Airborne lidar measurements of XCO$_2$ in synoptically active environment and associated comparisons with numerical simulations

International audience; Frontal boundaries have been shown to cause large changes in CO$_2$ mole-fractions, but clouds and the complex vertical structure of fronts make these gradients difficult to observe. It remains unclear how the column average CO$_2$ dry air mole-fraction (XCO$_2$) changes spatially across fronts, and how well airborne lidar observations, data assimilation systems, and numerical models without assimilation capture XCO$_2$ frontal contrasts (ΔXCO$_2$, i.e., warm minus cold sector average of XCO$_2$). We demonstrated the potential of airborne Multifunctional Fiber Laser Lidar (MFLL) measurements in heterogeneous weather conditions (i.e., frontal environment) to investigate the ΔXCO$_2$ during four seasonal field campaigns of the Atmospheric Carbon and Transport-America (ACT-America) mission. Most frontal cases in summer (winter) reveal higher (lower) XCO$_2$ in the warm (cold) sector than in the cold (warm) sector. During the transitional seasons (spring and fall), no clear signal in ΔXCO$_2$ was observed. Intercomparison among the MFLL, assimilated fields from NASA's Global Modeling and Assimilation Office (GMAO), and simulations from the Weather Research and Forecasting-—Chemistry (WRF-Chem) showed that (a) all products had a similar sign of ΔXCO$_2$ though with different levels of agreement in ΔXCO$_2$ magnitudes among seasons; (b) ΔXCO$_2$ in summer decreases with altitude; and (c) significant challenges remain in observing and simulating XCO$_2$ frontal contrasts. A linear regression analyses between ΔXCO$_2$ for MFLL versus GMAO, and MFLL versus WRF-Chem for summer-2016 cases yielded a correlation coefficient of 0.95 and 0.88, respectively. The reported ΔXCO2 variability among four seasons provide guidance to the spatial structures of XCO$_2$ transport errors in models and satellite measurements of XCO$_2$ in synoptically-active weather systems.