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

In this paper, a quantitative analysis of in situ and radiative measurement s concerning cirrus clouds is presented. These measurements were performed during the European Cloud and Radiative Experiment 1994 (EUCREX'94) as disc ussed in an earlier paper (Part I). The analyses are expressed in terms of cirrus microphysics structure. The complex microphysical structure of cirru s cloud is approximated by simple hexagonal monocrystalline particles (colu mns and plates) and by polycrystalline particles (randomized triadic Koch f ractals of second generation) both arbitrarily oriented in space (3D). The authors have also considered hexagonal plates randomly oriented in horizont al planes with a lilted angle of 15 degrees (2D). Radiative properties of c irrus cloud are analyzed, assuming that the cloud is composed of 3D ice cry stals, by way of an adding-doubling code. For the hypothesis of 2D ice crys tals, a modified successive order of scattering code has been used. The fir st order of scattering is calculated exactly using the scattering phase fun ction of 2D crystals; for the higher orders, it is assumed that the same pa rticles are 3D oriented. To explain the whole dataset, the most appropriate microphysics, in terms of radiative properties of cirrus clouds, is that o f the 2D hexagonal plates whose aspect ratio (length divided by diameter) i s 0.05.