Extended Abstract (36K)P2.18
The influence of Antarctic cloud and surface properties on cloud radiative forcing at the surface
Michael J. Pavolonis, CIMSS/Univ. of Wisconsin, Madison, WI; and J. R. Key
Surface cloud radiative forcing was calculated using cloud and surface properties from the International Satellite Cloud Climotology Project (ISCCP) D-series data set as input into a radiative transfer model. The period 1989 to 1991 and area between 60S and 90S latitude were examined in this study. The net cloud forcing at the surface was found to be positive for most of the year over the entire Antarctic continent. In addition, from about 80S to 90S clouds were determined to have a warming effect on the surface every month of the year. While clouds have a warming effect on the surface in the longwave and a cooling effect in the shortwave, the decrease in downwelling shortwave radiation due to reflection by clouds is much smaller at high latitudes because of larger solar zenith angles and the presence of a very bright surface, which increases the downwelling shortwave flux through multiple surface-cloud reflections. Model sensitivity studies were conducted to determine the relative importance of various cloud and surface properties on the longwave and shortwave cloud forcing. In the longwave, the cloud forcing at the surface was found to increase the most when mean cloud cover was increased. Changes in cloud top temperature, cloud optical depth, and surface temperature did not produce a large change in the longwave cloud forcing. Results using the ISCCP longwave cloud forcing support the longwave sensitivity tests. Model calculations indicate that shortwave cloud forcing is sensitive to changes in mean cloud cover, cloud optical depth, and surface albedo, especially at lower latitudes. Theoretically, shortwave cloud forcing should become less negative with decreasing cloud optical depth. However, calculations with the ISCCP data indicate that increases in mean cloud amount and decreases in surface reflectance dominate so that the shortwave cloud forcing still becomes more negative despite a decrease in cloud optical depth. Overall, any changes in mean cloud cover will have the greatest affect on the net surface cloud forcing and hence any surface warming or cooling due to the presence of clouds.
Poster Session 2, Climatology and Long-term Satellite Studies
Monday, 15 October 2001, 2:15 PM-4:00 PM
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