Evaluation of a regional atmospheric model using measurements of surface heat exchange processes from a site in Antarctica

A regional atmospheric climate model with a horizontal grid spacing of 55 km has been used to simulate the Antarctic atmosphere during an austral summer period. ECMWF re-analyses were used to force the atmospheric prognostic variables from the lateral boundaries. Sea surface temperatures and sea ice mask in the model were prescribed from observations. Parameterizations of the physical processes are taken from the ECHAM4 General Circulation Model. Before applying the model to Antarctic conditions, several adjustments have been made to the original code. In particular, a better correspondence between model output and measurements was accomplished by 1), the use of a fixed value of 0.8 for the surface albedo rather than applying an albedo that linearly rises with surface temperature, and 2), the use of the volumetric heat capacity and the thermal diffusivity of snow rather than employing the values for ice.

The model is evaluated for the period 14 to 19 January 1993 (P1) on the basis of an extensive data-set compiled from measurements made at a site (Svea) in Dronning Maud Land. This data-set contains boundary-layer temperature and specific humidity profiles, snow temperatures and surface heat fluxes. The surface fluxes were obtained from direct measurements combined with an energy balance model. The atmospheric temperature profiles simulated at the grid points corresponding most closely to Svea are in good agreement with the measured profiles, although the model slightly overestimates the vertical temperature gradient. The model probably underestimates the turbulent transport of heat and moisture to atmospheric layers above roughly 200 m. At Svea a cloudcover of less than 0.5 octas was observed during P1. The model overestimates the cloud cover, which results in an underestimation of shortwave and an overestimation of longwave radiative fluxes at the surface. The simulated values for the net radiative fluxes, the heat flux into the snow, and the turbulent heat fluxes correspond within 4 W m-2 to the fluxes that were inferred from measurements.