The direct fitting approach for total ozone column retrievals: a sensitivity study on GOME-2/MetOp-A measurements

We present a sensitivity study of the direct fitting approach to retrieve total ozone columns from the clear sky Global Ozone Monitoring Experiment 2/MetOp-A (GOME-2/MetOp-A) measurements between 325 and 335 nm in the period 2007–2010. The direct fitting of the measurement is based on adjusting the scaling of a reference ozone profile and requires accurate simulation of GOME-2 radiances. In this context, we study the effect of three aspects that introduce forward model errors if not addressed appropriately: (1) the use of a clear sky model atmosphere in the radiative transfer demanding cloud filtering, (2) different approximations of Earth's sphericity to address the influence of the solar zenith angle, and (3) the need of polarization in radiative transfer modeling. We conclude that cloud filtering using the operational GOME-2 FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A band) cloud product, which is part of level 1B data, and the use of pseudo-spherical scalar radiative transfer is fully sufficient for the purpose of this retrieval. A validation with ground-based measurements at 36 stations confirms this showing a global mean bias of −0.1 % with a standard deviation (SD) of 2.7 %. The regularization effect inherent to the profile scaling approach is thoroughly characterized by the total column averaging kernel for each individual retrieval. It characterizes the effect of the particular choice of the ozone profile to be scaled by the inversion and is part of the retrieval product. Two different interpretations of the data product are possible: first, regarding the retrieval product as an estimate of the true column, a direct comparison of the retrieved column with total ozone columns from ground-based measurements can be done. This requires accurate a priori knowledge of the reference ozone profile and the column averaging kernel is not needed. Alternatively, the retrieval product can be interpreted as an effective column defined by the total column averaging kernel. This interpretation relies much less on the a priori knowledge of the reference ozone profile; however, for its validation, measurements of the vertical ozone distribution are needed. The different manners of data interpretation are demonstrated for simulated and real measurements using on-ground ozone column and ozonesonde measurements for validation