1. Comparisons CH 4 SCIAMACHY / GROUND-BASED FTIR B. Dils, BIRA + HYMN partners Corinne Vigouroux, BIRA, HYMN progress meeting, 7-8 April 2008.

2. The FTIR stations for HYMN Stations (UFTIR) Ny-Alesund Kiruna Harestua Bremen (CH4) Zugspitze Jungfraujoch Izana Paramaribo St-Denis Collocation grid with SCIAMACHY data: Large grid = Lat ± 2.5° Lon ± 10° Small grid = Lat ± 2.5° Lon ± 5°

3. Previous work on FTIR / SCIA comparisons  Dils et al., ACP, 6, 1953-1976, 2006  European stations (as UFTIR, but different retrieval strategies !) + Egbert, Toronto, Wollongong, Lauder and Arrival Heights  3 Algorithms for SCIA data: 1) IMAP-DOAS (Frankenberg et al.) (only 2003)2) WFM-DOAS (Buchwitz et al.) 3) IMLM (Gloudemans et al.)  Scarce data in daily coincidence → Compare the SCIA data with a 3rd order polynomial fit through the FTIR data; therefore daily variability cannot be captured. → Large scatters (except IMAP-D: 1.1%, but worst correlation coefficient: 0.7)

4.  Dils et al., Proceedings of ACVE-3, Dec. 2006.  UFTIR stations only. Homogenize strategy: called this data set “UFTIR data”  WFM-DOAS (improved version)  2003 + 2004  Methodology: still polynomial fit (to compare the improvement with respect to previous version WFM-DOAS) → Improvement of the comparisons compared to old data sets WFM-D & UFTIR. Previous work on FTIR / SCIA comparisons

5. HYMN project  Data sets & methodology  UFTIR stations + Paramaribo and Reunion, using a common strategy ! → UFTIR or improved ?  IMAP-DOAS (new version)  2004 (+ ???)  Methodology : for the year 2004 & with new IMAP algorithm, more SCIA data are available → polynomial fit through FTIR data or direct comparisons ?

6. HYMN project  Today, we show :  Comparisons new IMAP-D vs WFM-D used lately in Dils et al., ACVE_3: → same UFTIR data set & same methodology (polynomial fit)  New IMAP-D, same UFTIR data, but considering direct comparisons of daily mean coincidences  Direct comparisons using new HYMN FTIR data set when available.

7.  Improvement in scatter and correlation coefficient   scat includes natural variability toward polynomial. 0.91 is close to the FTIR scatter = 0.85% (2004) CH 4 WFM-DIMAP-D BiasWeighted mean [(SCIA-FTIR)/FTIR] ± 3*std/sqrt(N) -3.50 ± 0.05%-1.23 ± 0.02 %  scat Weighted std of SCIA around the polynomial FTIR fit, shifted with the bias 1.40%0.91% RCorrelation coefficient between weighted monthly means 0.660.79 NNb coincidences1065146574 IMAP-DOAS & WFM-DOAS vs FTIR polynomial fit (UFTIR data)

8. IMAP-D & WFM-D: bias as a function of latitude IMAP-DOAS bias less pronounced and more homogeneous than for WFM-DOAS

9. Examples of CH 4 monthly time series in 2004

10. Polynomial fit or direct comparisons ? UFTIR data Polynomial fit UFTIR data Direct comparisons BiasScatterBias (N)Scatter Ny Alesund-0.320.81-0.17 (10)1.11 Kiruna-0.131.10-0.21 (37)1.07 Harestua-1.331.03-1.16 (12)1.17 Bremen-1.300.95-0.92 (21)1.34 Zugspitze-1.781.05-1.89 (45)1.42 Jungfraujoch-0.740.98-0.82 (62)1.11 Izaña-1.380.73-1.39 (53)0.77 Paramaribo St-Denis Both techniques are in agreement: to be done: check in detail if we see daily effects.

12. HYMN data set UFTIR Daily means HYMN data setChanges HYMN vs UFTIR Bias (N)ScatterBias (N)Scatter Ny Alesund-0.17 (10)1.11+0.66 (25)1.10Spectroscopy + new a priori profile Kiruna-0.21 (37)1.07 Harestua-1.16 (12)1.17 Bremen-0.92 (21)1.34+3.44 (10)1.06Spectroscopy + new a priori profile Zugspitze-1.89 (45)1.42+1.30 (47)1.30??? Jungfraujoch-0.82 (62)1.11-0.45 (62)1.29Spectroscopy Izaña-1.39 (53)0.77 Paramaribo+3.14 (20)1.21Spectroscopy + new a priori profile StDenis+1.56 (25)1.46Spectroscopy → Homogenization problem ?

13.  Today, we show :  Improvement of new IMAP-DOAS data  Methodologies in agreement. Final choice to be decided.  It seems that there is some problems of homogeneity with new HYMN FTIR data set: discussion in progress.  Questions for FTIR (next meeting May):  when should we deliver data for other years than 2004 ?  N2O, CO ?  ECMWF / NCEP ? (at St-Denis: ~ 1% bias on total columns, ~ 0.2% on tropospheric columns) Conclusions