Advances in studies of cloud overlap and its radiative transfer issues in the climate models

The latest advances in studies of cloud overlap treatments and its radiative transfer issues in the global climate models are summarized. The developments in this international challenging problem are described from aspects such as the formation of the cloud overlap model, the realization of the cloud overlap models in climate models, and the data and methods used to obtain observational consistent cloud overlap structure and the radiative transfer in overlapped clouds. To date, there have been an appreciable amount of achievements in studies of cloud overlap in climate models, as demonstrated by: scientific model (e.g. e-folding overlap) has been developed to describe cloud overlap; fast radiative transfer method for overlapped clouds (Monte Carlo Independent Column Approximation, i.e. McICA) has been invented and wildly used; as well as continuous 3D cloud satellite observation (e.g. CloudSat/CALIPSO) and cloud-resolving models provide plenty of data valuable for the exact description of cloud overlap structure in climate models. However, the present treatments of cloud overlap and its radiative transfer method are far from complete, and there remain many unsettled problems needed to be explored in the future.