On the Role of Cloud Amount in an Energy Balance Model of the Earth's Climate

H. M. Van Den Dool Royal Netherlands Meteorological Institute, De Bilt, The Netherlands

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Abstract

The influence of cloud amount on the earth's climate is studied with an energy balance climate model. Planetary albedo and infrared radiation are parameterized in terms of cloud amount and surface temperature. For the present climate a prescribed change in cloud amount (independent of latitude) leads to a negligible change in the global mean temperature (∂T̄/∂Ac≈0). For global temperatures lower than present ∂T̄/∂Ac, becomes positive rapidly; higher temperatures lead to. negative values of ∂T̄/∂Ac. The sensitivity of the global mean temperature to a 1% change in the solar constant (∂T̄/∂S) is ∼ 1.5 K. With reduced cloud amount ∂T̄/∂S becomes larger because the snow-ice feedback is active in the larger cloud-free portion; with increased cloud amount ∂T̄/∂S becomes smaller. Due to the strong absorption of solar radiation by clouds deep freeze solutions are possible only for very low values of the solar constant. The response of the model to changes in cloud amount or incoming radiation should be studied as a function of latitude. Two expressions that quantify the sensitivity to changes In the solar constant and cloud amount as a function of latitude are defined. If cloud amount is assumed to increase with temperature in a certain latitude belt and to decrease with temperature elsewhere, ∂T̄/∂S (as a function of latitude) and ∂T̄/∂S can change considerably.

Abstract

The influence of cloud amount on the earth's climate is studied with an energy balance climate model. Planetary albedo and infrared radiation are parameterized in terms of cloud amount and surface temperature. For the present climate a prescribed change in cloud amount (independent of latitude) leads to a negligible change in the global mean temperature (∂T̄/∂Ac≈0). For global temperatures lower than present ∂T̄/∂Ac, becomes positive rapidly; higher temperatures lead to. negative values of ∂T̄/∂Ac. The sensitivity of the global mean temperature to a 1% change in the solar constant (∂T̄/∂S) is ∼ 1.5 K. With reduced cloud amount ∂T̄/∂S becomes larger because the snow-ice feedback is active in the larger cloud-free portion; with increased cloud amount ∂T̄/∂S becomes smaller. Due to the strong absorption of solar radiation by clouds deep freeze solutions are possible only for very low values of the solar constant. The response of the model to changes in cloud amount or incoming radiation should be studied as a function of latitude. Two expressions that quantify the sensitivity to changes In the solar constant and cloud amount as a function of latitude are defined. If cloud amount is assumed to increase with temperature in a certain latitude belt and to decrease with temperature elsewhere, ∂T̄/∂S (as a function of latitude) and ∂T̄/∂S can change considerably.

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