1 / 21

GOME-2 Polarisation measurements

GOME-2 Polarisation measurements. Gijsbert Tilstra (KS/AK) Royal Netherlands Meteorological Institute (KNMI). CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010. Outline Introduction: polarisation and Stokes parameters Polarisation measurements by GOME-2

connor-hawkins
Download Presentation

GOME-2 Polarisation measurements

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. GOME-2 Polarisation measurements Gijsbert Tilstra (KS/AK) Royal Netherlands Meteorological Institute (KNMI) CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  2. Outline • Introduction: polarisation and Stokes parameters • Polarisation measurements by GOME-2 • Polarisation validation: special geometries method • Quality of GOME-2 polarisation • First results of GOME-2 AOT retrieval using polarisation data CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  3. 1 / Polarisation (picture: fully polarised light) Sunlight: unpolarised Atmospheric light: partly polarised; Stokes vector {I, Q, U, V} Q and U together describe the linear part of the polarisation, V describes circular polarisation (V≈0 for atmospheric light) Q = I0°– I90° U = I45°– I135° Q = P I cos 2 U = PIsin 2 P = degree of linear polarisation  = (angular) direction of linear polarisation CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  4. 2 / Polarisation measurements by GOME-2 The parallel (I0°) and perpendicular (I90°) components of the incoming light are measured using two Polarisation Measurement Devices: PMD-p and PMD-s End product: Stokes fraction Q/I (note: Q/I = P cos 2) GOME-1: • 3 PMD bands (broad) • only parallel component measured, main science channel information needed • footprint size = footprint size of main science channels = 320 х 40 km2 • number of observations per PMD band per orbit: ~1400 GOME-2: • 15 PMD bands (programmable) • parallel and perpendicular component both measured (PMD-p / PMD-s) • footprint size = footprint size of PMDs = 10 х 40 km2 • number of observations per PMD band per orbit: ~150.000 CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  5. 2 detector arrays on board: PMD-p and PMD-s, each containing 1024 detector pixels • First 748 pixels : not used • Next 20 pixels : swuv, currently not used • Last 256 pixels : 300–1200 nm (res. 2.8–40 nm) The PMD-p and PMD-s detector arrays are read out in 15 programmable bands: The first 7 PMD bands are positioned between 300 and 400 nm to help improve the polarisation correction of the radiance spectra in this part of the UV. GOME-1: one PMD (broadband, 295–397 nm). CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  6. Example of Stokes fraction Q/I measured by GOME-2: PMD λ (nm) 01 312 03 325 05 338 07 382 09 460 11 554 13 639 1 11 9 7 5 3 13 Typical for polarisation: strong dependence on scattering geometry CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  7. 3 / Validation: special geometries method Below 300 nm: single scattering in ozone layer  analytical P = Pss and = ss Above 300 nm: multiple scattering, surface reflection, … Direction of polarisation:  ≈ss for the majority of scenes POLDER data: Also supported by RT simulations So, above 300 nm we still have  ≈ss Schutgens et al., JGR 109, 2004 CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  8. ss = 45° or 135° : Q/I = P cos 2 ≈ P cos 2ss = 0 (irrespective of P!) Special geometries along the GOME-2 orbit for which this holds (blue points): (Q/I)ss Q/I PMD 1 Backscatter geometries: P ≈ 0  Q/I ≈ 0 (red points) CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  9. 4 / Quality of GOME-2 polarisation Current level-1b version: v4.4, high quality Stokes fractions Processor versions up to 3.3: many (software) bugs, low-quality Stokes fractions affecting the quality of the polarisation correction and the Earthshine spectra Steps forward in data processor: Verification/validation using special geometry approach, starting from v3.4, improvements, effects of instrument degradation, et cetera   CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  10. Processor version 3.4 / PMD band definition v1.0 14 June 2007, one orbit blue points : |cos(2ss)| ≤ 0.01 red points : 177° ≤ Θ ≤ 180° green points : (Q/I)ss • Wavelength dependent offset • Scan-angle dependence • Two branches exist Branches: Northern and Southern part of the orbit, separated by the principal plane (where the backscattering points are located). CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  11. Check: sunlight is unpolarised  By studying the ratio of the calibrated full spectrum solar irradiance 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 below 400 nm. The spectral misalignment between PMD-p and PMD-s is not the cause of the absolute calibration error (key data issue). PMD band definition 1.0: no misalignment between PMD-p and PMD-s foreseen.  EUMETSAT uploaded a new PMD band definition, v2.0, to the GOME-2 instrument, using a shift of 4 detector pixels in the UV. CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  12. Processor version 3.6 / PMD band definition v2.0 9 October 2007, one orbit blue points : |cos(2ss)| ≤ 0.01 red points : 177° ≤ Θ ≤ 180° green points : (Q/I)ss Clearly a strong overcorrection. The wavelength calibration of the two PMDs turned out to be incorrect, leading to an overestimation of the spectral misalignment of PMD-p w.r.t. PMD-s in PMD band definition v2.0.  EUMETSAT completely renewed the algorithm dealing with the wavelength calibration of the PMDs, resulting in a new PMD band definition, v3.1. CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  13. Processor version 3.8 / PMD band definition v3.1 5 February 2008, one orbit blue points : |cos(2ss)| ≤ 0.01 red points : 177° ≤ Θ ≤ 180° green points : (Q/I)ss • Offsets ≈ 0 (for exact nadir) • Scan-angle dependence • Branches have joined together Idea: scan-angle dependence of polarisation key data incorrect.  EUMETSAT: new polarisation key data obtain from PMD raw mode, using special geometries method. CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  14. Processor version 3.9 / PMD band definition v3.1 / New polarisation key data 26 March 2008, one orbit blue points : |cos(2ss)| ≤ 0.01 red points : 177° ≤ Θ ≤ 180° green points : (Q/I)ss 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. CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  15. Instrument degradation PMD-p / PMD-s: PMD band definition used:green : v1.0blue : v3.1 Accuracy of the method: 0.001–0.005 Trend due to degradation Relative changes in PMD-p and PMD-s throughput were not anticipated. This needs to be corrected some way. CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  16. Processor version 4.4 / PMD band definition v3.1 / New polarisation key data + ‘‘an additional time dependent correction to the key data meant to solve key data issues, the effects of instrument degradation, and other unknown issues’’ 30 October 2009, one orbit blue points : |cos(2ss)| ≤ 0.01 red points : 177° ≤ Θ ≤ 180° green points : (Q/I)ss Looks pretty much ok. CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  17. 5 / First results from GOME-2 AOT retrieval • Algorithm uses PMD 6–15 (Stokes fraction Q/I and reflectance R) • Spatial resolution: PMD footprint size (10 х 40 km2) • 29 aerosol models, with different aerosol types and layer heights • Currently in validation phase Sensitive to effects of instrument degradation Retrieval over land is still problematic An accurate PMD cloud filter is essential, this needs to be improved CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  18. AOT versus AAI: Does not look too bad High values in sunglint areas: ? (AOT: light grey = clouded, no result) (AAI: light grey = sunglint geometry) (sunglint filtered out) CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  19. 6 / Summary, conclusions • Polarisation retrieval of GOME-2 was fixed using a combination of in-flight calibration and “special geometries” validation method • GOME-2 AOT algorithm in validation phase CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  20. Extra slides (R) CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

  21. 0 / Validation techniques • Verification: comparison with single scattering 0 < Q/I < (Q/I)ss for most measurements • Satellite intercomparison: usually not possible • Limiting atmospheres • Special (validation) geometries CK/AK Science Meeting, KNMI, De Bilt, 09-03-2010

More Related