CloudCT: Spaceborne scattering tomography by a large formation of small satellites for improving climate predictions

The response of small convective clouds to changes of environmental conditions is a major source of uncertainty in climate predictions. Current satellites yield retrievals of cloud-tops via coarse 2D data, and cloud microphysics based on a 1D radiative model. This is incompatible with the 3D heterogeneous nature of small clouds. Observing and quantifying the microphysics of small clouds require a tomographic setup and a volumetric retrieval model based on 3D radiative transfer. Observations should be both multi-angular and simultaneous, because these clouds have a few minutes lifetime, and their morphology changes during advection. This joint requirement is inconsistent with common observational satellites, which are bulky, expensive and have a solitude viewpoint at any time. To meet the needs, we report on a new spaceborne mission, `CloudCT` funded by the European Research Council (ERC). The enabling technology for a tomography setup is state-of-the-art nanosatellites. CloudCT will comprise a formation of 10 nanosatellites tailored for the mission. Carrying multi-band cameras, they will coordinate to image cloud fields simultaneously from multiple directions. The satellite formation will autonomously self-organize its topology for best data acquisition and use advanced pointing to aim at the same clouds. The formation is planned to launch and start data collection in 2022. The data will be used for 3D scattering tomography, which we devise. We present initial simulations of cloud tomography, based on initial specifications of an orbit, formation, platforms and cameras.