GOME-2 Polarisation Study First results L.G. Tilstra (1,2), I. Aben (1), P. Stammes (2) (1) SRON;...

GOME-2 Polarisation Study

First results

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

(1)SRON; (2)KNMI

EUMETSAT, Darmstadt, 29-06-2007

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OverviewOverview

1.1. Q/I versus (Q/I)Q/I versus (Q/I)ssss

2.2. verification: forward scan versus backward scan pixelsverification: forward scan versus backward scan pixels

3.3. verification: PMD readouts versus “187.5 ms subpixels”verification: PMD readouts versus “187.5 ms subpixels”

4.4. special points where Q/I =P∙cos(2special points where Q/I =P∙cos(2χχ)= 0 owing to:)= 0 owing to:

- cos(2- cos(2χχssss) = 0) = 0

- backscatter geometry- backscatter geometry

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1: Q/I versus (Q/I)1: Q/I versus (Q/I)ssss

GOME-2 polarisation measurements: 256 PMD readouts per scan (in 15 spectral bands). A scan lasts exactly 6 seconds, of which 4.5 seconds in forward scan, and the remaining 1.5 seconds in backward scan. Data: one level-1B orbit (actually consisting of 3-min. “chuncks” of data) of 13-APR-2007.

Total number of NADIR scans: 593 scans = 151808 polarisation values per orbit.

IDEA: 0 ≤ Q/I ≤ (Q/I)ss

Geometry in the data product is given only 32 times per scan (for the 187.5 ms subpixels).

Verification: checked (Q/I)ss on the 32-per-scan grid in the GOME-2 product with our own calculations

Interpolated viewing and solar angles to the 256-readouts-per-scan grid

Calculated (Q/I)ss on the 256-readouts-per-scan grid

[sheet “1”]

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BAND 1 : 311 nmBAND 1 : 311 nm

distribution largely outside “physical regime”(where 0≤Q/I≤(Q/I)ss)

there is an offset problem

~70% of the data points outside interval [-1,1]

these data points ALWAYS have the same value of –2147.4836

reported wavelengths: same value when wrong

reported errors: ALWAYS equal to 0.065535

unphysicalregime

unphysicalregime

data from 13-APR-2007

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offset problem smaller

unphysicalregime

unphysicalregime


6


unphysicalregime

unphysicalregime


7


unphysicalregime

unphysicalregime


8


unphysicalregime

unphysicalregime


9


unphysicalregime

unphysicalregime


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distribution again outside “physical regime”, even for (Q/I)ss far from 0

unphysicalregime

unphysicalregime


11


similar

unphysicalregime

unphysicalregime


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looks pretty ok

unphysicalregime

unphysicalregime


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BAND 10 : 558 nmBAND 10 : 558 nm

Looks ok: not a lot of measurements in the unphysical regime, and spread around Q=0 is smaller than before

unphysicalregime

unphysicalregime


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BAND 11 : 621 nmBAND 11 : 621 nm

unphysicalregime

unphysicalregime


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BAND 12 : 749 nmBAND 12 : 749 nm

again somewhat out of physical regime…

“outliers” : rainbow?unphysical

regime

unphysicalregime


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BAND 13 : 761 nmBAND 13 : 761 nm

offset

“outliers”unphysicalregime

unphysicalregime


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BAND 14 : 795 nmBAND 14 : 795 nm

offset problem

outliersunphysicalregime

unphysicalregime


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BAND 15 : 842 nmBAND 15 : 842 nm

offset problems are quite severe

again a lot of outliers

BAND 15 was not working at all for older versions of the data (06-DEC-2006)

unphysicalregime

unphysicalregime


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Conclusion Q/I versus (Q/I)Conclusion Q/I versus (Q/I)ss:ss:

• At first sight, the data generally look okAt first sight, the data generally look ok• Data outside interval [-1,1] : algorithm gives up too soon?Data outside interval [-1,1] : algorithm gives up too soon?

BAND 01 (311 nm) : 69.8 % outBAND 02 (314 nm) : 38.5 % outBAND 03 (319 nm) : 27.7 % outBAND 04 (325 nm) : 21.0 % outBAND 05 (332 nm) : 18.5 % outBAND 06 (354 nm) : 17.2 % outBAND 07 (381 nm) : 16.8 % outBAND 08 (413 nm) : 14.0 % outBAND 09 (482 nm) : 12.6 % outBAND 10 (558 nm) : 12.8 % outBAND 11 (621 nm) : 13.1 % outBAND 12 (749 nm) : 12.8 % outBAND 13 (761 nm) : 13.0 % outBAND 14 (795 nm) : 13.4 % outBAND 15 (842 nm) : 13.6 % out

(and having a value of –2147.4836)

• Reported errors on Q/I are always equal to 0.065535 (for all measurements, for all bands)Reported errors on Q/I are always equal to 0.065535 (for all measurements, for all bands)• Offset-like problems can be seenOffset-like problems can be seen

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2: Verification: forward scan versus backward scan2: Verification: forward scan versus backward scan

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Sometimes the forward-scan measurement fails while the backward-scan measurement is ok; sometimes it is the backward-scan that is at fault.

Why are there so many errors and where do they come from? (TBD)

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)35.1/5.4()/()/(1

1

NIQIQN

i

forwardiN

BACKWARD

non-homogeneous but otherwise normal scene:

Binning:

rather inhomogeneous scene:

Good correlation for mildly inhomogeneous scene; spread probably also determined by improper binning of ratios instead of intensities. Inhomogeneous scene: the higher the wavelength, the worse the correlation.

In conclusion, no problems related to scan direction.

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3: Verification: PMD readouts versus 187.5 ms subpixels3: Verification: PMD readouts versus 187.5 ms subpixels

Binning:

Conclusion: As far as we can tell, there don’t appear to be any obvious problems in the mapping of the measured polarisation values of the PMD readouts (256 scan-1, 23.4 ms) to 187.5 ms subpixels (32 scan-1).

)832/256()/()/(1

1

NIQIQN

i

smalliN

BIG

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4: Special geometries where Q/I = 04: Special geometries where Q/I = 0

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

–– no physical link with data, selection of points determined by choice of no physical link with data, selection of points determined by choice of reference plane (which is the local meridian plane)reference plane (which is the local meridian plane)

–– 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 (!!)processor treats these situations in a special way (!!)

2.2. Backscatter situations (Backscatter situations (ΘΘ = 180°) = 180°)

++ does not depend on the definition of a reference planedoes not depend on the definition of a reference plane

++ 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°)

DependenciesDependencies: pixel number, VZA, SZA, VAZI, SAZI, RAZI, SCAT, CHI_SS: pixel number, VZA, SZA, VAZI, SAZI, RAZI, SCAT, CHI_SS

To be done: dependency on PMD-p and PMD-s intensityTo be done: dependency on PMD-p and PMD-s intensity

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4.1 cos(24.1 cos(2χχssss) = 0 : pixel number (index)) = 0 : pixel number (index) [sheet “2”][sheet “2”]

half-way orbit

symmetrical... geometrical effect…?

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4.1 cos(24.1 cos(2χχssss) = 0 : VZA) = 0 : VZA [sheet “3”][sheet “3”]

?

Viewing angle dependence?

Or just indirect?

noisy +

stable branch

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4.1 cos(24.1 cos(2χχssss) = 0 : SCATTERING ANGLE) = 0 : SCATTERING ANGLE [sheet “5”][sheet “5”]

Indirect dependence on VZA, SZA, …?

stable branch turns into noisy branch

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4.2 BACKSCATTERING (4.2 BACKSCATTERING (ΘΘ = 180°) = 180°): pixel number (index): pixel number (index) [sheet “6”][sheet “6”]

Mind the small range in pixel number

(compare with sheet “1”…)

Complete agreement with cos(2χss) = 0 method over the entire mutual pixel number range!

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4.2 4.2 ΘΘ = 180° = 180°: DIRECTION OF POLARISATION CHI_SS: DIRECTION OF POLARISATION CHI_SS [sheet “8”][sheet “8”]

We find no strange behaviour at the points where χss=45° or χss=135°.

data having this geometry, where cos(2χss) = 0 and (U/Q)ss does not exist, behave similar to other data, despite alternative treatment of data processor.

therefore, the cos(2χss) = 0 method appears to be a reliable tool.

χss=45° χss=135°

Clearly, more analyses are needed to sort out the problems…

GOME-2 Polarisation Study First results L.G. Tilstra (1,2), I. Aben (1), P. Stammes (2) (1) SRON;...

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