Abstract
Cloud-radiation interactions are crucial to climate modeling and will become increasingly important in numerical weather prediction as forecast range increases and as models are developed to predict secondary variables, such as surface temperature. This study explores the role of cirrus cloud optical thickness feedbacks in contributing to climate changes due to increasing atmospheric concentrations of carbon dioxide and other trace gases. For low and middle clouds, which are approximately black bodies for infrared radiative transfer, an increase in cloud optical thickness serves primarily to increase the cloud albedo. Thus, if a climate warming is accompanied by an increase in average atmospheric absolute humidity and hence in average cloud liquid water content, low and middle cloud albedo may increase and serve as a negative feedback which tends to reduce the surface temperature increase. Cirrus clouds, however, are not generally black in the infrared, and so an increase in cloud optical thickness may increase the cirrus greenhouse effect as well as the cirrus albedo. We test this hypothesis using a radiative-convective equilibrium model. Our major result is that cirrus cloud optical thickness feedbacks may indeed tend to increase the surface warming due to trace gas increases. However, this positive feedback from cirrus appears to be generally weaker than the negative feedbacks due to lower clouds. Our results thus confirm those of earlier research indicating that the net effect of cloud optical thickness feedbacks may be a negative feedback which might reduce by a factor of about two the surface warming expected from doubling atmospheric concentrations of carbon dioxide. All of these results are obtained under severely idealized assumptions, however, and should be regarded as tentative. Much further research will be required to determine the effect of cloud optical thickness feedbacks definitively.
