Investigation of the tidal variations in a 3-D dynamics-chemistry-transport model of the middle atmosphere
Résumé
The Monitoring of Stratospheric Depletion of Ozone Layer (MSDOL) system is a software tool that is used to assimilate continuously satellite ozone data into a 3-D dynamics-chemistry-transport mechanistic model of the middle atmosphere. Because atmospheric tides lead to variations in temperature, which then affect ozone amounts, we suspect that the ability of the model to converge toward the true 3-D ozone field should therefore depend on how well atmospheric tides are simulated in the model. In this paper, the MSDOL model run without ozone data assimilation was used to explore the representation of the diurnal and semidiurnal tidal components between 10 and 80 km. Comparison with the predictions from the Global-Scale Wave Model (GSWM) showed the model to be in good qualitative agreement with GSWM results, despite quantitative differences. These differences might be explained by the differences in the representation of the background atmosphere, tidal forcing and tidal dissipation in MSDOL and GSWM. A series of sensitivity simulations were then conducted by changing some input parameters. The sensitivity of the tidal amplitude to changes of the tidal forcing and the effect of variations in the structure of the background wind and temperature fields were described; in particular, it was shown that significant fluctuations of the diurnal amplitude (∼20%) could be obtained due to variable tidal forcing. The results here provide some hints on the way the model should be used in the future. Indeed, they suggest the need for updating the model with revised tidal forcing, and for assimilating some part of the real background atmosphere in order to provide real-time estimates of the tides and improve the accuracy of the ozone data assimilation result.