Observations of Greenhouse Gas Changes Across Summer Frontal Boundaries in the Eastern United States. Issue 5 (2nd March 2020) - Pal, Sandip
Abstract: Our knowledge about synoptic‐scale variations of atmospheric‐CO2 produced by interactions between midlatitude cyclones and regional‐scale surface fluxes remains limited due to the scarcity of observations. We synthesized observations of greenhouse gases (GHGs) with respect to frontal passages to understand how GHG distributions change vertically and horizontally during a synoptic event. We use the airborne in situ measurements of GHGs collected during the Atmospheric Carbon and Transport‐America summer 2016 field campaign. Using these measurements, we defined three metrics, (1) the differences in the GHG mole fractions across frontal boundaries in the atmospheric boundary layer (BL) and free troposphere (FT), (2) differences in the vertical contrasts in GHGs in warm and cold sectors, and (3) the size and magnitude of enhanced CO2 in the vicinity of frontal boundary. We found that frontal structures are clearly associated with spatially coherent and significant changes in GHG composition. Warm sector CO2 mole fractions [CO2 ] are higher than those in the cold sector. The cross‐frontal [CO2 ] contrasts are largest in the BL (5–30 ppm) with smaller differences in the FT (3–5 ppm). We found higher [CH4 ] in the BL in the warm sector than in the cold sector for 5 out of 11 cases. Analyses of vertical profiles revealed higher [CO2 ] in the FT than in the BL in the cold sector while opposite pattern in the warm sector. Average BL‐to‐FT [CO2 ] differences are 12 and −6 ppm in the warm and cold sectors, respectively. The observational analyses presented define new metrics involving horizontal and vertical GHG contrasts across fronts during summer which will be used to evaluate simulations of GHG transport. Key Points: We report the first comprehensive airborne observations of the horizontal and vertical variability in atmospheric greenhouse gases (CO2, CH4 ) associated with midlatitude cyclones Cold fronts contain coherent CO2 structures including cross‐frontal contrasts at all levels in the troposphere and a narrow band of enhanced CO2 along the frontal boundary Fronts showed lower [CO2 ] in the cold sector (north/west) and higher [CO2 ] in the warm sector (south/east), with larger contrasts in boundary layer than in free troposphere