Parameterizations of reflectance and effective emittance for satellite remote sensing of cloud properties

The interpretation of satellite-observed radiances to derive cloud optical depth and effective particle size requires radiative transfer calculations relating these parameters to the reflectance, transmittance, and emittance of the cloud. Such computations can be extremely time consuming when used in an operational mode to analyze routine satellite data. Adding–doubling (AD) radiative transfer models are used here to compute reflectance and effective emittance at wavelengths commonly used by operational meteorological satellite imagers for droplet effective radii ranging from 2 to 32 mm and for distributions of randomly oriented hexagonal ice crystals with effective diameters varying from 6 to 135 mm. Cloud reflectance lookup tables were generated at the typical visible-channel wavelength of 0.65 mm and the solar–infrared (SI) at wavelengths of 3.75 and 3.90 mm. A combination of four-point Lagrangian and linear interpolation between the model nodal points is the most accurate and economical method for estimating reflectance as a function of particle size for any set of solar zenith, viewing zenith, and relative azimuth angles. Compared to exact AD calculations, the four-point method retrieves the reflectance to within 63%–9 % for water droplets and ice crystals, respectively. Most of the error is confined to scattering angles near distinct features in the phase functions. The errors are reduced to;62 % for ice when the assessment is constrained to only those angles that are actually useful in satellite retrievals. Effectiv