Wednesday, 5 June 2002: 9:30 AM
Comprehensive diagnosis of atmospheric radiative heating rate profiles using a radiative transfer model and ISCCP D1 data
For the same total diabatic heating, the atmospheric dynamical response will differ for different horizontal and vertical distributions of that heating. Moreover, differing space-time correlations of heating perturbations with temperature variations will lead to different net effects. Although the overall effect of clouds on the planetary and surface radiative fluxes is not very large, being only about 10-20% of the total, they significantly alter both the horizontal and vertical distributions of the radiative heating on the same space-time scales as the atmospheric motions. Thus, since clouds are produced by these atmospheric motions, cloud-radiative feedback on the general circulation can only be diagnosed by determining the synoptic-scale perturbations of the horizontal and vertical distributions of radiative heating induced by clouds. Because of the sparsity of observations, diagnosis of cloud-radiative perturbations at synoptic scales, but covering the whole globe for many years, has not been done before. The first such detailed, synoptic scale, global, long-term diagnosis has now been completed by employing several different observation systems, including satellite observations of clouds, atmosphere and surface properties and conventional weather observations for more than a decade, and a detailed radiative transfer model. Previous analysis had already shown that, overall, clouds inhibit the radiative cooling of the tropical atmosphere and enhance it at higher latitudes, providing a positive feedback on the mean meridional circulation. This new analysis shows that the cloud effect at lower latitudes is a reduction of the radiative cooling below the 600 mb level and above the 400 mb level. In the polar regions in wintertime, clouds significantly enhance the cooling rate in the 400-700 mb layer and reduce it below. In summertime, clouds enhance the cooling at all levels up to the 400 mb level, but more strongly below the 700 mb level. These vertical shifts in the atmospheric radiative cooling reinforce the mean latitudinal effects by increasing the required vertical heat transport in the tropics and the large-scale sinking of air in the polar regions.
Zhang, Y.-C., W.B. Rossow and A.A. Lacis, 1995: Calculation of surface and top of atmosphere radiative fluxes from physical quantities based on ISCCP data sets: I. Method and sensitivity to input data uncertainties. J. Geophys. Res., 100, 1149-1165.
Rossow, W.B., and Y.-C. Zhang, 1995: Calculation of surface and top of atmosphere radiative fluxes from physical quantities based on ISCCP data sets: II. Validation and first results. J. Geophys. Res., 100, 1167-1197.