Gome Ozone Data Assimilation and the Ozone Mini-hole of 30 November 1999

Ozone transport models driven by high-quality analysed meteorological wind fields have been demonstrated to produce realistic ozone distributions. We describe a data assimilation system (TM3-DAM, version 3) which is currently in use to assimilate near-real time GOME ozone data, and which produces (three-day or longer) ozone forecasts. The model is a tracer transport model, called TM3, with a parameterised description of stratospheric ozone chemistry. The model is driven by meteorological fields from the ECMWF weather prediction model. The assimilation is based on near-real time level-2 ozone data from the GOME instrument on the ESA ERS-2 satellite. This data assimilation model is used to investigate the record low ozone values as observed over northwestern Europe by GOME, Brewer instruments and ozone sondes. Three possible causes, uplifting of the tropopause, chemical ozone destruction and horizontal transport, are investigated. The main cause of this event is found to be pole-ward advection of subtropical air with small ozone mixing ratios.