CERES TRMM SSF data have been used to explore the Iris hypothesis recently proposed by Lindzen et al. Measured radiation fluxes have been obtained from the CERES SSF products for each part of the 3.5 box climate model hypothesized by Lindzen. The dry region has the properties of that half of the tropical maritime areas with the warmest longwave fluxes. The determination of radiation properties for the clear moist and cloudy moist regions is explored in this study, using various cloud properties to obtain the best identification of these regions.
Although downwelling shortwave and upwelling longwave fluxes for the different regions are sensitive to the definition of the cloudy moist region, results show that the measured net radiation (shortwave minus longwave) is quite insensitive to that definition. Net radiation ranges between 41 and 56 W/m2 for each part of the 3.5 box partition. This is in contrast to the values of 123 W/m2 for cloudy moist, and 13 W/m2 for the dry region, which were somewhat subjectively assigned by Lindzen et al. Using the CERES-measured radiative properties, the tropical cloud feedback is at most 10% of that hypothesized by Lindzen. In half of the cases considered, a zero or small positive feedback is found. In the other half a small negative feedback is found. These results are basically consistent with previous studies using CERES ERBE-like data. They show that the Tropical Iris, if it exists, is much weaker than the one proposed by Lindzen et al.
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