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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 10 Jan 2019

Research article | 10 Jan 2019

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Measurement Techniques (AMT).

Comparison between the assimilation of IASI Level 2 retrievals and Level 1 radiances for ozone reanalyses

Emanuele Emili1, Brice Barret2, Eric Le Flochmoën2, and Daniel Cariolle1 Emanuele Emili et al.
  • 1CECI, Université de Toulouse, Cerfacs, CNRS, Toulouse, France
  • 2Laboratoire d’Aérologie, Université de Toulouse, CNRS, UPS, Toulouse, France

Abstract. The prior information used for Level 2 (L2) retrievals in the thermal infrared can influence the quality of the retrievals themselves and, therefore, their further assimilation in atmospheric composition models. In this study we evaluate the differences between assimilating L2 ozone profiles and Level 1 (L1) radiances from the Infrared Atmospheric Sounding Interferometer (IASI). We minimized potential differences between the two approaches by employing the same radiative transfer code (RTTOV) and a very similar setup for both the L2 retrievals (1D-Var) and the L1 assimilation (3D-Var). We computed hourly 3D-Var reanalyses assimilating respectively L1 and L2 data in the chemical transport model MOCAGE and compared the resulting O3 fields among each other and against ozonesondes. We also evaluated the joint assimilation of limb measurements from the Microwave Limb Sounder (MLS) on top of IASI to assess the impact of stratospheric O3 on tropospheric reanalyses. Results indicate that significant differences can arise between L2 and L1 assimilation, especially in regions where the L2 prior is biased (at the tropics and in the southern hemisphere in this study). L1 and L2 assimilation give instead very similar results in the northern hemisphere, especially when MLS measurements are used to constrain the stratospheric O3 column. We conclude this study by listing remaining issues that are common to both the L1 and L2 approaches and that deserve further research.

Emanuele Emili et al.
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Status: final response (author comments only)
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Emanuele Emili et al.
Emanuele Emili et al.
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Publications Copernicus
Short summary
We examine the differences between assimilating ozone profiles retrieved from IASI and the corresponding infrared spectra in a chemical transport model. The motivation was to quantify the impact of the retrieval's prior on ozone reanalyses. We found that significant differences arise between the two approaches, depending on the latitude. In general, superior results are obtained assimilating directly IASI radiances. This suggests interesting implications for highly coupled Earth system models.
We examine the differences between assimilating ozone profiles retrieved from IASI and the...