The Impact of Antarctic Cloud Radiative Properties on a GCM Climate Simulation*

Abstract

A sensitivity study to evaluate the impact upon regional and hemispheric climate caused by changing the optical properties of clouds over the Antarctic continent is conducted with the NCAR Community Model version 2 (CCM2). Sensitivity runs are performed in which radiation interacts with ice clouds with particle sizes of 10 and 40 μm rather than with the standard 10-μm water clouds. The experiments are carried out for perpetual January conditions with the diurnal cycle considered. The effects of these cloud changes on the Antarctic radiation budget are examined by considering cloud forcing at the top of the atmosphere and net radiation at the surface. Changes of the cloud radiative properties to those of 10-μm ice clouds over Antarctica have significant impacts on regional climate: temperature increases throughout the Antarctic troposphere by 1°–2°C and total cloud fraction over Antarctica is smaller than that of the control at low levels but is larger than that of the control in the mid- to upper troposphere. As a result of Antarctic warming and changes in the north–south temperature gradient, the drainage flows at the surface as well as the meridional mass circulation are weakened. Similarly, the circumpolar trough weakens significantly by 4–8 hPa and moves northward by about 4°–5° latitude. This regional mass field adjustment halves the strength of the simulated surface westerly winds. As a result of indirect thermodynamic and dynamic effects, significant changes are observed in the zonal mean circulation and eddies in the middle latitudes. In fact, the simulated impacts of the Antarctic cloud radiative alteration are not confined to the Southern Hemisphere. The meridional mean mass flux, zonal wind, and latent heat release exhibit statistically significant changes in the Tropics and even extratropics of the Northern Hemisphere. The simulation with radiative properties of 40-μm ice clouds produces colder surface temperatures over Antarctica by up to 3°C compared to the control. Otherwise, the results of the 40-μm ice cloud simulation are similar to those of the 10-μm ice cloud simulation.