Remote sensing datasets of water vapor isotopic composition are used along with objective measures of convective aggregation to better understand the impact of convective aggregation on the atmospheric hydrologic cycle in the global tropics ($30^{\circ}$N to $30^{\circ}$S) for the period 2015-2020. When convection is unaggregated, vertical velocity profiles are top-heavy, mixing ratios increase and water vapor $\delta D$ decreases as the mean precipitation rate increases, consistent with partial hydrometeor evaporation below anvils into a relatively humid atmospheric column. Aggregated convection is associated with bottom-heavy vertical velocity profiles and a positive correlation between mixing ratio and $\delta D$, a result that is consistent with isotopic enrichment from detrainment of shallow convection near the observation level. Intermediate degrees of aggregation do not display significant variation in $\delta D$ with mixing ratio or precipitation rate. Convective aggregation provides a useful paradigm for understanding the relationships between mixing ratio and isotopic composition across a range of convective settings. The results presented here may have utility for a variety of applications including the interpretation of paleoclimate archives and the evaluation of numerical simulations of convection.

Convective mixing in the lower free troposphere (LFT) and its response to climate change are at the heart of low-cloud feedbacks in projections of future warming, but are challenging to diagnose from observations. The stable isotopic composition of water vapor in the LFT is a sensitive recorder of shallow convective moistening, and can potentially provide independent constraints on shallow convective processes. In-situ and remote sensing measurements from the southeast Pacific marine stratocumulus region and an isotope-enabled general circulation model (GCM) are used along with Gaussian process regression (GPR) to explore the utility of stable isotope measurements and simulations for improved estimates of shallow convective moistening tendencies in marine stratocumulus settings. We train the GPR algorithm on conventional and isotopic fields from a GCM (LMDZ5B) from the SE Pacific marine stratocumulus region and assess the algorithm on out-of-sample GCM output. The GPR trained on isotopic fields yields better estimates of shallow convective moistening tendencies than GPR trained only on conventional meteorological fields. Climate change is not well-captured if the GPR is trained only on the control climate, but performs much better if the training data include samples from both cool and warm climates, and is also reasonably well-captured if the GPR is only trained on the warm climate. The GPR algorithm is applied to isotopic and conventional measurements from the SE Pacific and yields realistic estimates of shallow convective moistening tendencies. Linking machine learning with isotopic simulations and measurements provides a unique and potentially useful framework for bridging GCMs and observations.

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.