Parameterization schemes for terrestrial water clouds in the radiative transfer model GOMETRAN

Quasi-exact and approximate methods for the parameterization of clouds inside finite-differencing radiative transfer codes are introduced and exemplified for the radiative transfer model GOMETRAN. The quasi-exact parameterization scheme considers the cloud as a plane-parallel, homogeneous layer of finite vertical extent, while the approximative scheme, which holds for optically thick clouds, treats the cloud top as a non-Lambertian reflecting surface that bounds the atmosphere from below. GOMETRAN simulations, using both cloud models, are compared for various cloud scenarios with and without atmospheric absorption. Comparison of results from the quasi-exact parameterization with DISORT simulations yields differences of generally less than 5% for all cloud optical depths and all viewing geometries. Using this model, photon path lengths and light path enhancements inside cloud layers as well as the applicability of the developed methods to the interpretation of satellite measurements have been investigated.