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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/amt-2020-5
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-5
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 28 Feb 2020

Submitted as: research article | 28 Feb 2020

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This preprint is currently under review for the journal AMT.

A tropopause-based a priori for IASI-SOFRID Ozone retrievals: improvements and validation

Brice Barret1, Emanuele Emili2, and Eric Le Flochmoen1 Brice Barret et al.
  • 1Laboratoire d’Aérologie/OMP, Université de Toulouse, Toulouse, France
  • 2CECI, Université de Toulouse, CERFACS, CNRS, Toulouse, France

Abstract. The Metop/IASI instruments provide data for operational meteorology and document atmospheric composition since 2007. IASI Ozone (O3) data have been used extensively to characterize the seasonal and interrannual variabilities and the evolution of tropospheric O3 at the global scale. The SOFRID (SOftware for a Fast Retrieval of IASI Data) is a fast retrieval algorithm that provides IASI O3 profiles for the whole IASI period. Up to now SOFRID O3 retrievals (v1.5 and 1.6) were performed with a single a priori profile which resulted in important biases and probably a too low variability. For the first time we have implemented a dynamical a priori profile for spaceborne O3 retrievals which takes the pixel location, time and tropopause height into account for SOFRID-O3 v3.5 retrievals. In the present study we validate SOFRID-O3 v1.6 and v3.5 with ECC ozonesonde profiles from the global WOUDC database for the 2008–2017 period. Our validation is based on a thorough statistical analysis using Taylor diagrams. Furthermore we compare our retrievals with ozonesonde profiles both smoothed by the IASI averaging kernels and raw. This methodology is essential to evaluate the inherent usefulness of the retrievals to assess O3 variability and trends. The use of a dynamical a priori largely improves the retrievals concerning two main aspects: (i) it corrects high biases for low-tropospheric O3 regions such as the southern hemisphere (ii) it increases the retrieved O3 variability leading to a better agreement with ozonesonde data. Concerning UTLS and stratospheric O3 the improvements are less important and the biases are very similar for both versions. The SOFRID Tropospheric Ozone Columns (TOC) display no significant drifts (< 2.5 %) for the northern hemisphere and significant negative ones (9.5 % for v1.6 and 4.3 % for v3.5) for the southern hemisphere. We have compared our validation results to those of the FORLI retrieval software from the literature for smoothed ozonesonde data only. This comparison highlights three main differences: (i) FORLI retrievals contain more theoretical information about tropospheric O3 than SOFRID (ii) RMSDs are smaller and correlation coefficients are higher for SOFRID than for FORLI (iii) in the northern hemisphere, no significant temporal drift is detected in SOFRID contrarily to FORLI (∼ 8 %).

Brice Barret et al.

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Brice Barret et al.

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Short summary
The IASI satellite sensor are used to document the variability and evolution of tropospheric Ozone (O3). IASI O3 retrievals generally use a single a priori profile which can be responsible for biases and low variability. We have therefore implemented a dynamical a priori based on pixel location, date and tropopause height. Comparison with 10 years of global ozonesonde profiles shows large improvements in the retrieved tropospheric O3 with biases corrected and enhanced variabilities.
The IASI satellite sensor are used to document the variability and evolution of tropospheric...
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