1. Page 1ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 ENVISAT VALIDATION WORKSHOP GOMOS Recommendations by the ESL team : Service d’Aéronomie (CNRS), FMI, IASB Odile Fanton d’Andon ACRI-ST

2. Page 2ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 GOMOS : a new instrument on-board o Very accurate for ozone, immune to instrument drifts o Accuracy is different from one occultation to the other o Altitude sampling depends on the geometry of observation o Sensitive to limb illumination conditions Star ENVISAT Principle : Stellar occultations

3. Page 3ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 Derivation of vertical profiles of transmission spectra in UV, VIS and NIR spectral range (Level 1b) Retrieval of O3, air, NO2, NO3, aerosols (spectro A), O2 and H2O (spectro B) by minimising the difference between measured transmission and a model (Level 2)

4. Page 4ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 Ozone Secondary maximum Diurnal variations due to photochemistry reactions are observed. Altitude range is higher than expected

5. Page 5ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 Feedback and lessons from validation General lessons on the observations  Large difference between observations in dark or bright limb  Degraded results in twilight conditions  Large oscillations at the bottom of the profiles  Error estimates underestimated

6. Page 6ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 Ozone  Excellent mean profiles : no bias with lidar and bias less than 10% with other sources  Individual differences  up to 20 % Feedback and lessons from validation

7. Page 7ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 N = 65 DARK

8. Page 8ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 Air : Overall good agreement with ECMWF; high level of fluctuations. O2 : show bias – need scientific study of algorithm and/or s database H2O: very preliminary; realistic around UTLS since last spectral re-calibration (end of November – V4) Temperature (high resolution profile) very promising (agreement with ECMWF better than 5K in 70% of cases) Temperature (low resolution) : Criticism. Lack of knowledge about model contribution. Feedback and lessons from validation NO2 and NO3 retrieval may be improved by changes in the algorithm (DOAS like method)

9. Page 9ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 Scintillation effect (20-40 km) Below 20 km High fluctuations 1)Weak SNR (DC, RTS) 2)Vertical sampling (only minimal smoothing is applied) Typical GOMOS ozone profiles relative difference with external sources Lessons from GOMOS ESL team studies Excellent agreement Dark limb

10. Page 10ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 Lessons from GOMOS ESL team studies Scintillation : current scintillation model does not fully describe the observed features [20 km – 40 km]

11. Page 11ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 DC : Dark charge remains higher than expected and erratic (RTS) despite three successive cooling of the instrument.  Impact on O3 retrieval  Larger for dimmer stars  Larger below 20 km Lessons from GOMOS ESL team studies Current specification (not yet implemented in PDS) : Dark charge calibration is performed at orbit level

12. Page 12ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 Limb radiance / background correction is not optimal  Straylight is observed near the poles  Fraction of stellar flux in the external bands  Day side : Limb is not well characterised yet Lessons from GOMOS ESL team studies

13. Page 13ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 Main recommendations Instrument DC : Monitoring of DC is fundamental – methods to characterise or better remove RTS must be investigated Level 1b : Short term – emergency (6-12 months) 1. First priority is to upgrade the background correction in D/B limbs. 2. Improve the estimation of the reference stellar spectra 3. Characterise and correct for reflectivity of the optics 4. Solve inconsistency between intercalibration of spectrometers and photometers 5. Solve non-uniformity issues for spectro B (re-characterisation)

14. Page 14ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 Main recommendations Level 2 - Algorithms : Short term – emergency (test and specifications within 6 months) 1.Experiment and qualify alternative spectral inversion schemes 2.Qualify and adapt smoothing in vertical inversion 3.Improve the error estimates and quality indicators (to be continued) Medium term (within 12-18 months) 1.Specify modelling error 2.Experiment and qualify the full covariance matrix method 3.Compute averaging kernels and provide them in products Long term 1. Experiment coupling of spectrometers 2.Other species retrieval (OClO, …) 3. Study aerosol model variations and the use of a priori information

15. Page 15ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 Main recommendations Level 2 – algorithm tuning : Short term (2003) 1. Tuning of High resolution Temperature Profile and its error estimates 2. Check temperature effects in spectral inversion (p-loop) Level 2 – products updates : Short term (2003) 1.New temperature/density products : GOMOS only (Rayleigh + O2 + HRTP) and external ECMWF+MSIS profiles 2.Add HRTP product into the meteo product 3.Solar zenith angle at tangent point should be provided

16. Page 16ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 Main recommendations Validation : 1.Validation must continue 2.Supply reprocessed data to validating teams 3.DDS antenna is absolutely required to pursue the ESL activity 4.Frequent reprocessings are required during the whole next year 5.Validation via assimilation must continue Data dissemination : YES, after the red actions are implemented and validated in PDS

17. Page 17ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 First GOMOS assimilated ozone field

18. Page 18ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 N = 65 DARK

19. Page 19ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 Comparison with lidar measurements (Mauna Loa) O 3 vertical profiles (from 02967/ S009 / S143) :

20. Page 20ENVISAT Validation Review / GOMOS session - ESRIN – 13th December 2002 8Comparison with lidar measurements (Mauna Loa) Relative difference of vertical profiles (from 02967/ S009 / S143):