1. Eumetsat GOME-2 Error Assessment Study Interim Findings Presentation by B.Kerridge on behalf of Serco, RAL, IUP & SRON GSAG, ESRIN, 11/12th April 2002

2. Structure 1Introduction 2Approach 3Baseline Error Budget 4Interim Findings: (a) Sampling Options for Band 1 (b) Spatial Aliassing (c) Spectral Resolution & Slit-Function Shape (d) RTM Assumptions & Earth Curvature (e) Non-Lambertian Surface BRDF (f) Cloud Obscuration & Horizontal RI Gradients (g) Pointing & Geolocation 4Summary & Further Work

3. 1. Introduction GOME-2 Error Assessment Study commissioned by Eumetsat Scope: –Identify through quantitative retrieval simulations factors which will limit accuracies of trace gas columns and ozone profiles. –Recommend operational settings and, if necessary, other action to mitigate these. Consortium: –Serco Europe Ltd (Prime Contractor) –RAL (Technical Co-ordinator, Ozone Profile Analysis) –IUP (Trace Gas Column Analysis, RTM Calculations) –SRON (Assessment of Instrumental Errors) Final Presentation: –25th June at Eumetsat

4. 2. Approach Trace gas column / O 3 profile algorithms as used by IUP / RAL for GOME-1 flight data, with specific modifications for GOME-2. For O 3 profiles, sensitivity to -ranges and a priori explored. Simulations generally for: 12 geo-temporal scenarios (solar geometry from orbit propagator ) 2 surface albedos (0.05, 0.8) Variety of view angles (nadir, +/- 29 o, +/- 45 o (=1920km swath)) Linear retrieval diagnostics analysed. Specified errors for GOME-2 quantified by linear mapping of their spectral signatures for comparison with: (a) Estimated Standard Deviations (ESD = sqrt(S x ) ) (b) Baseline Error Budget

5. 3. Baseline Error Budget Error sources for GOME-1 reviewed. Baseline error budgets compiled for GOME-2 O 3 Profile Errors: –Radiometry: 2% of sun-normalised radiance –Polarisation correction: SRON prescription for GOME-2 –Degradation of scan-mirror reflectance: quantified but neglected –Surface pressure: 10hPa –Temperature profile: error covariance matrix from IASI retrieval –Aerosol: LOWTRAN “high” - “background” Trace Gas Column Errors: –Photon noise & read-out noise, with Ring x-section fitted. –Polarisation correction: error quantified but negligible. Clouds, spectroscopy and other instrument effects not included.

6. 4. Interim Findings (a) Sampling Options for Band 1 Baseline integration times: 12s for Band 1A; 0.1875s for Band 1B  Single Band 1A pixel: 1920km x 80km –Non-linear dependence of RT with view angle over +/- 45 o –RT errors at extreme view angles –Horizontal variability of stratospheric O 3 profile Flexibility to read out Band 1A at 1.5s (640km x 40km) and co- add Impact of increased read-out noise assessed, in frame of 1% noise floor Co-adding 1.5s Band 1A pixels to 12s (640km x 320km) or 24s (640km x 640km) yields similar esd to single 12s Band 1A pixel. Additional read-out noise insignificant also for co-addition of 0.1875s Band 1B pixels  No impediment to reading out Band 1A at 1.5s & Band 1B at 0.1875s

7. (b) Spatial Aliasing Five Landsat ETM+ images (~180km x 180km at 1m resolution). Ensemble of >350 spatially-aliassed signatures calculated (via  dependent surface albedos) for each of the12 geo-temporal scenarios. Ensemble min, max, mean (bias) and RMS examined. GOME-2 IFOV (0.29 o ~4km on ground) effectively filters high- frequency structure (ie spatially-aliassed noise).  Coarser resolution images (eg ATSR-2 1km x 1km) would suffice for a future comprehensive study, and are calibrated over full dynamic range. Trace Gas Columns Fitting windows small, so errors insensitive to low-frequency structure. Max errors (0.02% O 3, 2% NO 2, 1% BrO, 10% OClO); below the esds  No reason to reduce detector read-out time.

8. O 3 Profiles Bands 1 & 2 handled as separate steps. Band 1 sees low-frequency structure only at longest wavelengths. Errors <esd for Band 1 range 265-307nm, but sometimes exceed esd when Band 1 range extended to 265-314nm (nb albedo extremes).  Reduction in Band 1B detector read-out time desirable. Errors arise from non-linear RT dependence on surface albedo plus scene inhomogeneity (ie not eliminated for negligible read- out time). Only small interval (~100 of 1024 detector pixels) of Band 2 used, and 2 nd order polynomial fitted to log(sun-normalised radiance).  Band 2 retrieval insensitive to low-frequency structure from aliassing. Algorithms which use (a) sun-normalised radiances and (b) extensive intervals of Bands 1 and 2 simultaneously would be more vulnerable.

9. (c) Spectral Resolution & Slit-Function Shape O 3 Profiles Slit-function FWHM: Trade-off between increasing esd and decreasing undersampling error as Band 2 FWHM is increased (redistributing but conserving photons). On this criterion, increase from 2-3px (0.24-0.36nm) would be beneficial, however knowledge of shape would then be more critical. Slit-function Shape: Gaussian shape assumed in Band 2 FM for joint retrieval of: –Slit FWHM and wavelength registration from direct-sun spectra –O 3 and other variables from sun-normalised radiance spectra

10. Analysis (OG) for EQM indicates non-Gaussian and asymmetric shape Shape for defocused case broader and more wavelength dependent, although closer to Gaussian, than for focused case. Measurements of direct-sun and sun-normalised spectra synthesised for “true” shapes (focused, defocused, Gaussian) using high-res solar spectrum, adding Ring to backscattered spectrum before convolution. Gaussian used in retrieval scheme FM. Two simulations w.r.t. undersampling:  -shift (direct-sun - backscattered) retrieved (as GOME-1)  -shift not retrieved but mapped, after interpolation & re- gridding. Errors on retrieved O 3 small when “true” shape really is Gaussian (ie FHWM of Gaussian recovered by retrieval), but large when it is not.

11. Errors larger for defocus (wider) case than for focus case –because, although smaller in amplitude, the spectral signature due to erroneous shape is more correlated with structure of O 3 Huggins Band. O 3 errors due to incorrect shape can exceed 100% in troposphere. Sign, magnitude and height-dependence of this error are similar to bias found for GOME-1 (RAL data) from comparison with >2,000 sondes. Characterisation of shape with onboard line lamp will be limited by: –Absence of suitable lamp lines between 306 and 333nm –Discrete sampling of narrow lines by broad detector pixels  Characterisation of Band 2 -dependent slit-function shape at sub-px resolution vital for accurate GOME-2 tropospheric O 3 retrieval

12. Trace Gas Columns High res FTS absorption x-sections used. Simulation of FWHM increase due to defocusing from 2px to 5px (0.24-0.6nm Band 2 & 0.5-1.25nm Band 3).  Esds for O 3 (<1%) and BrO (<60%) increased by only factor ~1.1. NO 2 esd (<20% at 2px) increased by factor ~1.24 at 3px. Undersampling errors small for O 3 (<0.5%) and NO 2 (<2%), but substantial for BrO (<100%). Preliminary assessment also of slit opening (increased photon flux): –Desirable to reduce integration time <0.1875s, hence ground pixel size. –Detector saturation and fixed data rate are constraints. Sensitivity to shape not assessed.

13. (d) RTM Assumptions & Earth Curvature Assumption: fully-spherical RTM too computationally expensive for operational processing. Calculations by CDI RTM in pseudo-spherical and fully-spherical modes differenced and linearly-mapped. O 3 Profiles CDI (with GOME FM x-sections) also used to calculate K’s. Pseudo-spherical approximation (as implemented in CDI) can cause substantial errors at all altitudes (ie in ss- as well as ms- domain). Largest errors at largest SZAs, as expected. Errors small for nadir-view, and much larger at +/-45 o than at +/- 29 o  Correction scheme required for outer pixels of 1920km swath Caveat: errors estimated from non-linear simulations might be smaller.

14. Trace Gas Columns Errors on O 3, NO 2 and BrO slant-columns negligible (<<1%), provided solar geometry for ground rather than TOA. Air-mass factors??

15. (e) Non-Lambertian Surface BRDF Spectra calculated by CDI in fully-spherical mode using angular- dependent surface BRDFs and their Lambertian equivalents for: –dark land, bright land, ocean & snow (April 55 o N) & sunglint (5 o S) O 3 Profiles Quasi non-linear simulation: gross deviations in BRDF accommodated through surface albedo retrieval, as per GOME- 1 scheme. O 3 errors from Lambertian assumption 30% for sunglint. Sunglint occurs in eastward views, peaking near 960km swath edge. It will affect a substantial fraction of data in tropics south of equator. (Peak intensity and affected area depend on surface wind-speed.)

16. Trace Gas Columns Errors on O 3, NO 2 and BrO slant-columns <1% for April 55 o N. Errors on NO 2 airmass factors also negligible Sun-glint??

17. (f) Cloud Obscuration & Horizontal RI Gradients Issue: extent to which cloud obscuration or horizontal gradients in refractive index would be more serious for 1920km than 960km swath. Cloud: ATSR-2 forward view (55 o ) ~ GOME-2 extreme (45 o ) for 1920km ATSR-2 statistics analysed for GOME-1 80x40km px for one year  Forward/nadir differences not substantial, even for occurrence of totally cloud-free scenes (12% vs 14%). Horizontal Gradient in RI: LOS path-lengths calculated for non-refracting and refracting (w/wo 0.14K/km gradient 0-40km height) atmosphere with ray- tracing model  Differences negligible for extreme view angle for 1920km.

18. (g) Pointing & Geolocation Errors as built taken from EPS Geolocation & Co-registration Budget –Nadir: 1.6km along-track, 1.2km across-track –1920km swath edge: 3.1km along-track, 3.6km across-track Direct impact on viewing geometry quantified Trace Gas Columns Errors <1% and generally <0.5% O 3 Profiles Across-track errors generally <2% (even at 1920km swath edge) Along-track errors <<1%  Direct impact of pointing errors at these levels is negligible cf others.

19. 5. Summary and Further Work 1. Band 1 Sampling: –Integrate for 1.5s (1A) / 0.1875s (1B) and co-add; retain 1A/1B boundary. 2. Spatial Aliassing: –Errors on trace gas columns not significant –Errors on O 3 profiles ~ esds for Band 1 limit of 307nm, > esds for 314nm.  Reduction in read-out time desirable. 3. Pointing Errors: negligible direct impact 4. Spectral Resolution & Slit-Function Shape: –FWHM increase from 2-3px would reduce sensitivity to undersampling, but would increase sensitivity to errors in knowledge of shape. –Accurate knowledge of shape in Band 2 <350nm vital for O 3 profiles.  Pre-flight measurements required, since onboard line-lamp not adequate. –Could opening slit permit eg smaller ground pixel with 1920km swath?

20. 5. Swath-width: –Cloud obscuration & horizontal RI gradients not significant factors. –Errors from pseudo-spherical approximation negligible for trace gas columns but large for O 3 profiles in outer pixels of 1920km swath.  Correction scheme needed if CPU time too great for fully-spherical RTM. –Errors from Lambertian BRDF approximation negligible for trace gas columns and <5% for O 3 profiles except for sunglint, where they are large. Sunglint more pervasive for 1920km swath, but not decisively. Further study required to: (a) Address identified issues in greater depth (eg swath, slit-shape). (b) Address issues not covered by this study (eg spectroscopy, diffuser spectral structures, ).