Post on 05-Apr-2015
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
Stefan.Noel@iup.physik.uni-bremen.de
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/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
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/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
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
Pj = Polynomial (for broadband effects)
I0Ij
ij
ln (Ij/I0) = Pj - ij
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
Onion Peeling Approach (2)
• Assumption:– optical depth ij proportional to number density ni
– linearity limited by saturation effects correction required
• Basic formula (only H2O absorption):
ijln (Ij/I0)meas = Pj - (ij)ref csat(ni) ai
• (ij)ref from radiative transfer model (SCIATRAN)
• csat(ni) from retrieval run on simulated data (setting aj = 1)
• Start retrieval at top of atmosphere, then propagate downwards only one aj to be determined in one step
fit parameter
saturation correction factor
measured spectra
optical depths
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
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/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
Modelled Optical Depths (1)
layers at different altitudes
looking at layer
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
Modelled Optical Depths (2)
layer at 50 km
looking at different altitudes
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
Saturation Correction (1)
• Correction smooth above 25 km
• No useful results below 11 km
• Refraction plays minor role above ~15 km
Csat (z)
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
Saturation Correction (2)
• Saturation correction as function of water vapour density
• Uses reference water vapour profile
Csat(n)
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
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
aj
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
Application to SCIAMACHY
• Fit interval 926 – 969 nm (channel 5)• Absorbers fitted: H2O, O3 (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/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
Example: Fit Results (50 km)
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
Example: Fit Results (20 km)
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
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/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
SCIAMACHY vs. ACE (1)
• About 400 collocations with ACE in 2004 - 2007 (max. distance 500 km)
• SCIAMACHY densities systematically larger than ACE above ~25 km
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
SCIAMACHY vs. ACE (2)
• In 15-25 km mean relative deviation <10%
• At higher altitudes up to ~25%
• Small standard deviation of relative differences
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
SCIAMACHY vs. ACE (3)
• Good correlation especially at higher altitudes
• Below 40 km maximum correlation 0.5
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
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/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
SCIAMACHY vs. ECMWF (2)
• ECMWF data generally lower
• mean relative deviation~ 10-50%
• small standard deviation of relative differences
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
Statistical Analysis: SCIA vs. ECMWF (3)
• Correlation less good than with ACE
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
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 15-25 km)
• Somewhat higher deviations from ECMWF profiles
Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1
SADDU June 2008
Stefan.Noel@iup.physik.uni-bremen.de
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)