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Snow melt models

The three snow melt models which will be used in the following calculations were described more extensively in a paper about the current state of the mass balance of the Greenland and Antarctic ice sheets (Bugnion (1999)); a brief summary is presented here:

The model is computationally sufficiently efficient to be solved on a 20 km grid on the Greenland ice sheet and 40 km on Antarctica. The resolution used for Greenland has been shown by Glover (1999) to be sufficient to resolve the features of the melt zone on the margins of that ice sheet. The model is allowed to equilibrate with the 1990 climate by developing temperature and density structures appropriate for each location on the ice sheet before proceeding with the transient climate change calculations.

The conditions at Qamanârssûp Sermia on the Greenland ice sheet, as predicted by the MIT climate model (see the next section for details about the model), provide an example of the differences in behavior of the three models. The average summer temperature is 5.4o which leads the linear model to predict 0.51 . 5.4 + 0.93 = 3.68 m. of runoff. This location experiences 4 months with temperatures above the melting point for a total of 527 positive degree-days (PDD). Melting the winter's snow accumulation uses only 19 PDDs and 60% of that meltwater is assumed to refreeze. The remaining PDDs are used to melt ice for a total runoff of 4 m. The snowpack model relies on the surface energy balance to generate 3.27 m of meltwater, 8 cm of liquid water is added in the form of rain, 4 cm refreezes within the snow cover and 3.31 m contribute to the runoff from the ice sheet. The potential for refreezing is for the most part eliminated between July, when the winter's snow is melted and bare ice outcrops, and September, when temperatures drop below the melting point. Because the snowpack model is based on well established physical principles, it can be expected to respond in a believable way to substantial changes in atmospheric forcing. The results obtained with the simpler temperature based models will therefore be assessed by comparison to those obtained with the snowpack model.

Because of the size of the ice sheets, the response of the internal ice dynamics of Greenland and Antarctica to changes in the surface forcing will take place on time scales greater than a century and will therefore be neglected (Greve (1997), Huybrechts (1990a)). Dynamic changes which could take place over less than a century, such as the partial collapse of the West Antarctic ice sheet (M.Oppenheimer (1998)) or rapid local changes in glacier dynamics (Krabill et al. (1999)) are still difficult to model accurately and will be neglected in this analysis.


next up previous
Next: Climate Models Up: Changes in Sea-Level associated 21st Previous: Introduction
Veronique Bugnion
1999-10-19