A Comprehensive Validation of Global Chemistry Transport Models Against Observations for a Complete 4-year Simulation Period (1995 to 1998)

As part of the EU funded project TRADEOFF (Aircraft Emissions: Contributions of various climate compounds to changes in composition and radiative forcing - tradeoff to reduce atmospheric impact) several European chemistry transport (CTM) and general circulation models (GCM) have been evaluated against observed tracer concentrations. The study focuses on the upper troposphere/lower stratosphere (UT/LS) region aiming to better assess the capability of the models to estimate the impact of aircraft emissions upon the atmosphere. For this purpose a comprehensive database of insitu observations of several compounds related to O₃ photochemistry was established for the 4-year period 1995 to 1998, covering observations from the most relevant commercial and scientific aircraft measurement campaigns and O₃ soundings. Tables of the positions and times of all observations available in a given month were supplied to the modellers and used to interpolate at every model time step the simulated tracer fields to the observation points. In this way a direct comparison of the models with observations taking into account the specific meteorological conditions during the measurements has been achieved. The models were run with a common database of surface and aircraft emissions (except for hydrocarbons) such that any differences between the models can be ascribed only to the different formulations of transport and chemistry. The results were evaluated with respect to the capability of the models to accurately simulate the observed concentration levels and variability, and the seasonal, latitudinal, and vertical distributions. The main results of this study will be presented. >http://www.lapeth.ethz.ch/~dominik/tradeoff/</a>