Two‐ and four‐stream optical properties for water clouds and solar wavelengths

Abstract

Presented is a parameterization of the liquid water droplet optical properties for the solar spectrum. Two models are provided: a 4‐band model for use in general circulation models and a 31‐band model for use in higher spectral resolution investigations. The form of the parameterization is a simple extension of Slingo [1989], and the subdivision of wavelengths into bands is almost identical. The parameterization scheme is for all of the optical properties needed to perform 2‐ and 4‐stream calculations. The 4‐band model has the same spectral divisions as Slingo [1989], whereas the 31‐band model has finer resolution of the absorbing bands in the near infrared. The parameterized single‐scattering optical properties are accurate to within 0.5% compared to exact Mie calculations for both band models over the r eff range from 5 to 40 μm. The layer radiative properties, from both two‐ and four‐stream methods, are almost always within 1% accuracy compared to calculations using exact optical properties as inputs. Also, it is shown that using the Henyey‐Greenstein phase function to obtain the higher‐order moments, used in four‐stream calculations, results in substantial error in the layer radiative properties compared to using exact Mie inputs.