Impact of IASI thermal infrared measurements on global ozone reanalyses

Abstract

Abstract. The information content of thermal infrared measurements for tropospheric O3 estimation has already been well demonstrated. However, the impact of such measurements to constrain modelled ozone distributions within global assimilation systems is not yet unequivocal. A new tropospheric O3 reanalysis is computed for the year 2010 by means of assimilating measurements from the Infrared Atmospheric Sounding Interferometer (IASI) within the MOCAGE chemical transport model. The objective is to evaluate the impact of recent methodological improvements of the data assimilation scheme on the O3 distribution. The new O3 reanalysis (named IASI-r) and its precursor (IASI-a) have been validated against ozonesondes, compared to independent estimations of tropospheric O3 and to results from two state-of-the-art models based on detailed tropospheric chemistry (GEOS-CCM and C-IFS). The main difference between IASI-r and the former IASI-a concerns the treatment of IASI observations, with radiances being assimilated directly in IASI-r instead of intermediate Level 2 O3 retrievals. IASI-r is found to correct major issues of IASI-a, i.e. the neutral or negative impact of IASI assimilation in the extra-tropics and the presence of residual biases in the tropics. IASI-r compares also relatively well to the C-IFS reanalysis, which is based on more comprehensive chemical mechanism and the assimilation of several UV and microwave measurements.