Commonly-occurring stratification and synoptic tendencies lead to liquid clouds and warm precipitation processes in the PBL over large portions of the globe. The climate is so sensitive to these low-level clouds that they are identified in IPCC reports as major uncertainty sources for climate prediction; their representation in GCMs thus needs improvement. PBL clouds have therefore been scrutinized in numerous field campaigns over both ocean and land. The main method for measuring clouds in field campaigns is in-situ airborne probing and, though these data are invaluable, it is widely recognized that spatial and temporal sampling is innately poor. We then turn to remote sensing as a way of drastically improving spatial sampling since it delivers cloud properties over more than a line-of-flight through 3+1D space. The obvious tradeoff is, however, generally complicated connections between remotely-measured radiances and inherent cloud properties of real interest to cloud process modelers. Active remote sensing from below or above the clouds improves vastly over in-situ sampling, but its outcome remains confined to a “ribbon” of vertical profiles ordered in time (from below) or space (from above). Passive imaging has the complimentary problem of delivering a potentially wide horizontal swath of cloud properties, but integrated along the vertical. At least that is the conventional wisdom when it comes to the solar spectrum, where observed radiances from clouds are dominated by multiple scattering. Based on recent results from AirMSPI imaging at 20 km altitude, we challenge the perceived limitation of passive shortwave radiometry to deliver only column-integrated properties. We demonstrate that multi-pixel exploitation of multi-angle spectro-polarimteric imaging at solar wavelengths can be used to extract not only maps of microphysical properties but also 3D cloud structure for both PBL-topped stratiform layers and vertically-developed 3D clouds in convective regimes. A key realization is: airborne and space-based sensors offer radically different spatial and angular sampling opportunities with unique advantages in both cases. We look forward to future PBL-specific missions in space for their global reach. At the same time, there is a clear case for deploying high-altitude imagers in all future campaigns.

The science guiding the \EURECA campaign and its measurements are presented. \EURECA 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, \EURECA 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 \EURECA 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 \EURECA’s outreach activities, environmental impact, and guidelines for scientific practice.