Single-column model studies for improving the representation of Antarctic cloud cover

The Antarctic Plateau, being the coldest and driest location on earth, presents unique challenges to climate modeling simulation. Current GCMs continue to make significant errors with respect to cloud climatology and atmospheric radiation balance over Antarctica, and there is a need to improve the physical representation of clouds in this extreme environment. To test new parameterizations for cloud physics and radiation, as they are developed or refined from field data, we have adapted the Scripps Single Column Model (SCM). Because there are often large temperature variations in the lower Antarctic troposphere over a few hundred meters (i.e., strong temperature inversions), we resolved the SCM atmosphere into 53 pressure levels. To determine the realism of the SCM's simulation of cloud amount, we forced the SCM with ECMWF reanalyses from 1992-93, ran the model for a grid cell centered about the South Pole, and validated the cloud simulations with cloud observations from the South Pole Weather Office. We find that, generally speaking, prognostic cloud prediction schemes result in a more realistic cloud simulation than diagnostic schemes, although diagnostic schemes can be tuned to yield acceptable results. We also find that the model, forced with ECMWF reanalyses, provides realistic simulations of cloud amount as a function of wind direction in the lower troposphere: air masses reaching the South Pole from the directions of the Weddell or Ross Seas are warmer, more moist, and bring about more cloud cover, while the opposite is true for air masses reaching the South Pole from Ellsworth Land or Wilkes Land. Our ongoing work with the SCM involves (1) testing the latest available GCM cloud parameterizations, including one that can simulate clear-sky precipitation, and (2) evaluating the response of the SCM, with appropriate forcing, to increased greenhouse gas concentrations.