The Infrared Radiative Impact of Antarctic Clouds

Clouds exert a strong radiative impact on the surface and have complicated effects that are still not well understood, particularly in the Antarctic. The amount of supercooled liquid water in Antarctic clouds, for example, is still poorly constrained, due to the low number of observations on the continent. It is also not clear how the radiative properties of supercooled liquid in those clouds should be represented in climate models. In particular, the complex refractive index of liquid water is known to depend on temperature, but this dependence is typically ignored in climate models.

Here, we present cloud properties retrieved from Antarctic downwelling infrared radiance measurements made by the Polar Atmospheric Emitted Radiance Interferometer (PAERI), using the Cloud Atmospheric Radiation Retrieval Algorithm (CLARRA). Preliminary retrievals were made of cloud height, optical depth, thermodynamic phase, and effective radius at Amundsen-Scott South Pole Station for nearly a year in 2000 and at Dome C, Antarctica, during the summer of 2003. Accuracies of cloud height retrievals made with CLARRA using two methods (the Minimum Local Emissivity Variation method, or MLEV, and CO₂ slicing/sorting) were evaluated by comparison to cloud heights determined by micropulse lidar for 4 months at South Pole station. Preliminary results suggest that CO₂ slicing/sorting is more accurate; this is attributed to the greater sensitivity of the MLEV method to errors in water vapor amount combined with the high uncertainty in available South Polar atmospheric water vapor profiles. At South Pole, we find that clouds are typically thin and near the surface, in keeping with prior work. The mode of the effective radii of liquid droplets (~4 μm) and ice particles (~15 μm in summer, ~12 μm in winter) at South Pole are found to be smaller than typical Arctic values (~9 μm for liquid and 17 to 25 μm for ice). Although ice cloud was found to dominate year-round, significant supercooled liquid water was present in the summer. We further find that the impact of ignoring the temperature dependence of the complex refractive index of supercooled liquid cloud to be as large as 1.6 W/m².