Radiation Profiles in Extended Water Clouds. II: Parameterization Schemes

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  • 1 CSIRO Division of Atmospheric Physics, Aspendale, Victoria, Australia
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Abstract

The shortwave absorption, albedo and longwave emissivity of water clouds are parameterized for use in operational and climatic models of the atmosphere. The parameterization also provides the shortwave heating and longwave cooling rates within the cloud. The scheme presented in this paper assumes a prior knowledge of the broadband spectral fluxes incident on the cloud and further assumes that the atmospheric models will provide the surface albedo, solar zenith angle, cloud temperature and total vertical liquid water path. The last parameter was chosen because it likely to be available in atmospheric circulation models and both observational and theoretical evidence suggest that it is strongly related to the radiative properties of clouds (Paltridge, 1974; Platt, 1976).

The parameterization of shortwave radiation resembles a two-stream approximation which has been “tuned” to match the results from a detailed theoretical model. The longwave scheme simply involves the parameterization of effective emissivity. Both schemes have been tested and the errors investigated. The shortwave radiative properties of clouds when compared against calculations can generally be estimated by the parameterized scheme to within 5% of the incident flux at the cloud top. The longwave cooling rates are well within 0.5°C h−1 of the theoretical beating rate profiles. The errors in longwave cooling and shortwave absorption are much smaller than the uncertainties that may arise from variations of cloud liquid water distribution.

Abstract

The shortwave absorption, albedo and longwave emissivity of water clouds are parameterized for use in operational and climatic models of the atmosphere. The parameterization also provides the shortwave heating and longwave cooling rates within the cloud. The scheme presented in this paper assumes a prior knowledge of the broadband spectral fluxes incident on the cloud and further assumes that the atmospheric models will provide the surface albedo, solar zenith angle, cloud temperature and total vertical liquid water path. The last parameter was chosen because it likely to be available in atmospheric circulation models and both observational and theoretical evidence suggest that it is strongly related to the radiative properties of clouds (Paltridge, 1974; Platt, 1976).

The parameterization of shortwave radiation resembles a two-stream approximation which has been “tuned” to match the results from a detailed theoretical model. The longwave scheme simply involves the parameterization of effective emissivity. Both schemes have been tested and the errors investigated. The shortwave radiative properties of clouds when compared against calculations can generally be estimated by the parameterized scheme to within 5% of the incident flux at the cloud top. The longwave cooling rates are well within 0.5°C h−1 of the theoretical beating rate profiles. The errors in longwave cooling and shortwave absorption are much smaller than the uncertainties that may arise from variations of cloud liquid water distribution.

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