Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 [email protected] SADDU June 2008 S. Noël, K.Bramstedt,

Institut für Umweltphysik/Fernerkundung Physik/ElektrotechnikFachbereich 1

[email protected]

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 -


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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


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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


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• 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


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• 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


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Modelled Optical Depths (1)

layers at different altitudes

looking at layer


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Modelled Optical Depths (2)

layer at 50 km

looking at different altitudes


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Saturation Correction (1)

• Correction smooth above 25 km

• No useful results below 11 km

• Refraction plays minor role above ~15 km

Csat (z)


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Saturation Correction (2)

• Saturation correction as function of water vapour density

• Uses reference water vapour profile

Csat(n)


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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


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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


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Example: Fit Results (50 km)


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Example: Fit Results (20 km)


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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


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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


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• In 15-25 km mean relative deviation <10%

• At higher altitudes up to ~25%

• Small standard deviation of relative differences


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• Good correlation especially at higher altitudes

• Below 40 km maximum correlation 0.5


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SCIAMACHY vs. ECMWF (1)

• ECMWF water vapour data for times/locations of SCIAMACHY collocations with ACE

• SCIAMACHY data typically higher


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SCIAMACHY vs. ECMWF (2)

• ECMWF data generally lower

• mean relative deviation~ 10-50%

• small standard deviation of relative differences


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Statistical Analysis: SCIA vs. ECMWF (3)

• Correlation less good than with ACE


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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


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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)

Documents

Institut für Umweltphysik/Fernerkundung Physik/Elektrotechnik Fachbereich 1 [email protected] SADDU June 2008 S. Noël, K.Bramstedt,