Model for an investigation of radiative transfer in cloudy atmosphere

Abstract

Parameterization of droplet size distributions using a composition of three gamma functions is suggested to simulate a wide spectrum of cloud droplets including drizzle and rain. It should be stressed out that this parameterization is based on extensive material (accumulated in the Russian Central Aerological Observatory) of aircraft observations in clouds and dependent on the cloud type, temperature and other factors. Effective computation technique using the geometric optics approach for the calculations of the parameters of electromagnetic scattering on spherical polydispersions and the database containing these parameters for a wide set of aerosol and cloud mediums are described. The parameterization, computation technique and the database are applied to create different optical atmospheric models with scattering properties rather close to reality for theoretical investigations of the atmospheric radiative transfer. Some examples of cloud radiation forcing studies are also considered in this paper. Their principal feature was the use of the original effective line-by-line and Monte-Carlo codes to take into account rigorously both the cloud/molecular scattering and a detailed spectral structure of water vapor, ozone, carbon dioxide and oxygen. It has been found: (a) variations of the mean droplet radius depending on the cloud temperature and other factors lead to essential changes in shortwave fluxes (tens of W/m 2), cloud radiation absorption (tens of W/m 2) and the Liquid Water Path (LWP) (by several times); (b) In continental clouds short wave fluxes mainly depend on small droplets. The existence of large (drizzle) drops perturbs slightly the fluxes and rather strongly affects the liquid water path and the cloud radiation absorption; (c) The solar radiation absorption and scattering by cloud media are generally defined only by the extinction coefficient and the effective radius and are slightly affected by other details of cloud size droplet distributions. These statements are in good agreement with other investigations. Thus, these parameterization and database should be good enough for the cloud optical properties simulation in the majority of practical applications. Moreover, the developed codes may be recommended for exact simulation of atmospheric radiative transfer in satellite and full-scale experiments, for testing and improving the radiation codes being used in climate models, etc.