Abstract. The science guiding the EUREC4A campaign and its measurements is presented. EUREC4A comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic – eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC4A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air–sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC4A explored – from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview of EUREC4A's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at https://doi.org/10.25326/165 (Stevens, 2021), and a film documenting the campaign is provided as a video supplement.

Abstract. The science guiding the EUREC4A campaign and its measurements are presented. EUREC4A comprised roughly five weeks of measurements in the downstream winter trades of the North Atlantic – eastward and south-eastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC4A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or, or the life-cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso (200 km) and larger (500 km) scales, roughly four hundred hours of flight time by four heavily instrumented research aircraft, four global-ocean class research vessels, an advanced ground-based cloud observatory, a flotilla of autonomous or tethered measurement devices operating in the upper ocean (nearly 10000 profiles), lower atmosphere (continuous profiling), and along the air-sea interface, a network of water stable isotopologue measurements, complemented by special programmes of satellite remote sensing and modeling with a new generation of weather/climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC4A explored – from Brazil Ring Current Eddies to turbulence induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview EUREC4A's outreach activities, environmental impact, and guidelines for scientific practice.

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15During the peak period of hurricane activity in the summer of 2010, vertical profiles of 16 ozone using ozonesondes were taken downstream of tropical cyclones in the Western and 17Eastern Atlantic Ocean basin at Barbados and Cape Verde. Measurements are taken for 18 tropical cyclones Danielle, Earl, Fiona, Gaston, Julia and Igor. The measurements show 19an increase in ozone mixing ratios with air originating from the tropical cyclones at 5-10 20 km altitude. We suggest that observed lightning activity associated tropical cyclones and 21the subsequent production of NO X followed by upper level outflow and subsidence ahead 22of the tropical cyclones and aged continental outflow from West Africa thunderstorms 23 produced observed increases in ozone mixing ratios. Hurricane Danielle showed the 24 largest changes in ozone mixing ratio with values increasing from 25 ppb to 70 ppb 25 between 22 and 25 August in the middle troposphere, near 450 hPa; warming and drying 26in the middle and lower troposphere. Measurements of ozone mixing ratios in Cape 27Verde show higher ozone mixing ratios prior to the passage of tropical storm Julia but 28 low ozone mixing ratios and high relative humidity up to 300 hPa when the storm was in 29 close proximity. This is due most likely the vertically transported from the marine 30 boundary layer. 31 32 33 34 3