Interactions between Ice-Albedo, Lapse-Rate and Cloud-Top Feedbacks: An Analysis of the Nonlinear Response of a GCM Climate Model

Abstract

This paper examines the interactions between ice-albedo, lapse-rate and cloud-top feedbacks with the aid of GCM climate experiments published by Wetherald and Manabe (1975). First we establish that the long-wave modification effect (the so-called “greenhouse effect”) of clouds depends largely on the temperature difference T_sc between surface and cloud tops. If T_sc changes with a change in the surface temperature T_s, then the longwave modification effect of clouds would change which would result in a modification of the initial change in T_s. This feedback between the longwave modification effect of clouds and T_s, is referred to as the cloud top feedback in this paper. The sign of this feedback is considered positive (negative) when it amplifies (decreases) an initial change in T_s and it is shown that the sign is determined by the sign of dT_sc/dT_s. In the GCM climate experiments of Wetherald and Manabe (1975), dT_sc/dT_s < 0 at low latitudes and dT_sc/dT_s > 0 at high latitudes; consequently, cloud tops exert a positive feedback at high latitudes and a negative feedback at low latitudes. We then demonstrate that the interaction between cloud-top, ice-albedo and lapse-rate feedbacks induces a nonlinear response of surface temperature changes in a GCM climate model. The significance of the H₂O e-type absorption to the surface energy budget of the tropics is stressed. The results suggest that the e-type absorption will be important in determining the sensitivity of the hydrological cycle to changes in surface temperature.