of the Penn State/ NCAR MM5 mesoscale model. Several modifications are made to the MM5 physical parameterizations, mainly to compensate for an overestimation of atmospheric moisture content at low levels. Initial and boundary conditions are obtained from ECMWF January and July monthly climatologies and held fixed. These alterations of MM5, combined with an unchanging diurnal radiation cycle, allow us to generate model states that represent climatological January and July. The glacier is modeled as a constant- temperature deep ice layer below a surface layer that responds to external forcings. Even with 30 km resolution, the atmospheric model does not represent the height of the Patagonian Icefields accurately enough. Therefore, a coupler is used to calculate turbulent heat fluxes at the observed altitude of the Patagonian Icefields for driving the glacier model.

The effect of the icefield on regional climate is generally small and localized to within 60 kilometers of its boundaries. In both January and July, the high precipitation rates that maintain the Patagonian Icefields are primarily attributable to orographic forcing. On the leeward side of the icefields, precipitation rates are an order of magnitude less than on the windward side, consistent with local observations. Simulation of doubled- CO₂ January and July conditions produces equilibrium line altitudes 200- 500 meters higher than present- day conditions because of increased sensible and latent heating of the ice surface, with no changes in precipitation rates significant enough to produce similarly strong effects. This suggests that the ELAs of the Patagonian glaciers are currently increasing at an average rate of a few meters per year.