Characterization of Regional-Scale CO₂ Transport Uncertainties in an Ensemble with Flow-Dependent Transport Errors

Inference of CO₂ surface fluxes using atmospheric CO₂ observations in atmospheric inversions depends critically on accurate representation of atmospheric transport. Here we characterize regional-scale CO₂ transport uncertainties due to uncertainties in meteorological fields using a mesoscale atmospheric model and an ensemble of simulations with flow-dependent transport errors. During a 1-month summer period over North America, transport uncertainties yield an ensemble spread in instantaneous CO₂ at 100 m above ground level comparable to the CO₂ uncertainties resulting from 48% relative uncertainty in 3-hourly natural CO₂ fluxes. Temporal averaging reduces transport uncertainties but increases the influence of CO₂ uncertainties from the lateral boundaries. The influence of CO₂ background uncertainties is especially large for column-averaged CO₂. These results suggest that transport errors and CO₂ background errors limit regional atmospheric inversions at two distinct timescales and that the error characteristics of transport and background errors should guide the design of regional inversion systems.