DETERMINATION OF LONGWAVE ANISOTROPIC EMISSION FACTORS FROM COMBINED BROAD-BAND AND NARROW-BAND RADIANCE MEASUREMENTS

The conversion of measured radiances into radiative fluxes requires ap plication of angular corrections: in the Earth Radiation Budget Experi ment (ERBE), the longwave anisotropic emission factors (AEFs) were tab ulated for different viewing zenith angles, seasons, latitude bands, a nd scene types, including four different cloud-cover classes. An alter native approach is investigated using simultaneous infrared atmospheri c window (10.5-12.5 mum) and broadband longwave (LW) measurements. Suc h measurements will be available from the ScaRaB (Scanner for Radiatio n Balance) instrument whose launch is planned to occur in 1993. Using a radiative transfer model to simulate the combined measurements, the AEF is parametrized as a function of viewing zenith angle and a single other variable-atmospheric pseudoabsorptance-defined as the normalize d difference between the broadband LW radiance and the integrated Plan ck emission at the 11.5-mum brightness temperature. For validation of the parameterization with existing satellite data, simultaneous colloc ated NOAA-9 ERBE Advanced Very High Resolution Radiometer data were us ed for broad- and narrowband radiances. The comparison between fluxes corrected with the parameterized AEF and those corrected with the ERBE AEF shows that the parameterization provides more realistic AEFs as a function of scene brightness temperature, which is related to cloud-t op 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 a nisotropy of semitransparent cirrus was also considered. Model and dat a show that although not explicitly treated in this procedure, the par ameterization gives good results. This parameterization may also be ad apted for somewhat different wavelength bands as in the NASA CERES (Cl ouds and the Earth's Radiant Energy System) project.