OZONE DIURNAL-VARIATIONS OBSERVED BY UARS AND THEIR MODEL SIMULATION

Several years of ozone measurements from the Microwave Limb Sounder on board the Upper Atmosphere Research Satellite are analyzed using a two -dimensional Fourier series in day of year and time of day. Because of limited temporal coverage near local noon, only the diurnal and semid iurnal components are included. Data are investigated in detail at 28 degrees N in the middle stratosphere to lower mesosphere, where the da ta are considered most reliable. The observations show that ozone is a maximum in the afternoon at 3 mbar and a minimum in the afternoon at 1 mbar and above with a narrow transition zone of reduced diurnal vari ation in between. This strong dependency on altitude in the transition from a maximum in the afternoon to a minimum in the afternoon, couple d with the small percentage changes in ozone, imposes strict requireme nts on the data and on the analysis of the data. Comparisons are made with results from a photochemical box model run at 11 levels between 0 .46 mbar and 21.5 mbar for 28 degrees N at spring equinox and near the solstices. This is the first time that a data analysis and model comp arison of this kind has been made, leading to the identification of re latively small diurnal variations, especially in the transition zone. In the middle stratosphere the model results are in poor agreement wit h the observations because of the influence of stratospheric dynamics which are neglected in the model runs. In the upper stratosphere the m odel shows the expected underprediction of absolute ozone amounts, alt hough the percentage change from the midnight value is in excellent ag reement with the observations and in particular correctly simulates th e diurnal variation in the transition zone between 3 and 1 mbar. Model sensitivity studies are performed to determine the effects of major r eaction rate changes and simplified tidal effects.