Specular scattering in cirrus clouds

Abstract

The radiative properties of cirrus clouds are of wide interest because of the visually striking effects they produce, the significant global coverage of cirrus and thin cirrus, and their intrinsic complexity. Within a limited class of crystal shape and size, hydrodynamic forces tend to orient the crystals with their long axis nearly horizontal, which can produce a narrow, intense specular reflection. First-principles scattering calculations are especially difficult because the scatterers are diverse in shape, size and tilt distributions. However, a geometric optics approximation to the single-crystal BRDF combined with plausible distributions of flutter-angle can be used to both simulate the appearance of the specular radiance feature, and extract microphysical cloud information from imagery of the specular point. This paper will review an empirical approach to radiative-transfer in the specular layer and integrate two different models for the specular BRDF into a cloud radiance simulation code. This will then be used to illustrate the appearance of the subsun and Bottlinger's ring for various spectral bands. The extraction of microphysical information on the specular layer from vis - near-ir cloud imagery will then be discussed.