10-years operational gome/ers-2 total column products: the...

ESA Atmospheric Science Conference, Frascati 8-12 May 2006

10-years operational GOME/ERS-2 total column products: the GDP 4.0 validation

D. Balis(1), J.-C. Lambert(2), M. Van Roozendael(2), R. Spurr(3), D. Loyola(4), Y. Livschitz(4), P.Valks(4), V. Amiridis(1), P. Gerard(2), J. Granville(2),

and C. Zehner(5)

(1) Aristotle University of Thessaloniki, Thessaloniki, Greece (2) Belgian Institute for Space Aeronomy, IASB-BIRA, Brussels, Belgium

(3) RT Solutions, Inc., Cambridge MA 02138, United States (4) German Aerospace Center, DLR, Oberpfaffenhofen, Wessling, Germany

(5) European Space Agency/ESRIN, Via Galileo Galilei CP. 64, 00044 Frascati, Italy


Outline

Main features of GOME Data Processor v4.0Validation data setsError sources in ground-based comparisonsOzone column validationNitrogen dioxide column validationConclusions


Summary of GDP v4.0DOAS -style retrieval algorithm (least squares Beer-Lambert fitting for the effective

slant column of ozone, followed by a NN-based AMF computation

Non-linear fitting (shift and squeeze) of radiance spectra to improve wavelength registration

Cross-correlation for the Level 1b irradiance spectrum in the region of the DOAS fitting window (325-335 nm)

Temperature dependence of Huggins-band ozone absorption taken into account by fitting the effective temperature from GOME spectra

New algorithm for correction of molecular Ring

NN-based iterative computation of the air mass factor AMF (at 325.5nm) and vertical column density VCD; radiative transfer calculations based on the use of a column classified ozone profile climatology (improved TOMS v8)

Two new algorithms OCRA for cloud fraction and ROCINN for cloud-top information

more tomorrow by Loyola et al.

(Details in van Roozendael et al., JGR in press).


Selection of validation data sets

Dobson and Brewer total ozone data records, as deposited at the WOUDC in Toronto, Canada (http://www.woudc.org)

UV-visible DOAS, Dobson and Brewer total ozone data records acquired as part of the NDACC (former NDSC) (http://www.ndsc.ws)

Occasionally, comparisons showed some inconsistencies between nearby stations or unreasonable small correlation between satellite and GB data. Results from international intercomparison campaigns of Dobson instruments explained some of these inconsistencies.

These stations have been presently excluded from the summary validation statistics.

Finally 41 Brewer, 61 Dobson, and 27 UV-visible DOAS instruments have been used for the comparisons with GDP3, GDP4 and TOMSv8.


Direct sun ground-based measurements have been studied separately. In the summary plots only these measurements have been used, since their accuracy for both Dobson and Brewer instruments is close to 1%.


Error sources in ground-based comparisonsIn standard WOUDC and NDACC data:For Dobsons: The temperature dependence of the ozone absorption coefficients used in the retrievals might account for a seasonal variation in the error of up to ±1.7% and for a systematic errors of up to 4% (can increase in extreme cold conditions)

For Brewers: The effect is smaller because of the different wavelengths used.

Both might suffer from long-term drifts associated with calibration changes. Additional stray-light problems for SZA>75°, mainly for single Brewers and Dobsons, less for double Brewers.

Dobson: A pair 305.5 and 325.4D pair 317.6 and 338.8 Brewer: MS4=306.3/316.8, MS5=310.1/316.8,

MS6=313.5/316.8, MS7320.1/316.8

For UV-visible: The use of DOAS and of the Chappuis band minimizes the temperature dependency. The use of standard AMF in real-time data introduces seasonalities as well as a meridian dependence well documented.


Error sources in ground-based comparisons Differences in horizontal smoothing : collocation criteria are based on spatial windowing

varying from 100 to 600 km and temporal windows from 1 hour to 1 day.

GOME pixel 320x40km2

Brewer and Dobson direct-sun air masses are located between the measuring station and a maximum distance of 150 km in the direction of the sun.

Zenith-sky air masses extend over several hundred kilometers in the azimuth of the rising and setting sun.

Longitude East

1 2 3 4 5 6 7 8 9

Latit

ude

Nor

th

43

44

45

46

47

48

49

50

51

SAOZLongitude

1 2 3 4 5 6 7 8 9

-5

0

5

10

15

Latitude43 44 45 46 47 48 49 50 51

(GO

ME-

SAO

Z)/S

AO

Z [

%]

-5

0

5

10

15

Comparison of the SAOZ sunset data (at Jungfraujoch) with all GOME pixels acquired within a radius of 500 km around the station would generate a systematic offset of nearly 5% and a 1σ scatter of 6%. Considering only GOME measurements with footprints overlapping by more than 25% with the effective air mass probed by SAOZ, then the mean agreement for this unstable day falls down to 2.5% and its 1σ scatter down to about 1%.


Ozone column validation/ Test case studies in the Alps


Ozone column validation/ Global comparisons

-90-80-70-60-50-40-30-20-10 0 10 20 30 40 50 60 70 80 90

Latitude (deg)

-8

-6

-4

-2

0

2

4

6

8

(SAT

-gro

und

base

d)/S

AT

GDP3-Dobs GDP3-BrewGDP4-DobsGDP4-Brew

-90-80-70-60-50-40-30-20-10 0 10 20 30 40 50 60 70 80 90

Latitude (deg)

-8

-6

-4

-2

0

2

4

6

8

(SAT

-gro

und

base

d)/S

AT

TOMSv7-DobsTOMSv7-BrewTOMS v8-DobsTOMSv8-Brew


Ozone column validation/ Seasonal dependence


Ozone column validation/ Long-term stability

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005-15

-10

-5

0

5

10

15

(SAT

-GRO

UND)

/SAT

Northern HemipshereGDP4-BrewerGDP3-BrewerTOMSv8-Brewer

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005-15

-10

-5

0

5

10

15

(SAT

-GRO

UND)

/SAT

Northern HemipshereGDP4-DobsonGDP3-DobsonTOMSv8-Dobson

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005-15

-10

-5

0

5

10

15

(SAT

-GRO

UND)

/SAT

Southern HemipshereGDP4-DobsonGDP3-DobsonTOMSv8-Dobson


Ozone column validation/ Dependence on SZA

Polar day technique applied


Ozone column validation/ Dependence on ozone column


Ozone column validation/ Dependence on fractional cloud cover

The main change in cloud fraction dependence is a clear reduction of the dispersion under clear skies, most likely thanks to the new molecular Ring effect correction and improved AMF calculations in GDP 4.0.


NO2 column validation: (1) no tropospheric pollution

GDP 2.7

GDP 3.0GDP 4.0


NO2 column validation: (2) incl. tropospheric pollution


Conclusions (1)New GDP 4.0 ozone column data have undergone thorough validation against

WMO/GAW ground-based networks (Dobson, Brewer, NDACC/UVVIS) and TOMSv8, using accurate comparison techniques

Ozone columns have almost no offset w.r.t. GAW/NDACC networks and TOMSv8 data [GDP3.0: -1.89 (Brewer), -2.06 (Dobson), GDP4.0: -0.17 (Brewer), 0.11 (Dobson), TOMSv8: -1.5 (Brewer), -0.3 (Dobson)]

There is no significant latitude dependence in GDP4.0 consistent with TOMSv8.

There is still a seasonal dependence in the middle and high latitudes, more evident in the Dobson comparisons. It is similar with GDP3.0 in amplitude (1.5%) but not in phase.

In the middle-to-high latitudes during the winter period GDP4 overestimates ozoneby 2%, which appears to be a SZA dependence. Otherwise there is not evident any significant SZA dependence.

The various algorithms (satellites as well as ground-based) deals differently with the temperature dependence of Huggins absorption band. This may account for part of the observed seasonalities.

Comparisons between GDP4.0 and ground-based data do not show any trend till mid 2003, which is not the case with TOMSv8.


Conclusions (2)New GDP 4.0 NO2 column data have undergone thorough validation against

NDACC/UVVIS ground-based network and satellites (HALOE, POAM-II/III), using accurate comparison techniques.

Where relevant, modelling data have been used for a better interpretation of diurnal cycle and tropospheric pollution effects.

Away from tropospheric pollution, GDP 4.0 total NO2 agrees with other stratospheric systems within a few 10 14 molec.cm-2 (max. ±15%).

In case of pollution, residual NO2 (GOME total column - stratospheric column) is consistent with the predicted behaviour of tropospheric NO2. Accurate estimation of tropospheric column is a challenge.

There are no large differences between GDP 4.0 and GDP 3.0. total NO2.

More details in Balis et al., JGR (in press)…

10-years operational gome/ers-2 total column products: the...

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