Tuesday, 15 January 2002
Satellite observations of long-term changes in tropical cloud and outgoing longwave radiation from 1985 to 1998
This report presents the observational long-term changes in clouds and outgoing longwave radiation in the tropics (20-20N) from 1985 to 1998. The tropical cloud information is derived from the observation of the Stratospheric Aerosol and Gas Experiment (SAGE) II, while the outgoing longwave radiation is obtained from the measurement of the Earth Radiation Budget Experiment (ERBE). Both the SAGE II and ERBE instruments were launched on board the Earth Radiation Budget Satellite (ERBS) in October 1984. The satellite data analysis indicates negative trends of high-altitude uppermost cloud (optical depth greater than 0.02 to 0.03) occurrence between approximately 11 km and 16 km with a peak of 2.5%/decade at 13.5 km and positive trends of low-level uppermost cloud (below about 4 km) up to 2%/decade in the tropics. The negative trend of high-altitude uppermost clouds appears to result from a lowering of cloud height in the upper troposphere. By using an existing cloud longwave radiative forcing model in conjunction with the meteorological information from the National Center for Environmental Prediction (NCEP), the SAGE II tropical cloud occurrence record has been translated into a time series of cloud longwave radiative forcing (CLRF). The present analysis shows that the derived SAGE II CLRF trend is consistent with the ERBE/ERBS OLR observation.
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