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The trade wind subcloud layer is an important structural component of the atmosphere. Its thermodynamic variability has long been characterized using simple frameworks, of which mixed-layer theory is the simplest kind. Past studies qualitatively support such a description, yet the adequacy of mixed-layer theory as a quantitative description has not been tested. Here we use observations collected during the Elucidating the Role of Clouds–Circulation Coupling in Climate (EUREC4A) field campaign to test this framework and evaluate our understanding of the trade wind subcloud layer. We find evidence for a transition layer separating the mixed-layer and subcloud-layer tops. The presence of such a finitely thick transition layer with vertical gradients complicates the application of mixed-layer theory, which assumes an abrupt gradient, or “jump” at the subcloud-layer top. This ambiguity introduces effective parameters and motivates their estimation through a Bayesian methodology. Results from this Bayesian inversion further reflect a finite-depth entrainment zone. We find that subcloud-layer moisture and heat budgets close for synoptic variability and a monthly campaign mean, yielding a campaign-mean residual of 3.6 W m−2 for moisture and 2.9 W m−2 for heat. Surface wind speed variability influences the subcloud-layer depth and fluxes, yet thermodynamic variability above the subcloud-layer top emerges as the primary control on subcloud-layer moisture and heat variability. Given that this simple theoretical framework can explain observed variability, it offers an appealing framework for evaluating larger-scale models that must parameterize the processes regulating this fundamental part of the atmosphere.

A variety of empirical estimates have been published for the lower bounds on aerosol radiative forcing, clustered around −1.0 or −2.0 W m−2. The reasons for obtaining such different constraints are not well understood. In this study, we explore bounds on aerosol radiative forcing using a Bayesian model of aerosol forcing and Earth’s multi-time-scale temperature response to radiative forcing. We first demonstrate the ability of a simple aerosol model to emulate aerosol radiative forcing simulated by 10 general circulation models. A joint inference of climate sensitivity and effective aerosol forcing from historical surface temperatures is then made over 1850–2019. We obtain a maximum likelihood estimate of aerosol radiative forcing of −0.85 W m−2 (5%–95% credible interval from −1.3 to −0.50 W m−2) for 2010–19 relative to 1750 and an equilibrium climate sensitivity of 3.4°C (5%–95% credible interval from 1.8° to 6.1°C). The wide range of climate sensitivity reflects difficulty in empirically constraining long-term responses using historical temperatures, as noted elsewhere. A relatively tight bound on aerosol forcing is nonetheless obtained from the structure of temperature and aerosol precursor emissions and, particularly, from the rapid growth in emissions between 1950 and 1980. Obtaining a 5th percentile lower bound on aerosol forcing around −2.0 W m−2 requires prescribing internal climate variance that is a factor of 5 larger than the CMIP6 mean and assuming large, correlated errors in global temperature observations. Ocean heat uptake observations may further constrain aerosol radiative forcing but require a better understanding of the relationship between time-variable radiative feedbacks and radiative forcing.

Individual paintings by artists including Vincent van Gogh and Edvard Munch have been shown to depict specific atmospheric phenomena, raising the question of whether longer-term environmental change influences stylistic trends in painting. Anthropogenic aerosol emissions increased to unprecedented levels during the 19th century as a consequence of the Industrial Revolution, particularly in Western European cities, leading to an optical environment having less contrast and more intensity. Here, we show that trends from more figurative to impressionistic representations in J.M.W. Turner and Claude Monet’s paintings in London and Paris over the 19th century accurately render physical changes in their local optical environment. In particular, we demonstrate that changes in local sulfur dioxide emissions are a highly statistically significant explanatory variable for trends in the contrast and intensity of Turner, Monet, and others’ works, including after controlling for time trends and subject matter. Industrialization altered the environmental context in which Turner and Monet painted, and our results indicate that their paintings capture changes in the optical environment associated with increasingly polluted atmospheres during the Industrial Revolution.

Anthropogenic aerosols exert a cooling influence that offsets part of the greenhouse gas warming. Due to their short tropospheric lifetime of only several days, the aerosol forcing responds quickly to emissions. Here, we present and discuss the evolution of the aerosol forcing since 2000. There are multiple lines of evidence that allow us to robustly conclude that the anthropogenic aerosol effective radiative forcing (ERF) - both aerosol- radiation interactions (ERFari) and aerosol-cloud interactions (ERFaci) - has become less negative globally, i.e. the trend in aerosol effective radiative forcing changed sign from negative to positive. Bottom-up inventories show that anthropogenic primary aerosol and aerosol precursor emissions declined in most regions of the world; observations related to aerosol burden show declining trends, in particular of the fine-mode particles that make up most of the anthropogenic aerosols; satellite retrievals of cloud droplet numbers show trends in regions with aerosol declines that are consistent with these in sign, as do observations of top-of-atmosphere radiation. Climate model results, including a revised set that is constrained by observations of the ocean heat content evolution show a consistent sign and magnitude for a positive forcing relative to the year 2000 due to reduced aerosol effects. This reduction leads to an acceleration of the forcing of climate change, i.e. an increase in forcing by 0.1 to 0.3 W m(-2), up to 12 % of the total climate forcing in 2019 compared to 1750 according to the Intergovernmental Panel on Climate Change (IPCC)., Atmospheric Chemistry and Physics, 22 (18), ISSN:1680-7375, ISSN:1680-7367

Anthropogenic aerosols exert a cooling influence that offsets part of the greenhouse gas warming. Due to their short tropospheric lifetime of only up to several days, the aerosol forcing responds quickly to emissions. Here we present and discuss the evolution of the aerosol forcing since 2000. There are multiple lines of evidence that allow to robustly conclude that the anthropogenic aerosol effective radiative forcing – both aerosol-radiation and aerosol-cloud interactions – has become globally less negative, i.e. that the trend in aerosol effective radiative forcing changed sign from negative to positive. Bottom-up inventories show that anthropogenic primary aerosol and aerosol precursor emissions declined in most regions of the world; observations related to aerosol burden show declining trends, in particular of the fine-mode particles that make up most of the anthropogenic aerosols; satellite retrievals of cloud droplet numbers show trends consistent in sign, as do observations of top-of-atmosphere radiation. Climate model results, including a revised set that is constrained by observations of the ocean heat content evolution show a consistent sign and magnitude for a positive forcing relative to 2000 due to reduced aerosol effects. This reduction leads to an acceleration of the forcing of climate change, i.e. an increase in forcing by 0.1 to 0.3 W m-2, up to 12 % of the total climate forcing in 2019 compared to 1750 according to IPCC.

Shallow trade cumulus clouds cool the planet and fuel the large-scale circulation. Their unknown response to climate change is a major source of uncertainty in climate projections. Differing changes in cloudiness near the base of the cumulus layer with warming control the spread in simulated trade cumulus cloud feedbacks in models, with high climate sensitivity models showing a strong negative coupling between lower-tropospheric mixing and cloudiness. However, such a mixing-desiccation mechanism has never been tested with observations. Here we present novel measurements of the convective mass flux, cloud fraction and relative humidity at cloud base from the recent EUREC4A field campaign and find the dynamical control of cloudiness through the mass flux to overwhelm the thermodynamic response to humidity. Because the mesoscale vertical velocity controls the mass flux as much as entrainment does, the mass flux ends up being uncorrelated to relative humidity, which opposes the mixing-desiccation hypothesis. The magnitude, variability, and coupling of mass flux and cloudiness differs drastically between climate models and the EUREC4A observations. Models that have particularly strong trade cumulus feedbacks tend to exaggerate the dependence of cloudiness on humidity rather than the mass flux, and also exaggerate variability in cloudiness. The process-based constraints presented here render those strongly positive trade cumulus feedbacks unrealistic, for the first time supporting and explaining a weak trade cumulus feedback at the relevant process scale.

The transition layer in the trades has long been observed and simulated, but its origins remain little investigated. It is often associated with an about 150 m deep layer at the top of the subcloud layer that acts as a barrier to overlying convection. Using extensive observations from the EUREC4A field campaign, we propose a reconceptualization of the transition layer. Strong jumps at the mixed layer top, as expected from the theory of cloud-free convective boundary layers, are only found rarely and when they occur, they tend to occur in large cloud-free areas. We show that small clouds with their bases around 600 m maintain the transition layer, in analogy with the maintenance of the trade-wind inversion by deeper clouds. From this analysis also emerges the potential for an alternate view of entrainment mixing, which is based on the ability to detrain condensate into the transition layer and induce gentle sinking motion through negative buoyancy. Mixed layer theory and Paluch mixing diagrams are also used to gain inferences into entrainment mixing.

Cloud free skies are rare in the trades. We analyze conditions in which cloud-free conditions prevail. For this purpose Raman water vapor measurements from the Barbados Cloud Observatory, complemented by ship-based measurements during EUREC4A are used to explore water vapor variability in the marine boundary layer. We explore the consistency of the inferred cloud base height with estimates of temperature and water vapor from the lidar signal, and examine the co-variability of these quantities. After having established the properties of these measurements, we seek to use them as well as others, to explain in what ways periods of cloud-free conditions are maintained, investigating the hypothesis that only when the wind stills is it simply sunny.

The trades form an important link in the atmospheric energy supply, transporting moisture and momentum to the deep tropics and influencing the global hydrological cycle. Trade-wind cumuli are the most ubiquitous cloud type over tropical oceans, yet models disagree in simulating their response to warming. Our study takes advantage of extensive in-situ soundings performed during the EUREC4A campaign, which took place in the downstream trades of the North Atlantic in winter 2020. We employ 1068 dropsondes made in a ca. 2deg x 2deg area to close the moisture and energy budgets of the subcloud layer and atmospheric column. Our motivation for closing moisture and energy budgets using EUREC4A data is two-fold. First, we try to understand which large-scale environmental factors control variability in subcloud layer moisture and moist static energy, given their influence on setting convective potential. Second, we quantify the interplay between clouds and their environment through an energetic lens. The cloud radiative effect emerges as a residual from the total column moist static energy budget, yielding an energetic estimate of clouds. We quantify how this cloud radiative effect compares with coincident satellite and geometric (i.e. cloud fraction) estimates of cloudiness, varies on different scales, and relates to large-scale environmental conditions.

This study investigates the role of radiative processes in shaping the spatial distribution of shallow clouds, using in-situ measurements retrieved during the EUREC4A field campaign. Horizontal gradients in atmospheric radiative cooling above the boundary layer had been advanced as important drivers of shallow circulation and low-level winds, through their effect on surface pressure gradients. Modeling studies first recognized their importance in idealized simulations of deep convection in radiative-convective equilibrium, then found a weaker role for idealized cases of very shallow convection; but recent work using remote-sensing data argued for their importance in strengthening the circulation close to the margin between dry and moist regions, on synoptic scales, arguing for a possible significance for these radiative effects on observed cloud structures.Here we investigate cases of intermediate scale, observed during the EUREC4A field campaign, where shallow convection extends vertically up to 4 km, and whose spatial organization can be described on mesoscales as “fish” or “flower” patterns. We perform careful radiative transfer calculations, using state-of-the-art spectroscopic data and over two thousand of dropsondes and radiosondes launched, to capture the fine details of radiative cooling profiles usually missed by satellite measurements. The large number of sondes allows us to sample radiative cooling information for the organization pattern of interest and analyze it in conjunction with the direct wind and humidity measurements. We also use geostationary estimates of precipitable water in clear-sky in order to cross-check the sonde data, and connect them to the organization pattern and to the position of the moist margin.Our results target the following relationships previously identified in idealized simulations: (a) between horizontal gradients in moisture and in top-of-the-boundary-layer radiative cooling, (b) between these radiative cooling gradients and surface wind anomalies across the moist margin, and (c) between the strength of surface winds as a function of the distance from the moist margin. These results will allow us to test the importance of radiative transfer processes in a real case of shallow convective organization.

Low‐cloud based emergent constraints have the potential to substantially reduce uncertainty in Earth’s equilibrium climate sensitivity, but recent work has shown that previously developed constraints fail in the latest generation of climate models, suggesting that new approaches are needed. Here, we investigate the potential for emergent constraints to reduce uncertainty in regional cloud feedbacks, rather than the global‐mean cloud feedback. Strong relationships are found between the monthly and interannual variability of tropical clouds, and the tropical net cloud feedback. These relationships are combined with observations to substantially narrow the uncertainty in the tropical cloud feedback and demonstrate that the tropical cloud feedback is likely >0Wm−2K−1. Promising relationships are also found in the 90°–60°S and 30°–60°N regions, though these relationships are not robust across model generations and we have not identified the associated physical mechanisms., Key Points Global low‐cloud‐based emergent constraints on equilibrium climate sensitivity fail in CMIP6Strong relationships are found between unforced cloud variability and long‐term cloud feedbacks in several regionsRegional emergent constraints suggest the tropical cloud feedback is likely positive

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.

How will low-level clouds respond to global warming? We approach this question by first investigating the spread of climate sensitivity and cloud feedbacks in CMIP6 models. We stratify the cloud response by circulation regime and focus in greater detail on the cloud response in tropical regimes of subsidence and weak ascent (i.e., their vertical structure in the present-day and future climate, how cloud profile changes relate to changes in cloud-controlling factors). This CMIP6 model analysis dovetails with an observational analysis of low cloud responses from the EUREC4A field campaign. We seek to employ a simple model of low cloud behavior, constrained with observations from EUREC4A and longer time series from the Barbados Cloud Observatory, to better constrain the range of low cloud behavior spanned by CMIP6 models.

Shallow cumulus clouds in the trade-wind regions cool the planet by reflecting solar radiation. The response of trade cumulus clouds to climate change is a key uncertainty in climate projections1–4. Trade cumulus feedbacks in climate models are governed by changes in cloud fraction near cloud base5,6, with high-climate-sensitivity models suggesting a strong decrease in cloud-base cloudiness owing to increased lower-tropospheric mixing5–7. Here we show that new observations from the EUREC4A (Elucidating the role of cloud-circulation coupling in climate) field campaign8,9 refute this mixing-desiccation hypothesis. We find the dynamical increase of cloudiness through mixing to overwhelm the thermodynamic control through humidity. Because mesoscale motions and the entrainment rate contribute equally to variability in mixing but have opposing effects on humidity, mixing does not desiccate clouds. The magnitude, variability and coupling of mixing and cloudiness differ markedly among climate models and with the EUREC4A observations. Models with large trade cumulus feedbacks tend to exaggerate the dependence of cloudiness on relative humidity as opposed to mixing and also exaggerate variability in cloudiness. Our observational analyses render models with large positive feedbacks implausible and both support and explain at the process scale a weak trade cumulus feedback. Our findings thus refute an important line of evidence for a high climate sensitivity10,11.

The response of trade cumulus clouds to warming remains a major source of uncertainty for climate sensitivity. Recent studies have highlighted the role of the cloud–convection coupling in explaining this spread in future warming estimates. Here, using observations from an instrumented site and an airborne field campaign, together with high-frequency climate model outputs, we show that i) over the course of the daily cycle, a cloud transition is observed from deeper cumuli during nighttime to shallower cumuli during daytime, ii) the cloud evolution that models predict from night to day reflects the strength of cloud sensitivity to convective mass flux and exhibits many similarities with the cloud evolution they predict under global warming, and iii) those models that simulate a realistic cloud transition over the daily cycle tend to predict weak trade cumulus feedback. Our findings thus show that the daily cycle is a particularly relevant testbed, amenable to process studies and anchored by observations, to assess and improve the model representation of cloud–convection coupling and thus make climate projections more reliable.

As part of the EUREC4A (Elucidating the role of cloud–circulation coupling in climate) field campaign, which took place in January and February 2020 over the western tropical Atlantic near Barbados, the French SAFIRE ATR42 research aircraft (ATR) conducted 19 flights in the lower troposphere. Each flight followed a common flight pattern that sampled the atmosphere around the cloud base level, at different heights of the subcloud layer, near the sea surface and in the lower free troposphere. The aircraft's payload included a backscatter lidar and a Doppler cloud radar that were both horizontally oriented; a Doppler cloud radar looking upward; microphysical probes; a cavity ring-down spectrometer for water isotopes; a multiwavelength radiometer; a visible camera; and multiple meteorological sensors, including fast rate sensors for turbulence measurements. With this instrumentation, the ATR characterized the macrophysical and microphysical properties of trade-wind clouds together with their thermodynamical, turbulent and radiative environment. This paper presents the airborne operations, the flight segmentation, the instrumentation, the data processing and the EUREC4A datasets produced from the ATR measurements. It shows that the ATR measurements of humidity, wind and cloud base cloud fraction measured with different techniques and samplings are internally consistent; that meteorological measurements are consistent with estimates from dropsondes launched from an overflying aircraft (the High Altitude and LOng Range Research Aircraft, HALO); and that water-isotopic measurements are well correlated with data from the Barbados Cloud Observatory. This consistency demonstrates the robustness of the ATR measurements of humidity, wind, cloud base cloud fraction and water-isotopic composition during EUREC4A. It also confirms that through their repeated flight patterns, the ATR and HALO measurements provided a statistically consistent sampling of trade-wind clouds and of their environment. The ATR datasets are freely available at the locations specified in Table 11.