On the parameterization of the radiative properties of broken clouds

Abstract

A one-dimensional radiative transfer scheme is presented which accounts for the effects of broken cloud in the solar and infrared radiation field. The fractional cloud amount is explicitly considered in the two-stream model by treating clouds as the boundary condition between two adjacent atmospheric layers. The scheme accounts for absorption and scattering by gases and aerosols assuming realistic atmospheres. The radiative properties of broken clouds are included in a parameterized form making use of results from three-dimensional radiative transfer models. The radiative characteristics of a cloud field are represented by an average finite cloud whose size or optical thickness grows with increasing cloud amount. This growth is described by a simple mathematical model, and its use yields qualitative agreement between model results and observations for solar radiation.

The scheme is then applied to calculate the net radiative effect of broken cloud. Since the cloud size growth with cloud amount implies a non-linear relation between the fractional cloud amount and the radiative properties of the cloud field, the net radiative effect of cloud depends on cloud amount. The idealized model shows that the albedo effect (increase of solar reflection with cloud amount) of broken cloud is smaller than that of a plane-parallel cloud for cloud amounts less than about 0.7, while the opposite is true for larger cloud amounts. The greenhouse effect (reduction of the outgoing long-wave flux) of broken cloud is larger than that of a plane-parallel cloud for small cloud amount and smaller for large cloud amount.

An application of the radiation scheme to compute bispectral curves of visible albedo versus thermal brightness temperature shows that broken cloud layers and unbroken layers with variable optical depths show a similar shape of the bispectral curve.