Abstract

Empirical orthogonal functions are used together with standard statistical metrics to evaluate the ability of models with different spatial resolutions to reproduce observed patterns of ozone (O₃) in the eastern United States in the summer of 1995. We examine simulations with the regional MAQSIP model (horizontal resolution of 36 km² and the global GEOS-CHEM model(2^o x 2.5^o and 4^o x 5^o). As the model resolution coarsens, the ability to resolve local O₃ maxima (O₃ >= 90 ppbv) is compromised but the spatial correlation improves. This result shows that the synoptic-scale processes modulating O₃ concentrations are easier to capture in models than the processes occurring on smaller scales. Empirical orthogonal functions (EOFs) derived from the observed O₃ fields reveal similar modes of variability when averaged onto the three model horizontal resolutions. The EOFs appear to represent (1) an east-west pattern associated with frontal passages, (2) a midwest-northeast pattern associated with migratory high pressure systems, and (3) a southeast stagnation pattern linked to westward extension of the Bermuda High. All models capture the east-west and southeast EOFs but the midwest-northeast EOF is misplaced in GEOS-CHEM. GEOS-CHEM captures the principal components of the observational EOFs when the model fields are projected onto these EOFs, implying that it can resolve the contribution of the EOFs to the observed variance. We conclude that coarse-resolution global models can successfully simulate the synoptic conditions leading to high-O₃ episodes in the eastern United States.