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.

To date, there is no long-term, stable, and uncertainty-quantified dataset of upper tropospheric humidity (UTH) that can be used for climate research. As intermediate step towards the overall goal of constructing such a climate data record (CDR) of UTH, we produced a new fundamental climate data record (FCDR) on the level of brightness temperature for microwave humidity sounders that will serve as basis for the CDR of UTH. Based on metrological principles, we constructed and implemented the measurement equation and the uncertainty propagation in the processing chain for the microwave humidity sounders. We reprocessed the level 1b data to obtain newly calibrated uncertainty quantified level 1c data in brightness temperature. Three aspects set apart this FCDR from previous attempts: (1) the data come in a ready-to-use NetCDF format; (2) the dataset provides extensive uncertainty information taking into account the different correlation behaviour of the underlying errors; and (3) inter-satellite biases have been understood and reduced by an improved calibration. Providing a detailed uncertainty budget on these data, this new FCDR provides valuable information for a climate scientist and also for the construction of the CDR.

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.

Free-tropospheric water vapor strongly impacts the Earth's outgoing longwave radiation (OLR) and therefore plays a key role in controlling the clear-sky response of the climate system to an increase in greenhouse gases. It is now widely accepted that this response is described by a warming and moistening of the atmosphere that is implied if the relative humidity (RH) and lapse rate were to depend on temperature alone, which corresponds to a warming at approximately constant RH (e.g., Held & Soden, 2000;Po-Chedley et al., 2019;Romps, 2014). This reduces the radiative response compared to a warming at constant absolute humidity, and can be described as a positive water-vapor-lapse-rate feedback. While general circulation models (GCMs) agree on this basic response (e.g., Bony et al., 2006;Soden & Held, 2006), there is still an appreciable inter-model spread in the magnitude of the water-vapor-lapse-rate feedback. This spread, which primarily originates from the tropics, contributes a non-negligible (about 30%) uncertainty to the climate sensitivity (Vial et al., 2013).

Abstract. As part of the EUREC4A (Elucidating the role of cloud–circulation coupling in climate) field campaign, the German research aircraft HALO (High Altitude and Long Range Research Aircraft), configured as a cloud observatory, conducted 15 research flights in the trade-wind region east of Barbados in January and February 2020. Narrative text, aircraft state data, and metadata describing HALO's operation during the campaign are provided. Each HALO research flight is segmented by timestamp intervals into standard elements to aid the consistent analysis of the flight data. Photographs from HALO's cabin and animated satellite images synchronized with flight tracks are provided to visually document flight conditions. As a comprehensive product from the remote sensing observations, a multi-sensor cloud mask product is derived and quantifies the incidence of clouds observed during the flights. In addition, to lower the threshold for new users of HALO's data, a collection of use cases is compiled into an online book, How to EUREC4A, included as an asset with this paper. This online book provides easy access to most of EUREC4A's HALO data through an intake catalogue. Code and data are freely available at the locations specified in Table 6.

data record to earlier years. The new definition has the additional advantage that UTH can be calculated directly from given atmospheric profiles of humidity and temperature without a detour via radiative transfer simulations. Detailed uncertainty information in CDRs derived from satellite-based Earth observations are needed to support the application of the data in climate research 7,8. Providing such information on CDR level (level 2 or 3) has mainly been constrained by the availability of uncertainty information in the underlying FCDRs (level 1) in the past. Within the FIDUCEO project four new versions of such FCDRs were created, among them the FIDUCEO Microwave FCDR used as input for our UTH CDR. The FCDR includes information on observational uncertainty on pixel level, which is the result of rigorous uncertainty analyses based on metrological principles 9. These uncertainties are propagated to the spatially and temporally averaged quantities in the UTH CDR. Depending on the spatial and temporal correlation behaviour of the underlying error sources, uncertainties are divided into three different classes, enabling the user to propagate them to spatial and temporal averages of the data. The FIDUCEO UTH CDR is validated against an exisiting microwave UTH data record provided by the Satellite Application Facility on Climate Monitoring (CM-SAF). Differences in monthly tropical mean UTH do not exceed 2% RH and can be attributed to differences in the underlying FCDR and in the CDR processing in approximately equal parts. The structure of this paper is as follows: The Methods chapter introduces the satellite instruments, the UTH retrieval method and the new definition of UTH. Furthermore, a detailed description of the CDR processing is provided. This is followed by the Data Records chapter, which includes a description of the CDR data file format as well as the satellite missions and time periods covered. The subsequent chapter Technical Validation consists of an evaluation of the UTH retrieval performance, the comparison of our CDR with the CM-SAF UTH CDR and a description of uncertainties not estimated in the CDR.

Abstract. Instruments on satellites for Earth observation on polar orbits usually employ a two-point calibration technique, in which deep space and an onboard calibration target provide two reference flux levels. As the direction of the deep-space view is in general close to the celestial equator, the Moon sometimes moves through the field of view and introduces an unwelcome additional signal. One can take advantage of this intrusion, however, by using the Moon as a third flux standard, and this has actually been done for checking the lifetime stability of sensors operating at visible wavelengths. As the disk-integrated thermal emission of the Moon is less well known than its reflected sunlight, this concept can in the microwave range only be used for stability checks and intercalibration. An estimate of the frequency of appearances of the Moon in the deep-space view, a description of the limiting factors of the measurement accuracy and models of the Moon's brightness, and a discussion of the benefits from complementing the naturally occurring appearances of the Moon with dedicated spacecraft maneuvers show that it would be possible to detect photometric lifetime drifts of a few percent with just two measurements. The pointing accuracy is the most crucial factor for the value of this method. Planning such observations in advance would be particularly beneficial, because it allows observing the Moon at welldefined phase angles and putting it at the center of the field of view. A constant phase angle eliminates the need for a model of the Moon's brightness when checking the stability of an instrument. With increasing spatial resolution of future microwave sensors another question arises, viz. to what extent foreground emission from objects other than the Moon will contaminate the flux entering the deep-space view, which is supposed to originate exclusively in the cosmic microwave background. We conclude that even the brightest discreet sources have flux densities below the detection limit of microwave sensors in a single scan.

Free-tropospheric water vapor strongly impacts the Earth's outgoing longwave radiation (OLR) and therefore plays a key role in controlling the clear-sky response of the climate system to an increase in greenhouse gases. It is now widely accepted that this response is described by a warming and moistening of the atmosphere that is implied if the relative humidity (RH) and lapse rate were to depend on temperature alone, which corresponds to a warming at approximately constant RH (e.g., Held & Soden, 2000;Po-Chedley et al., 2019;Romps, 2014). This reduces the radiative response compared to a warming at constant absolute humidity, and can be described as a positive water-vapor-lapse-rate feedback. While general circulation models (GCMs) agree on this basic response (e.g., Bony et al., 2006;Soden & Held, 2006), there is still an appreciable inter-model spread in the magnitude of the water-vapor-lapse-rate feedback. This spread, which primarily originates from the tropics, contributes a non-negligible (about 30%) uncertainty to the climate sensitivity (Vial et al., 2013).