Sensitivity of Atmospheric Radiative Heating Rate Profiles to Variations of Cloud Layer Overlap

Three different cloud overlap schemes are applied to the International Satellite Cloud Climatology Project (ISCCP) vertical distribution of clouds in the radiative transfer model from the National Aeronautics and Space Administration Goddard Institute for Space Studies climate GCM to study the sensitivity of radiative fluxes and atmospheric radiative heating rate profiles to variations in cloud vertical structure. This study differs from previous ones because the ISCCP dataset constrains the total column optical thickness of the clouds at each location, a fact that is used to constrain cloud overlap occurrence. Moreover, this study considers the effects of cloud vertical structure on both shortwave (SW) and longwave (LW) fluxes at the top of the atmosphere, at the surface, and in the atmosphere. The in-atmosphere net fluxes are decomposed further into vertical profiles of radiative heating and cooling rates. The results show that the changes in the top-of-atmosphere (TOA) and surface (SRF) radiative fluxes vary among the different schemes, depending on the part of the atmosphere–surface system and spectral band (SW and LW) considered, but that the magnitudes of the changes generally are small. The scheme without a total optical thickness constraint produces opposite-signed changes in fluxes (except for the SRF LW flux) and the profile of atmospheric radiative heating rate in comparison with the schemes with the constraint. The constraint on total optical thickness eliminates nearly all of the effects on the total TOA and SRF radiation budget, significantly reducing the frequency of layer overlap occurrence and thereby reducing the effect of overlap on the radiative heating rate profiles. Even when the assumptions are changed to produce a frequency of occurrence of multilayer clouds that is similar to other estimates, the resulting changes in the radiative heating rate profile are quantitatively small. The magnitude of these changes is similar to the magnitude of the total overall cloud effect, however, making the layer overlap critical to accurate determinations of the shape of the radiative heating rate profiles.