Robert S. Fausto

Title: Improving Surface Boundary Conditions for Large-Scale Ice Sheet Models of Greenland

A thesis submitted for the degree of Doctor of Philosophy (PhD)
on August 17, 2009.

Centre for Ice and Climate (CIC) and
Copenhagen Global Change Initiative (COGCI)

Niels Bohr Institute (NBI), University of Copenhagen
and Geological Survey of
Denmark and Greenland (GEUS)

Supervisors:
Sigfus J. Johnsen
Carl E. Bøggild
Christoph Mayer
Andreas P. Ahlstrøm

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Abstract

Improving Surface Boundary Conditions for Large-Scale Ice Sheet Models of Greenland

Improving surface boundary conditions for large-scale ice sheet models of Greenland is the main focus of this thesis. Near-surface air temperature (2m) over the Greenland Ice Sheet (GrIS) is parameterized using data from automatic weather stations (AWS) located on land and on the ice sheet.

The parameterization is expressed in terms of mean annual temperatures and mean July temperatures both depending linearly on altitude, latitude and longitude. The temperature parameterization is compared to a previous study and it is shown to have a better agreement with observations.The temperature parameterization is tested in a positive degree day (PDD) model to simulate the present (1996-2006) mean melt area extent of the GrIS.

The model accounts for ¯rn warming, rainfall, and refreezing of melt water, with di®erent PDD-factors for ice and snow under warm and cold climate
conditions. The simulated melt area extent is found to have a reasonable agreement with satellite-derived observations. Snow pack changes during the melt season are often not incorporated in modelling studies of the surface mass balance (SMB) of the GrIS. Densi¯cation of snow accelerates when meltwater is present due to percolation and subsequent refreezing and needs to be incorporated in ice sheet models for ablation calculations.

In this thesis, simple parameterizations used to calculate surface melt, snow densi¯cation and meltwater retention are included as surface boundary conditions in a large-scale ice sheet model of Greenland. Coupling the snow densi¯cation and meltwater retention processes achieves a separation of volume and mass changes of the surface layer in order to determine the surface melt contribution to runo®. Experiments for present-day conditions show that snow depth at the onset of melting, mean annual near-surface air temperature and the mean density of the annual snow layer are key factors controlling the quantity and spatial distribution of meltwater runo® above the equilibrium line on the GrIS.

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