Cloud 3D Structure and Radiation (C3SAR) - A new initiative to account for 3d effects in climate research

Starting end of 2024, the cooperative German research unit C3SAR aims at improving our understanding the role of clouds in the climate system by investigating the impact of their 3dimensionality. This shall be achieved by a unique and comprehensive combination of 3D-modelling and observation of clouds and their radiative effects. The initiative is working towards a physically-based correction of biases in climate modelling and cloud remote sensing, that have been a result from oversimplifications of the complex geometrical and microphysical nature of clouds in previous work. Along this goal, characteristic 3D cloud structures for relevant cloud regimes from cloud resolving modelling and from synergistic satellite observations will serve as input to radiative transfer modelling of different complexity to quantify the consequences for cloud structure simplifications and to establish physically based cloud-radiation correlations. Long-term high-quality ground-based cloud and radiation observations over Germany in combination with novel techniques for the simultaneous measurement of spectral radiance provide the validation of these relations by means of radiative closure studies. Current and new generations of satellite sensors will provide the corresponding closure at the top of the atmosphere. A large field campaign in summer 2026 in Lindenberg, Germany (C3SAR-X), will bring modelling, ground-based and satellite-based remote sensing and in-situ radiation measurements together in a synergetic closure study in order to validate our ability to observe, understand and model the cloud radiative effects and thus to reduce a major source of uncertainty in predicting the future climate. For the second phase of the research unit, it is planned to extend this approach to more globally distributed test-beds in the framework of large international observational networks, to improve the regimebased cloud-radiation relations and apply those to climate modelling and the new generations of satellite sensors. This article presents the joint strategy of the research unit and invites interested working groups worldwide to a joint seminar series and to participate in the C3SAR-X field campaign and its interpretation as well as in follow-up campaigns. Specifically, measurements are planned for the second phase in the tropical region at the Cape Verde Atmospheric Observatory CVAO and on the German research vessel Polarstern II in the polar regions and the North and South Atlantic Ocean.