Impact of higher-resolution meteorological data on ARAC predictions during ETEX

The European Tracer Experiment (ETEX) consisted of two principal phases. In the first phase, conducted in realtime, participants made predictions of the concentration of a released gas at many locations across Europe, acting in a simulated emergency response mode. In the second phase, participants were not under time restrictions, and all were provided with access to identical high-quality meteorological data from the European Centre for Medium-Range Weather Forecasts (ECMWF). The Atmospheric Release Advisory Capability (ARAC) participated in both phases of ETEX, achieving much better results in Phase II. In addition to using the ECMWF data, ARAC made other changes to its system for Phase II, including using newer versions of its models and using revised values of input model parameters. This paper examines the significance of each of these factors. (author)