Retrieval of cloud optical properties over the South Pole from AVHRR data

Recent GCM studies have shown that the specification of cloud microphysical properties over the Antarctic Plateau influences the tropospheric circulation throughout the Southern Hemisphere. Our knowledge of cloud optical and microphysical properties over the Antarctic continent remains incomplete, due to a scarcity of in situ data, and also due to the difficulties with retrieving this information from satellite remote sensors. Unique environmental features that foil traditional techniques for retrieving cloud properties from sensors such as the Advanced Very High Resolution Radiometer (AVHRR) include (1) very high surface albedo, which renders backscattered radiance at conservative scattering wavelengths insensitive to cloud opacity, and (2) strong temperature inversions or isothermal conditions in the lower troposphere, which can make cloud detection difficult. In this study, we use a discrete ordinates radiative transfer model to simulate the radiances in AVHRR channels 3, 4, and 5 (3.7, 11, and 12 microns, respectively) as they would be measured from space over the Antarctic Plateau in cloudy or clear-sky conditions. We find that the albedo of a pristine snow surface at 3.7 microns is not negligible, and therefore our atmospheric radiative transfer model must include a surface scattering layer to represent a snowpack. Analysis of AVHRR channel 3 and 4 brightness temperatures over cloud-free scenes reveals consistency with snow grain sizes larger than 100 microns. For these larger snow grain sizes, the AVHRR channel 3 radiance (solar backscatter plus thermal emission) is insensitive to snow grain size, and therefore our model can be used with confidence to retrieve cloud optical properties and to distinguish them from surface effects. In contrast, the AVHRR channel 3 radiance is very sensitive to scattering particle size, in the size range 5 - 70 microns, which is the expected range of ice cloud effective particle radius. We have first compared our retrieval results with a recently published set of ground-based observations based on Fourier Transform Infared (FTIR) radiance measurements mad at the South Pole throughout 1992, and have found good consistency. We are proceeding with our retrievals for multiple years over the South Pole, where rawinsonde data are available to better constrain the temperature profile in our radiative transfer model.