Assimilation of stratospheric ozone in the chemical transport model STRATAQ

We describe a sequential assimilation approach useful for assimilating tracer measurements into a three- dimensional chemical transport model (CTM) of the strato- sphere. The numerical code, developed largely according to Khattatov et al. (2000), uses parameterizations and sim- plifications allowing assimilation of sparse observations and the simultaneous evaluation of analysis errors, with reason- able computational requirements. Assimilation parameters are set by using χ 2 and OmF (Observation minus Forecast) statistics. The CTM used here is a high resolution three- dimensional model. It includes a detailed chemical pack- age and is driven by UKMO (United Kingdom Meteorolog- ical Office) analyses. We illustrate the method using assim- ilation of Upper Atmosphere Research Satellite/Microwave Limb Sounder (UARS/MLS) ozone observations for three weeks during the 1996 antarctic spring. The comparison of results from the simulations with TOMS (Total Ozone Mapping Spectrometer) measurements shows improved total ozone fields due to assimilation of MLS observations. More- over, the assimilation gives indications on a possible model weakness in reproducing polar ozone values during spring- time.