GOME-2 Polarisation Study — Final Presentation L.G. Tilstra (1,2), I. Aben (1), P. Stammes (2) (1)...

GOME-2 Polarisation Study — Final Presentation

L.G. Tilstra (1,2), I. Aben (1), P. Stammes (2)

(1)SRON; (2)KNMI

EUMETSAT, Darmstadt, 28-11-2008

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Overview

1) Introduction of validation techniques

2) Evolution of the data processor

3) Task 1 – Special geometries analysis

4) Task 2 – Limiting atmospheres analysis

5) Solar irradiance measured by the PMDs

6) Errors on Stokes fractions and main science channel radiance

7) Task 3 – PMD raw mode

8) Summary

9) Recommendations

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1) Validation of GOME-2 polarisation data (Stokes fraction Q/I)

A. Inspect so-called “butterfly diagrams” of Q/I versus (Q/I)ss

B. Focus on special geometries along the orbit where Q/I = 0

C. Limiting atmospheres approach

D. Focus on the solar irradiance (sunlight is unpolarised)

Available techniques:

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A. Butterfly diagrams

In general, we expect 0 < Q/I < (Q/I)ss for most measurements.

Exceptions are:

near-backscattering geometries

sunglint geometries

rainbow geometries (Θ between 130° and 155°)(v4.0)

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B. Special geometries where Q/I = 0

A. Situations where cos(2χss) = 0 [or: χss=45° or 135°]

++ many situations are found, along virtually the entire orbit (because of the many situations are found, along virtually the entire orbit (because of the large range of viewing angles and the small pixel sizes in scan direction)large range of viewing angles and the small pixel sizes in scan direction)

++ very high accuracy (for each day of data)very high accuracy (for each day of data)

–– these are special situations where (U/Q)these are special situations where (U/Q)ssss is undetermined, and the data is undetermined, and the data

processor treats these situations in a special way by setting U/I = 0 (!!)processor treats these situations in a special way by setting U/I = 0 (!!)

B. Backscatter situations (Θ = 180°)

++ rainbow and sunglint situations are automatically filtered outrainbow and sunglint situations are automatically filtered out

–– situations are only found “around the equator” (situations are only found “around the equator” (φφ––φφ00≈180°)≈180°)

–– situations occur for a very small range of viewing anglessituations occur for a very small range of viewing angles

The results from approach (B) agree completely with those of approach (A)

Q/I = P·cos (2χ) (P = degree of polarisation, χ = direction of polarisation)

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C. Limiting atmospheres PMD 14 (~756 nm):

Very low scattering optical thickness (~0.02);

mostly single scattering processes.

The depolarised limit is reached for clouds.

The polarised limit is obtained in the case of a

“black surface” (ocean and sea).

For the depolarised limit we have Q/I≈0.

For the polarised limit we have Q/I≈(Q/I)ss.

PMD 2 (~317 nm):

Higher scattering optical thickness.

Single, but also multiple scattering processes.

Black surface: soil, vegetation, ocean, or sea.

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2)2) Evolution of the data processor Evolution of the data processor

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“Butterfly diagrams”:

(rainbow geometries removed)

(v4.0)

General behaviour ok Scatter for the shorter wavelengths

Scatter is caused by very low signal

levels. Happens rarely. Could even

be corrected by bringing the Q/I back

to their single scattering value.

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Processor version 3.4 /

PMD band definition v1.0

14 June 2007, one orbit

1. Wavelength dependent offset

2. Scan-angle dependence

3. Two branches exist

blue points : |cos(2χss)| ≤ 0.01

red points : 177° ≤ Θ ≤ 180°

green points : (Q/I)ss

Branches: Northern and Southern part of the orbit, separated by the principal plane (where the backscattering points are located).

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9 October 2007, one orbit


red points : 177° ≤ Θ ≤ 180°


Introduction of PMD band definition v2.0 has a large effect, especially in the UV. Offsets have changed, but there is clearly a strong overcorrection.

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By studying the ratio of the (calibrated) solar irradiance spectra measured by PMD-p and PMD-s, and by shifting PMD-s w.r.t. PMD-p by 0, 1, 2, 3, and 4 detector pixels, we found a misalignment of ~2 detector pixels in the UV more appropriate than the ~4 detector pixels shift in PMD band definition v2.0.

Cause of the overcorrection:

The wavelength calibration of the PMDs turned out to be incorrect, leading to an overestimation of the spectral misalignment of PMD-p w.r.t. PMD-s, and hence an overcorrecting by PMD band definition v2.0.

EUMETSAT improved the wavelength calibration of the PMDs, resulting in a new PMD band definition, v3.1.

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5 February 2008, one orbit


red points : 177° ≤ Θ ≤ 180°


1. Offsets ≈ 0 (for exact nadir)

2. Scan-angle dependence

3. Branches have joined together

Idea: scan-angle dependence of polarisation key data incorrect.EUMETSAT: new polarisation key data obtain from PMD raw mode.

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red points : 177° ≤ Θ ≤ 180°



PMD band definition v3.1 /

New polarisation key data

26 March 2008, one orbit

Scan-angle dependence better, but

can still be improved. There are still

systematic offsets, but only for the

longer wavelengths (so, not caused by

spectral misalignment PMDs).

Processor version 4.0: sign of U/I.

Effect on Q/I small, large effect on

radiance main science channels.

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3)3) Task 1 – Special geometry analysis Task 1 – Special geometry analysis

yellow : 3.7 a

red : 3.8 a + b 02/2008

green : 3.9 b + c 03/2008

blue : 4.0 b + c + d 06/2008

Data since 01-01-2008 (10 months)

Every 3rd day is processed, in total 95 days

PMD band definition v3.1 is active since the

beginning of March 2008

a) PMD band definition v1.0

b) spectral calibration fixed

c) PMD band definition v3.1 + new key data

d) change in sign Stokes parameter U

Special geometries: results

[recent near-real time data]


[recent near-real time data]

yellow : 3.7 a

red : 3.8 a + b 02/2008

green : 3.9 b + c 03/2008

blue : 4.0 b + c + d 06/2008

a) PMD band definition v1.0

b) spectral calibration fixed

c) PMD band definition v3.1 + new key data

d) change in sign Stokes parameter U


Every 3rd day is processed, in total 95 days




[reprocessed data set v4.0]

PMD band definition used:

green : v1.0

blue : v3.1 03/2008

There is a (small) dependence on scanner

angle (except for PMD 15: large scan-

angle dependence)


Every 6th day is processed, in total 99 days



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PMD 8:

Looks ok: no transition occurs when going from data measured with PMD band definition v1.0 to data measured with PMD band definition v3.1.


green : v1.0

blue : v3.1

Accuracy of the method:

0.001–0.005

Trend due to degradation?

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PMD 7:

Clear transition in going from PMD band definition v1.0 to PMD band definition v3.1 (improvement) ; small scan-angle dependence, increasing with time.


green : v1.0

blue : v3.1


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PMD 12:

In this case, the situation seems to have worsened. However, the wavelengths in the two PMD band definitions are very different (744 versus 589 nm).


green : v1.0

blue : v3.1

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PMD 1:

Transition (improvement). Deviating behaviour (at a scanner angle of about –40 degrees) is caused by measurement 245 in each (backward) scan. Reported.


green : v1.0

blue : v3.1


(reset pixels: 241-244)

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PMD 15:

The error was reported and the bug was traced down by EUMETSAT. The bug will be fixed in the next version of the GOME-2 data processor (v4.1).


green : v1.0

blue : v3.1

This large scan-angle

dependence was not present

in version 3.9.

(version 3.9 should give the

same results as version 4.0

for the special geometries)

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4)4) Task 2 – Limiting atmospheres analysis Task 2 – Limiting atmospheres analysis

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Limiting atmospheres: results for recent near-real time data

Slopes of linear fits

O : polarised limit

◊ : depolarised limit

Data since 01-01-2008

Every 3rd day is processed,

in total 95 days

PMD band definition v3.1 is

active since the beginning

of March 2008

? temporal behaviour ?

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Limiting atmospheres: results for recent near-real time data

Intercepts of linear fits

Intercepts of

polarised and

depolarised fit

agree nicely

v4.0: PMD 15 ?

(data since 01-01-2008)

O : polarised limit


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Limiting atmospheres: results for reprocessed data set (v4.0)

Slopes of linear fits

O : polarised limit


Data since 01-01-2007

Every 6th day is processed,

in total 99 days

PMD band definition v3.1 is

active since the beginning

of March 2008

? periodic behaviour ?

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Limiting atmospheres: results for reprocessed data set (v4.0)

Intercepts of linear fits

PMD band definition

v3.1 yields better

results: initial

wavelength mismatch

between PMD-p and

PMD-s smaller

v4.0: PMD 15 noisy

(data since 01-01-2007)

O : polarised limit


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Polarised limit: comparison with simulations of limiting atmospheres

Blue circles: simulated Q/I for PMD 14, based on input parameters of GOME-2 measurements, including ozone column, viewing and solar angles, for a fixed surface albedo of 0.002 (ocean).

Dependence on surface albedo: even very small changes in the surface albedo already cause a different position of the limit. Polarised limit is very sensitive.

756 nm

756 nm

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Polarised limit: comparison with simulations of limiting atmospheres

Blue circles: simulated Q/I for PMD 3, based on input parameters of GOME-2 measurements, including ozone column, viewing and solar angles, for a fixed surface albedo of 0.005 (ocean).

325 nm

Dependence on surface albedo: even large changes in the surface albedo have little effect on the location of the limit. Polarised limit is not very sensitive in the UV.Also soil and vegetation surfaces contribute.

325 nm

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5)5) Solar irradiance measured by the PMDs Solar irradiance measured by the PMDs

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4) Solar irradiance measurements by the PMDs

Irradiances of PMDs p and s should be the same; their intensity ratio should be 1

Effect of distance

between Earth and

Sun is divided out

Improvement with

new PMD band

definition v3.1

Temporal behaviour:

different degradation

of PMD-p and PMD-s

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Difference in solar irradiance, for all 15 PMD bands:

The shape of the ratio cannot be caused by degradation (alone). Key data issue?

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Degradation of PMD-p:

corrected for distance Earth-Sun, normalised to 1 at the start of the time series

Strong degradation for

the shorter wavelength

PMDs (1-8):

Rate of ~20% per year

v4.0

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Degradation of PMD-s:

corrected for distance Earth-Sun, normalised to 1 at the start of the time series

Again strong

degradation for the

shorter wavelengths

Looks like PMD-s is

less stable than PMD-p,

and also more

responsible for the

periodic behaviour of

their ratio than PMD-p

v4.0

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6)6) Errors on Stokes fractions and main science channel radiances

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Systematic offsets found for the PMD bands:(average over 2½ months)

very consistent results

depolarised limit agrees

with polarised limit

polarised limit less

accurate for long

wavelengths

Error bars:

limiting atmospheres: no

error assumed in slope

(statistical error)

special geometries:

error larger because of

scan-angle dependence

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Effect of errors in the Stokes fractions on the radiances of the main science channels

Errors are below ~0.5% for all

wavelengths

Shorter wavelength: smaller errors

despite higher sensitivity to

polarisation

Sensitivity of the main science channels to

polarisation (Stokes fraction Q/I) : mu_2

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7)7) Task 3 – PMD raw mode Task 3 – PMD raw mode

Earth reflectance

Stokes fractions

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PMD raw mode:

In PMD raw mode, GOME-2 measures the spectrum at the full spectral

resolution, using all 256 detector pixels, at the expense of the spatial

coverage, which is 1/16th of the normal coverage. The first 15

measurements in a scan are skipped, and the 16th is recorded (at the full

spectral grid). This is repeated 16 times in each scan.

Simulations:

The simulations are performed by the

polarised radiative transfer model

“DAK” (v3.1). The ozone columns are

determined from SCIAMACHY

assimilated total ozone columns.

Surface albedo values are obtained

from a LER database. Above 400 nm, a

very high accuracy of the LER values is

required, which is not available.

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Selection of cloud-free scenes using AVHRR data (provided by EUMETSAT)

(left:) AVHRR cloud fraction

Two orbit parts: right part θ0 > 90°,

left part mostly over sea

(right:) cloud fraction < 0.05

Yellow circle in Alaska: suitable

GOME-2 PMD raw measurement

with cloud fraction ~0.00

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Scene 1 – Alaska (land)

(west-viewing geometry)

Clear signatures of vegetation.

Fair agreement, even for the longer

wavelengths. In the UV ~3% off.

Below ~320 nm the values are too

high (probably straylight).

Red points : GOME-2

Blue points : simulations

Green lines : Q/I=0 and Q/I=(Q/I)ss

Good agreement, keeping in mind

that we know that there should be

offsets. Offsets roughly as large as

expected. Noise at the shorter

wavelengths: low signal levels.

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Poor agreement for the longer

wavelengths. Surface albedo

clearly not correct.

Shape looks reasonable, but the

deviation is not as expected for the

longer wavelengths. A too high

value for the surface albedo leads

to enhanced depolarisation.

Scene 2 – Canada (land)

(east-viewing geometry)

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Scene 3 – Alaska II (land)

(west-viewing geometry)

Fair agreement for the longer

wavelengths. In the UV ~4–7% off.

Very similar to Scene 1.

Good agreement, offsets are similar

to offsets found for Scene 1 (and

those were roughly similar to the

offsets for the PMD bands found

from the special geometry and

limiting atmospheres analyses).

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Scene 4 – Special geometry scene

(cloudy, ocean)

There are oscillations at the longer

wavelengths. Also seen in Scenes 1–3.

Reasonable agreement, offsets follow

the shape of the offsets found before.

All in all: error on Q/I for the PMD raw

mode is smaller than 0.05 (above 320

nm), and most likely similar to the

errors found for the PMD band data.

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8) Summary (1/2)

Special geometry and limiting atmospheres analyses have consistent results

(Stokes fraction).

Both analyses show a very clear improvement with every new data processor

version.

In particular, there was a large improvement with the introduction of PMD band

definition v3.1 and new (polarisation) key data.

The scan-angle dependence has been reduced, but is still there to some degree

see recommendations.

Behaviour of PMD 15 since processor version 4.0: bug will be fixed in v4.1.

Solar PMD data show periodic oscillations suggesting problems in the (diffuser)

calibration.

The solar irradiance spectra measured by PMD-p and PMD-s show a rate of

degradation ~20% per year in the UV.

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8) Summary (2/2)

At the moment, the degradation of PMD–p and PMD-s is very similar The

effect on the Stokes fractions (so far) is small. The decision to use two

orthogonal PMDs (instead of one in combination with the main science

channels for GOME-1) is paying off.

(Relative) degradation correction for the PMDs may be necessary to obtain

reliable Stokes fractions in the future.

This degradation correction is probably scanner-angle dependent.

Some time-dependent periodic behaviour is also visible in the Stokes fractions.

PMD raw mode Stokes fraction and Earth reflectance look reasonable, and copy

the validation results of the PMD band data.

Systematic offsets in Stokes fractions are below 0.02 for wavelengths below

520 nm, and below 0.04 for the longer wavelengths.

Effect of errors in Stokes fractions on main science channel radiance /

reflectance is below the 0.5%.

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9) Recommendations:

The scan-angle dependence, which was reduced by already a lot, can and

needs to be further reduced (ongoing work at EUMETSAT).

Implement a correction for the Stokes fractions (i.e., for the ratio of PMD-p and

PMD-s signals) based on the special geometry approach (scan-angle

dependent). This then automatically corrects for instrument degradation,

temporal variations and scan-angle dependencies.

Acknowledgements :

We greatly acknowledge support from EUMETSAT (R.Lang, Y.Livschitz,

R.Munro). In particular also the very quick analyses, testing and

implementation of improvements in the operational data processor.

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Limiting atmospheres: how to determine the limits (1)

Some of the practical problems:

– filter out rainbow situations (130° ≤ Θ ≤ 155°)

– filter out “suspicious” pixel 245 for each scan:

note that this is the pixel following reset pixels 241–244

PMD radiance is approximately a factor 5 too high (why?)

Q/I looks ok, but sometimes “saturation-like” effects occur

– outliers for short wavelength PMD bands: signals too low?

PMD 1, 14-07-2008 (1 day of data)

~2 million measurements, version 4.0

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Limiting atmospheres: how to determine the limits (2)

Approach:

– horizontal bins with cell size 0.02 (optimum)

– per horizontal cell, determine the histogram using vertical bins of 0.01

– determine the two edges of this distribution function

– assign a weight to these edges, based on the number of measurements

– fit a linear function through the points, using the weights

depolarised limit

polarised limit

PMD 5, all limits found in 2008

(some more fine tuning may further improve the results)

GOME-2 Polarisation Study — Final Presentation L.G. Tilstra (1,2), I. Aben (1), P. Stammes (2) (1)...

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