As a cloud moves through environmental air, cloudy and cloud free air mix at the cloud edges in the entrainment process. Entrainment is a key cloud process central to understanding cloud morphology and microphysical processes such as precipitation formation. After decades of research, no reliable formulation exists that allows to describe and understand entrainment in terms of cloud- and environmental physical quantities ("the entrainment puzzle"). Differences in representation of entrainment in climate models are a main cause for spread in climate sensitivity estimates. The realistic treatment of entrainment in climate models will help to reduce a large part of the uncertainty in predictions of climate sensitivity. This project aims at Solving The Entrainment Puzzle (STEP): - By identification of the main physical parameters that govern the amount of air entrained into and detrained from clouds using controlled laboratory measurements. - By translating the experimental results into a new mathematical formulation to use in Large Eddy Simulations of clouds using computational fluid dynamics modelling. In a unique approach, STEP will combine observations from a novel turbulent cloud chamber, computational fluid dynamics, and Large Eddy Simulations to develop a new reliable entrainment calculation scheme. Solving the entrainment puzzle will allow quantifying the entrainment processes reliably and thus deliver the basis for climate model improvements. The STEP project builds on my expertise in cloud microphysics and dynamics gained at various international institutes and will be accomplished at a leading international research institution. The host will provide me with excellent complementary training and career development opportunities. Through various dissemination and communication activities, STEP will contribute to the visibility and excellence of the European Science Area, addressing one of the Europe 2020 strategy main targets "Climate Change and Energy".