Determination of Longwave Anisotropic Emission Factors from Combined Broad-and Narrowband Radiance Measurements

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  • 1 Laboratoire de Météorologie Dynamique du CNRS, Ecole Polytechnique, Palaiseau, France
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Abstract

The conversion of measured radiances into radiative fluxes requires application of angular corrections; in the Earth Radiation Budget Experiment (ERBE), the longwave anisotropic emission factors (AEFS) were tabulated for different viewing zenith angles, seasons, latitude bands, and scene types, including four differment cloud-cover classes. An alternative approach is investigated using simultaneous infrared atmospheric window (10.5-12.5 µm) and broadband longwave (LW) measurements. Such measurements will be available from the ScaRaB (Scanner for Radiation Balance) instrument whose launch is planned to occur in 1993.

Using a radiative transfer model to simulate the combined measurements, the AEF is parameterized as a function of viewing zenith angle and a single other variable—atmospheric pseudoabsorptance—defined as the normalized difference between the broadband LW radiance and the integrated Planck emission at the 11.5-µm brightness temperature. For validation of the parameterization with existing satellite data, simultaneous collocated NOAA-9 ERBE Advanced Very High Resolution Radiometer data were used for broad- and narrowband radiances. The comparison between fluxes corrected with the parameterized AEF and those corrected with the ERRE AEF shows that the parameterization provides more realistic AEFs as a function of scene brightness temperature, which is related to cloud-top height. Analysis of classified cloud data indicates that there are only a few extreme cases in which additional anisotropy due to broken clouds will affect the usefulness of this parameterization. Enhanced anisotropy of semitransparant cirrus was also considered. Model and data show that although not explicitly treated in this procedure, the parameterization gives good results. This parameterization may also be adapted for somewhat different wavelength bands as in the NASA CERES (Clouds and the Earth's Radiant Energy System) project.

Abstract

The conversion of measured radiances into radiative fluxes requires application of angular corrections; in the Earth Radiation Budget Experiment (ERBE), the longwave anisotropic emission factors (AEFS) were tabulated for different viewing zenith angles, seasons, latitude bands, and scene types, including four differment cloud-cover classes. An alternative approach is investigated using simultaneous infrared atmospheric window (10.5-12.5 µm) and broadband longwave (LW) measurements. Such measurements will be available from the ScaRaB (Scanner for Radiation Balance) instrument whose launch is planned to occur in 1993.

Using a radiative transfer model to simulate the combined measurements, the AEF is parameterized as a function of viewing zenith angle and a single other variable—atmospheric pseudoabsorptance—defined as the normalized difference between the broadband LW radiance and the integrated Planck emission at the 11.5-µm brightness temperature. For validation of the parameterization with existing satellite data, simultaneous collocated NOAA-9 ERBE Advanced Very High Resolution Radiometer data were used for broad- and narrowband radiances. The comparison between fluxes corrected with the parameterized AEF and those corrected with the ERRE AEF shows that the parameterization provides more realistic AEFs as a function of scene brightness temperature, which is related to cloud-top height. Analysis of classified cloud data indicates that there are only a few extreme cases in which additional anisotropy due to broken clouds will affect the usefulness of this parameterization. Enhanced anisotropy of semitransparant cirrus was also considered. Model and data show that although not explicitly treated in this procedure, the parameterization gives good results. This parameterization may also be adapted for somewhat different wavelength bands as in the NASA CERES (Clouds and the Earth's Radiant Energy System) project.

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