An ensemble assessment of regional ozone model uncertainty with an explicit error representation

In this paper, we examine the extent to which an ensemble generated from a single air quality model correctly represents the global uncertainty of ozone simulations with a regional scale chemistry-transport model and whether it is suitable for ozone data assimilation. An ensemble of 30 members is constructed from a reference simulation in which the model parameters that are the most uncertain for determining ozone concentrations are randomly perturbed. Comparisons of the simulated ensemble using the model CHIMERE with observations are examined over the summer season both at the surface and higher in the troposphere. Although the ensemble overestimates vertical and horizontal correlation between errors, the average model error is well represented both at the surface and in the vertical dimension (after an adjustment for the latter). A variability of about 7–8 ppb is found. The results found in this study with respect to tropospheric ozone model errors and error correlation lengths both from ensemble simulations and model to observation comparisons are of particular interest for data assimilation when constructing model error covariance matrices.