Abstract
The conversion of measured broad-band radiances into radiative fluxes requires application of angular corrections: In the Earth Radiation Budget Experiment (ERBE), the longwave anisotropic emission factors (AEF) were tabulated for different viewing zenith angles, seasons, latitude bands and scence types including four different cloud cover classes. Investigating an alternative approach for the ScaRaB (Scanner for Radiation Balance) instrument, we use a radiative transfer model to parametrize the AEF as a function of viewing zenith angle and a single other variable - ‘atmospheric pseudo-absorptance’ - extracted from simultaneously measured broad-band longwave (LW) and infrared (IR) atmospheric window (10.5–12.5μm) radiances. For validation, we use simultaneous co-located NOAA-9 AVHRR-ERBE data. Compared to the ERBE AEF's the ScaRaB parametrization provides more realistic AEF's 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 our parametrization. We also consider enhanced anisotropy of semitransparent cirrus. Model and data show that although not explicitly treated in our procedure, the parametrization gives good results.
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