Evaluation of tropospheric ozone columns derived from assimilated GOME ozone profile observations

Abstract. Tropospheric O3 column estimates are produced and evaluated from spaceborne O3 observations by the subtraction of assimilated O3 profile observations from total column observations, the so-called Tropospheric O3 ReAnalysis or TORA method. Here we apply the TORA method to six years (1996–2001) of ERS-2 GOME/TOMS total O3 and ERS-2 GOME O3 profile observations using the TM5 global chemistry-transport model with a linearized O3 photochemistry parameterization scheme.

Free running TM5 simulations show good agreement with O3 sonde observations in the upper-tropospheric and lower stratospheric (UTLS). Assimilation of GOME O3 profile observations improves the comparisons in the tropical UTLS region but slightly degrades the model-to-sonde comparisons in the extra-tropical UTLS for both short day-do-day variability as well as for monthly means. We suggest that this degradation is related to the large ground pixel size of the GOME O3 measurements (960×100 km) in combination with retrieval and calibration errors. The assimilation of GOME O3 profile observations does counter the gradual multiyear mid-latitude stratospheric O3 accumulation caused by the overstrong stratospheric meridional circulation in TM5.

The evaluation of daily and monthly tropospheric O3 columns obtained from total column observations and using the TORA methodology shows realistic residuals within the tropics but unrealistically large deviations outside of the tropics, although average biases remain small for the monthly means. The findings of this paper suggest that improvements can be expected by using O3 observations from present-day instruments like MetOp/GOME-2 and EOS-AURA/OMI.