Differing regional responses to a perturbation in solar cloud absorption in the SKYHI general circulation model

Abstract

In this study we perform an idealized experiment to investigate the effect of solar absorption in clouds on climate using a general circulation model with prescribed sea surface temperatures, focusing on the manner of regional changes during the northern summer season. The response arising from this type of perturbation is akin to "semidirect" effects of absorbing aerosols, namely, dissipation of clouds owing to a high aerosol absorption in the cloud layers. In the experiment, we apply a similar perturbation to all low-cloud layers, reducing their single-scattering albedo to a value of 0.99, which enables us to isolate the effect of such solar absorption from other aerosol related influences. We find that in both midlatitude and equatorial regions, the reduction in low-cloud single-scattering albedo causes a reduction in low-cloud amount and a warming of the surface. However, the dynamical response of the system varies from one continental region to another. In the midlatitude regions of the United States and Europe/east Asia, the diabatic heating perturbation leads to the dissipation of low clouds, an increase in shortwave flux to the surface, an increase in horizontal heat advection, and an increase in atmospheric stability. In the tropical region of North Africa, the diabatic heating perturbation translates into an increase in convection, a decrease in stability, an increase in middle- and high-level clouds, and a reduction in shortwave flux to the surface. In agreement with previous studies, these results demonstrate the distinctive response of the tropical versus midlatitude regions to a similar solar perturbation. Copyright 2006 by the American Geophysical Union.