Efficient Calculation of Infrared Fluxes and Cooling Rates Using the Two-Stream Equations

J. M. Edwards Hadley Centre for Climate Prediction and Research, U.K. Meteorological Office, Bracknell, Berkshire, United Kingdom

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Abstract

The calculation of infrared radiative fluxes and cooling rates using the two-stream equations is discussed. It is argued that at infrared wavelengths the two-stream equations are best viewed as an approximation to the differential radiance, the difference between the actual radiance and the local isotropic Planckian radiance. In this formulation the diffusivity factor and the forward- and backward-scattering fractions are related to the differential radiance. By considering the corresponding differential fluxes, approximate methods for representing overlapping gaseous absorption are derived. These methods offer substantial savings in computing time compared to an explicit representation of the overlap.

Abstract

The calculation of infrared radiative fluxes and cooling rates using the two-stream equations is discussed. It is argued that at infrared wavelengths the two-stream equations are best viewed as an approximation to the differential radiance, the difference between the actual radiance and the local isotropic Planckian radiance. In this formulation the diffusivity factor and the forward- and backward-scattering fractions are related to the differential radiance. By considering the corresponding differential fluxes, approximate methods for representing overlapping gaseous absorption are derived. These methods offer substantial savings in computing time compared to an explicit representation of the overlap.

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