A SIMPLE METHOD OF INCLUDING LONGWAVE RADIATION IN A TROPOSPHERIC NUMERICAL PREDICTION MODEL

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  • 1 Naval Postgraduate School, Monterey, Calif. 2
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Abstract

A simple method of computing longwave radiative cooling in the troposphere associated with water vapor is described. The procedure may readily be incorporated into a tropospheric numerical prediction model. Radiation from ozone and carbon dioxide is not considered. However, influences of arbitrary vertical distributions of cloud and moisture are included.

Average annual cooling rates along a meridional cross section are calculated for a cloudless atmosphere. The results agree fairly well with the total radiative cooling (longwave and shortwave) as given by Manabe and Möller except in the lower troposphere at low latitudes. Here shortwave absorption by water vapor is appreciable.

The three-dimensional distribution of longwave radiative cooling is also computed in a case of a developing cyclone for comparison with that of release of latent heat. The largest cooling occurs at cloud top and can be a significant fraction of the amount of energy released as latent heat in the upper troposphere. Computations also show that in this case study the longwave cooling tends to reduce the available potential energy, especially in the upper troposphere.

Synoptic-scale precipitation amounts resulting from destabilization of clouds by longwave cooling are computed. These range up to 1.4 mm in 12 hr. This destabilizing effect may be important in explaining the nocturnal maximum of precipitation over the sea. It may also contribute significantly to cyclone development.

Abstract

A simple method of computing longwave radiative cooling in the troposphere associated with water vapor is described. The procedure may readily be incorporated into a tropospheric numerical prediction model. Radiation from ozone and carbon dioxide is not considered. However, influences of arbitrary vertical distributions of cloud and moisture are included.

Average annual cooling rates along a meridional cross section are calculated for a cloudless atmosphere. The results agree fairly well with the total radiative cooling (longwave and shortwave) as given by Manabe and Möller except in the lower troposphere at low latitudes. Here shortwave absorption by water vapor is appreciable.

The three-dimensional distribution of longwave radiative cooling is also computed in a case of a developing cyclone for comparison with that of release of latent heat. The largest cooling occurs at cloud top and can be a significant fraction of the amount of energy released as latent heat in the upper troposphere. Computations also show that in this case study the longwave cooling tends to reduce the available potential energy, especially in the upper troposphere.

Synoptic-scale precipitation amounts resulting from destabilization of clouds by longwave cooling are computed. These range up to 1.4 mm in 12 hr. This destabilizing effect may be important in explaining the nocturnal maximum of precipitation over the sea. It may also contribute significantly to cyclone development.

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