Current climate warming is accelerating mass loss from glaciers and ice sheets. In Greenland, the rates of mass changes are now dominated by changes in surface mass balance (SMB) due to increased surface melting. To improve the future sea-level rise projections, it is therefore crucial to have an accurate estimate of the SMB, which depends on the representation of the processes occurring within the snowpack.
Here, we present the recent developments we made to apply the snow module implemented in the land surface model ORCHIDEE to glaciers and ice sheets. Our analysis mainly concerns the model’s ability to represent ablation-related processes of the Greenland ice sheet. At the regional scale, our results are compared to the outputs of the regional atmospheric model MAR and to MODIS albedo retrievals. Using different albedo parameterizations, we performed several simulations forced by the MAR model over the 2000–2019 period. Our results reveal a strong sensitivity of the modelled SMB components to the albedo parameterization. Results inferred with albedo parameters obtained using a manual tuning approach present very good agreement with the MAR outputs. Conversely, with the albedo parameterization used in the standard ORCHIDEE version, runoff and sublimation were underestimated. We also tested the impact of albedo parameters coming from a previous data assimilation experiment using MODIS products on the ablation processes. While these parameters greatly improve the modelled albedo over the entire ice sheet, they degrade the other model outputs compared to those obtained with the manually tuned approach. This is likely due to the model overfitting to the calibration albedo dataset without any constraint applied to the other processes controlling the state of the snowpack. This underlines the need to perform a “multi-objective” optimization using auxiliary observations related to internal snowpack processes. Although there is still room for further improvements, the developments reported in the present study constitute an important advance in assessing the Greenland SMB with possible extension to mountain glaciers or the Antarctic ice sheet.
Authors: Sylvie Charbit, Christophe Dumas, Fabienne Maignan, Catherine Ottlé, Nina Raoult, Xavier Fettweis, et Philippe Conesa