Abstract

Clouds reduce the degree of linear polarization (DOLP) of skylight relative to that of a clear sky. Even thin subvisual clouds in the “twilight zone” between clouds and aerosols produce a drop in skylight DOLP long before clouds become visible in the sky. In contrast, the angle of polarization (AOP) of light scattered by a cloud in a partly cloudy sky remains the same as in the clear sky for most cases. In unique instances, though, select clouds display AOP signatures that are oriented 90° from the clear-sky AOP. For these clouds, scattered light oriented parallel to the scattering plane dominates the perpendicularly polarized Rayleigh-scattered light between the instrument and the cloud. For liquid clouds, this effect may assist cloud particle size identification because it occurs only over a relatively limited range of particle radii that will scatter parallel polarized light. Images are shown from a digital all-sky-polarization imager to illustrate these effects. Images are also shown that provide validation of previously published theories for weak (2%) polarization parallel to the scattering plane for a 22° halo.

© 2008 Optical Society of America

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References

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  1. N. J. Pust and J. A. Shaw, "Dual-field imaging polarimeter using liquid crystal variable retarders," Appl. Opt. 45, 5470-5478 (2006).
    [CrossRef]
  2. I. Pomozi, G. Horvath, and R. Wehner, "How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation," J. Exp. Biol. 204, 2933-2942 (2001).
  3. G. Horvath, A. Barta, J. Gal, B. Suhai, and O. Haiman, "Ground-based full-sky imaging polarimetry of rapidly changing skies and its use for polarimetric cloud detection," Appl. Opt. 41, 543-559 (2002).
    [CrossRef]
  4. M. L. Brines and J. L. Gould, "Skylight polarization patterns and animal orientation," J. Exp. Biol. 96, 69-91 (1982).
  5. I. Koren, L. A. Remer, Y. J. Kaufman, Y. Rudich, and J. V. Martins, "On the twilight zone between clouds and aerosols," Geophys. Res. Lett. 34, L08805 (2007).
    [CrossRef]
  6. J. S. Tyo, D. L. Goldstein, D. B. Chenault, and J. A. Shaw, "Review of passive imaging polarimetry for remote sensing applications," Appl. Opt. 45, 5453-5469 (2006).
    [CrossRef]
  7. J. A. North and M. J. Duggin, "Stokes vector imaging of the polarized sky-dome," Appl. Opt. 36, 723-730 (1997).
  8. R. L. Lee, Jr., "Digital imaging of clear-sky polarization," Appl. Opt. 37, 1465-1476 (1998).
    [CrossRef]
  9. K. J. Voss and Y. Liu, "Polarized radiance distribution measurements of skylight. I. System description and characterization," Appl. Opt. 36, 6083-6094 (1997).
    [CrossRef]
  10. Y. Liu and K. J. Voss, "Polarized radiance distribution measurement of skylight. II. Experiment and data," Appl. Opt. 36, 8753-8764 (1997).
    [CrossRef]
  11. T. W. Cronin, E. J. Warrant, and B. Greiner, "Celestial polarization patterns during twilight," Appl. Opt. 45, 5582-5589 (2006).
    [CrossRef]
  12. A. Marshak, G. Wen, J. A. Coakley, Jr., L. A. Remer, N. G. Loeb, and R. F. Cahalan, "A simple model for the cloud adjacency effect and the apparent bluing of aerosols near clouds," J. Geophys. Res. 113, D14S17(2008), doi:10.1029/2007JD009196.
    [CrossRef]
  13. I. Koren, L. Oreopoulos, G. Feingold, L. A. Remer, and O. Altaratz, "How small is a small cloud," Atmos. Chem. Phys. Discuss. 8, 6379-6407 (2008).
  14. P. Goloub, M. Herman, H. Chepfer, J. Riedl, G. Brogniez, P. Couvert, and G. Seze, "Cloud thermodynamical phase classification from the POLDER spaceborne instrument," J. Geophys. Res. 105, 14747-14759 (2000).
    [CrossRef]
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    [CrossRef]
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    [CrossRef]
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    [CrossRef]
  18. A. Kokhanovsky, "Optical properties of terrestrial clouds," Earth Sci. Rev. 64, 189-241 (2004).
  19. G. W. Kattawar and G. N. Plass, "Degree and direction of polarization of multiple scattered light. 1: homogeneous cloud layers," Appl. Opt. 11, 2851-2865 (1972).
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  22. A. A. Kokhanovsky, "The determination of the effective radius of drops in water clouds from polarization measurements," Phys. Chem. Earth B 25, 471-474 (2000).
  23. W. G. Egan, S. Israel, M. Sidran, E. E. Hindman, W. R. Johnson, and V. S. Whitehead, "Optical properties of continental haze and cumulus and orographic clouds based on space shuttle polarimetric observations," Appl. Opt. 32, 6841-6852(1993).
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2008

A. Marshak, G. Wen, J. A. Coakley, Jr., L. A. Remer, N. G. Loeb, and R. F. Cahalan, "A simple model for the cloud adjacency effect and the apparent bluing of aerosols near clouds," J. Geophys. Res. 113, D14S17(2008), doi:10.1029/2007JD009196.
[CrossRef]

I. Koren, L. Oreopoulos, G. Feingold, L. A. Remer, and O. Altaratz, "How small is a small cloud," Atmos. Chem. Phys. Discuss. 8, 6379-6407 (2008).

2007

I. Koren, L. A. Remer, Y. J. Kaufman, Y. Rudich, and J. V. Martins, "On the twilight zone between clouds and aerosols," Geophys. Res. Lett. 34, L08805 (2007).
[CrossRef]

2006

2004

A. Kokhanovsky, "Optical properties of terrestrial clouds," Earth Sci. Rev. 64, 189-241 (2004).

2002

2001

L. C. Labonnote, G. Brogniez, J. C. Buriez, and M. Doutriaux-Boucher, "Polarized light scattering by inhomogeneous hexagonal monocrystals: validation with ADEOS-POLDER measurements," J. Geophys. Res. 106, 12139-12155 (2001).
[CrossRef]

I. Pomozi, G. Horvath, and R. Wehner, "How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation," J. Exp. Biol. 204, 2933-2942 (2001).

2000

P. Goloub, M. Herman, H. Chepfer, J. Riedl, G. Brogniez, P. Couvert, and G. Seze, "Cloud thermodynamical phase classification from the POLDER spaceborne instrument," J. Geophys. Res. 105, 14747-14759 (2000).
[CrossRef]

A. A. Kokhanovsky, "The determination of the effective radius of drops in water clouds from polarization measurements," Phys. Chem. Earth B 25, 471-474 (2000).

1998

F. M. Breon and P. Goloub, "Cloud droplet effective radius from spaceborne polarization measurements," Geophys. Res. Lett. 25, 1879-1882 (1998).

R. L. Lee, Jr., "Digital imaging of clear-sky polarization," Appl. Opt. 37, 1465-1476 (1998).
[CrossRef]

1997

1996

A. Macke, J. Mueller, E. Raschke, "Single scattering properties of atmospheric ice crystals," J. Atmos. Sci. 53, 2813-2825 (1996).
[CrossRef]

1995

Y. Takano and K. N. Liou, "Radiative transfer in cirrus clouds. Pt. III: Light scattering by irregular ice crystals," J. Atmos. Sci. 52, 818-837 (1995).
[CrossRef]

1993

1991

1983

1982

M. L. Brines and J. L. Gould, "Skylight polarization patterns and animal orientation," J. Exp. Biol. 96, 69-91 (1982).

1979

1972

Altaratz, O.

I. Koren, L. Oreopoulos, G. Feingold, L. A. Remer, and O. Altaratz, "How small is a small cloud," Atmos. Chem. Phys. Discuss. 8, 6379-6407 (2008).

Barta, A.

Breon, F. M.

F. M. Breon and P. Goloub, "Cloud droplet effective radius from spaceborne polarization measurements," Geophys. Res. Lett. 25, 1879-1882 (1998).

Brines, M. L.

M. L. Brines and J. L. Gould, "Skylight polarization patterns and animal orientation," J. Exp. Biol. 96, 69-91 (1982).

Brogniez, G.

L. C. Labonnote, G. Brogniez, J. C. Buriez, and M. Doutriaux-Boucher, "Polarized light scattering by inhomogeneous hexagonal monocrystals: validation with ADEOS-POLDER measurements," J. Geophys. Res. 106, 12139-12155 (2001).
[CrossRef]

P. Goloub, M. Herman, H. Chepfer, J. Riedl, G. Brogniez, P. Couvert, and G. Seze, "Cloud thermodynamical phase classification from the POLDER spaceborne instrument," J. Geophys. Res. 105, 14747-14759 (2000).
[CrossRef]

Buriez, J. C.

L. C. Labonnote, G. Brogniez, J. C. Buriez, and M. Doutriaux-Boucher, "Polarized light scattering by inhomogeneous hexagonal monocrystals: validation with ADEOS-POLDER measurements," J. Geophys. Res. 106, 12139-12155 (2001).
[CrossRef]

Cahalan, R. F.

A. Marshak, G. Wen, J. A. Coakley, Jr., L. A. Remer, N. G. Loeb, and R. F. Cahalan, "A simple model for the cloud adjacency effect and the apparent bluing of aerosols near clouds," J. Geophys. Res. 113, D14S17(2008), doi:10.1029/2007JD009196.
[CrossRef]

Chenault, D. B.

Chepfer, H.

P. Goloub, M. Herman, H. Chepfer, J. Riedl, G. Brogniez, P. Couvert, and G. Seze, "Cloud thermodynamical phase classification from the POLDER spaceborne instrument," J. Geophys. Res. 105, 14747-14759 (2000).
[CrossRef]

Coakley, Jr., J. A.

A. Marshak, G. Wen, J. A. Coakley, Jr., L. A. Remer, N. G. Loeb, and R. F. Cahalan, "A simple model for the cloud adjacency effect and the apparent bluing of aerosols near clouds," J. Geophys. Res. 113, D14S17(2008), doi:10.1029/2007JD009196.
[CrossRef]

Couvert, P.

P. Goloub, M. Herman, H. Chepfer, J. Riedl, G. Brogniez, P. Couvert, and G. Seze, "Cloud thermodynamical phase classification from the POLDER spaceborne instrument," J. Geophys. Res. 105, 14747-14759 (2000).
[CrossRef]

Cronin, T. W.

Doutriaux-Boucher, M.

L. C. Labonnote, G. Brogniez, J. C. Buriez, and M. Doutriaux-Boucher, "Polarized light scattering by inhomogeneous hexagonal monocrystals: validation with ADEOS-POLDER measurements," J. Geophys. Res. 106, 12139-12155 (2001).
[CrossRef]

Duggin, M. J.

J. A. North and M. J. Duggin, "Stokes vector imaging of the polarized sky-dome," Appl. Opt. 36, 723-730 (1997).

Egan, W. G.

Feingold, G.

I. Koren, L. Oreopoulos, G. Feingold, L. A. Remer, and O. Altaratz, "How small is a small cloud," Atmos. Chem. Phys. Discuss. 8, 6379-6407 (2008).

Gal, J.

Goldstein, D. L.

Goloub, P.

P. Goloub, M. Herman, H. Chepfer, J. Riedl, G. Brogniez, P. Couvert, and G. Seze, "Cloud thermodynamical phase classification from the POLDER spaceborne instrument," J. Geophys. Res. 105, 14747-14759 (2000).
[CrossRef]

F. M. Breon and P. Goloub, "Cloud droplet effective radius from spaceborne polarization measurements," Geophys. Res. Lett. 25, 1879-1882 (1998).

Gould, J. L.

M. L. Brines and J. L. Gould, "Skylight polarization patterns and animal orientation," J. Exp. Biol. 96, 69-91 (1982).

Greiner, B.

Haiman, O.

Herman, M.

P. Goloub, M. Herman, H. Chepfer, J. Riedl, G. Brogniez, P. Couvert, and G. Seze, "Cloud thermodynamical phase classification from the POLDER spaceborne instrument," J. Geophys. Res. 105, 14747-14759 (2000).
[CrossRef]

Hindman, E. E.

Horvath, G.

G. Horvath, A. Barta, J. Gal, B. Suhai, and O. Haiman, "Ground-based full-sky imaging polarimetry of rapidly changing skies and its use for polarimetric cloud detection," Appl. Opt. 41, 543-559 (2002).
[CrossRef]

I. Pomozi, G. Horvath, and R. Wehner, "How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation," J. Exp. Biol. 204, 2933-2942 (2001).

Israel, S.

Johnson, W. R.

Kattawar, G. W.

Kaufman, Y. J.

I. Koren, L. A. Remer, Y. J. Kaufman, Y. Rudich, and J. V. Martins, "On the twilight zone between clouds and aerosols," Geophys. Res. Lett. 34, L08805 (2007).
[CrossRef]

Kokhanovsky, A.

A. Kokhanovsky, "Optical properties of terrestrial clouds," Earth Sci. Rev. 64, 189-241 (2004).

Kokhanovsky, A. A.

A. A. Kokhanovsky, "The determination of the effective radius of drops in water clouds from polarization measurements," Phys. Chem. Earth B 25, 471-474 (2000).

Konnen, G. P.

Koren, I.

I. Koren, L. Oreopoulos, G. Feingold, L. A. Remer, and O. Altaratz, "How small is a small cloud," Atmos. Chem. Phys. Discuss. 8, 6379-6407 (2008).

I. Koren, L. A. Remer, Y. J. Kaufman, Y. Rudich, and J. V. Martins, "On the twilight zone between clouds and aerosols," Geophys. Res. Lett. 34, L08805 (2007).
[CrossRef]

Labonnote, L. C.

L. C. Labonnote, G. Brogniez, J. C. Buriez, and M. Doutriaux-Boucher, "Polarized light scattering by inhomogeneous hexagonal monocrystals: validation with ADEOS-POLDER measurements," J. Geophys. Res. 106, 12139-12155 (2001).
[CrossRef]

Lee, Jr., R. L.

Liou, K. N.

Y. Takano and K. N. Liou, "Radiative transfer in cirrus clouds. Pt. III: Light scattering by irregular ice crystals," J. Atmos. Sci. 52, 818-837 (1995).
[CrossRef]

K. N. Liou, An Introduction to Atmospheric Radiation(Academic, 1980).

Liu, Y.

Loeb, N. G.

A. Marshak, G. Wen, J. A. Coakley, Jr., L. A. Remer, N. G. Loeb, and R. F. Cahalan, "A simple model for the cloud adjacency effect and the apparent bluing of aerosols near clouds," J. Geophys. Res. 113, D14S17(2008), doi:10.1029/2007JD009196.
[CrossRef]

Lynch, D.

Macke, A.

A. Macke, J. Mueller, E. Raschke, "Single scattering properties of atmospheric ice crystals," J. Atmos. Sci. 53, 2813-2825 (1996).
[CrossRef]

Marshak, A.

A. Marshak, G. Wen, J. A. Coakley, Jr., L. A. Remer, N. G. Loeb, and R. F. Cahalan, "A simple model for the cloud adjacency effect and the apparent bluing of aerosols near clouds," J. Geophys. Res. 113, D14S17(2008), doi:10.1029/2007JD009196.
[CrossRef]

Martins, J. V.

I. Koren, L. A. Remer, Y. J. Kaufman, Y. Rudich, and J. V. Martins, "On the twilight zone between clouds and aerosols," Geophys. Res. Lett. 34, L08805 (2007).
[CrossRef]

Mueller, J.

A. Macke, J. Mueller, E. Raschke, "Single scattering properties of atmospheric ice crystals," J. Atmos. Sci. 53, 2813-2825 (1996).
[CrossRef]

North, J. A.

J. A. North and M. J. Duggin, "Stokes vector imaging of the polarized sky-dome," Appl. Opt. 36, 723-730 (1997).

Oreopoulos, L.

I. Koren, L. Oreopoulos, G. Feingold, L. A. Remer, and O. Altaratz, "How small is a small cloud," Atmos. Chem. Phys. Discuss. 8, 6379-6407 (2008).

Plass, G. N.

Pomozi, I.

I. Pomozi, G. Horvath, and R. Wehner, "How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation," J. Exp. Biol. 204, 2933-2942 (2001).

Pust, N. J.

Raschke, E.

A. Macke, J. Mueller, E. Raschke, "Single scattering properties of atmospheric ice crystals," J. Atmos. Sci. 53, 2813-2825 (1996).
[CrossRef]

Remer, L. A.

I. Koren, L. Oreopoulos, G. Feingold, L. A. Remer, and O. Altaratz, "How small is a small cloud," Atmos. Chem. Phys. Discuss. 8, 6379-6407 (2008).

A. Marshak, G. Wen, J. A. Coakley, Jr., L. A. Remer, N. G. Loeb, and R. F. Cahalan, "A simple model for the cloud adjacency effect and the apparent bluing of aerosols near clouds," J. Geophys. Res. 113, D14S17(2008), doi:10.1029/2007JD009196.
[CrossRef]

I. Koren, L. A. Remer, Y. J. Kaufman, Y. Rudich, and J. V. Martins, "On the twilight zone between clouds and aerosols," Geophys. Res. Lett. 34, L08805 (2007).
[CrossRef]

Riedl, J.

P. Goloub, M. Herman, H. Chepfer, J. Riedl, G. Brogniez, P. Couvert, and G. Seze, "Cloud thermodynamical phase classification from the POLDER spaceborne instrument," J. Geophys. Res. 105, 14747-14759 (2000).
[CrossRef]

Rudich, Y.

I. Koren, L. A. Remer, Y. J. Kaufman, Y. Rudich, and J. V. Martins, "On the twilight zone between clouds and aerosols," Geophys. Res. Lett. 34, L08805 (2007).
[CrossRef]

Seze, G.

P. Goloub, M. Herman, H. Chepfer, J. Riedl, G. Brogniez, P. Couvert, and G. Seze, "Cloud thermodynamical phase classification from the POLDER spaceborne instrument," J. Geophys. Res. 105, 14747-14759 (2000).
[CrossRef]

Shaw, J. A.

Sidran, M.

Suhai, B.

Takano, Y.

Y. Takano and K. N. Liou, "Radiative transfer in cirrus clouds. Pt. III: Light scattering by irregular ice crystals," J. Atmos. Sci. 52, 818-837 (1995).
[CrossRef]

Tinbergen, J.

Tyo, J. S.

Voss, K. J.

Warrant, E. J.

Wehner, R.

I. Pomozi, G. Horvath, and R. Wehner, "How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation," J. Exp. Biol. 204, 2933-2942 (2001).

Wen, G.

A. Marshak, G. Wen, J. A. Coakley, Jr., L. A. Remer, N. G. Loeb, and R. F. Cahalan, "A simple model for the cloud adjacency effect and the apparent bluing of aerosols near clouds," J. Geophys. Res. 113, D14S17(2008), doi:10.1029/2007JD009196.
[CrossRef]

Whitehead, V. S.

Appl. Opt.

G. W. Kattawar and G. N. Plass, "Degree and direction of polarization of multiple scattered light. 1: homogeneous cloud layers," Appl. Opt. 11, 2851-2865 (1972).

W. G. Egan, S. Israel, M. Sidran, E. E. Hindman, W. R. Johnson, and V. S. Whitehead, "Optical properties of continental haze and cumulus and orographic clouds based on space shuttle polarimetric observations," Appl. Opt. 32, 6841-6852(1993).

K. J. Voss and Y. Liu, "Polarized radiance distribution measurements of skylight. I. System description and characterization," Appl. Opt. 36, 6083-6094 (1997).
[CrossRef]

R. L. Lee, Jr., "Digital imaging of clear-sky polarization," Appl. Opt. 37, 1465-1476 (1998).
[CrossRef]

Y. Liu and K. J. Voss, "Polarized radiance distribution measurement of skylight. II. Experiment and data," Appl. Opt. 36, 8753-8764 (1997).
[CrossRef]

G. Horvath, A. Barta, J. Gal, B. Suhai, and O. Haiman, "Ground-based full-sky imaging polarimetry of rapidly changing skies and its use for polarimetric cloud detection," Appl. Opt. 41, 543-559 (2002).
[CrossRef]

G. P. Konnen and J. Tinbergen, "Polarimetry of a 22° halo," Appl. Opt. 30, 3382-3400 (1991).

J. S. Tyo, D. L. Goldstein, D. B. Chenault, and J. A. Shaw, "Review of passive imaging polarimetry for remote sensing applications," Appl. Opt. 45, 5453-5469 (2006).
[CrossRef]

N. J. Pust and J. A. Shaw, "Dual-field imaging polarimeter using liquid crystal variable retarders," Appl. Opt. 45, 5470-5478 (2006).
[CrossRef]

T. W. Cronin, E. J. Warrant, and B. Greiner, "Celestial polarization patterns during twilight," Appl. Opt. 45, 5582-5589 (2006).
[CrossRef]

Atmos. Chem. Phys. Discuss.

I. Koren, L. Oreopoulos, G. Feingold, L. A. Remer, and O. Altaratz, "How small is a small cloud," Atmos. Chem. Phys. Discuss. 8, 6379-6407 (2008).

Earth Sci. Rev.

A. Kokhanovsky, "Optical properties of terrestrial clouds," Earth Sci. Rev. 64, 189-241 (2004).

Geophys. Res. Lett.

F. M. Breon and P. Goloub, "Cloud droplet effective radius from spaceborne polarization measurements," Geophys. Res. Lett. 25, 1879-1882 (1998).

I. Koren, L. A. Remer, Y. J. Kaufman, Y. Rudich, and J. V. Martins, "On the twilight zone between clouds and aerosols," Geophys. Res. Lett. 34, L08805 (2007).
[CrossRef]

J. Atmos. Sci.

Y. Takano and K. N. Liou, "Radiative transfer in cirrus clouds. Pt. III: Light scattering by irregular ice crystals," J. Atmos. Sci. 52, 818-837 (1995).
[CrossRef]

A. Macke, J. Mueller, E. Raschke, "Single scattering properties of atmospheric ice crystals," J. Atmos. Sci. 53, 2813-2825 (1996).
[CrossRef]

J. Exp. Biol.

I. Pomozi, G. Horvath, and R. Wehner, "How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation," J. Exp. Biol. 204, 2933-2942 (2001).

M. L. Brines and J. L. Gould, "Skylight polarization patterns and animal orientation," J. Exp. Biol. 96, 69-91 (1982).

J. Geophys. Res.

L. C. Labonnote, G. Brogniez, J. C. Buriez, and M. Doutriaux-Boucher, "Polarized light scattering by inhomogeneous hexagonal monocrystals: validation with ADEOS-POLDER measurements," J. Geophys. Res. 106, 12139-12155 (2001).
[CrossRef]

P. Goloub, M. Herman, H. Chepfer, J. Riedl, G. Brogniez, P. Couvert, and G. Seze, "Cloud thermodynamical phase classification from the POLDER spaceborne instrument," J. Geophys. Res. 105, 14747-14759 (2000).
[CrossRef]

A. Marshak, G. Wen, J. A. Coakley, Jr., L. A. Remer, N. G. Loeb, and R. F. Cahalan, "A simple model for the cloud adjacency effect and the apparent bluing of aerosols near clouds," J. Geophys. Res. 113, D14S17(2008), doi:10.1029/2007JD009196.
[CrossRef]

J. Opt. Soc. Am.

Phys. Chem. Earth B

A. A. Kokhanovsky, "The determination of the effective radius of drops in water clouds from polarization measurements," Phys. Chem. Earth B 25, 471-474 (2000).

Other

K. N. Liou, An Introduction to Atmospheric Radiation(Academic, 1980).

J. A. North and M. J. Duggin, "Stokes vector imaging of the polarized sky-dome," Appl. Opt. 36, 723-730 (1997).

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Figures (9)

Fig. 1
Fig. 1

Clear-sky polarization at 450 nm (26 June 2006, 5:55 pm MDT = UTC 6 h ). Black areas denote regions that are overexposed, underexposed, uncalibrated, or—in the case of the AOP—undefined due to lack of measurable polarized light. For AOP to be considered defined, a threshold of DOLP > 1 % was chosen.

Fig. 2
Fig. 2

Cloud polarization in a partially cloudy sky at 450 nm (23 June 2006, 5:55 pm MDT = UTC 6 h ). Black areas denote regions that are overexposed, underexposed, uncalibrated, or—in the case of the AOP—undefined due to lack of measurable polarized light. For AOP to be considered defined, a threshold of DOLP > 1 % was chosen.

Fig. 3
Fig. 3

Maximum degree of polarization for 23 June, 26 June, and 11 September 2006 at 700 nm . The sky was completely cloud free on 26 June and 11 September, while on 23 June the sky was perceived to be cloud free except in the gray-shaded time when a few small, localized clouds were visible (see Fig. 2). The flat spot in the graph of 11 September data occurs when the Sun never attained that height. The extra x-axis hash shows the maximum solar elevation for 23 June.

Fig. 4
Fig. 4

Polarization for a cloud that changes the AOP at 700 nm (24 May 2006, 5:21 pm MDT = UTC 6 h ). Black areas denote regions that are overexposed, underexposed, uncalibrated, or—in the case of the AOP—undefined due to lack of measurably polarized light. For AOP to be considered defined, a threshold of DOLP > 1 % was chosen.

Fig. 5
Fig. 5

Scattering contributors to the polarimetric ground measurement. The plane of the paper is the scattering plane. The polarized portion of the Rayleigh scattered components (A and D) is always polarized perpendicular to the scattering plane, while the polarized component of the Mie scattering from the cloud can potentially be polarized either parallel or perpendicular to the scattering plane. θ is the scattering angle. ( θ = 0 is forward scattering.)

Fig. 6
Fig. 6

Areas of spherical particle sizes that scatter a polarization that is parallel to the scattering plane (shown in the gray areas). The mode radius of the cloud droplet size distribution (the most frequent radius size of the distribution) is labeled on the x axis. (Distribution parameter μ = 20 .)

Fig. 7
Fig. 7

Same as Fig. 6 but with a wider cloud droplet distribution function ( μ = 5 ).

Fig. 8
Fig. 8

Intensity image of a 22 ° halo at 11:24:13 MDT = UTC 6 on 12 April 2006 ( 450 nm ).

Fig. 9
Fig. 9

DOLP and AOP images of a 22 ° halo at 11:24:13 MDT = UTC 6 on 12 April 2006 ( 450 nm ). Areas of low DOLP ( 1 % ) are masked in black. Black areas denote regions that are overexposed, underexposed, uncalibrated, or—in the case of the AOP—undefined owing to the lack of measurably polarized light. For AOP to be considered defined, a threshold of DOLP > 1 % was chosen.

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