Radiative transfer in water and ice clouds in the visible and infrared region

The radiance and polarization are calculated at six wavelengths from 0.7 micro to 6.05 micro for the photons reflected from and transmitted through two model clouds representing water and ice clouds with modal radii of 12 micro and 50 micro, respectively. The single scattering matrix is obtained from the Mie theory for spherical particles from the measured values of the complex index of refraction for water and ice. Multiple scattering to all orders is taken into account by a Monte Carlo technique which computes the exact three-dimensional paths of the photons. The upward and downward radiance and polarization are given as a function of optical thickness for each cloud model. The mean optical path of the photon, the cloud albedo, and the flux at the lower and upper boundaries are also given. The reflected radiance is considerably less for the ice than for the nimbostatus (water) model at all angles at most infrared wavelengths, particularly at wavelengths of 1.7 micro, 2.1 micro, and 3.5 micro. The polarization of the reflected photons is often very much greater for the ice than for the nimbostratus model, particularly at wavelengths of 1.7 micro, 2.1 micro, and 3.5 micro. These differences may be used in order to discriminate between water and ice clouds from measurements of the reflected radiation.