Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 S. Noël, K.Bramstedt, A. Rozanov IFE/IUP, University of Bremen, Germany Water Vapour Profiles from SCIAMACHY Solar Occultation Measurements - Preliminary Results -

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Introduction SCIAMACHY occultation measurements are performed once per orbit in northern latitudes Vertical scans over the sun during sunrise Different scan strategies above ~100 km New method has been developed to derive water vapour profiles from SCIAMACHY occultation data Simple onion peeling approach using DOAS techniques

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Onion Peeling Approach (1) Absorption of whole atmosphere can be written as sum of absorption of individual altitude layers: ij = optical depth for absorber in layer i (only) and observation tangent height j P j = Polynomial (for broadband effects) I0I0 IjIj i j ln (I j /I 0 ) = P j - ij

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Onion Peeling Approach (2) Assumption: –optical depth ij proportional to number density n i –linearity limited by saturation effects correction required Basic formula (only H 2 O absorption): i j ln (I j /I 0 ) meas = P j - ( ij ) ref c sat (n i ) a i ( ij ) ref from radiative transfer model (SCIATRAN) c sat (n i ) from retrieval run on simulated data (setting a j = 1) Start retrieval at top of atmosphere, then propagate downwards only one a j to be determined in one step fit parameter saturation correction factor measured spectra optical depths

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Onion Peeling Approach (3) Advantages: –Simple DOAS-like approach –Uses pre-calculated data base; no individual radiative transfer model calculations required –Numerically very fast Limitations: –Dependence of optical depths and saturation correction on model atmosphere –Saturation correction not only function of density but also light path (but: no significant changes in results when using n(z)) –Multiplicative saturation correction factor may not be sufficient –Downward propagation of errors

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Modelled Optical Depths (1) layers at different altitudes looking at layer

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Modelled Optical Depths (2) layer at 50 km looking at different altitudes

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Saturation Correction (1) Correction smooth above 25 km No useful results below 11 km Refraction plays minor role above ~15 km C sat (z)

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Saturation Correction (2) Saturation correction as function of water vapour density Uses reference water vapour profile C sat (n)

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Self Consistency Very good consistency above 30 km Max. ~5% offsets precision limit Minor dependence on refraction Use RTM data base without refraction for retrieval ajaj

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Application to SCIAMACHY Fit interval 926 – 969 nm (channel 5) Absorbers fitted: H 2 O, O 3 (handled similarly) Retrieval altitude grid: –0 – 50 km –1 km steps until 25 km; 2.5 km steps above (matching RTM altitude grid) Use subset of SCIAMACHY measurements: –Currently only state 49 used (nominal scan) –Only upward scans & 4 readouts close to solar centre –Only tangent altitudes below 60 km –Ratio to spectra above atmosphere for matching readouts / position on sun SCIAMACHY spectra are interpolated to retrieval grid Retrieved profiles are interpolated back to SCIAMACHY altitude grid Only useful results above ~ 15 km Reference

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Example: Fit Results (50 km)

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Example: Fit Results (20 km)

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Example: Retrieved Profile Comparison with matching ACE-FTS & ECMWF data shows quite good agreement below ~25 km SCIAMACHY data tend to be higher at higher altitudes

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 SCIAMACHY vs. ACE (1) About 400 collocations with ACE in (max. distance 500 km) SCIAMACHY densities systematically larger than ACE above ~25 km

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 SCIAMACHY vs. ACE (2) In km mean relative deviation <10% At higher altitudes up to ~25% Small standard deviation of relative differences

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 SCIAMACHY vs. ACE (3) Good correlation especially at higher altitudes Below 40 km maximum correlation 0.5

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 SCIAMACHY vs. ECMWF (1) ECMWF water vapour data for times/locations of SCIAMACHY collocations with ACE SCIAMACHY data typically higher

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 SCIAMACHY vs. ECMWF (2) ECMWF data generally lower mean relative deviation ~ 10-50% small standard deviation of relative differences

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Statistical Analysis: SCIA vs. ECMWF (3) Correlation less good than with ACE

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Summary First results for SCIAMACHY water vapour profiles using an onion peeling approach look promising. Estimated precision of method ~5% Agreement with ACE data within about 10-25% (best between km) Somewhat higher deviations from ECMWF profiles

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 SADDU June 2008 Open Issues Improvement of saturation correction: –Parameterisation of saturation? –Non-linear correction? Field-of-View integration / altitude shift? Optimisation of altitude grid Optimisation of fitting window Different reference atmosphere? Comparison with optimal estimation method (K. Bramstedt)