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International audience; We use 148 airborne vertical profiles of CO2 for frontal cases from the summer 2016 Atmospheric Carbon and Transport—America (ACT America) campaign to evaluate the skill of 10 global CO2 in situ inversion models from the version 7 Orbiting Carbon Observatory-2 (OCO 2) Model Intercomparison Project (MIP). Model errors (model posterior—observed CO2 dry air mole fractions) were categorized by region (Mid Atlantic, Midwest, and South), frontal sector (warm or cold), and transport model (predominantly Tracer Model 5 (TM5) and Goddard Earth Observing System—Chemistry [GEOS Chem]). All inversions assimilated the same CO2 observations. Overall, the median inversion profiles reproduce the general structures of the observations (enhanced/depleted low level CO2 in warm/cold sectors), but 1) they underestimate the magnitude of the warm/cold sector mole fraction difference, and 2) the spread among individual inversions can be quite large (>5 ppm). Uniquely in the Mid Atlantic, inversion biases segregated according to atmospheric transport model, where TM5 inversions biases were -3 to -4 ppm in warm sectors, while those of GEOS Chem were +2 to +3 ppm in cold sectors. The large spread among the mean posterior CO2 profiles is not explained by the different atmospheric transport models. These results show that the inversion systems themselves are the dominant cause of this spread, and that the aircraft campaign data are clearly able to identify these large biases. Future controlled experiments should identify which inversions best reproduce mid latitude CO2 mole fractions, and how inversion system components are linked to system performance.

Atmospheric transport model errors are a major contributor to uncertainty in CO2 inverse flux estimates. Our study compares CO2 mole fraction observations from the North American Carbon Program Mid-Continental Intensive (MCI) field campaign and modeled mole fractions from two atmospheric transport models: the global Transport Model 5 from NOAA's CarbonTracker system and the mesoscale Weather Research and Forecasting model. Both models are coupled to identical CO2 fluxes and lateral boundary conditions from CarbonTracker (CT2009 release). Statistical analyses were performed for two periods of 2007 using observed daily daytime average mole fractions of CO2 to test the ability of these models to reproduce the observations and to infer possible causes of the discrepancies. TM5-CT2009 overestimates midsummer planetary boundary layer CO2 for sites in the U.S. corn belt by 10 ppm. Weather Research and Forecasting (WRF)-CT2009 estimates diverge from the observations with similar magnitudes, but the signs of the differences vary from site to site. The modeled mole fractions are highly correlated with the observed seasonal cycle (r ≥ 0.7) but less correlated in the growing season, where weather-related changes in CO2 dominate the observed variability. Spatial correlations in residuals from TM5-CT2009 are higher than WRF-CT2009 perhaps due to TM5's coarse horizontal resolution and shallow vertical mixing. Vertical mixing appears to have influenced CO2 residuals from both models. TM5-CT2009 has relatively weak vertical mixing near the surface limiting the connection between local CO2 surface fluxes and boundary layer. WRF-CT2009 has stronger vertical mixing that may increase the connections between local surface fluxes and the boundary layer.

[1] Although boundary layer processes are important in climate, weather and air quality, boundary layer climatology has received little attention, partly for lack of observational data sets. We analyze boundary layer climatology over Europe and the continental U.S. using a measure of boundary layer height based on the bulk Richardson number. Seasonal and diurnal variations during 1981–2005 are estimated from radiosonde observations, a reanalysis that assimilates observations, and two contemporary climate models that do not. Data limitations in vertical profiles introduce height uncertainties that can exceed 50% for shallow boundary layers (