INFERENCE OF CIRRUS CLOUD PROPERTIES USING SATELLITE-OBSERVED VISIBLEAND INFRARED RADIANCES .1. PARAMETERIZATION OF RADIANCE FIELDS

Current techniques for deriving cirrus optical depth and altitude from visible (0.65 mum) and infrared (11.5 mum) satellite data use radiati ve transfer calculations based on scattering phase functions of spheri cal water droplets. This study examines the impact of using phase func tions for spherical droplets and hexagonal ice crystals to analyze rad iances from cirrus. Adding-doubling radiative transfer calculations ar e used to compute radiances for different cloud thicknesses and height s over various backgrounds. These radiances are used to develop parame terizations of top-of-the-atmosphere visible reflectance and infrared emittance utilizing tables of reflectance as a function of cloud optic al depth, viewing and illumination angles, and microphysics. This para meterization, which includes Rayleigh scattering, ozone absorption, va riable cloud height, and an anisotropic surface reflectance, reproduce s the computed top-of-the-atmosphere reflectances with an accuracy of +/-6% for four microphysical models: 10-mum water droplet, small symme tric crystal, cirrostratus, and cirrus uncinus. The accuracy is twice that of previous models. Bidirectional reflectance patterns from theor etical ice-crystal clouds are distinctly different from those of the t heoretical water-droplet clouds. In general, the ice-crystal phase fun ctions produce significantly larger reflectances than the water-drople t phase function for a given optical depth. A parameterization relatin g infrared emittance to visible optical depth is also developed. The e ffective infrared emittances computed with the adding-doubling method are reproduced with a precision of +/-2%. Infrared scattering reduces emittance by an average of 5%. Simulated cloud retrievals using the pa rameterization indicate that optical depths and cloud temperatures can be determined with an accuracy of approximately 25% and approximately 6 K for typical cirrus conditions. Retrievals of colder clouds over b righter surfaces are not as accurate, while those of warmer clouds ove r dark surfaces will be more reliable. Sensitivity analyses show that the use of the water-droplet phase function to interpret radiances fro m a theoretical cirrostratus cloud will significantly overestimate the optical depth and underestimate cloud height by 1.5-2.0 km for nomina l cirrus clouds (temperature of 240 K and visible optical depth of app roximately 1). The parameterization developed here is economical in te rms of computer memory and is useful for both simulation and interpret ation of cloud radiance fields.