Cooperative Institute for Research in Environmental Sciences

Evaluation of Polar MM5 simulations of Antarctic atmospheric circulation

Evaluation of a complete annual cycle of nonhydrostatic mesoscale model simulations of the Antarctic atmospheric circulation is presented. The year-long time series are compiled from a series of overlapping short-duration (72 h) simulations of the atmospheric state with the first 24 h being discarded for spinup reasons, and the 24-72-h periods used for model evaluation. The simulations are generated with the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5), which is modified for polar applications, and is referred to as the Polar MM5. With a horizontal resolution of 60 km, the Polar MM5 has been run for the period of January 1993-December 1993, creating short-term simulations from initial and boundary conditions provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) Tropical Ocean Global Atmosphere (TOGA) operational analyses. The model output is compared with observations from automatic weather stations, upper-air data, and global atmospheric analyses as well as climatological maps over timescales from diurnal to annual. In comparison with the observations, the evaluation shows that simulations with the Polar MM5 capture both the large- and regional-scale circulation features with generally small bias in the modeled variables. For example, the differences between the observations and simulations at the 500-hPa level are usually less than 2degreesC for temperature and dewpoint temperature, and 20 m for geopotential height. On the annual timescale the largest errors in the model simulations are the deficient total cloud cover and precipitation, and the colder near-surface temperature over the interior of the Antarctic plateau. The deficiencies in the cloud prediction and precipitation simulation 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 timescale a persistent positive pressure bias is found in the model simulations, caused by the interaction between the gravity waves and the model upper boundary condition. The observed synoptic variability of the pressure, temperature, wind speed, wind direction, and water vapor mixing ratio, as well as the diurnal cycles of temperature, wind speed, and mixing ratio, are reproduced by the Polar MM5 with reasonable accuracy.