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.

中文翻译：

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夏季中纬度地区二氧化碳沿额线的分布，成因和沿边界的迁移

天气天气系统是大气CO ₂摩尔分数中空间梯度的主要驱动因素。在额叶通道期间，来自不同区域的空气团在额叶边界汇合，从而在CO ₂摩尔分数中产生明显的梯度。我们定量描述了中纬度冷锋通道期间CO ₂摩尔分数的大气传输，并探讨了各种来源的CO _2的影响。我们在这里集中讨论2016年8月4日在内布拉斯加州林肯上空的冷锋通道，该飞机在大气碳与美国运输运动期间被飞机观测到。沿前边界放置了一条带较高CO ₂的空气带。观察到的和模拟的CO ₂差异正面的浓度高达25 ppm。使用天气研究和天气预报模型以及化学方法在云分辨率（3 km）处进行了数值模拟，并结合了CarbonTracker（CT‐NRTv2017x）的CO ₂表面通量和边界条件，以探索前部的大气传输。模型结果表明，对流层高层的额叶CO ₂差异可以用北美以外的大量流入来解释。这种差异在大气边界层和对流层下部被大陆表面通量修正，在这种情况下，主要由生物和矿物燃料通量决定。发现水平和垂直对流是CO ₂传输的原因沿额边界的摩尔分数。我们表明，冷锋通道会导致大型的CO ₂传输事件，包括垂直对流的重大贡献，而大陆中部的锋面边界是由复杂的CO ₂混合气形成的。