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.

Quantification of factors influencing cloud responses to warming across the full range of relevant scales.• First measurements linking clouds and warm-rain processes to meso and sub-mesoscale motions in the lower atmosphere and upper ocean.• Novel application of remote sensing, isotopologue measurements, and un-crewed vehicles (to study the remote marine environment.

Abstract. The Fast Infrared Hygrometer (FIRH), employing open-path tunable diode laser absorption spectroscopy at the wavelengths near the 1364.6896 nm line, was adapted to perform contactless humidity measurements at the Turbulent Leipzig Aerosol Cloud Interaction Simulator (LACIS-T), a unique turbulent moist-air wind tunnel. The configuration of the setup allows for scanning from outside the walls of the wind tunnel and at various positions without the need for repeated optics adjustments. We identified three factors which significantly influence the measurement – self-broadening of the absorption line, interference in the glass windows and parasitic absorption in the ambient air outside the wind tunnel – and developed correction methods which satisfactorily account for these effects. The comparison between FIRH and a reference hygrometer (dew-point mirror MBW 973) indicated a good agreement within the expected errors across the wide range of water vapour concentration 1.0–6.1×1017 cm−3 (equivalent to dew-point temperature of −5.4 to +21 ∘C at the temperature of 23 ∘C). High temporal resolution (∼2 kHz) allowed for studying turbulent fluctuations in the course of intensive mixing of two air streams which had the same mean velocity but differed in temperature and humidity, also including the settings for which the mixture can be supersaturated. The obtained results contribute to improved understanding and interpretation of cloud formation studies conducted in LACIS-T by complementing the previous characterizations of turbulent velocity and temperature fields inside the wind tunnel.

<p>UltraFast Thermometers (UFT’s), developed successively at the Institute of Geophysics, University of Warsaw, allow for airborne measurements of temperature fluctuations in turbulent atmosphere with the resolution better than 1 cm, which provides insight into small-scale turbulent mixing in clouds, atmospheric boundary layer and free atmosphere.</p><p><span>In recent years new versions of UFT thermometers (UFT-2 family) were used in two measurement campaigns: ACORES and EUREC</span><sup><span>4</span></sup><span>A. In ACORES UFT-2 was deployed on the helicopter-borne measurement platform ACTOS used to sample marine stratocumulus clouds with the resolution reaching 3 mm. During the EUREC</span><sup><span>4</span></sup><span>A campaign similar 20 kHz temperature time series have been collected with the UFT-2b deployed onboard the BAS Twin Otter aircraft (average speed of </span><span>6</span><span>0 m/s) in the subtropical low atmosphere, in and between trade wind warm cumulus clouds. Data, resolving scales down the dissipation range,</span><span> allow to</span> <span>estimate</span> <span>directly</span> <span>the temperature </span><span>dissipation rate (TD) in cloud interiors, cloud shells, air spaces between the clouds, </span><span>and</span><span> in the atmospheric boundary layer.</span></p><p><span><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.bcad0e8be76268767790561/sdaolpUECMynit/2202SME&app=m&a=0&c=f286e6c560bb8ccb42931ce4fe1a4934&ct=x&pn=gnp.elif&d=1" alt=""></span></p><p><span>Fig. 1. Example of the UFT-2 0.5 s long measurements from EUREC<sup>4</sup>A campaign: high-resolution plots of temperature (red) and TD (blue) before and during penetration through a cumulus cloud.</span></p><p>Until now, no experimental data on temperature dissipation in free atmosphere and in clouds from in situ measurements have ever been published. Such data may help to understand cloud microphysical processes with phase changes. Classically, during turbulent mixing of air masses, temperature is considered a passive scalar. In clouds, in the course of condensation/evaporation heat is released/absorbed, which may affect fine-scale fluctuations of temperature. Thus, statistical properties of temperature dissipation should differ from situations in which the temperature is just a passive scalar. Examples of temperature fluctuations and associate TD records (see Fig. 1), characteristic to the various atmospheric conditions, will be presented and discussed.</p><p>Acknowledgements: This project has received funding from Polish National Science Center (NCN) under grant agreement 2018/30/M/ST10/00674.</p>