AVHRRデータから求めた1987年の東南極大陸上の雲の分布特性

Abstract

Polar cloud climatology is an urgent issue to be solved for the study of global climate from satellite data. However, detection of clouds in the polar regions involves many difficulties on account of the high albedo and low temperature of the snow and ice covered ground surface. Discrimination of clouds was done using AVHRR split window channel data. Brightness temperature differences of 11μm (ch 4) and 12μm (ch 5) were one of the indices of thin clouds; the correlation of the brightness temperature difference and the brightness temperature itself was used. Cloud amounts thus derived were well explained by the all-sky camera data measured at the ground. The cloud analysis was done from daily NOAA-9 data for thirteen months from January 1987 to January 1988, received and processed at Syowa Station, Antarctica.Spatial and temporal distribution characteristics of clouds over the East Antarctic continent are discussed. Annual mean cloud fraction ranged from 50% to less than 10% according to the region, being larger near the coast and smaller over the high interior plateau. Within the interior, cloud amounts were liable to be higher over the western slope facing the Weddell Sea compared to the eastern slope. Semiannual variations were seen in most of the area related to the behavior of the disturbances. In July to September, cloud amounts in the interior increased in some regions, and in November to January, increased in most regions; however, not much variation was seen in the coastal area. The analyzed area was divided into three regions of different variation characteristics, related to the topography. An oscillation of about 7 or 15 days in the time variation of clouds was noticeable in most regions. Comparing the brightness temperature for clear and cloudy sky, the radiative effect of clouds at the top of the atmosphere was found to be negative (cooling) in winter in the interior, and small positive (heating) in the longwave in summer months over the whole area.