11.5
Cloud and surface radiative properties over the Antarctic Plateau from AVHRR data
Dan Lubin, SIO/Univ. Of California, La Jolla, CA; and J. Berque and R. C. J. Somerville
Recent Antarctic climate modeling studies have demonstrated the need to understand the microphysical and radiative properties of cloud cover over the Antarctic continent. Suitable ground based or in situ observations are rare, but GCM simulations show that an uncertainty in cloud phase or range of particle size can lead to large uncertainties in the simulated radiation balance, surface pressure fields, and wind fields over the entire Southern Ocean as well as over Antarctica. AVHRR data provide a useful way to extend the limited ground based observations to longer timescales and larger geographic areas. Radiances emitted to space in the two middle infrared channels (4 and 5), and radiances both backscattered and emitted to space in the 3.7-micron channel (3) are sensitive to the optical depth and effective particle size of ice clouds, and are also sensitive to the effective grain size of clean snow on the Antarctic Plateau. The albedo of clean Antarctic snow at 3.7 microns is small (of order 5%), but is sufficient to increase the apparent AVHRR channel 3 brightness temperature by 10-20 K (depending on snow grain size) over that of channel 4 or 5. Because clouds tend to be optically thin over the Antarctic Plateau, it is important to use a discrete ordinates radiative transfer formulation that couples the surface to the atmosphere by treating the snow surface as a scattering/absorbing layer; the radiative properties of the snow surface can influence the top-of-atmosphere radiances through an optically thin cloud. It is necessary to first characterize the optical properties of the snow surface before using AVHRR infrared data to retrieve cloud properties.
Session 11, Climate Forcing (Parallel with Sessions 10 & 12)
Wednesday, 17 January 2001, 1:30 PM-5:00 PM
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