A surface mass balance model for the Greenland Ice Sheet

Abstract

[1] A surface mass balance model aimed at being coupled to a Global Circulation Model (GCM) for future climate prediction is described and tested for the Greenland Ice Sheet. The model builds on previous modeling designed to be forced by automatic weather station data, and includes surface energy balance as well as processes occurring near the surface such as water percolation and refreezing. Surface albedo is calculated with a new scheme that differentiates the timescale for aging of wet and dry snow and incorporates the effect of a thin layer of water and/or fresh snow at the surface. The model was driven with automatic weather station data from two sites located in the ablation zone in the Kangerlussuaq area (West Greenland), and calculated reasonable annual mass balance values (within 10% in seven out of eight cases) for four individual and consecutive years (1998)(1999)(2000)(2001), using both measured and calculated albedo. This implies that the albedo parameterization is adequate and climate feedbacks affecting the mass balance are well captured. The model was then applied to a distributed 20-km-resolution grid covering the whole ice sheet, and forced with 10 years of the European Centre for Medium-range Weather Forecast (ECMWF) reanalysis (ERA-40) data. With the aim of coupling the model to a GCM, this study focuses on the ability to model the interannual variability in mass balance rather than to assess the present state of balance of the ice sheet. Modeled spatial and temporal wet zone extent compares well with information derived from passive microwave satellite data.

Impact of model physics on estimating the surface mass balance of the Greenland ice sheet

“…Whereas the PDDM calculates the mass balance as a direct response to temperature change, the EBSM mass balance depends strongly on the surface albedo value, and the expansion of the runoff limit inland can be explained for the most part by surface albedo feedbacks. These depend on the snowpack and surface properties [Brun et al, 1992;Oerlemans and Knap, 1998;Warren, 1982;Zuo and Oerlemans, 1996], as well as on the ice sheet geometry (which affects the speed of meltwater transport) [Bougamont et al, 2005;Knap and Oerlemans, 1996;Zuo and Oerlemans, 1996]. The surface albedo value drops as melt increases, removing snow from the surface.…”

“…However, the PDDM parameters used here have been derived from field observations and are relatively realistic [Braithwaite, 1995;Lefebre et al, 2002]. The EBSM was originally tuned to match in-situ observations of mass balance along the K-Transect [Bougamont et al, 2005] and ETH-Camp [Greuell and Konzelmann, 1994] in southwest Greenland. Moreover, the EBSM tunable parameters expected to have the greatest impact on mass balance affect mostly the melt generation (via the surface albedo as well as the timescale for runoff formation), and less so the refreezing processes.…”

“…The list of tunable parameters is longer for the EBSM. Parameters have identical values to Bougamont et al [2005], with the exception of ice and old snow albedo (a ice and a old respectively), the characteristic scale for surficial water (w*), and two constants used to calculate the timescale for runoff to occur (c 2 and c 3 in equation 13, Bougamont et al [2005]). New values are summarized in Table 1.…”

“…The EBSM used here is validated and described by Bougamont et al [2005]. Physically-based equations are used to estimate the energy available for melt, and refreezing is evaluated using a scheme that models the evolution of the snowpack through time.…”

Sensitivity of Greenland Ice Sheet surface mass balance to surface albedo parameterization: a study with a regional climate model

“…Modeled surface mass balance (SMB) of the GrIS is highly sensitive to surface albedo (Bougamont and Bamber, 2005;Fitzgerald et al, 2012;Tedesco et al, 2011), owing to the positive melt-albedo feedback, which results in lower albedo in high-melt years (Stroeve, 2007). Following recent warming, a persistent drop in GrIS surface albedo has been observed in satellite data and climate models (Box et al, 2012).…”