The radiative properties of inhomogeneous boundary layer cloud: Observations and modelling

The importance of internal inhomogeneities in the microphysical structure of stratocumulus in relation to the radiative properties of various cloud fields is studied using aircraft measurements and a 3-D Monte Carlo model. This study is unique in that in situ microphysical data and observations of cloud thickness have been used in the initialization of a Monte Carlo model, and the resulting model predictions of reflectance and albedo have been compared with in situ aircraft radiometric observations of the same cloud. The model predicts a reduction in reflectance and albedo for the 3-D cloud field compared to a homogeneous cloud layer with the same average microphysical properties. The in situ observations confirm these simulations. The Monte Carlo model predicts that internal inhomogeneities lead to a reduction in cloud absorptance and an increase in cloud transmittance. A further study of the importance of the distribution of clouds within a domain was carried out using a Monte Carlo model where the cloud fraction is kept constant while the separation of the cloud elements is varied. These results reveal a large sensitivity of the radiative properties to the distribution of the cloud, and that this sensitivity increases as the separation of the cloud elements decreases.