GOME OZONE DATA ASSIMILATION AND THE OZONE MINI-HOLE OF 30 NOVEMBER 1999 Henk Eskes, Peter van Velthoven, Ghada El Serafy, and Hennie Kelder KNMI (Royal Netherlands Meteorological Institute), PO Box 201, 3730 AE De Bilt, The Netherlands Email: eskes@knmi.nl Abstract. 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. THE MODEL TRANSPORT The tracer transport model (TM3) has been adapted from the global Tracer transport Model TM2 [Heimann, 1995], and calculates the horizontal and vertical transport of tracer masses. It is driven by the meteorological fields (wind, surface pressure, temperature) from the European Centre for Medium-Range Weather Forecasts (ECMWF) model. These fields are updated every 6 hours. Since October 1999 the operational forecasts are performed on 60 vertical model levels, with a top layer at 0.1 hPa (the mesosphere). Compared to the 31 level ECMWF model version, operational until 1999 and with a top level at 10 hPa, this implies an enormous improvement in resolving the stratospheric flow. In this study the TM3 model is driven by the 60 level version. The horizontal resolution is currently 2.5 degrees. We use the second moments scheme [Prather, 1986] for the 3D advection. Apart from the ozone amount in each cell, this scheme explicitly defines three slopes and six second derivative fields in each cell. This results in a resolution which is effectively higher than the grid size would suggest. These first and second derivatives provide information about the variation of ozone inside a grid cell, and the ozone distributions are plotted on a 1.25 by 1.25 degree grid. LINEAR CHEMISTRY The production and loss of ozone in the stratosphere is described by the chemistry parametrisation developed by [Cariolle, 1986], see also [Jeuken, 1999]. The mixing ratio of ozone (χ) is modified according to, dχ dt = 〈S〉 + 〈 ∂S ∂χ 〉(χ −〈χ〉) +〈 ∂S ∂T 〉(T −〈T 〉)+ 〈 ∂S ∂ Φ 〉(Φ −〈Φ〉) (1) The right-hand side consists of a source term, a driving force towards an equilibrium ozone mixing ratio 〈χ〉, a response to a change in temperature T and a radiation term depending on the amount of ozone 〈Φ〉 above the current position. The relaxation time (related to 〈 ∂S ∂χ 〉) depends strongly on height, being of the order of months in the lower stratosphere, and