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

Abstract

Abstract 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.