Short-wave optical properties of precipitating water clouds

Water cloud drop-size distributions for non-precipitating clouds are taken from various sources. The optical depth, single-scattering co-albedo and asymmetry parameter are calculated using Mie theory. By averaging over four spectral intervals in the short-wave regime, a linear parametrization for the co-albedo and the asymmetry parameter is made as a function of the cloud-layer effective drop radius, r(e), in each interval; r(e) can be related to the cloud-layer liquid-water content (LWC). The optical depth is parametrized quadratically on r(e)(-1). This gives an excellent fit. These parametrizations are useful for general-circulation model (GCM) and weather-forecast models, which produce estimates of cloud-layer LWC. Large-drop effects are studied by adding typical raindrop size distributions to the stratus- and cumulonimbus-type cloud-drop distributions. The r(e) of these precipitating clouds can be related to the rain intensity, R. It turns out that (1) the asymmetry parameter is insensitive to R, (2) the optical depth follows the quadratic parametrization on r(e)(-1) and (3) the co-albedo increases nearly linearly with r(e) (or R), so absorption of a cloud layer increases with R. A cloud layer with large drops can thus absorb more solar radiation than one without large drops. A simple four-band GCM-type parametrization of the large-drop effect is presented.