Seven years of IASI ozone retrievals from FORLI: validation with independent total column and vertical profile measurements

This paper presents an extensive inter-comparison and validation for the ozone (O3) product measured by the two Infrared Atmospheric Sounding Interferometers (IASI) launched onboard the Metop-A and Metop-B satellites in 2006 and in 2012, respectively. IASI O3 total columns and vertical profiles obtained from Fast Optimal Retrievals on Layers for IASI (FORLI-O3) v20140922 software (running up until recently) are validated against independent observations during the period 2008–2014 on a global scale. On average for the period 2013–2014, IASI-A and IASI-B TOCs retrieved using FORLI are consistent, with IASI-B providing slightly lower values with a global difference of only 0.2±0.8 %. The comparison between IASI-A and IASI-B O3 vertical profiles shows differences within ±2 % over the entire altitude range. Global validation results for seven years of IASI TOCs from FORLI against GOME-2/Metop-A, Dobson and Brewer data show that, on average, IASI overestimates the UV data by 5–6 % with the largest differences found in the Southern high latitudes. The comparison with UV-vis SAOZ measurements shows a mean bias between IASI and SAOZ TOCs of 2–4 % in the mid-latitudes and tropics, and 7 % at the polar circle. Part of the discrepancies found at high latitudes can be attributed to the limited information content in the observations, due to low brightness temperatures. The comparison with ozonesonde vertical profiles (limited to 30 km) shows that on average IASI with FORLI processing underestimates O3 by ~5–15 % in the troposphere while it overestimates O3 by ~10–40 % in the stratosphere depending on the latitude. In the Northern middle latitudes, the bias varies within ±20 % for the entire altitude range. The largest relative differences are found in the tropical tropopause region; this can be explained by the low O3 amounts leading to large relative errors. In this study, we also evaluate an updated version of FORLI-O3 retrieval software (v20151001), using look-up tables recalculated to cover a larger spectral range using the latest HITRAN spectroscopic database (HITRAN 2012), and implementing numerical corrections. The assessement of the new O3 product with the same set of observations as that used for the validation exercise shows a correction of ~4 % for the TOC positive bias when compared to the UV ground-based and satellite observations, bringing the overall global comparison to ~1–2 % on average. This improvement is mainly associated with a decrease in the retrieved O3 concentration in the stratosphere (above 30 hPa/25 km) as shown by the comparison with ozonesonde data.