A Solar Radiation Model for Use in Climate Studies

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  • 1 Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, Maryland
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Abstract

A solar radiation routine has been developed for use in climate studies. It includes the absorption and scattering due to ozone, water vapor, oxygen, carbon dioxide, clouds, and aerosols. Rayleigh scattering is also included. The UV and visible region (λ < 0.69 μm) is grouped into four bands. An effective coefficient for ozone absorption and an effective cross section for Rayleigh scattering are computed for each band. In the near-infrared region (λ > 0.69 μm), the broadband parameterization is used to compute the absorption by water vapor in a clear atmosphere, and the k-distribution method is applied to compute fluxes in a scattering atmosphere. The reflectivity and transmissivity of a scattering layer are computed analytically using the delta-four-stream discrete-ordinate approximation. The two-stream adding method is then applied to compute fluxes for a composite of clear and scattering layers. Compared to the results of high spectral resolution and detailed multiple-scattering calculations, fluxes and heating rate are accurately computed to within a few percent.

The high accuracy of flux and heating rate calculations is achieved with a reasonable amount of computing time. With the UV and visible region grouped into four bands, this solar radiation routine is useful not only for climate studies but also for studies on the photolysis in the upper atmosphere and the photosynthesis in the biosphere.

Abstract

A solar radiation routine has been developed for use in climate studies. It includes the absorption and scattering due to ozone, water vapor, oxygen, carbon dioxide, clouds, and aerosols. Rayleigh scattering is also included. The UV and visible region (λ < 0.69 μm) is grouped into four bands. An effective coefficient for ozone absorption and an effective cross section for Rayleigh scattering are computed for each band. In the near-infrared region (λ > 0.69 μm), the broadband parameterization is used to compute the absorption by water vapor in a clear atmosphere, and the k-distribution method is applied to compute fluxes in a scattering atmosphere. The reflectivity and transmissivity of a scattering layer are computed analytically using the delta-four-stream discrete-ordinate approximation. The two-stream adding method is then applied to compute fluxes for a composite of clear and scattering layers. Compared to the results of high spectral resolution and detailed multiple-scattering calculations, fluxes and heating rate are accurately computed to within a few percent.

The high accuracy of flux and heating rate calculations is achieved with a reasonable amount of computing time. With the UV and visible region grouped into four bands, this solar radiation routine is useful not only for climate studies but also for studies on the photolysis in the upper atmosphere and the photosynthesis in the biosphere.

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