Remote sensing of cirrus radiative parameters during EUCREX'94. case study of 17 April 1994. Part II: Microphysical models

Abstract

In this paper, a quantitative analysis of in situ and radiative measurements concerning cirrus clouds is presented. These measurements were performed during the European Cloud and Radiative Experiment 1994 (EUCREX'94) as discussed in an earlier paper (Part I). The analyses are expressed in terms of cirrus microphysics structure. The complex microphysical structure of cirrus cloud is approximated by simple hexagonal monocrystalline particles (columns and plates) and by polycrystalline particles (randomized triadic Koch fractals of second generation) both arbitrarily oriented in space (3D). The authors have also considered hexagonal plates randomly oriented in horizontal planes with a tilted angle of 15°(2D). Radiative properties of cirrus cloud are analyzed, assuming that the cloud is composed of 3D ice crystals, by way of an adding-doubling code. For the hypothesis of 2D ice crystals, a modified successive order of scattering code has been used. The first order of scattering is calculated exactly using the scattering phase function of 2D crystals; for the higher orders, it is assumed that the same particles are 3D oriented. To explain the whole dataset, the most appropriate microphysics, in terms of radiative properties of cirrus clouds, is that of the 2D hexagonal plates whose aspect ratio (length divided by diameter) is 0.05.