Observations of Synoptic and Meso Scale Variability in Greenhouse Gases across Fronts in Four Seasons

Diverse meteorological processes associated with mid-latitude cyclones (e.g. cold and warm fronts, dry lines, meso-scale convective systems) play distinct roles in distributing atmospheric greenhouse gases (GHG, e.g., CO₂, CH₄) and other tracer atmospheric constituents (e.g. aerosols, water vapor, O₃). However, our understanding of the horizontal structures and vertical variability in GHGs across frontal boundaries remains limited. The Atmospheric Carbon and Transport–America (ACT-America, a NASA Earth Venture Suborbital project) mission collected the first measurements of atmospheric GHGs and state variables across many different fronts within the atmospheric boundary layer (ABL) and free troposphere (FT) covering all four seasons (summer 2016, winter 2017, fall 2017, and spring 2018) in three regions of the United States (Mid-Atlantic, Mid-West, and South). By deploying two NASA aircraft (C-130 and B-200), we collected more than 850 hours of high-resolution (5s) airborne in-situ and remote sensing measurements with sufficient spatial resolution (500m) to detect GHG distributions, and adequate spatial coverage (up to 1800 km within a single research flight) to cover a large fraction of the GHG and meteorological structures associated with the mid-latitude storms. In this work, we report on the magnitude of GHG frontal structure in the ABL and FT using measurements from 39 research flights conducted across frontal environments, and discuss the mechanisms governing the frontal contrasts over the three regions and four seasons.

Results indicate that for all seasons except for summer, we observed higher CO₂ in the cold sector than in the warm sector. We also found higher GHG frontal contrasts in the ABL compared to the FT as well as larger case-to-case variability in GHG frontal contrasts in the ABL compared to the FT in all four seasons. Additionally, we found the magnitude of CO₂ frontal contrast in the ABL was higher in summer (5-30 ppm) than during the other three seasons (2-8 ppm). A synthesis of all frontal research flights across the four seasons showed a higher seasonal cycle amplitude (peak to trough) in the cold sector air mass (25 ppm) than the warm sector air mass (7 ppm) presumably the result of the combined roles of synoptic scale transport, meso-scale processes, and different seasonal patterns of underlying sources and sinks of CO₂. We will show how the results of GHG frontal contrasts in the ABL compare with those in the pre- and post-frontal air masses observed by the US continental tower network. Our results confirm that the ACT-America field campaigns successfully sampled a wide range of synoptic systems across the four seasons and that these observational findings are generally representative of the eastern half of the U.S. These results will help us to improve our understanding of atmospheric transport of greenhouse gases by mid-latitude cyclones and the associated regional carbon fluxes.