Radiative Properties of Finite Inhomogeneous Cirrus Clouds: Monte Carlo Simulations

Abstract

The Monte Carlo method is used to study the impact of various cloud morphologies (roughness, voids, waves, and horizontal spreading) on radiative properties of finite, thin, model cirrus clouds. The cloud-top reflectance is calculated for various cloud-top structures and is compared to reflectance of finite homogeneous cloud of the same ice crystal size and ice water content. Cloud roughness, voids, and waves generally decrease cloud reflectance as well as cloud absorption. Although the local horizontal variations in the reflectance can be quite large (several hundred percent), variation in the total reflectance, integrated over the top surface of a cloud, is in the range of a few percent. For overhead incident radiation, the decrease in cloud reflectance due to the considered morphological changes remains under 5%, as compared to a finite homogeneous cloud. A comparable reduction in cloud reflectivity is achieved by about a 5% increase of the effective size of ice crystals. The reflectivity of an optically thin cloud is primarily determined by the total ice content and the effective ice crystal size. The morphological structure of a cloud plays a secondary role.