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

V. Ramanathan National Center for Atmospheric Research, Boulder, Colo. 80307

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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 Tsc between surface and cloud tops. If Tsc changes with a change in the surface temperature Ts, then the longwave modification effect of clouds would change which would result in a modification of the initial change in Ts. This feedback between the longwave modification effect of clouds and Ts, 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 Ts and it is shown that the sign is determined by the sign of dTsc/dTs.

In the GCM climate experiments of Wetherald and Manabe (1975), dTsc/dTs < 0 at low latitudes and dTsc/dTs > 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.

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 Tsc between surface and cloud tops. If Tsc changes with a change in the surface temperature Ts, then the longwave modification effect of clouds would change which would result in a modification of the initial change in Ts. This feedback between the longwave modification effect of clouds and Ts, 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 Ts and it is shown that the sign is determined by the sign of dTsc/dTs.

In the GCM climate experiments of Wetherald and Manabe (1975), dTsc/dTs < 0 at low latitudes and dTsc/dTs > 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.

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