Analysis of the impact of clouds and water vapor on the seasonal cycle of the terrestrial radiation budget. Implications for climate sensitivity.

Summary The estimation of climate sensitivity to natural or anthropogenic forcing is linked to our understanding of the mechanisms that regulate radiative exchange between the earth and space. Using satellite observations erbe (earth radiation budget experiment) and ssm/i (special sensor microwave imager) and numerical climate simulation models (the lmd model and the arpege model developed by meteo-france), seasonal variations of the terrestrial radiation budget are analyzed and the respective roles of temperature, water vapor and clouds in these variations are examined. The ability of general circulation models to simulate the seasonal cycle of water vapor, clear-sky radiative fluxes and clouds radiative forcing is evaluated, and the models are used in turn to interpret some observed characteristics. This allows to identify more precisely the factors that modulate the greenhouse effect on seasonal and interannual scales and under climate change scenarios. This study shows the essential role of the variability of the vertical atmospheric structure (in particular the relative humidity profile and the vertical temperature gradient) in the observable variations of the climate as well as in its global sensitivity, and shows the need to differentiate these effects in the analysis of the climate sensitivity at various time scales. Finally, this study highlights the importance of validating seasonal and interannual variations in the vertical atmospheric structure for the application of climate simulation models to future scenarios.