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Weather and climate models are challenged by uncertainties and biases in simulating Southern Ocean (SO) radiative fluxes that trace to a poor understanding of cloud, aerosol, precipitation, and radiative processes, and their interactions. Projects between 2016 and 2018 used in situ probes, radar, lidar, and other instruments to make comprehensive measurements of thermodynamics, surface radiation, cloud, precipitation, aerosol, cloud condensation nuclei (CCN), and ice nucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase clouds common to this pristine environment. Data including soundings were collected from the NSF–NCAR G-V aircraft flying north–south gradients south of Tasmania, at Macquarie Island, and on the R/V Investigator and RSV Aurora Australis. Synergistically these data characterize boundary layer and free troposphere environmental properties, and represent the most comprehensive data of this type available south of the oceanic polar front, in the cold sector of SO cyclones, and across seasons. Results show largely pristine environments with numerous small and few large aerosols above cloud, suggesting new particle formation and limited long-range transport from continents, high variability in CCN and cloud droplet concentrations, and ubiquitous supercooled water in thin, multilayered clouds, often with small-scale generating cells near cloud top. These observations demonstrate how cloud properties depend on aerosols while highlighting the importance of dynamics and turbulence that likely drive heterogeneity of cloud phase. Satellite retrievals confirmed low clouds were responsible for radiation biases. The combination of models and observations is examining how aerosols and meteorology couple to control SO water and energy budgets.

The quantification of methane concentrations in air is essential for the quantification of methane emissions, which in turn is necessary to determine absolute emissions and the efficacy of emission mitigation strategies. These are essential if countries are to meet climate goals. Large-scale deployment of methane analyzers across millions of emission sites is prohibitively expensive, and lower-cost instrumentation has been recently developed as an alternative. Currently, it is unclear how cheaper instrumentation will affect measurement resolution or accuracy. To test this, the Wireless Autonomous Transportable Methane Emission Reporting System (WATCH4ERS) has been developed, comprising four commercially available sensing technologies: metal oxide (MOx,), Non-dispersion Infrared (NDIR), integrated infrared (INIR), and tunable diode laser absorption spectrometer (TDLAS). WATCHERS is the accumulated knowledge of several long-term methane measurement projects at Colorado State University’s Methane Emission Technology Evaluation Center (METEC), and this study describes the integration of these sensors into a single unit and reports initial instrument response to calibration procedures and controlled release experiments. Specifically, this paper aims to describe the development of the WATCH4ERS unit, report initial sensor responses, and describe future research goals. Meanwhile, future work will use data gathered by multiple WATCH4ERS units to 1. better understand the cost–benefit balance of methane sensors, and 2. identify how decreasing instrumentation costs could increase deployment coverage and therefore inform large-scale methane monitoring strategies. Both calibration and response experiments indicate the INIR has little practical use for measuring methane concentrations less than 500 ppm. The MOx sensor is shown to have a logarithmic response to methane concentration change between background and 600 ppm but it is strongly suggested that passively sampling MOx sensors cannot respond fast enough to report concentrations that change in a sub-minute time frame. The NDIR sensor reported a linear change to methane concentration between background and 600 ppm, although there was a noticeable lag in reporting changing concentration, especially at higher values, and individual peaks could be observed throughout the experiment even when the plumes were released 5 s apart. The TDLAS sensor reported all changes in concentration but remains prohibitively expensive. Our findings suggest that each sensor technology could be optimized by either operational design or deployment location to quantify methane emissions. The WATCH4ERS units will be deployed in real-world environments to investigate the utility of each in the future.

The Southern Ocean plays a critical role in the global climate system by mediating atmosphere–ocean partitioning of heat and carbon dioxide. However, Earth system models are demonstrably deficient in the Southern Ocean, leading to large uncertainties in future air–sea CO₂ flux projections under climate warming and incomplete interpretations of natural variability on interannual to geologic time scales. Here, we describe a recent aircraft observational campaign, the O₂/N₂ Ratio and CO₂ Airborne Southern Ocean (ORCAS) study, which collected measurements over the Southern Ocean during January and February 2016. The primary research objective of the ORCAS campaign was to improve observational constraints on the seasonal exchange of atmospheric carbon dioxide and oxygen with the Southern Ocean. The campaign also included measurements of anthropogenic and marine biogenic reactive gases; high-resolution, hyperspectral ocean color imaging of the ocean surface; and microphysical data relevant for understanding and modeling cloud processes. In each of these components of the ORCAS project, the campaign has significantly expanded the amount of observational data available for this remote region. Ongoing research based on these observations will contribute to advancing our understanding of this climatically important system across a range of topics including carbon cycling, atmospheric chemistry and transport, and cloud physics. This article presents an overview of the scientific and methodological aspects of the ORCAS project and highlights early findings.

Stratocumulus clouds over the Southern Ocean have fewer droplets and are more likely to exist in the predominately supercooled phase than clouds at similar temperatures over northern oceans. One likely reason is that this region has few continental and anthropogenic sources of cloud‐nucleating particles that can form droplets and ice. In this work, we present an overview of aerosol particle types over the Southern Ocean, including new measurements made below, in and above clouds in this region. These measurements and others indicate that biogenic sulfur‐based particles >0.1 μm diameter contribute the majority of cloud condensation nuclei number concentrations in summer. Ice nucleating particles tend to have more organic components, likely from sea‐spray. Both types of cloud nucleating particles may increase in a warming climate likely to have less sea ice, more phytoplankton activity, and stronger winds over the Southern Ocean near Antarctica. Taken together, clouds over the Southern Ocean may become more reflective and partially counter the region's expected albedo decrease due to diminishing sea ice. However, detailed modeling studies are needed to test this hypothesis due to the complexity of ocean‐cloud‐climate feedbacks in the region. Plain Language Summary Clouds over the Southern Ocean tend to have less droplets and ice crystals than similar clouds over northern oceans due to fewer sources of cloud‐nucleating aerosol particles in the region. In this work, we present an overview of aerosol particle types over the Southern Ocean, including new measurements made below, in and above clouds. These measurements indicate that while sea‐spray‐derived salts do provide cloud nuclei, the majority of aerosol particles that influence summertime clouds in this region are biogenic—that is, derived from ocean microorganisms, with the ocean region near Antarctica being a large summertime source. These cloud‐nucleating particles may increase in a warming climate likely to have less sea ice and more phytoplankton activity near Antarctica. These additional particles could make low clouds reflect more light and offset a portion of the warming expected due to diminishing sea ice in a future climate. Key Points Biogenic sulfate dominates the number concentration of 0.1–0.5 microns diameter particles and cloud condensation nuclei (CCN) over the summertime Southern Ocean Biogenic organics are a key component of ice nucleating particles over the Southern Ocean As Antarctic climate changes, increased biological activity could partially offset warming effects of sea‐ice loss via influences on CCN

Methane emissions from end-use installations in residential natural gas systems remain poorly quantified, despite their importance to both safety and climate policies worldwide. While distribution networks and appliances have received research attention, interior piping between the meter and appliances represents a critical knowledge gap. To address this gap, a systematic survey of 473 residential systems in Saarlouis, Germany, was conducted using standardized pressure decay tests (DVGW G 600). Measurements were performed during the installation of gas regulators necessitated by a grid pressure increase from 23 mbar to 55 mbar above ambient. This provided a unique opportunity to assess whole-system leakage under controlled conditions without installation modifications. Leak rates were standardized to reference pressure and converted to methane emissions using measured gas composition, using a linear pressure scaling as a provisional approximation valid for the small pressure differences in the applied test conditions. A total of 411 (86.9%) installations showed no detectable leak rate (LDL: 0.2 Lh−1). However, seven systems (1.5%) exceeded 1 Lh−1, and one surpassed the unacceptable threshold of 5 Lh−1. Mean emissions across all systems were 0.067 [0.041, 0.098] gh−1, with smaller installations showing higher volume-normalized rates. Critically, fewer than 1.48% of systems contributed more than 46% of total emissions, demonstrating a strongly skewed, heavy-tailed distribution. Scaled nationally using Monte Carlo methods accounting for sampling uncertainty and skewed distributions, residential interior piping contributes 12.30 [8.11, 18.55] Ggyear−1 to Germany’s methane emissions. These results emphasize the need to include residential leak rates in emission inventories and highlight the efficiency potential of targeted mitigation strategies focused on high-emitting installations under evolving EU methane regulations.

In the western US and similar topographic regions across the world, precipitation in the mountains is crucial to local and downstream freshwater supplies. Atmospheric aerosols can impact clouds and precipitation by acting as cloud condensation nuclei (CCN) and ice-nucleating particles (INPs). Previous studies suggest that there is increased aerosol variability in these regions due to their complex terrain, but none of these studies have quantified the extent of this variability. In the fall of 2021, Handix Scientific contributed to the Surface Atmosphere Integrated Field Laboratory (SAIL), funded by the US Department of Energy (DOE) and situated in the East River watershed (ERW; Colorado, USA), by deploying SAIL-Net, a novel network of six aerosol measurement nodes spanning the horizontal and vertical domains of SAIL. The ERW is a topographically diverse region, meaning individual measurement sites can miss important observations of aerosol–cloud interactions. Each measurement node included a small particle counter (the Portable Optical Particle Spectrometer – POPS), a miniature CCN counter (CloudPuck), and a filter sampler (the Time-Resolved Aerosol Particle Sampler – TRAPS) for INP analysis. SAIL-Net studied the spatiotemporal variability in aerosols and the usefulness of dense measurement networks in complex terrain. After the project's completion in the summer of 2023, we analyzed the data to explore these topics. We found increased variability compared to a similar study over flat land. This variability was correlated with the elevation of the sites, and the extent of the variability changed seasonally. These data and analyses serve as a valuable resource for continued research into the role of aerosols in the hydrologic cycle and as the foundation for designing measurement networks in complex terrain.

This work addresses the need for improved measurements of condensed water in clouds from research aircraft. First, new spectral line fitting codes are shown to improve retrievals of (H216O) in evaporated clouds by traditional near-infrared tunable diode laser spectroscopy. These codes are then applied to measurements of cloud water contents (CWC) with the University of Colorado Closed-Path Laser Hygrometer (CLH-2) during a series of flights in 2018 into clouds of varying types over the Southern Ocean. By comparing with observations of CWCs from cloud probes ranging from hot wires to droplet imagers, it is found that measurements of CWC using bulk water are generally accurate, but they can also be adversely affected by inlet artifacts such as icing, especially in the presence of mixed phases of water. Because the same can be true of other methods, there is currently no one instrument that accurately measures CWC under all conditions.Second, using knowledge gained from improvements in optics and electronics for CWC measurements, alongside improved retrievals of (H216O) abundances using sophisticated line-fitting methods developed for those CWC measurements, it is demonstrated through a series of laboratory studies that traditional hygrometer measurements of multiple isotopologues of water may be possible on research aircraft with distributed feedback lasers operating in the near-infrared (1.4 μm and 2.6 μm). It is proposed to conduct these measurements as the ratio of absorbances of pairs of adjacent absorption lines of two isotopologues (e.g., (H218O) and (H216O)) to characterize fractionation that can be used to elucidate important issues related to properties of clouds, such as liquid and ice-water contents and precipitation, and the microphysics of cloud particle formation. Through such measurements it should be possible to reduce outstanding uncertainties in cloud processes and the hydrological cycle, such as precipitation efficiency related to the aerosol indirect effect, mixed-phase cloud microphysics, and atmospheric transport and mixing in the vicinity of clouds. In addition, augmenting existing TDLAS instruments with the capability to detect isotopologue pairs will allow for characterization of inlet artifacts, such as condensation on sample lines and icing on inlet surfaces.

Our ability to forecast epidemics far into the future is constrained by the many complexities of disease systems. Realistic longer-term projections may, however, be possible under well-defined scenarios that specify the future state of critical epidemic drivers. Since December 2020, the U.S. COVID-19 Scenario Modeling Hub (SMH) has convened multiple modeling teams to make months ahead projections of SARS-CoV-2 burden, totaling nearly 1.8 million national and state-level projections. Here, we find SMH performance varied widely as a function of both scenario validity and model calibration. We show scenarios remained close to reality for 22 weeks on average before the arrival of unanticipated SARS-CoV-2 variants invalidated key assumptions. An ensemble of participating models that preserved variation between models (using the linear opinion pool method) was consistently more reliable than any single model in periods of valid scenario assumptions, while projection interval coverage was near target levels. SMH projections were used to guide pandemic response, illustrating the value of collaborative hubs for longer-term scenario projections. The US COVID-19 Scenario Modeling Hub produced medium to long term projections based on different epidemic scenarios. In this study, the authors evaluate 14 rounds of projections by comparing them to the epidemic trajectories that occurred, and discuss lessons learned for future similar projects.

In Spring 2021, the highly transmissible SARS-CoV-2 Delta variant began to cause increases in cases, hospitalizations, and deaths in parts of the United States. At the time, with slowed vaccination uptake, this novel variant was expected to increase the risk of pandemic resurgence in the US in summer and fall 2021. As part of the COVID-19 Scenario Modeling Hub, an ensemble of nine mechanistic models produced 6-month scenario projections for July–December 2021 for the United States. These projections estimated substantial resurgences of COVID-19 across the US resulting from the more transmissible Delta variant, projected to occur across most of the US, coinciding with school and business reopening. The scenarios revealed that reaching higher vaccine coverage in July–December 2021 reduced the size and duration of the projected resurgence substantially, with the expected impacts was largely concentrated in a subset of states with lower vaccination coverage. Despite accurate projection of COVID-19 surges occurring and timing, the magnitude was substantially underestimated 2021 by the models compared with the of the reported cases, hospitalizations, and deaths occurring during July–December, highlighting the continued challenges to predict the evolving COVID-19 pandemic. Vaccination uptake remains critical to limiting transmission and disease, particularly in states with lower vaccination coverage. Higher vaccination goals at the onset of the surge of the new variant were estimated to avert over 1.5 million cases and 21,000 deaths, although may have had even greater impacts, considering the underestimated resurgence magnitude from the model.