EUREC4A-UK: Elucidating the role of cloud-circulation coupling in climate

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EUREC4A-UK is a programme of observational and modelling research which aimsto study the detailed aerosol and cloud processes in the life cycle of shallow tradecumulus clouds and the two-way interactions between the cloud processes and thelarge-scale dynamics. The different responses of these clouds to warming in globalclimate models (GCM) explain most of the inter-model differences, yet the physicsof these responses remains poorly constrained. The programme is focussed onthe participation of UK scientists and the BAS Twin Otter aircraft in EUREC4A(Elucidating the Role of Clouds-Circulation Coupling in Climate). EUREC4A isa coordinated international campaign that aims to address the current lack of understanding ofthe processes controlling the response of trade-wind cumulus clouds to changing environmentalconditions in a warmer climate. The goal of EUREC4A is to examine the interplay between theclouds, atmospheric circulations and climate sensitivity. EUREC4A-UK will make a unique and self-contained contribution to the international programme by: (i) providing observational facilities whichare needed as part of the coordinated field campaign; (ii) conducting and leading the analysis ofthe aerosols, cloud microphysics and boundary-layer processes in the life cycle of shallow tradecumulus clouds; (iii) placing the analysis in the context of the EUREC4A problems by modelling thetwo-way interactions between the cloud processes and the large-scale dynamics; and (iv) applyingthe results by testing the new convection scheme in the UM and using the improved model to determinethe dominant processes controlling the cloud fields. International partners will complement the re-search with a focus on observing and modelling the macrophysical properties and the environmentof trade-cumulus clouds in order to determine: (i) what controls the convective mass flux, mesoscaleorganization and depth of shallow-cumulus clouds; (ii) how the trade-cumulus cloud fraction varieswith turbulence, convective mixing and large-scale circulations; and (iii) the impact this variation hason atmospheric radiation.The radiative properties of the trade-wind cumulus clouds that are ubiquitous over the tropical oceans have a major influence on the Earth's radiation budget. The response to global warming of these clouds is therefore critical for global mean cloud feedbacks. It is the differing response to warming that explains most of the spread of climate sensitivity in climate models. Hence, a better understanding is required of the mechanisms that control the low-level cloud fraction. The urgency of the research is made clear by the fact that the World Climate Research Programme endorses the EUREC4A field project which supports the Grand Challenge on Clouds, Circulation and Climate Sensitivity.There is a clear need for EUREC4A-UK because the aerosol,cloud and precipitation processes influence the macrophysical properties of the clouds in differentenvironments. For example, the vertical distribution of raincan affect the concentration and size of cloud drops in the upper detrainment layers, which influ-ences cloud radiative properties. The intensity of rain and evaporation of raindrops influences thestrength of gust fronts and hence secondary cloud-production. However, model calculations of therate of production of rain and hence the quantity of rain are uncertain due to the complex interactionsof aerosols, entrainment, turbulence and giant cloud condensation nuclei (GCCN). These processesdepend on the environment conditions, controlled by the large-scale dynamics. Equally, the aerosol-cloud-precipitation processes can influence the larger-scale dynamics, for example through radiativetransfer. Indeed there are many interactions between processes on a range of scales that need to beunderstood and represented in models.