RADIATIVE PROPERTIES OF FINITE INHOMOGENOUS CIRRUS CLOUDS - MONTE-CARLO SIMULATIONS

The Monte Carlo method is used to study the impact of various cloud mo rphologies (roughness, voids, waves, and horizontal spreading) on radi ative properties of finite, thin, model cirrus clouds. The cloud-top r eflectance is calculated for various cloud-top structures and is compa red to reflectance of finite homogeneous cloud of the same ice crystal size and ice water content. Cloud roughness, voids, and waves general ly decrease cloud reflectance as well as cloud absorption. Although th e local horizontal variations in the reflectance can be quite large (s everal hundred percent), variation in the total reflectance, integrate d over the top surface of a cloud, is in the range of a few percent. F or 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 reflect ivity is achieved by about a 5% increase of the effective size of ice crystals. The reflectivity of an optically thin cloud is primarily det ermined by the total ice content and the effective ice crystal size. T he morphological structure of a cloud plays a secondary role.