Ozone Forecasts of the Stratospheric Polar Vortex–Splitting Event in September 2002

The Southern Hemisphere major warming event in September 2002 has led to a breakup of the vortex in the middle and higher stratosphere and to a corresponding splitting of the ozone hole. Daily 3D ozone forecasts, produced at the Royal Netherlands Meteorological Institute (KNMI) with a tracer transport and assimilation model based on the ECMWF dynamical forecasts, provided an accurate prediction of this event a week prior to the actual breakup of the vortex. The ozone forecast model contains parameterizations for gas phase and heterogeneous chemistry. Initial states for the forecast are obtained from the assimilation of near-real-time ozone data from the Global Ozone Monitoring Experiment (GOME) on European Space Agency (ESA) Remote Sensing Satellite-2 (ERS-2). In this paper, the ozone forecasts and analyses are discussed as produced before, during, and after the event. These fields are compared with ground-based Dobson, ozonesonde, and Total Ozone Mapping Spectrometer (TOMS) observations. The total ozone comparisons show that the location of the vortex edge is generally well described by the 5–7-day forecasts in September and October. The GOME assimilation compared with TOMS shows a good correspondence concerning vortex location and ozone features but also reflects clear differences in the average ozone amount between the two retrieval schemes. The assimilation system produces realistic ozone profiles, apart from a systematic underestimation of ozone around 150 hPa inside the vortex in August–October.