David Noone

The sources of air and water vapor in the tropical trade‐wind Atlantic subcloud boundary layer (SBL) are analyzed using in situ measurements of temperature, specific humidity, and stable water isotopologues (HDO and H ₂ ¹⁸ O) from shipboard observations in January‐February 2020. The coldest cold pools had a higher maximum D and coincided with precipitation. The hydrogen isotope ratio (D) of water vapor was greater within 9 (64% of 14) cold pools than in their surroundings. The strongest D change at the arrival (or front) of a cold pool was ‰ (−72‰ to −66‰). The isotopic composition, potential temperature, and specific humidity can be explained by evaporation of hydrometeors in convective downdrafts. In these cases, evaporation of enriched hydrometeors dominates over depletion of atmospheric vapor by removal of enriched precipitation. Mixtures of ambient subcloud air with 20% mass fraction of air, having water vapor with the isotope composition of the hydrometeors ( ¹⁸ O _L = +0.7‰; D _L = +15.4‰), explain the observed enrichment of vapor in the cold pools ( ¹⁸ O = −9.6‰; D = −66.6‰). As the strength of a cold pool decays, the isotopically enriched vapor from the downdrafts is diluted by surface and entrainment fluxes. Isotopes of water vapor in the wakes dilute ∼30× faster than the residence time of water vapor in the subcloud layer, suggesting the cold pool wakes are shallow and originate from concentrated evaporative downdrafts.

Describing the processes that regulate the flows and exchanges of water within the atmosphere and between the atmosphere and Earth’s surface is critical for understanding environmental change and predicting Earth’s future accurately. The heavy-to-light hydrogen and oxygen isotope ratios of water provide a useful lens through which to evaluate these processes due to their innate sensitivity to evaporation, condensation, and mixing. In this review, we examine how isotopic information advances our understanding about the origin and transport history of moisture in the atmosphere and about convective processes—including cloud mixing and detrainment, precipitation formation, and rain evaporation. Moreover, we discuss how isotopic data can be used to benchmark numerical simulations across a range of scales and improve predictive skill through data assimilation techniques. This synthesis of work illustrates that, when paired with air mass thermodynamic properties that are commonly measured and modeled (such as specific humidity and temperature), water’s isotope ratios help shed light on moist processes that help set the climate state.

Tropical Pacific seawater and precipitation stable oxygen isotope data aid in understanding modern oceanic and atmospheric interactions, and these data are particularly valuable as they are archived in isotope‐based paleoclimate records. However, the absence of modern seawater isotope time series limits the ability to identify the atmospheric influences on these data, precluding robust paleoclimate interpretations. We present a new 10 year sub‐monthly record of seawater and precipitation stable oxygen isotope values (δ ¹⁸ O _sw and δ ¹⁸ O _p ) from Koror, Palau. Our dataset indicates that temporally, δ ¹⁸ O _sw is strongly influenced by local δ ¹⁸ O _p. Both monthly δ ¹⁸ O _sw and δ ¹⁸ O _p are highly correlated with outgoing longwave radiation across the tropical Pacific, reflecting a Walker Circulation imprint on the surface ocean. Changes in the Palau δ ¹⁸ O _sw —salinity relationship correspond to NINO3.4 variability, indicating a difference in how these variables record El Niño Southern Oscillation (ENSO) information, but demonstrating the utility of δ ¹⁸ O _sw to reconstruct ENSO variability in the western tropical Pacific.

MethaneSAT is a joint American and New Zealand satellite mission, which involves partnership between the Environmental Defense Fund (EDF), MethaneSAT LLC and New Zealand government. MethaneSAT’s primary mission is to detect and quantify methane (CH4) emissions from both point and area sources from the global oil and gas production industry in support of emissions reductions. MethaneSAT will target specific 200 km x 200 km regions and map CH4 within those regions at 100 m x 400 m resolution with unprecedented precision. The Aotearoa New Zealand team’s aim is to develop and test the ability of the satellite to detect agricultural CH4 emissions. New Zealand is an ideal place to develop this capability due to its large CH4 emissions, 85% of which are from agricultural sources. We will present results of modelled atmospheric CH4 concentrations for agricultural targets in New Zealand, emission estimates from the agricultural targets and CH4 measurements collected during a shakedown field campaign, in preparation for the MethaneSAT launch in 2024.We use 1.5 km spatial resolution, New Zealand specific bottom-up CH4 fluxes and the Numerical Atmospheric dispersion Modelling Environment (NAME III), driven by meteorological input from the New Zealand Convective Scale Model (NZCSM, 1.5 km spatial resolution) Numerical Weather Prediction (NWP) model to create modelled agricultural XCH4 (column averaged) enhancements. The MethaneSAT-like targets are created for different scenarios to assess the changes in the XCH4 enhancements relative to meteorological conditions and bottom-up fluxes. We use the modelled agricultural XCH4 fields to test operational methods that are being developed for Level 4 products (i.e., emissions) in an Observing System Simulation Experiments (OSSE) framework and adapt them for diffuse agricultural sources. We will present results of modelled XCH4 scans for the main agricultural targets across New Zealand and the application of the MethaneSAT Level 4 methods (i.e., Geostatistical Inversion Framework, Divergence Integral Method) for agricultural sources.The 2023 New Zealand MethaneSAT pre-launch shakedown field campaign took place over ten days in Waikato, a region with New Zealand’s strongest agricultural CH4 emissions. The campaign involved the deployment of two EM27/SUN portable spectrometers and in situ CH4 samplers. One EM27/SUN was at a fixed location for the duration of the campaign, while the second instrument was positioned up or downwind to measure enhancements of XCH4. Four remote sites were used, with measurements collected on multiple occasions and under different meteorological conditions. Typical XCH4 enhancements of between 3 and 8 ppb were observed while side-by-side measurements with the two spectrometers yielded a minimum detection limit of 0.3 ppb. Ground based and airborne in situ measurements were also collected to provide additional context to the measured enhancements. The measured XCH4 enhancements aligned with agricultural XCH4 estimates from the modelling framework.

Land surface models (LSMs) play a crucial role in elucidating water and carbon cycles by simulating processes such as plant transpiration and evaporation from bare soil, yet calibration often relies on comparing LSM outputs of landscape total evapotranspiration (ET) and discharge with measured bulk fluxes. Discrepancies in partitioning into component fluxes predicted by various LSMs have been noted, prompting the need for improved evaluation methods. Stable water isotopes serve as effective tracers of component hydrologic fluxes, but data and model integration challenges have hindered their widespread application. Leveraging National Ecological Observation Network measurements of water isotope ratios at 16 US sites over 3 years combined with LSM‐modeled fluxes, we employed an isotope‐enabled mass balance framework to simulate ET isotope values (δET) within three operational LSMs (Mosaic, Noah, and VIC) to evaluate their partitioning. Models simulating δET values consistent with observations were deemed more reflective of water cycling in these ecosystems. Mosaic exhibited the best overall performance (Kling‐Gupta Efficiency of 0.28). For both Mosaic and Noah there were robust correlations between bare soil evaporation fraction and error (negative) as well as transpiration fraction and error (positive). We found the point at which errors are smallest (x‐intercept of the multi‐site regression) is at a higher transpiration fraction than is currently specified in the models. Which means that transpiration fraction is underestimated on average. Stable isotope tracers offer an additional tool for model evaluation and identifying areas for improvement, potentially enhancing LSM simulations and our understanding of land‐surface hydrologic processes.

Cloud condensation and hydrometeor evaporation fractionate stable isotopes of water, enriching liquid with heavy isotopes; whereupon updrafts, downdrafts, and rain vertically redistribute water and its isotopes in the lower troposphere. These vertical water fluxes through the marine boundary layer affect low cloud climate feedback and, combined with isotope fractionation, are hypothesized to explain the depletion of tropical precipitation at higher precipitation rates known as the “amount effect.” Here, an efficient and numerically stable quasi‐analytical model simulates the evaporation of raindrops and enrichment of their isotope composition. It is applied to a drop size distribution and subcloud environment representative of Atlantic trade cumulus clouds. Idealized physics experiments artificially zero out selected processes to discern the separate effects on the isotope ratio of raindrops, of exchange with the environment, evaporation, and kinetic molecular diffusion. A parameterization of size‐dependent molecular and eddy diffusion is formulated that enriches raindrops much more strongly (+5‰ for deuterated water [HDO] and +3.5‰ for O) than equilibrium evaporation as they become smaller than 1 mm. The effect on evaporated vapor is also assessed. Rain evaporation enriches subcloud vapor by +12‰ per mm rain (for HDO), explaining observations of enriched vapor in cold pools sourced by evaporatively cooled downdrafts. Drops smaller than 0.5 mm evaporate completely before falling 700 m in typical subtropical marine boundary layer conditions. The early and complete evaporation of these smaller drops in the rain size distribution enriches the vapor produced by rain evaporation.

Evaporation decreases the mass and increases the isotope composition of falling drops. Combining and integrating the dependence of the evaporation on the drop diameter and on the drop-environment humidity difference, the square of drop diameter is found to decrease with the square of vertical distance below cloud base. Drops smaller than 0.5 mm evaporate completely before falling 700 m in typical subtropical marine boundary layer conditions. The effect on the isotope ratio of equilibration with the environment, evaporation, and kinetic molecular diffusion is modeled by molecular and eddy diffusive fluxes after Craig and Gordon (1965), with a size-dependent parameterization of diffusion that enriches small drops more strongly, and approaches the rough aerodynamic limit for large drops. Rain shortly approaches a steady state with the subcloud vapor by exchange with a length scale of 40 m. Kinetic molecular diffusion enriches drops up to as they evaporate by up to +5~\permil~for deuterated water (HDO) and +3.5~\permil~for H$_2$$^{18}$O. Rain evaporation enriches undiluted subcloud vapor by +12~\permil~per mm rain, explaining enrichment of vapor in evaporatively cooled downdrafts that contribute to cold pools. Microphysics enriches the vapor lost by the early and complete evaporation of smaller drops in the distribution. Vapor from hydrometeors is more enriched than it would be by Rayleigh distillation or by mixtures of liquid rain and vapor in equilibrium with rain.