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A rigorous evaluation of five global Chemistry-Transport and twoChemistry-Climate Models operated by several different groups in Europe, was performed. Comparisons were made of the models with trace gasobservations from a number of research aircraft measurement campaigns during the four-year period1995-1998. Whenever possible the models were run over the same four-year period and at each simulation time step theinstantaneous tracer fields were interpolated to all coinciding observation points. This approach allows for a very closecomparison with observations and fully accounts for the specific meteorological conditions during the measurement flights. This isimportant considering the often limited availability and representativity of such trace gas measurements. A new extensivedatabase including all major research and commercial aircraft measurements between 1995 and 1998, as well as ozonesoundings, was established specifically to support this type of direct comparison. Quantitative methods were applied to judgemodel performance including the calculation of average concentration biases and the visualization of correlations and RMSerrors in the form of so-called Taylor diagrams. We present the general concepts applied, the structure and content of thedatabase, and an overall analysis of model skills over four distinct regions. These regions were selected to represent variousatmospheric conditions and to cover large geographical domains such that sufficient observations are available for comparison. The comparisonof model results with the observations revealed specific problems for each individual model. This study suggests the furtherimprovements needed and serves as a benchmark for re-evaluations of such improvements. In general all models showdeficiencies with respect to both mean concentrations and vertical gradients of important trace gases. These include ozone, CO andNOx at the tropopause. Too strong two-way mixing across the tropopause issuggested to be the main reason for differences between simulated and observed CO and ozone values. The generally poor correlationsbetween simulated and measured NOx values suggest that in particular theNOx input by lightning and the convective transport from the polluted boundary layer are still not welldescribed by current parameterizations, which may lead to significant differences in the spatial and seasonal distributionof NOx in the models. Simulated OH concentrations, on the otherhand, were found to be in surprisingly good agreement with measured values

atmospheric chemistry and physics

An evaluation of the performance of chemistry transport models by comparison with research aircraft observations. Part 1: Concepts and overall model performance