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ENVISAT VALIDATION WORKSHOP

ENVISAT VALIDATION WORKSHOP. GOMOS Recommendations by the ESL team : Service d’Aéronomie (CNRS), FMI, IASB. Odile Fanton d’Andon ACRI-ST. GOMOS : a new instrument on-board. Star. ENVISAT. Principle : Stellar occultations. Very accurate for ozone, immune to instrument drifts

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ENVISAT VALIDATION WORKSHOP

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  1. ENVISAT VALIDATION WORKSHOP GOMOS Recommendations by the ESL team : Service d’Aéronomie (CNRS), FMI, IASB Odile Fanton d’Andon ACRI-ST

  2. GOMOS : a new instrument on-board Star ENVISAT Principle : Stellar occultations • Very accurate for ozone, immune to instrument drifts • Accuracy is different from one occultation to the other • Altitude sampling depends on the geometry of observation • Sensitive to limb illumination conditions

  3. 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. Ozone Secondary maximum Diurnal variations due to photochemistry reactions are observed. Altitude range is higher than expected

  5. 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. Feedback and lessons from validation Ozone • Individual differences  up to 20 % • Excellent mean profiles : no bias with lidar and bias less than 10% with other sources

  7. DARK N = 65

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

  9. Lessons from GOMOS ESL team studies Typical GOMOS ozone profiles relative difference with external sources Dark limb Excellent agreement Scintillation effect (20-40 km) • Below 20 km • High fluctuations • Weak SNR (DC, RTS) • Vertical sampling • (only minimal • smoothing is applied)

  10. Lessons from GOMOS ESL team studies Scintillation : current scintillation model does not fully describe the observed features [20 km – 40 km]

  11. Lessons from GOMOS ESL team studies 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 Current specification (not yet implemented in PDS) : Dark charge calibration is performed at orbit level

  12. Lessons from GOMOS ESL team studies 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

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

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

  16. Main recommendations Validation : Validation must continue Supply reprocessed data to validating teams DDS antenna is absolutely required to pursue the ESL activity Frequent reprocessings are required during the whole next year Validation via assimilation must continue Data dissemination : YES, after the red actions are implemented and validated in PDS

  17. First GOMOS assimilated ozone field

  18. DARK N = 65

  19. O3 vertical profiles (from 02967/ S009 / S143) : Comparison with lidar measurements (Mauna Loa)

  20. 8Comparison with lidar measurements (Mauna Loa) Relative difference of vertical profiles (from 02967/ S009 / S143):

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