Information content of satellite remote-sensing measurements: photopolarimetric vs intensity-only

1. Information content of satellite remote-sensing measurements: photopolarimetric vs intensity-only 2009 CLARREO Meeting at LaRC

2. Why the CLARREO solar instrument needs to be an SI-traceable photopolarimeter with • high radiometric accuracy/precision • high polarization accuracy/precision • thorough multi-angle coverage • broad multispectral coverage 2009 CLARREO Meeting at LaRC

3. Do we need to detect global climate change with CLARREO? 2009 CLARREO Meeting at LaRC

4. CLARREO objective To obtain and archive a benchmark climate record that is on-orbit SI traceable, and can also serve as a calibration reference for operational satellites • CLARREO measurements must be fully loaded with information content sufficient to attribute climate change and to constrain models. • Measurements need to be sensitive to specific model parameters (remote-sensing justification of CLARREO measurements) in order to constrain climate models and attribute climate change. • CLARREO measurements must be capable of improving the retrieval accuracy of operational instruments. • CLARREO measurements must be capable of improving the accuracy of measurements with operational instruments. 2009 CLARREO Meeting at LaRC

5. CLARREO science objectives • Aerosol direct effect, its long-term trends and attribution • Aerosol semi-direct effect, its long-term trends and attribution • Land albedo change, its long-term trends and attribution • Aerosol indirect effect, its long-term trends and attribution ↓ The short-wave CLARREO instrument must be a self-sufficient climate-monitoring instrument. 2009 CLARREO Meeting at LaRC

6. CLARREO science objectives Recalibration of short-wave radiance-only instruments will not solve the climate-monitoring problem 2009 CLARREO Meeting at LaRC

7. Comparisons of MODIS and MISR pixel-level retrievals • A MODIS-Terra level-2 aerosol pixel and a MISR level-2 aerosol pixel are defined as “fully compatible” if they • are located within the narrower MISR swath; • have been collocated spatially to ±3.3 km and temporally to ±3 min; • have been determined to be “cloud-free” by both cloud-screening procedures; and • have been identified as suitable for aerosol retrieval and have been taken through the standard MODIS and MISR retrieval routines, thereby resulting in specific AOT and AE values. 2009 CLARREO Meeting at LaRC

8. Comparisons of MODIS and MISR pixel-level retrievals Scatter plots of fully compatible MISR vs. MODIS-Terra AOTs for January 2006. The straight dotted line depicts the 1-to-1 perfect agreement. J. Quant. Spectrosc. Radiat. Transfer109, 2376 (2008) 2009 CLARREO Meeting at LaRC

9. Comparisons of MODIS and MISR pixel-level retrievals MISR vs. MODIS-Terra Ångström exponents (AEs). The straight dotted lines depict the 1-to-1 perfect agreement. J. Quant. Spectrosc. Radiat. Transfer109, 2376 (2008) 2009 CLARREO Meeting at LaRC

10. Comparisons of MODIS and MISR pixel-level retrievals J. Quant. Spectrosc. Radiat. Transfer110, in press (2009) 2009 CLARREO Meeting at LaRC

11. Comparisons of MODIS and MISR pixel-level retrievals What makes MODIS and MISR mediocre Vis/SWIR remote sensing instruments is not poor radiance calibration but their inherent insensitivity to aerosol and surface properties. J. Quant. Spectrosc. Radiat. Transfer110, in press (2009) 2009 CLARREO Meeting at LaRC

12. Retrievals with aircraft prototype of APS • Spectral AOT values retrieved from precise polarimetric measurements agree exceedingly well with those measured by ground-based sunphotometers over an AOT range from 0.05 to more than 1. • The absence of spectrally-dependent biases demonstrates the reliability of the size distribution estimate for both small and large modes of a bimodal aerosol distribution. • In situ and retrieved size distributions also agree extremely well (difference in aerosol effective radius of less than 0.04 µm). 2009 CLARREO Meeting at LaRC

13. Polarized radiances A beam of light is fully characterized by four polarized radiances (Stokes parameters) having the same dimension of W m–2 sr–1 : I −I ≤Q ≤I −I ≤ U ≤I −I ≤ V ≤I V is usually small and difficult to measure and carries little information. 2009 CLARREO Meeting at LaRC

14. Polarization is very sensitive to aerosol particle size and refractive index 2009 CLARREO Meeting at LaRC

15. Polarimetry Q and U can be measured in much the same way as I. However, |Q| and |U| are often (much) smaller than I, which often causes a problem. Example: I = 1 ΔI = ±0.02 Q = 0.02 ΔQ = ±0.02 U = –0.01 ΔU = ±0.02 Q and U cannot be measured. 2009 CLARREO Meeting at LaRC

16. Polarimetry However, the ratios −1 ≤ Q/I ≤ 1 −1 ≤ U/I ≤ 1 can be measured extremely accurately. Astrophysicists can measure them to ±0.000001. The Glory APS will measure them to ±0.001. Example: I = 1 ΔI = ±0.02 Q = 0.02 ΔQ = ±0.001 U = –0.01 ΔU = ±0.001 Q and U can be measured quite accurately despite their small absolute values. 2009 CLARREO Meeting at LaRC

17. Polarimetry A high-radiometric-accuracy polarimeter is a high-polarization-accuracy polarimeter. Example: I = 1 ΔI = ±0.002 Q = 0.02 ΔQ = ±0.002 U = –0.01 ΔU = ±0.002 Q and U are measured with useful accuracy. 2009 CLARREO Meeting at LaRC

18. “Must haves” of an SI-traceable self-sufficient SW photopolarimeter Precise polarimetry particle size distribution, refractive index, and shape (~0.001): Wide scattering particle size distribution, refractive index, and shape angle range (i + p): Multiple (~60) (i) cloud particle size via rainbow angle angles (i + p): (ii) particle size, refractive index, and shape (iii) ocean surface roughness (iv) aerosol retrievals in cloud-contaminated pixels (v) aerosol retrievals above clouds (semi-direct effect) Wide spectral range (i) separation of submicron and supermicron particles (i + p): (ii) spectral refractive index  chemical composition 1370 nm (i + p): characterization of thin cirrus clouds 2200 nm (p): (i) characterization of the land surface contribution at visible wavelengths (ii) cloud particle sizing 2009 CLARREO Meeting at LaRC

19. Role of polarization in retrieval cross-calibration 2009 CLARREO Meeting at LaRC

20. SI-traceable conversion of radiances into fluxes IPCC, 2007 • Multi-angle photopolarimetry will provide: • Better choice of BRDF model based on polarimetric retrievals • Thorough quantitative checks of CERES BRDF models 2009 CLARREO Meeting at LaRC

21. SI-traceable conversion of radiances into fluxes 2009 CLARREO Meeting at LaRC

22. Conclusions • The short-wave CLARREO instrument must be a self-sufficient climate-monitoring instrument. • A high-accuracy multi-angle multispectral photopolarimeter is the logical instrument with fully tested capability to provide the detailed information content necessary to attribute climate change and constrain models. • This instrument is necessary to improve the retrieval accuracy of operational SW instruments. 2009 CLARREO Meeting at LaRC

23. Nadir-looking imaging spectrometer: (i) ~200 (600?) units of information per pixel (ii) nonzero swath (iii) is not a stand-alone climate instrument (iv) cannot be used to convert radiances into fluxes (v) cannot be used to cross-calibrate retrievals Multiangle filter polarimeter (i) ~250  9  3 = 6,750 units of information per pixel (ii) zero swath for science measurement (iii) is a stand-alone climate instrument (iv) can be used to convert radiances into fluxes (v) can be used to cross-calibrate retrievals Multiangle spectropolarimeter: (i) ~250  200  3 = 150,000 units of information per pixel (ii) zero swath for science measurement (iii) is a stand-alone climate instrument (iv) can be used to convert radiances into fluxes (v) can be used to cross-calibrate retrievals Multiangle imaging (i) ~250  200  3 = 150,000 units of information per pixel spectropolarimeter: (ii) nonzero swath (iii) is a stand-alone climate instrument (iv) can be used to convert radiances into fluxes (v) can be used to cross-calibrate retrievals Trade space 2009 CLARREO Meeting at LaRC

24. Role of polarization accuracy Range of acceptable retrievals for different levels of polarimetric and radiometric accuracy. 2009 CLARREO Meeting at LaRC

25. Retrievals with aircraft prototype of APS • Low-altitude reflectance and polarized reflectance bare soil vegetation • Measurements of two different surface types at 410, 470, 555, 670, 865, 1590 and 2250 nm (blue, mauve, turquoise, green, red, magenta, black) with different viewing geometries. • Solid lines are bare soil, dashed lines are vegetation. These are single aggregated scans of a single pixel. Imperfection are primarily due to yaw. • Reflectances of different surface types show significant variations in color. Polarized reflectance of different surface types is dominated by geometry. 2009 CLARREO Meeting at LaRC

26. Polarized observations of clouds are sensitive to the cloud droplet size distribution (rainbow), the cloud top pressure (side scattering in the blue UV) and aerosols above the cloud (side scattering in the red/NIR/SWIR). Retrievals with aircraft prototype of APS 2009 CLARREO Meeting at LaRC

27. Retrievals with aircraft prototype of APS BEFLUX is the ground based estimate of the total solar directional hemispheric reflectance at the DoE ARM SGP CF. The RSP estimate of DHR comes from a single snapshot (i.e. instantaneous) while the MODIS processing stream uses sixteen days of data to reduce the effects of aerosols, clouds and increase angular sampling. 2009 CLARREO Meeting at LaRC

28. 2009 CLARREO Meeting at LaRC

29. Role of polarization in cross-calibration of measurements • Three critical problems: • The need to cross-calibrate polarimeters • Polarization sensitivity of the CLARREO SW instrument • Polarization sensitivity of operational radiometers 2009 CLARREO Meeting at LaRC

30. Role of polarization in cross-calibration • To cross-calibrate polarimeters, the CLARREO SW instrument should have ~0.2% radiance and ~0.1% polarization accuracy 2009 CLARREO Meeting at LaRC

31. Role of polarization in radiance calibration and radiance calibration transfer The voltage response of any radiometer can be expressed as follows: Since Vinc is usually very small, we can simplify this formula as follows: The CALARREO SW instrument must be a polarimeter in order to be an SI-traceable radiance cross-calibrator. 2009 CLARREO Meeting at LaRC

32. Role of polarization in radiance calibration and radiance calibration transfer Assume that M12is known. M12Q/I = M12 q must be known to ~0.001. If M12 is 0.1 then q must be known to 0.01. If M12 is 0.05 then q must be known to 0.02. If M12 is 0.01 then q must be known to 0.1. 2009 CLARREO Meeting at LaRC

33. Role of polarization in radiance calibration and radiance calibration transfer Assume that M12is unknown. M12Q/I = M12 q must be known to ~0.001. Since q can be 100%, M12 must be known to ~0.001 This means that Δq = 0.001q/M12 If M12 = 0.1 and q = 0.2 then Δq = 0.002 If M12 = 0.05 and q = 0.2 then Δq = 0.004 If M12 = 0.01 and q = 0.2 then Δq = 0.02 2009 CLARREO Meeting at LaRC

34. Role of polarization in radiance calibration transfer The CLARREO photopolarimeter can provide Qinc and Uinc even when the view angles of the two instruments are different 2009 CLARREO Meeting at LaRC

35. Nadir-looking imaging spectropolarimeter: (i) contiguous spectral coverage (ii) is not a stand-alone climate instrument (iii) cannot be used to convert radiances into fluxes (iv) cannot be used to cross-calibrate retrievals Multiangle filter polarimeter: (i) narrow-filter instrument (ii) is a stand-alone climate instrument (iii) can be used to convert radiances into fluxes (iv) can be used to cross-calibrate retrievals Multiangle spectropolarimeter: (i) contiguous spectral coverage (ii) is a stand-alone climate instrument (iii) can be used to convert radiances into fluxes (v) can be used to cross-calibrate retrievals Multiangle imaging (i) contiguous spectral coverage spectropolarimeter: (ii) is a stand-alone climate instrument (iii) can be used to convert radiances into fluxes (iv) can be used to cross-calibrate retrievals Trade space 2009 CLARREO Meeting at LaRC

36. Conclusions • The short-wave CLARREO instrument must be a self-sufficient climate-monitoring instrument. • A high-accuracy multi-angle multispectral photopolarimeter is the logical instrument with fully tested capability to provide the detailed information content necessary to attribute climate change and constrain models. • This instrument is necessary to improve the retrieval accuracy of operational SW instruments. • This instrument is necessary to improve the radiometric accuracy of operational satellite measurements. 2009 CLARREO Meeting at LaRC

37. Conclusion • The CLARREO solar instrument does indeed need to be an SI-traceable photopolarimeter with • high radiometric accuracy/precision • high polarization accuracy/precision • thorough multi-angle coverage • broad multispectral coverage 2009 CLARREO Meeting at LaRC

38. The right strategy: fully exploit the information content of the reflected sunlight Classification of passive remote sensing techniques by 1. Spectral range 2. Scattering geometry range 3. Number of Stokes parameters Hierarchy of existing/planned instruments: AVHRR  MODIS, MISR, VIIRS  Glory APS • The measurement approach developed for the Glory mission is to use multi-angle multi-spectral polarimetric measurements because: • Polarization is a relative measurement that can be made extremely accurately. • Polarimetric measurements can be accurately and stably calibrated on orbit. • The variation of polarization with scattering angle and wavelength allows aerosol and cloud particle size, refractive index and shape to be determined. • Appropriate analysis tools are available. 2009 CLARREO Meeting at LaRC