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

Abstract. 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 between MAR and RACMO2 are attributed to differences

in advection of precipitation with snowfall particles being likely advected too

far inland in MAR.