Richard Siddans RAL

1. PREMIER CORSA StudyTask 6.1: Update on joint-linear simulations Richard Siddans RAL CCN4 Final Meeting 18 Dec 2012,ESTEC

2. Status at PM3 telcon • Focus: combined retrievals from PREMIER + Metop-SG IASI + Sentinel 5 • Profile retrievals simulated at 1.5km resolution • Results integrated to estimate error on lower tropospheric (0-6km in mid-latitude case) column averaged vmr. • Nadir simulations based on realistic spectral selections, retrieval schemes and fit precision but some limiting errors ignored in particular surface T + emissivity, surf/atm T constrast, residual cloud, spectroscopic consistency etc • Results sensitive to choice of prior covariance.

3. Prior Constraints • 1000% + 1km correlation length • “standard case”: 300% (20% CH4) + 1km correlation length • 300% (20% CH4) + 6 km correlation length • ECMWF: MACC background covariance • GLOBAL: MACC variation from global mean during Aug 2008 • GZM: MACC variation from 10 deg zonal mean • GCOR: Correlations from GLOBAL, std.devs set to “standard” • Effect of applying 1% constraint on strat O3 tested. • GAQMON: GEM-AQ in “monsoon” region (3 days, high space, time resolution) • MACMON: MACC in same region, whole month, 100km resolution, 1 sample per day. Using model based covariance (except GCOR) leads to the result that “a priori” meets requirements for O3, CH4.

4. Following discussion BJK, MvW, JL Reviewed / increased prior errors for O3+CH4 in LT. Applied model constraints in UT and Stratosphere (to see how PREMIER improves) Modified constraints on strat. + UT NO2 21 December 2011 CORSA 4

5. Revised tropospheric prior for O3 & CH4 • Requirements start from definition of relevant dynamic range • defined focusing mainly on UT: O3 : 150 ppbv (10 - 160 ppbv) => threshold 30 ppbv (5 points); target 15 ppbv (10 points) CH4 : 250 ppbv (1.75 - 2 ppmv) => threshold 50 ppbv (5 points); target 25 ppbv (10 points) • Simulations repeated assuming prior std.dev = ~2 x model monsoon std.dev (capture outliers). • 50 ppb for O3 • 100 ppb for CH4 • simple prior error scaled to give these values when 6km length used • These close to half relevant dynamic range (lower for O3, but can argue LT O3 dynamic range smaller than UT) • Scaled errors applied to “standard” & monsoon covariances

8. Summary for O3 & CH4 In general: nadir retrievals sensitive to prior knowledge. This sensitivity almost totally removed using PREMIER PREMIER definitely improves over use of the model covariances to constraint UT+stratosphere For CH4 using100 ppbv prior error in LT give similar results for 3 covariances(6km correlation length, MACMON & GAQMON cases). For O3, slightly more variationin the three cases with 50ppb prior error: NB PREMIER Combination only requires 1 nadir sounder (May consider assumptions in IASI retrievals too optimistic?) 21 December 2011 CORSA 8

9. Consolidation of NO2 simulations • Threshold requirement 0.4 ppbv, targer 0.2 ppbv • S5 has one piece of information on NO2 • Error on LT column controlled by prior assumption on upper layers (those these can be reasonably quite tight cf LT variations) • Previous simulations showed S5 error ~0.4 ppbv if stratosphere constrained to within 1% (assumed by model) • Info from MvW now used to consolidate S5 case: • UT variability estimated ~2ppbv • Error in slant column from model constraint estimated by KNMI to be ~0.3 x 1015 molec/cm2: • Leads to 0.5 ppbv error on LT column

10. New: Including IRLS instrumental errors • Linear simulation results provided by Oxford (AD) • Solution covariance Sx • Linearly mapped error profiles x, in matrix Dx • For ECMWF mid-latitude profile • 1km vertical grid from 5-50km • 100% a priori constraint on constituents. • IRLS covariance used to directly define prior from 5km and above • Cannot adjust prior in these altitudes (and get correct estimates of mapped errors) without full solution covariance (including co-retrieved continua etc) • Can use either solution covariance or Sx + DxtDx= Sx + Sm • Then IRLS errors mapped onto joint retrieval • x_joint = ( I – A ) x_irls • Levels below can be constrained as before • Here show 6km correlation length + revised std.dev for O3,CH4

11. O3: Sx Sx+Sm

12. GOSAT/ IASI status for CH4

13. CH4: Sx Sx+Sm

14. New vs old errors for CH4 and O3

15. Summary / Next steps • Mapping of additional limb errors degrades performance of limb/nadir combination, particularly for CH4. • O3 still outperforms nadir combination by factor 2 if mapped errors added to solution covariance to give total error prior to combination • Is current measurement error inflation optimum ? • May not be appropriate to include all systematic errors (so CH4 may improve) • Next step • Include MWLS with errors • In particular assess CO • Nadir still “only” noise + realistic fit range + error floor + comtaminant species, T profile, Tsurf etc • No clear what additional errors should be reasonably included

17. 22/2/2013: Update of CH4 results based on latest input from Anu • This mainly only improves the result when on Sx used. • When Sx+Sm used, then still no significant improvement over S5+IASI from premier for CH4. • NB I have also tested approach in which purely only diagonal elements of Sm used: This gives very similar results, so mitigation of errors by using Sm, is not strongly dependent on assuming the “correct” vertical correlations, only on the std-deviations.

18. CH4: Sx Sx+Sm

19. New vs old errors for CH4 and O3 New figs in red