On the shortwave radiative properties of stratiform water clouds



The development of a comprehensive radiation scheme for computing shortwave fluxes and heating rates in a cloudy stratified atmosphere is described. The scheme uses the Delta-Eddington multiple scattering method and treats Rayleigh scattering, absorption by water vapour and ozone, and scattering and absorption by the cloud drops. The molecular absorption data were obtained from the Lowtran 3B subroutine and incorporated using the exponential sum fitting of transmissions technique. Five versions of the scheme were used to examine the effect of spectral resolution on the results and comparisons are shown with previous work. The short-wave properties of clouds were examined using 24 spectral bands between 0.25 and 4μm. A significant dependence of the properties on the drop size distribution was found, in agreement with previous work. A simple parametrization of the single scattering properties of the cloud drops in terms of liquid water content and equivalent radius was examined and shown to produce satisfactory results, thus circumventing the need for time-consuming Mie theory computations. This allows the scheme to be used in comparisons with aircraft measurements of layer clouds, within which both liquid water content and drop size distribution are functions of height.