Scaling issues in glaciology: addressing subgrid topography

Le Morzadec, Kevin (2016) Scaling issues in glaciology: addressing subgrid topography. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Ice sheets are a key component of the earth system and disturbances of their state affects various aspects of human activity (e.g. due to sea level rise). Glaciological studies take advantage of paleo-data to improve our knowledge about ice sheet processes. To investigate ice sheet evolution over the timescale of a glacial cycle, 3-D ice sheet models (ISM) are typically run at grid resolutions of five to tens of kilometres. This introduces to-date unquantified errors in sub-grid (SG) ice transport and surface mass balance (SMB, the difference between ice surface accumulation and ablation). Here, I document the impact of ISM resolution on SMB and ice flux in regions of rough topography and develop parametrizations to reduce the associated ISM grid resolution dependency. For inland regions, I develop a new flow line SG model which uses hypsometric curves, a statistical summary of the topography, to describe the variability of SMB. The 1-D mass transport for the SG model is computed with the shallow ice approximation. I test this model against high resolution simulations from the 3-D ISSM model over regions of 30km by 60 km. Using SG topographic information improves the SMB and flux representation, however, depending on the regional topographic characteristics, the new SG model simulates ice volumes 45% lower to 15% higher than simulated by the ISSM. An ensemble of last glacial cycle simulations for the North American ice complex shows increases of up to 35m eustatic sea level equivalent in ice volume with inclusion of the SG model. For coastal regions, glacial valley geometry and density impact ice drainage. Decreasing the model resolution from 1km to 5, 10 and 25km overestimates the ice drainage and creates some lags in the timing of ice growth and decay. Modifying ice flow parameters reduces the excess of drainage on the order of 20% at 5 and 10 km resolution and by 5% at 25 km resolution. Simulations at 25km resolution still have a lag of one to three thousand years in growing ice after an interstadial period.

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