Articles | Volume 18, issue 9
Atmos. Chem. Phys., 18, 6939–6958, 2018
Atmos. Chem. Phys., 18, 6939–6958, 2018

Research article 17 May 2018

Research article | 17 May 2018

Multi-year assimilation of IASI and MLS ozone retrievals: variability of tropospheric ozone over the tropics in response to ENSO

Hélène Peiro1, Emanuele Emili1, Daniel Cariolle1,2, Brice Barret3, and Eric Le Flochmoën3 Hélène Peiro et al.
  • 1CECI, Université de Toulouse, Cerfacs, CNRS, Toulouse, France
  • 2Météo-France, Toulouse, France
  • 3Laboratoire d'Aérologie, Université de Toulouse, CNRS, UPS, Toulouse, France

Abstract. The Infrared Atmospheric Sounder Instrument (IASI) allows global coverage with very high spatial resolution and its measurements are promising for long-term ozone monitoring. In this study, Microwave Limb Sounder (MLS) O3 profiles and IASI O3 partial columns (1013.25–345 hPa) are assimilated in a chemistry transport model to produce 6-hourly analyses of tropospheric ozone for 6 years (2008–2013). We have compared and evaluated the IASI-MLS analysis and the MLS analysis to assess the added value of IASI measurements.

The global chemical transport model MOCAGE (MOdèle de Chimie Atmosphérique à Grande Echelle) has been used with a linear ozone chemistry scheme and meteorological forcing fields from ERA-Interim (ECMWF global reanalysis) with a horizontal resolution of 2°  ×  2° and 60 vertical levels. The MLS and IASI O3 retrievals have been assimilated with a 4-D variational algorithm to constrain stratospheric and tropospheric ozone respectively. The ozone analyses are validated against ozone soundings and tropospheric column ozone (TCO) from the OMI-MLS residual method. In addition, an Ozone ENSO Index (OEI) is computed from the analysis to validate the TCO variability during the ENSO events.

We show that the assimilation of IASI reproduces the variability of tropospheric ozone well during the period under study. The variability deduced from the IASI-MLS analysis and the OMI-MLS measurements are similar for the period of study. The IASI-MLS analysis can reproduce the extreme oscillation of tropospheric ozone caused by ENSO events over the tropical Pacific Ocean, although a correction is required to reduce a constant bias present in the IASI-MLS analysis.

Short summary
Tropospheric ozone (O3) is the third most important greenhouse gas. Several sensors on satellites measure O3 concentration like the infrared sounding IASI of MetOp satellites with a spatiotemporal coverage never reached so far. The assimilation of tropospheric IASI data within the chemistry transport model MOCAGE gives a good representation of O3 variability over the tropics linked with ENSO and the Walker circulation. IASI assimilation can contribute to the long-term monitoring of O3.
Final-revised paper