Midlatitude stratosphere – troposphere exchange as diagnosed by MLS O3 and MOPITT CO assimilated fields
1CNRM-GAME, Météo-France and CNRS URA 1357, Toulouse, France
2Laboratoire d'Aérologie, Université de Toulouse, CNRS/INSU, Toulouse, France
3CNRM, Direction de la Météorologie Nationale, Casablanca, Morocco
4University of Maryland, Baltimore County, USA
5CERFACS, Toulouse, France
6Laboratoire de l'Atmosphère et des Cyclones, Université de La Réunion, France
Abstract. This paper presents a complete characterization of a very deep stratospheric intrusion which occurred over the British Isles on 15 August 2007. The signature of this event is diagnosed using ozonesonde measurements over Lerwick, UK (60.14° N, 1.19° W) and is also well characterized using meteorological analyses from the global operational weather prediction model of Météo-France, ARPEGE. Modelled as well as assimilated fields of both ozone (O3) and carbon monoxide (CO) have been used in order to better document this event. The paper also presents a demonstration of the capability of O3 and CO assimilated fields to better describe a stratosphere-troposphere exchange (STE) event in comparison with the free run modelled O3 and CO fields. O3 and CO from Aura/MLS and Terra/MOPITT instruments, respectively, are assimilated into the three-dimensional chemical transport model MOCAGE of Météo-France using a variational 3-D-FGAT (First Guess at Appropriate Time) method within the MOCAGE-PALM assimilation system. The usefulness of assimilated MOPITT CO data in a STE study is demonstrated in this novel result. The study shows that the use of the model MOCAGE gives consistent 3-D fields capable of describing the synoptic evolution of the event. However, modelled O3 and CO vertical distributions do not provide a quantitative evaluation of the intrusion. Although the assimilation of MLS data improves the distribution of O3 above the tropopause compared to the free model run, it is not sufficient to reproduce the stratospheric intrusion event well. Conversely, assimilated MOPITT CO allows a better description of the stratospheric intrusion event. Indeed, the horizontal distribution of the CO assimilated field is consistent with meteorological analyses. Moreover, the vertical distribution of the CO assimilated field is in accordance with the potential vorticity distribution and reveals a deeper intrusion from the lower stratosphere downward to the mid-troposphere compared to the O3 assimilated field. This study clearly demonstrates the capability of the assimilation of MOPITT CO to improve the CO distribution in the upper troposphere and lower stratosphere region. In addition, the behaviour of CO assimilated field is consistent with the synoptic evolution of the meteorological conditions. Therefore, the results of this study open the perspectives for using MOPITT CO in the STE studies.