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.

The mechanisms controlling the influence of the sea surface temperature (SST) structure on the surface winds are studied by means of numerical simulations run with a nonhydrostatic fully compressible state‐of‐the‐art numerical model in a realistic midlatitudes setup, leading to the 9 October 2014 Genoa heavy rainfall event. Starting from a simulation with high‐resolution submesoscale eddy‐permitting SST field, the surface temperature boundary conditions are changed to enhance, reduce, or smooth the SST gradient. It is found that the marine atmospheric boundary layer responds to the submesoscale SST forcing structures over time scales of the order of hours. In particular, through the downward momentum mixing mechanism, the presence of SST horizontal gradients impacts the spatial structure of the surface wind convergence, which can displace the convective heavy rain bands that develop over the sea.

Tropical cyclones generate a large and wide cold wake along their trajectories, which conditions the subsequent evolution of the tropical cyclone themselves. The cold wakes persist for weeks, impacting both the upper ocean, the air‐sea fluxes, and the atmosphere. The study by Z. Ma et al. (2020, https://doi.org/10.1029/2020GL088873) for the first time analyzes a composite of remotely sensed data sets to show that cold wakes modify surface winds and reduce cloud coverage and rainfall. These results contribute to shedding light on the mechanisms at the origin of the air‐sea feedbacks, which can differ at different latitudes depending on the stability of the marine atmospheric boundary layer. The work stimulates further research to assess whether the cloud cover anomalies induced by tropical cyclones significantly modify the radiative budget of the Earth.

Hyperspectral imagers enable the collection of high-resolution spectral images exploitable for the supervised classification of habitats and objects of interest (OOI). Although this is a well-established technology for the study of subaerial environments, Ecotone AS has developed an underwater hyperspectral imager (UHI) system to explore the properties of the seafloor. The aim of the project is to evaluate the potential of this instrument for mapping and monitoring benthic habitats in shallow and deep-water environments. For the first time, we tested this system at two sites in the Southern Adriatic Sea (Mediterranean Sea): the cold-water coral (CWC) habitat in the Bari Canyon and the Coralligenous habitat off Brindisi. We created a spectral library for each site, considering the different substrates and the main OOI reaching, where possible, the lower taxonomic rank. We applied the spectral angle mapper (SAM) supervised classification to map the areal extent of the Coralligenous and to recognize the major CWC habitat-formers. Despite some technical problems, the first results demonstrate the suitability of the UHI camera for habitat mapping and seabed monitoring, through the achievement of quantifiable and repeatable classifications.

The Mediterranean region is frequently struck by severe rainfall events causing numerous casualties and several million euros of damages every year. Thus, improving the forecast accuracy is a fundamental goal to limit social and economic damages. Numerical Weather Prediction (NWP) models are currently able to produce forecasts at the km scale grid spacing but unreliable surface information and a poor knowledge of the initial state of the atmosphere may produce inaccurate simulations of weather phenomena. The STEAM (SaTellite Earth observation for Atmospheric Modelling) project aims to investigate whether Sentinel satellites constellation weather observation data, in combination with Global Navigation Satellite System (GNSS) observations, can be used to better understand and predict with a higher spatio-temporal resolution the atmospheric phenomena resulting in severe weather events. Two heavy rainfall events that occurred in Italy in the autumn of 2017 are studied—a localized and short-lived event and a long-lived one. By assimilating a wide range of Sentinel and GNSS observations in a state-of-the-art NWP model, it is found that the forecasts benefit the most when the model is provided with information on the wind field and/or the water vapor content.

Air and sea interact on a wide range of scales, shaping climate and influencing weather. The direct effect of sea surface temperature (SST) structures on the extratropical atmosphere at the daily time‐scale is generally masked by the large variability associated with atmospheric dynamics. With 25 years of daily SST and surface wind observational products, obtained with data from buoys, satellite and atmospheric analysis in the Mediterranean, we show that strong surface wind convergence preferentially occurs when the air encounters a cold SST front. The mechanism responsible for the influence of ocean fronts on surface winds is rooted in the thermal disequilibrium that emerges at the air–sea interface, where cold water enhances the stability of the boundary layer, decoupling surface winds from the stronger winds aloft. Surface convergence drives upward motion which, under appropriate conditions, favours cloud formation. Thus, these results suggest that weather forecast models need to properly represent the small‐scale ocean thermal structures, which could affect rainfall.