Carbon Dioxide Distribution, Origins, and Transport Along a Frontal Boundary During Summer in Mid‐Latitudes

Abstract

Synoptic weather systems are a major driver of spatial gradients in atmospheric CO2 mole fractions. During frontal passages, air masses from different regions meet at the frontal boundary creating significant gradients in CO2 mole fractions. We quantitatively describe the atmospheric transport of CO2 mole fractions during a mid‐latitude cold front passage and explore the impact of various sources of CO2. We focus here on a cold front passage over Lincoln, Nebraska on August 4th, 2016 observed by aircraft during the Atmospheric Carbon and Transport‐America campaign. A band of air with elevated CO2 was located along the frontal boundary. Observed and simulated differences in CO2 across the front were as high as 25 ppm. Numerical simulations using Weather Research and Forecasting Model with Chemistry at cloud resolving resolutions (3 km), coupled with CO2 surface fluxes and boundary conditions from CarbonTracker (CT‐NRTv2017x), were performed to explore atmospheric transport at the front. Model results demonstrate that the frontal CO2 difference in the upper troposphere can be explained largely by inflow from outside of North America. This difference is modified in the atmospheric boundary layer and lower troposphere by continental surface fluxes, dominated in this case by biogenic and fossil fuel fluxes. Horizontal and vertical advection are found to be responsible for the transport of CO2 mole fractions along the frontal boundary. We show that cold front passages lead to large CO2 transport events including a significant contribution from vertical advection, and that midcontinent frontal boundaries are formed from a complex mixture of CO2 sources.

Joint CO2 Mole Fraction and Flux Analysis Confirms Missing Processes in CASA Terrestrial Carbon Uptake Over North America

“…The 8/4/2016 frontal case shows a narrow, elevated [CO 2 ] band at the frontal boundary (∼25 ppm difference in [CO 2 ] across the front). We examined this frontal case with a 3 × 3-km (cloud-resolving) resolution model and showed that this elevated [CO 2 ] band has a maximum width of ∼200 km and a length of over 800 km extending from northeastern Kansas to northeastern Iowa (Samaddar et al, 2021). Figure 2 shows that the frontal boundaries are associated not only with elevated [CO 2 ] but also with highly variable [CO 2 bio].…”

Atmospheric Carbon and Transport – America (ACT-America) Datasets: Description, Management, and Delivery

“…An elevated CO 2 band (416 ppm) was found at the frontal boundary, similar to results from the summer-2016 campaign (Figure 3a). This phenomenon appears closely tied to biological CO 2 fluxes (Samaddar et al, 2021), perhaps particularly respiratory fluxes (Hu et al, 2021). CO and CH 4 mole fractions were higher in the cold sector than in the warm sector (Figures 3d and 3e), likely indicative of large CH 4 emissions from animal agriculture in the upper Midwest and perhaps larger upwind anthropogenic CO sources in the upper Midwest (analyses in preparation).…”

Atmospheric Carbon and Transport – America (ACT‐America) Data Sets: Description, Management, and Delivery

“…An elevated CO2 band (416 ppm) was found at the frontal boundary, similar to results from the summer-2016 campaign (Figure 3a). This phenomena appears closely tied to biological CO2 fluxes (Samaddar et al, 2021), perhaps particularly respiratory fluxes (Hu et al, 2021). CO and CH4 mole fractions were higher in the cold sector than in the warm sector (Figure 3d and 3e), likely indicative of large CH4 emissions from animal agriculture in the upper Midwest and perhaps larger upwind anthropogenic CO sources in the upper Midwest (analyses in preparation).…”

“…Gaudet et al (2020), for example, evaluated the skill of ten global CO2 inversion models from the OCO-2 MIP using 148 airborne vertical profiles of CO2 for frontal cases from the ACT-America Summer 2016 campaign. High resolution models (Samaddar et al, 2021;Hu et al, 2021) of the elevated CO2 band observed along the frontal boundary shown in Pal et al (2020a) have been evaluated using ACT-America airborne data, as has a newly developed global CO2 transport model (Zheng et al, 2020). As noted previously, OCO-2 XCO2 variability at local to synoptic scales has been evaluated using ACT-America lidar and in situ observations (Bell et al, 2020).…”

“…The ACT-America team is analyzing how GHG distributions change vertically and horizontally across frontal boundaries (e.g., Pal et al, 2020a), and is comparing these observations to numerical simulations (Gerken et al, 2021;Samaddar et al, 2021). The 20 June 2019 RF described in Section 1.1 is an excellent illustration of the information available in one such flight.…”