Validation of GOME-2 MetopA and MetopB ozone profiles M. Hess 1, W. Steinbrecht 1, L. Kins 1, O. Tuinder 2 1 DWD, 2 KNMI.

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1 Validation of GOME-2 MetopA and MetopB ozone profiles M. Hess 1, W. Steinbrecht 1, L. Kins 1, O. Tuinder 2 1 DWD, 2 KNMI Introduction The GOME-2 instruments on the MetopA and MetopB satellites measure nadir UV radiances which are used to retrieve trace gas concentrations, including ozone profiles (van Peet et al., 2013). These data are validated in the framework of O3MSAF on a long-term basis from the beginning, i.e. since the launch of MetopA in October Ground-based lidar and microwave ozone profile measurements at NDACC stations are used for validating stratospheric ozone profiles. Ozone sondes are used for validation in the lower atmosphere, but are not covered in this paper. In total, 9 stations (4 lidar and 5 microwave) are used for the validation. Here, we present the statistics of deviations between ground-based and satellite measurements at exemplary stations. The trend of the deviations over time is evaluated and validation results for GOME-2 on MetopA and MetopB are compared. Comparison of ground-based ozone with corresponding GOME-2 data Comparisons are performed on the basis of individual ground based measurements. GOME-2 measurements are taken from an area of 200 km around the location and within 2 hours of the microwave ground measurement, or within 12 hours in case of lidar measurements, which usually take place during the night before the satellite morning overpass. The relative differences (right panel in Fig. 1) with GOME-2 averaging kernels applied to the ground based data, are used for the statistical evaluation in the following figures. Figure 1: ozone profile measured by a microwave instrument at Bern, compared to a number of corresponding GOME-2 measurements (left panel). The other panels show the absolute and relative differences between ground based and GOME-2 data, respectively. The green curves are for the original microwave results, the red ones for microwave results with GOME-2 averaging kernel applied to them. For the blue curves, the GOME-2 a priori has also been applied to the ground based results. Geneva, September

2 Scatter plots for 13 months of MetopB data at Hohenpeißenberg This is a direct comparison of all data from December 2012 to December For both coarse resolution (CR) and high resolution (HR) GOME-2 profiles, the coincidence between satellite and lidar is best between 30 and 40 km. Comparisons are not as good below 25 km (see also validation of GOME-2 profiles using ozone-sondes). Figure 2: Scatter plots between ground-based and GOME-2 ozone values in different altitude regimes. Shown are the high resolution (HR) and coarse resolution (CR) results. Linear fits are calculated with both GOME-2 (red) and Lidar (green) data as independent variable, to give an optical impression of regression quality. MetopA and MetopB time-series of deviations from ground based instruments Time series of monthly mean values reveal a pronounced seasonal dependence with minima during winter below 25 km, and a weaker seasonal dependence above with maxima during winter. Beginning at 30 km, there is an increase of the GOME-2/MetopA differences over time, due to the increasing degradation of the GOME-2 sensors, which causes too low GOME-2 ozone concentrations. This decrease can reach more than 50 % per decade in the differences at higher altitudes. The overlap of the blue and green curves demonstrates the good agreement between our former and current comparison algorithms. These differ mainly in the selection of corresponding measurements, but the new algorithm has been rewritten from scratch. The red curve indicates that differences between GOME-2/B and ground-based instruments are similar to those of GOME-2/A at their beginning in 2007 (i.e. before degradation became visible for the higher altitude layers), or even smaller. Geneva, September

3 Figure 3: The blue curves show monthly mean values of the percent differences between GOME-2/MetopA ozone measurements and corresponding ground-based measurements, from January 2007 until June The green curves show corresponding results with our new, slightly different algorithm for January 2012 to December The red curves are results for GOME-2/MetopB from December 2012 to December 2013, with the new algorithm. Dependence of the deviations on Total Ozone and Solar Zenith Angle The retrieval of ozone profiles from GOME-2 data may, amongst other things, be influenced by scan angle, solar zenith angle, cloud fraction, and total ozone. Differences between satellite and ground based measurements may also be influenced by the coincidence of both in space and time. Therefore, the influence of all these quantities on the satellite ground based differences is monitored as well. In most cases no dependence was seen, but apparently (cf. Figure 4) something happens for solar zenith angles (SZA) larger than about 60, and total ozone values larger than 400 DU. For SZA, this seems to be due to the GOME-2 algorithm (it is visible in microwave measurements as well), while the total ozone dependence is not seen in the microwave results (microwave cases are not shown here). Bias at different stations The bias between GOME-2 ozone profiles and ground-based profiles depends not only on altitude (cf. Fig. 1 and 3), but also to some degree on station and instrument. There are some systematic deviations between the different ground based instruments, reaching up to 5 or 10 %. The best agreement (same GOME-2 bias) is seen for lidar instruments between 30 and 40 km, while the microwave instruments differ by up to 10 % even for stations lying as close together as Bern and Payerne. Geneva, September

4 Figure 4: Dependence of satellite - ground based ozone concentration differences on total ozone (integral over retrieved profile) and solar zenith angle, respectively. Figure 5: Average percent differences between GOME-2 and ground based retrieved ozone concentrations at different altitudes. Dashed lines belong to lidar instruments, solid lines to microwave instruments. Geneva, September

5 Conclusion at the beginning of GOME-2 life time, profiles agree within 30% with ground-based references, both for MetopA and MetopB there is a small annual cycle in GOME-2 bias the bias increases with altitude long-term drift (up to 50% per decade!) of operational MetopA ozone profile data current operational profiles are not suited for ozone trend detection at altitudes in question.. Stability 1% per decade or better would be required (WMO, 2010) long-term validation of satellite ozone profiles is essential! References Peet, J.C.A. van, R.J. van der A, O.N.E. Tuinder, E. Wolfram, J. Salvador, P.F. Levelt and H.M. Kelder, Ozone ProfilE Retrieval Algorithm (OPERA) for nadir-looking satellite instruments in the UV-VIS; Atmospheric Measurement Techniques, 2014, 7, 3, , doi: /amt WMO: Scientific Assessment of Ozone Depletion: 2010, Global Ozone Research and Monitoring Project Report No. 52, World Meteorological Organization, Geneva, Switzerland, 2011, Chapter Geneva, September