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BACKSCATTERING OF SPECTRAL IRRADIANCE BY BUBBLES: HYCODE

BACKSCATTERING OF SPECTRAL IRRADIANCE BY BUBBLES: HYCODE. Marlon R. Lewis 1,2 ,Bruce Johnson 2 ,Alex Hay 2. 1 Satlantic Inc. 2 Department of Oceanography Dalhousie University. Satlantic Inc. Dalhousie Univ. . Background. Phase Function.

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BACKSCATTERING OF SPECTRAL IRRADIANCE BY BUBBLES: HYCODE

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  1. BACKSCATTERING OF SPECTRAL IRRADIANCE BY BUBBLES: HYCODE Marlon R. Lewis1,2 ,Bruce Johnson2 ,Alex Hay2 1Satlantic Inc. 2Department of Oceanography Dalhousie University

  2. Satlantic Inc. Dalhousie Univ. Background Phase Function Hypothesis: Bubbles injected into the upper ocean account for much of the backscattering of visible light, and variations in the backscattering coefficient in time and space. Blue Green Spectral Reflectance Surface Radiance Amplitude and Angular Distribution

  3. HyCode 2000 REMUS data analysis: (Catherine Brown1, Yannick Huot1, Mike Purcell2, Marlon Lewis1, and John Cullen1 Mapping coastal bio-optical properties with an autonomous underwater vehicle (AUV) and a spectral inversion model. Honorable Mention Student Award! Temp. Fluoro. KLu(555) KD(555)

  4. Spectral derivative, coupled w/ assumption re sun + sky spectral shape.

  5. Bubbles are: • Ubiquitous (although their concentrations vary over >4 orders of magnitude) • Generated largely by breaking waves (although biological processes can help through development of supersaturated soln., and • production of bubbles, particularly in sediment) • Are stabilized by the adsorption of organic material which occurs within seconds of formation. • Are spherical, and rather simple optically so Mie approach is valid. Wind= 10 12 16 ms-1 Side View Top View (Anguelova,1998)

  6. Background: • Bubbles are refringent, hence “hard scatterers”. • Size distribution and number density uncertain, and variable. • Organic coating forms on bubbles within minutes, particularly in productive waters - this stabilizes bubbles that would otherwise collapse, and permits much smaller bubbles to persist than would be predicted on the basis of physics of “clean” bubbles. Bubble backscattering: Mie Computations Measurement of number-size distribution

  7. Zhang, X., M.R. Lewis, M. Li, B. Johnson, and G. Korotaev. 2002. The volume scattering function of natural bubble populations. Limnol. Oceanogr. 47: 1273-1282. Bubble diagnostic? Bubbles are optically “hard”, and scatter light strongly, particularly in the backward direction. Organic coatings increase backscatter by 4x.

  8. HYCODE 2001: OVERVIEW VSM: Volume- scattering function continuous, joint w/ MHI & OSU. HHS: Above-water hyperspectral upwelling radiance (overflights) SPAR: Surface hyperspectral upwelling radiance & downward irradiance, acoustics, camera, CT, GTD, tilt. HyperPro: Sub-surface hyperspectral profiles (radiance/irradiance) + CT, Tilt SQM: Intercalibration/ comparison (McLean)

  9. STATION OVERVIEW Stations occupied from offshore blue to inshore brown waters. • Data Status: • HyperPro: Processed, available (WOODS) • SPAR Optics: Processed, available as above. • HHS: Processed: Under evaluation w/Bissett, Steward. • VSM: Processed, available. • SQM: Data all processed, report available.

  10. HyperPro: Provided useful hyperspectral downwelling irradiance/ upwelling radiance to >1% light level.

  11. SPAR buoy provided hyperspectral upwelling radiance (0.6 m) and downwelling irradiance (above surface), along with multi-frequency acoustics, bubble camera, CT, tilts.

  12. SPAR: Multifrequency Acoustic View of Bubble Cloud (ship wake) 1.5 MHz 2 MHz . . . . 4 MHz 300 kHz 350 kHz . . . . . . 600 kHz

  13. SQM: Overall, agreement between numerous instruments is excellent (see McLean report) 1. 48 independent instruments intercompared (including PHILLS (NRL)). 2. Hyperspectral and multispectral radiance sensors w/in 4%, in-water irradiance w/in 1%. 3. Above-water irradiance sensors more variable (9%). 4. PHILLS intercomparison with standard reference better than 7% w/ some roll-off near edges of target (WFOV). 5. Some processing uncertainties remain with other sensors (ASD, HOBILabs).

  14. VSM: Deployed for continuous sampling (w/ OSU ac-9, others). 850 VSF observations over wide range of optical water types. Lee, M., and M.R. Lewis. 2003. A New Method for the Measurement of the Optical Volume Scattering Function in the Upper Ocean. J. Atmos. Ocean. Tech., in press.

  15. See Mobley et al. 2001 Boss and Pegau 2001

  16. “Unnatural Bubbles”: Zhang, X., M.R. Lewis, W.P. Bissett, B. Johnson, and D. Kohler2Optical Influence of Ship Wakes (to go toApplied Optics) ‘Blue’ water ‘Brown’ water

  17. PHILLS2 Conclusion: Model (Mie + Hydrolight) does a good job of predicting enhanced wake reflectance and spectral shifts in blue water and green water environments. We do not have a model that works at all well for the waters just off RUMFS.

  18. Natural Bubble Injection (Zhang et al. 2002, L&O) 

  19. 15:00 29 July 18:30 30 July 21:30 30 July 22:40 31 July

  20. Conclusions 1. Non-phytoplankton particles are needed to explain the optical measurement of VSF. 2. Small bubbles and particles are optically important. The shape of the natural VSF is largely determined by them. 3. Coated bubbles provide better fit with model (and coatings are physically required in nature) better than clean bubbles.

  21. Components of albedo: • Fresnel reflectance from surface (direct solar disk + sky radiance) • Contributions from the ocean interior - the “water-leaving radiance” or “underlight”

  22. Rather weak dependence of Fresnel reflectance on winds. Hydrolight Sim. W/ cardoidial sky.

  23. Whitecaps and foam enhance reflectance as winds increase: Foam coverage from Moore et. al., 2000, + Hydrolight

  24. Quite strong dependence of water-leaving irradiances on wind speeds. ‘Surface’ of ocean not well defined! “Our” bubble models + Hydrolight

  25. Bubbles dominate above ~20 ms-1

  26. From Jin, Charlock and Rutledge (2002) year long albedo measurements in support of CERES experiment: Measured albedo less than model estimates, even when unrealistic (too high backscatter) particle phase function used (Petzold). “Sensitivity tests indicate that the uncertainties in aerosol optical properties, chlorophyll concentration, wind speed or foams are not the primary factors for the model-observation differences in the ocean surface albedo, whereas the scattering by air bubbles and/or by suspended materials have the potential to significantly reduce or eliminate the model-observation differences in the ocean surface reflection.”

  27. Hyperspectral Determination of Nitrate in the far UV: See: Johnson and Coletti, 2002, DSR

  28. Nitrate on SeaBird CTD * Autoanalyzer nitrate

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