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17 septembre 2019
Estimation of the Antarctic surface mass balance using the regional climate model MAR (1979–2015) and identification of dominant processes

Fig. 1. Modelled vs. observed SMB for sectors and transects over Antarctica. MAR SMB is in blue and green and observed SMB is in black. SMB values are first averaged on MAR grid cells then along a chosen grid direction or by elevation bins. Distance along the transect starts at the coast.

The Antarctic ice sheet mass balance is a major component of the sea level budget and results from the difference of two fluxes of a similar magnitude: ice flow discharging in the ocean and net snow accumulation on the ice sheet surface, i.e. the surface mass balance (SMB). Separately modelling ice dynamics and SMB is the only way to project future trends. In addition, mass balance studies frequently use regional climate models (RCMs) outputs as an alternative to observed fields because SMB observations are particularly scarce on the ice sheet.

Here we evaluate new simulations of the polar RCM MAR forced by three reanalyses, ERA-Interim, JRA-55, and MERRA-2, for the period 1979–2015, and we compare MAR results to the last outputs of the RCM RACMO2 forced by ERA-Interim. We show that MAR and RACMO2 perform similarly well in simulating coast-to-plateau SMB gradients, and we find no significant differences in their simulated SMB when integrated over the ice sheet or its major basins. More importantly, we outline and quantify missing or underestimated processes in both RCMs. Along stake transects, we show that both models accumulate too much snow on crests, and not enough snow in valleys, as a result of drifting snow transport fluxes not included in MAR and probably underestimated in RACMO2 by a factor of 3. Our results tend to confirm that drifting snow transport and sublimation  fluxes are much larger than previous model-based estimates and need to be better resolved and constrained in climate models. Sublimation of precipitating particles in low-level atmospheric layers is responsible for the significantly lower snowfall rates in MAR than in RACMO2 in katabatic channels at the ice sheet margins. Atmospheric sublimation in MAR represents 363 Gt yr−1 over the grounded ice sheet for the year 2015, which is 16 % of the simulated snowfall loaded at the ground. This estimate is consistent with a recent study based on precipitation radar observations and is more than twice as much as simulated in RACMO2 because of different time residence of precipitating particles in the atmosphere. The remaining spatial differences in snowfall are attributed to differences in advection of precipitation with snowfall particles being likely advected too far inland in MAR.

 

Authors : C. Agosta, C. Amory, C. Kittel, A. Orsi, V. Favier, H. Gallée, M.R. van den Broeke, J.T.M. Lenaerts, J.M. van Wessem, W.J. van de Berg, and X. Fettweis.

 

Maj : 17/09/2019 (188)

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