SCIAMACHY products quality and recommendations

1. SCIAMACHY products quality and recommendations Based on presentations and discussions during this workshop Presented by Ankie Piters

2. Level 1 Main features (nadir radiance): • Polarisation correction is wrong • ch7+8: • time dependent dark current • correction for transmission loss (ice) • PPG, etalon • wavelength calibration can be improved • Unrealistic features near channel boundaries • Geolocation: limb tangent height 1-2 km shift; geometry issues(?) Accuracy: • Solar spectrum (irradiance): 8-10% larger than Kurucz in all channels • Reflectance: • Ch 1&2: 5-10% smaller than GOME • Ch 3-5: 10-25% smaller than GOME • Ch 6-8: unknown • Limb and occultation radiance: unknown

3. Level 1 (continued) Effect on level 2: • Errors in calibration propagate into all level 2 products, O3 and NO2 columns to a lesser extent. Level 2 products from ch 7&8 are physically incorrect. Recommendations: • give highest priority to solving the calibration and level 0-1 processing problems • Calibration issues for limb and occultation radiances should be identified and resolved • Calibration should be made operational in line with planning, including m-factors, regular optimized updates of sun-spectrum, dark currents, etc

4. Level 2: O3 columns Preliminary comparisons mainly based on SV 3.53; few co-locations Main features: • AMF <= 0 for large solar zenith angles (>~90 degr) • Viewing angle dependance • Wrong geolocations in meteo-products • Wrong trace gas absorption cross sections and Ring spectrum used Current accuracy: • For Oct/Nov 5-10% smaller than comparative measurements, better for high latitudes (SV 3.54: increase of ~8% in tropics; 2 orbits with SV 4.00 yield similar results to SV 3.53) • temporal variation in the Antarctic ozone hole is well reproduced Current precision: • Approximately 3%

5. Level 2: O3 columns (continued) Recommendations: • Fix identified algorithm/implementation bugs. • Update the algorithms such that products are consistent with GDP v 3.0 or better. • Include viewing geometry and cloud information in meteo product

6. JC Lambert et al.

7. Level2: NO2 columns Preliminary comparisons mainly based on SV 3.53: few co-locations Main features: • AMF <= 0 for large solar zenith angles (>~90 degr) • Wrong trace gas absorption cross sections and Ring spectrum used Current accuracy: • <~30 N: offset -1015 mol/cm-2 • >30 N: offset up to +3.5*1015 mol/cm-2 • SV 3.54 and 4.00 seem to be worse in SH, better in NH, TBC (only 2 orbits) Recommendations: • Fix identified algorithm/implementation bugs. • Update the algorithms such that products are consistent with GDP v 3.0 or better. • Check use of proper climatology data base for AMF calculation

8. Level 2: columns of CH4, CO, N2O, H2O, CO2 Main features: • Wildly varying values: operational product orders of magnitude off, CO2=0 everywhere • Different orbital phases have different behaviour in average and scatter (jumps) • Strong correlation with albedo for CO and CH4 Current accuracy: • Current accuracy for these operational products can not be determined • Scientific retrievals of CH4 show physically consistent values, but are still underestimated Recommendation: • Improve the calibration, in particular the dark signal determination • Identify possible errors in the level 1-2 algorithms making use of the knowledge obtained from the scientific retrievals

9. Scientific retrieval Uni Bremen

10. Level 2: columns of SO2, BrO, OClO, H2CO These products were not in SV 3.51 - 3.53 (cal/val distribution): SV 3.54 contained SO2, BrO, H2CO; SV 4.0 contained only SO2. Main features: • BrO too large (factor 10) • SO2 mostly negative (SV 3.54) • H2CO right order of magnitude Recommendations: • Make these products available for verification

11. Level 2: clouds and AAI • AAI algorithm works well, but values sensitive to level 1 quality • Operational cloud cover fraction and cloud-top pressure show unphysical patterns • Scientific retrievals of cloud cover fraction and cloud-top pressure yield physically consistent values Recommendations: • Improve calibration / level 0-1 processing • Fix algorithm and implementation bugs in level 1-2 cloud cover algorithm • Retrieve cloud-top pressures with one of the scientifically available algorithms, instead of using climatology

12. Limb profiles One OL-ASCII file with O3 and NO2 profiles has been delivered; Verification has barely started. Main features: • Values below O3 peak are too low (sometimes negative) • Altitude shift, also in scientific retrievals (see level 1) Accuracy: • Current accuracy of the operational product is unknown • Scientific retrievals of O3, NO2, and BrO profiles appear to be physically consistent Recommendations: • Make OL limb profiles available and start verification

13. Conclusions: current status of level 2

14. Conclusions: general • Verification and validation up to now have been hampered by: • Lack of data distribution towards cal/val teams, SODAP team, and IECF • Slow implementation of problem solutions • The operational products do not meet the required accuracy. • Scientific retrievals show that this is in many cases a result of errors in the calibration, in the operational algorithms and their implementation. • Release of the current operational products is likely to damage the scientific credibility and subsequent usage of SCIAMACHY data.

15. Recommendations • Solve problems associated with calibration and level 0-1 processing. One specific issue is related to frequent update of calibration and degradation parameters (IECF). • Identify and select a suitable data set from SCIAMACHY co-located with validation measurements, to be reprocessed after every processor upgrade for validation. • Update the ATBD, provide specification of changes between data releases, provide documentation about the data bases used by the processors (e.g. spectroscopy, climatology, etc). • Identify possible errors in the level 1-2 algorithm making use of the knowledge obtained from the successful scientific retrievals • Achieve a significant improvement in accuracy of level 1 and 2 products before releasing operational data to the general (i.e. non cal/val) scientific community.