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International audience; Three-dimensional chemical transport models (CTMs) and coupled chemistry-climate models (CCMs) have been widely used to study the dynamical and chemical processes which control polar ozone depletion. Despite generally good agreement between modelled and observed loss, there are still some uncertainties in our understanding and limitations of the models. For example, the accuracy of modelled springtime ozone loss depends critically on the transport, chemistry and treatment of polar stratospheric clouds (PSCs). There is also increasing evidence that descent of NOx from the mesosphere can impact ozone at high latitudes, even in the absence of halogen-catalysed loss. CCMs used for future predictions need to accurately include these processes.At the time of writing (late February 2016) the current Arctic winter has been cold with extensive PSC ac- tivity. The local maximum modelled O3 loss is currently ∼50% (www.see.leeds.ac.uk/tomcat), which is similar to that from previous cold years with record ozone depletion (e.g. 2010/11). This is interesting in its own right, but will also provide new meteorological conditions under which to test modelled loss against observations.We will present results from the TOMCAT/SLIMCAT off-line 3-D CTM and the NCAR Whole Atmo- sphere Community Climate Model (WACCM), which we have recently extended by including mesospheric ion chemistry. Both models are forced or nudged by the ECMWF ERA-Interim 4D-var reanalyses. First we will focus on the cold Arctic winter/spring 2015/16 and quantify the amount of chemical ozone loss using both models and satellite observations. Then we will investigate the chorine activation and denitrification as well as mesospheric NOx/HOx impact on stratospheric ozone. Other sensitivities experiments will be also discussed.

During the ten years of its operations, Envisat/GOMOS (Global Ozone Monitoring by Occultation of Stars) instrument has successfully demonstrated the capabilities of stellar occultation technique for atmospheric profiling. In this presentation we show highlights of GOMOS measurements: vertical profiles of O3, NO2, NO3 and aerosols, including polar mesospheric clouds and polar stratospheric clouds, using UV-VIS wavelengths and O2 and H2O using two near-IR channels. High vertical resolution temperature profiles are obtained using the time delay of the red and the blue photometers of GOMOS. The two photometers are also used for studying turbulence and gravity waves. Constituents with weak spectral signatures, such as OClO and Na, can be detected using temporal averaging. GOMOS measurements provide global coverage with about 200-400 daily measurements. Vertically the measurements extend from 10 km to 100 km with varying valid altitude range depending on constituent. Stellar occultation technique ensures high vertical resolution of 2-4 km, very accurate altitude registration and relatively simple data retrieval. The self-calibrating feature of the occultation technique is particularly suitable for long term trend analysis and thus crucial for studying the climate-chemistry interactions.

International audience

MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is operating on the ENVIronmental SATellite (ENVISAT) since March 2002. After two years of nearly continuous limb scanning measurements, at the end of March 2004, the instrument was stopped due to problems with the mirror drive of the interferometer. Operations with reduced maximum path difference, corresponding to both a reduced-spectral-resolution and a shorter measurement time, were resumed on January 2005. In order to exploit the reduction in measurement time, the measurement scenario was changed adopting a finer vertical limb scanning. The change of spectral resolution and of measurement scenario entailed an update of the data processing strategy. The aim of this paper is the assessment of the differences in the quality of the MIPAS ozone data acquired before and after the stop of the operations. Two sets of MIPAS ozone profiles acquired in 2003–2004 (full-resolution measurements) and in 2005–2006 (reduced-resolution measurements) are compared with collocated ozone profiles obtained by GOMOS (Global Ozone Monitoring by Occultation of Stars), itself also onboard ENVISAT. The continuity of the GOMOS data quality allows to assess a possible discontinuity of the MIPAS performances. The relative bias and precision of MIPAS ozone profiles with respect to the GOMOS ones have been compared for the measurements acquired before and after the stop of the MIPAS operations. The results of the comparison show that, in general, the quality of the MIPAS ozone profiles retrieved from reduced-resolution measurements is comparable or better than that obtained from the full-resolution dataset. The only significant change in MIPAS performances is observed at pressures around 2 hPa, where the relative bias of the instruments increases by a factor of 2 from the 2003–2004 to 2005–2006 measurements.