Seasonal and diurnal ozone variations:: Observations and modeling

Ozone mixing ratios observed by the Bordeaux microwave radiometer between 1995 and 2002 in an altitude range 25 - 75 km show diurnal variations in the mesosphere and seasonal variations in terms of annual and semi-annual oscillations (SAO) in the stratosphere and in the mesosphere. The observations with 10 - 15 km altitude resolution are presented and compared to photochemical and transport model results. Diurnal ozone variations are analyzed by averaging the years 1995- 1997 for four representative months and six altitude levels. The photochemical models show a good agreement with the observations for altitudes higher than 50 km. Seasonal ozone variations mainly appear as an annual cycle in the middle and upper stratosphere and a semi-annual cycle in the mesosphere with amplitude and phase depending on altitude. Higher resolution ( 2 km) HALOE ( halogen occultation experiment) ozone observations show a phase reversal of the SAO between 44 and 64 km. In HALOE data, a tendancy for an opposite water vapour cycle can be identified in the altitude range 40 - 60 km. Generally, the relative variations at all altitudes are well explained by the transport model ( up to 54 km) and the photochemical models. Only a newly developed photochemical model (1-D) with improved time-dependent treatment of water vapour profiles and solar flux manages to reproduce fairly well the absolute values.