PNNL

Abstract

Frontal boundaries have been shown to cause large changes in CO2 mole-fractions, but clouds and the complex vertical structure of fronts make these gradients difficult to observe. It remains unclear how the column average CO2 dry air mole-fraction (XCO2) changes spatially across fronts, and how well airborne lidar observations, in-situ data driven assimilation system (here Global Modeling and Assimilation Office, GMAO), and a numerical model without any optimization by in-situ data (WRF-Chem) capture XCO2 frontal contrasts (?XCO2, i.e., warm minus cold sector average of XCO2). We demonstrated the potential of airborne Multifunctional Fiber Laser Lidar (MFLL) measurements in heterogeneous weather conditions (i.e., frontal environment) to investigate the ?XCO2 during four seasonal field campaigns of the Atmospheric Carbon and Transport-America (ACT-America) mission. Most frontal cases in summer (winter) reveal higher (lower) XCO2 in the warm (cold) sector than in the cold (warm) sector. During the transitional seasons (spring and fall), no clear signal in ?XCO2 was observed. Intercomparison among the MFLL, WRF-Chem and GMAO (1) yielded similar sign of ?XCO2 though with varying ?XCO2 magnitudes among seasons; (2) evinced that ?XCO2 in summer decreases with altitude; and (3) illustrated how challenging it is to observe and simulate the ?XCO2. A linear regression analyses between ?XCO2 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 ?XCO2-variability among seasons provide guidance to the spatial structures of XCO2 transport errors in models and satellite measurements of XCO2 in synoptically-active weather systems.