Effects of changes in the ECMWF physics on the surface mass balance of Antarctica predicted by a regional atmospheric climate model

In recent years the regional climate model RACMO has been used and adapted for climate and mass balance studies over polar regions such as Greenland and Antarctica. RACMO2.1, as used in current studies, uses the ECMWF physics package of cycle 23r4. Recently the model has been updated to RACMO2.3 in which the new physics package of cycle 33r1 has been implemented. This mainly constitutes significant changes in the cloud convection and prognostic schemes, radiation schemes as well as changes in the parameterization of precipitation and the turbulent fluxes of moisture and heat.

Here the first results of the impact of these changes on the surface mass balance (SMB) of Antarctica are presented. RACMO2.3 gives, integrated over the entire ice sheet (including ice shelves) for the period 1979-2009, a SMB of ≈ 2570 Gt y^-1 , higher by ≈150 Gt y^-1 than that of RACMO2.1. Especially for the East Antarctic plateau, for which SMB values were known to be underestimated in RACMO2.1, the new cycle gives higher values up to 50 Gt y^-1 locally and on average ≈15 Gt y^-1, equal to an increase of 25%.

These changes are attributed to (respectively) locally increased precipitation in areas of steep topography like the Antarctic Peninsula and a decrease of autumn/winter (snowdrift) sublimation rates more inland due to decreased near-surface wind speeds. Moreover the downward longwave radiation in East Antarctica, which like the SMB was known to be underestimated, has among other influences increased due to an almost uniformly increased cloud cover. Together with a decrease of the sensible heat flux the changes in the cloud and radiation schemes have a positive effect on the modelled SMB and energy balance of Antarctica.

Other related changes are the increased two meter temperatures across the grounded ice sheet as well as decreased melt fluxes over the floating ice shelves and coastal regions.