Validation of the S5P TROPOMI L2_O3_PR ozone profile data entails quantitative comparisons to independent reference measurements collected from ground-based monitoring networks, cross-validation with other satellite instruments, assessment of the retrieved information content based on the analysis of the associated averaging kernels, and visual inspections of daily maps of S5P ozone data and associated parameters.
Comparison results between ground-based reference measurements and coincident TROPOMI L2_O3_PR pixels (overpass pixel on the same day with qa_value > 0.5) are obtained through the versatile Multi-TASTE validation system at BIRA-IASB, as part of both S5P MPC and S5PVT CHEOPS-5p validation activities. Prior to their comparison to S5P data, ground-based measurements – acquired at higher vertical resolution than S5P profile data – are convolved with the averaging kernels associated with the S5P retrievals to account for vertical smoothing differences (see Rodgers and Connor, 2003, Calisesi et al. 2005, Keppens et al., 2019). The difference of L2_O3_PR ozone number density values with respect to ozonesonde measurements is reported as a function of a selection of influence quantities, together with the information content diagnostics: vertical sensitivity, altitude registration offset, and averaging kernel full width at half maximum (FWHM). The geographical distribution of the ozonesonde stations indicates the domain of applicability of the validation results.
For the routine validation of the S5P/TROPOMI ozone profiles, the automated validation server (AVS) deployed within the MPC VDAF facility is used. This facility collects S5P ozone profile data and correlative measurements to identify suitable co-locations, compare the co-located data, and produce S5P data quality indicators. The VDAF-AVS produces curtain plots (ozone number density as a function of altitude and time) of the satellite data at a selection of ground-based ozonesonde stations, together with curtain plots showing the difference between S5P and ground-based data. The VDAF-AVS also provides statistical estimates of the bias and dispersion of S5P data with respect to the ground-based measurements.
Comparison to other satellite data extend ground-based validation to the global domain and increase the number of data comparisons. For stratospheric ozone, comparisons to limb and solar occultation sounders (MLS, OMPS-limb, ACE-FTS) are appropriate. For tropospheric ozone, comparisons can be made to OMI and OMPS-nadir, where the OMPS-nadir measurements have the best spatial and temporal co-registration with TROPOMI.
Analysis of information content:
The information content of the S5P ozone profile data is assessed through algebraic analysis of the associated averaging kernel matrix (AK) generated by the same S5P processing algorithm. The row sums of the AK matrix indicate the vertical sensitivity of the S5P ozone profile retrieval (Rodgers, 2000). The trace of the AK kernel matrix gives the Degree of Freedom of the Signal (DFS), to be understood here as the amount of vertical sub-columns with independent ozone information from each other. The Full Width at Half Maximum (FWHM) of the AK corresponding to a given altitude gives an indication of the vertical resolution of the retrieved profile at this altitude. This effective resolution of the retrieved information is not the numerical resolution of the vertical grid used for the retrieval process, usually much higher than the true, physical resolution of the retrieved information. The true altitude registration of the retrieved profile information at a given altitude of calculation can be estimated as the barycentre of the associated AK at this calculation altitude.
Daily global maps:
The S5P MPC VDAF creates daily global maps of the six partial columns provided in the ozone profile product, together with the integrated total column. The latter is compared with the daily global map of the TROPOMI total column retrieval to assess their mutual consistency. Daily global maps easily allow identifying data gaps, retrieval artefacts, along-orbit striping, and other large-scale features that are not typically detected through comparison with respect to point-like ground-based data.
Parameter correlation checks:
Using the in-house PyCAMA software, correlation checks are performed by KNMI on a broad selection of satellite data parameters within the orbit files. These checks provide a view on single-orbit features, correlations between retrievals of subsequent pixels, the appropriateness of the data flagging, etc. Relevant results can be found on the TROPOMI Portal for Level-2 Quality Control.
- Calisesi, Y., Soebijanta, V. T., and van Oss, R. Regridding of remote soundings: Formulation and application to ozone profile comparison. J. Geophys. Res. 110, https://doi.org/10.1029/2005JD006122, 2005.
- Keppens, A., Compernolle, S., Verhoelst, T., Hubert, D., and Lambert, J.-C. Harmonization and comparison of vertically resolved atmospheric state observations: Methods, effects, and uncertainty budget, Atmos. Meas. Tech. 12, https://doi.org/10.5194/amt-12-4379-2019, 2019.
- Rodgers, C. D. Inverse Methods for Atmospheric Sounding. World Scientific, 2000.
Rodgers, C. D. and Connor, B. J. Intercomparison of remote sounding instruments. J. Geophys. Res. 108, https://doi.org/10.1029/2002JD002299, 2003.