Abstract

Evaluation of a complete annual cycle of 72h nonhydrostatic mesoscale model simulations of the Antarctic atmospheric circulation is presented. The simulations are generated with the Pennsylvania State University-National Center for Atmospheric Research fifth-generation mesoscale model (MM5), which is modified for polar applications, and is referred to as the Polar MM5. With a horizontal resolution of 60km, the Polar MM5 has been run for the period of January 1993 through December 1993 in a year-long series of short-term forecasts from initial and boundary conditions provided by the operational analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF). The model output is compared with observations from automatic weather stations, upper air data, global atmospheric analyses as well as climatological maps over time scales from diurnal to annual. The evaluation shows that simulations with the Polar MM5 accurately capture both the large and regional scale circulation features with minimal bias in the modeled variables. On the annual time scale the largest error in the model simulations is the deficient total cloud cover and precipitation, and the colder near-surface temperature over the interior of the Antarctic plateau. The deficiencies in the total cloud cover and precipitation simulations follow from low-level dry biases found in the Polar MM5 simulations, and the cold bias is related to the low predicted downward longwave radiation under clear skies in the radiation parameterization scheme. The deficient predicted precipitation also reflects the limited ability of Polar MM5 to represent clear sky precipitation. On the seasonal time scale a persistent positive pressure bias is found in the model simulations, and it is caused by the interaction between the gravity waves and the upper boundary condition. The observed synoptic variability of the pressure, temperature, wind speed, wind direction, and mixing vapor ratio, as well as the diurnal cycle of temperature, wind speed, and mixing ratio are reproduced by the Polar MM5 with reasonable accuracy.