During the last decade modelling of cloud radiative transfer has revealed that the reflectivity of inhomogeneous cloud fields is smaller than the reflectivity of equivalent plane parallel cloud fields. Progresses in 3D radiative transfer modelling have significantly improved estimates of the Earth's radiative energy balance in global circulation models (GCM) and meteorological models. However, even with the most accurate radiative transfer calculation methods, such as Monte Carlo simulation models, an absolute agreement between observed and modelled cloud reflectivity is hardly obtained. One of the main reasons therefore is the difficulty of describing real cloud fields in radiative transfer models. I will present a modified description of the cloud optical depth for boundary-layer cloud fields that was derived from cloud thermodynamic properties and cloud observations of microphysical and macrophysical properties (ASTEX, BICEP). Results show that clouds obtained with the modified description reproduce realistic cloud physical properties for well-mixed boundary-layer cloud fields. Using the modified description for e.g. stratocumulus above sea, modelled cloud reflectivities agree well with observed cloud reflectivities.