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

[摘要] We present a sensitivity study of the direct fitting approach to retrieve total ozone columns from the clear skyGlobal Ozone Monitoring Experiment 2/MetOp-A (GOME-2/MetOp-A) measurements between 325 and335 nm in the period 2007–2010. The direct fitting of the measurement is based on adjusting thescaling of a reference ozone profile and requires accurate simulation ofGOME-2 radiances. In this context, we study the effect of three aspects thatintroduce forward model errors if not addressed appropriately: (1) the use ofa clear sky model atmosphere in the radiative transfer demanding cloudfiltering, (2) different approximations of Earth's sphericity to address theinfluence of the solar zenith angle, and (3) the need of polarization inradiative transfer modeling. We conclude that cloud filtering using theoperational GOME-2 FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A band) cloud product, which is part of level 1B data, andthe use of pseudo-spherical scalar radiative transfer is fully sufficient forthe purpose of this retrieval. A validation with ground-based measurements at36 stations confirms this showing a global mean bias of −0.1 % with astandard deviation (SD) of 2.7 %. The regularization effect inherent to theprofile scaling approach is thoroughly characterized by the total columnaveraging kernel for each individual retrieval. It characterizes the effectof the particular choice of the ozone profile to be scaled by the inversionand is part of the retrieval product. Two different interpretations of thedata product are possible: first, regarding the retrieval product as anestimate of the true column, a direct comparison of the retrieved column withtotal ozone columns from ground-based measurements can be done. This requiresaccurate a priori knowledge of the reference ozone profile and the columnaveraging kernel is not needed. Alternatively, the retrieval product can beinterpreted as an effective column defined by the total column averagingkernel. This interpretation relies much less on the a priori knowledge of thereference ozone profile; however,for its validation, measurements of the verticalozone distribution are needed. The different manners of data interpretationare demonstrated for simulated and real measurements using on-ground ozonecolumn and ozonesonde measurements for validation.

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