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BRDF correction and IOP retrieval from angular Rrs: A non-Case 1 approach

BRDF correction and IOP retrieval from angular Rrs: A non-Case 1 approach. ZhongPing Lee 1 , Deric Gray 2 , Bertrand Lubac 2 , Alan Weidemann 2 , Robert Arnone 2 , Paul Martinolich 2 1 Northern Gulf Institute, Mississippi State University,

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BRDF correction and IOP retrieval from angular Rrs: A non-Case 1 approach

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  1. BRDF correction and IOP retrieval from angular Rrs: A non-Case1 approach ZhongPing Lee1, Deric Gray2, Bertrand Lubac2, Alan Weidemann2, Robert Arnone2, Paul Martinolich2 1Northern Gulf Institute, Mississippi State University, Stennis Space Center, MS 39529; zplee@ngi.msstate.edu 2Naval Research Lab, Stennis Space Center, MS 39529

  2. 400 nm 640 nm Water-leaving radiance in the Sun plane, zenith dependence (arrow length indicates radiance value) BRDF background: Bottom line: Water-leaving radiance changes with angle.

  3. Case1 approach to correct angular variation (BRDF): BRDF correction data flow: Ω: {θSun,θv,φ} Advantage: [Chl] is the only in-water property required. Rrs(Ω0) Rrs(Ω) [Chl]  Conditions: Require waters to follow the Case1 bio-optical relationships, e.g. fixed CDOM:Chl and bp:Chl dependences. Minor: Interpolate among 7 wavelengths and 6 [Chl].

  4. Non-Case1 approach: Objective: Obtain BRDF correction and IOP retrieval without the Case-1 assumption, i.e., not limited by CDOM:Chl ratio or bp:Chl ratio. a. Most (> 90%) of the G (~ f/Q) variation is resulted from change of bb/(a+bb) (e.g. Gordon et al 1988); b. Minor (<50%) variation is due to Ω (θSun,θv,φ), especially in the remote-sensing domain; c. Minor (<30%) variation is due to phase function. G(0) (sr-1) bb/(a+bb)

  5. Critical information required for IOP retrieval and BRDF Correction: • Angular geometry (Ω) • Particle phase function G depends on phase function Need G values to analytically invert Rrs.

  6. Particle phase function bb/b ratio: Phase function [sr-1] Scattering angle [deg] G {Gw,g0,g1} Table functions of wind, Ω, and bb/b.

  7. IOP retrieval and BRDF correction, & examples: Remote-sensing domain Nadir range Azimuth range Strong Sun glint region bb/b; Phase function Rrs(Ω0) Rrs(Ω) G(Ω) {a,bbp} & G(Ω0)

  8. Example 1: Knowing exactly the phase function Rrs(Ω) compared with Rrs(Ω0) Rrs(ΩΩ0) compared with Rrs(Ω0) Rrs comparison Rrs(Ω) Rrs(Ω)  Rrs(Ω0) Rrs(Ω0) Rrs(Ω0) 75o 15o

  9. Example 1: Knowing exactly the phase function Rrs(Ω) compared with Rrs(Ω0) Rrs(ΩΩ0) compared with Rrs(Ω0) Rrs comparison Rrs(Ω) Rrs(Ω)  Rrs(Ω0) 75o Rrs(Ω0) Rrs(Ω0) 90o

  10. Example 1: Knowing exactly the phase function Rrs(Ω) compared with Rrs(Ω0) Rrs(ΩΩ0) compared with Rrs(Ω0) Rrs comparison 75o Rrs(Ω) Rrs(Ω)  Rrs(Ω0) 165o Rrs(Ω0) Rrs(Ω0)

  11. Example 1: Knowing exactly the phase function Comparison of total absorption atot [m-1] atot [m-1] Comparison of backscattering coefficient bbp [m-1] IOP comparison bbp [m-1] 75o 15o

  12. Example 1: Knowing exactly the phase function Comparison of total absorption atot [m-1] atot [m-1] Comparison of backscattering coefficient bbp [m-1] IOP comparison bbp [m-1] 75o 90o

  13. Example 2: Incorrect phase function Rrs(Ω) is generated with bb/b = 0.5%; Rrs(Ω) inversion used G of bb/b = 1.5%. Rrs(ΩΩ0) compared with Rrs(Ω0) Rrs(Ω)  Rrs(Ω0) Rrs comparison Rrs(Ω0) of bb/b = 0.5%. bb/b; Phase function Rrs(Ω0) Rrs(Ω) G(Ω) {a,bbp} & G(Ω0) 30o 15o

  14. Example 2: Incorrect phase function Rrs(Ω) is generated with bb/b = 1.0%; Rrs(Ω) inversion used G of bb/b = 2.5%. Rrs(ΩΩ0) compared with Rrs(Ω0) Rrs(Ω)  Rrs(Ω0) Rrs comparison Rrs(Ω0) of bb/b = 1.0%. bb/b; Phase function Rrs(Ω0) Rrs(Ω) G(Ω) {a,bbp} & G(Ω0) 60o 15o

  15. Example 2: Incorrect phase function Rrs(Ω) is generated with bb/b = 1.0%; Rrs(Ω) inversion used G of bb/b = 2.5%. Comparison of total absorption atot [m-1] atot [m-1] Comparison of backscattering coefficient bbp [m-1] IOP comparison bbp [m-1] bb/b; Phase function Rrs(Ω0) Rrs(Ω) G(Ω) {a,bbp} & G(Ω0) 60o 15o

  16. Example 2: Incorrect phase function Rrs(Ω) is generated with bb/b = 0.5%; Rrs(Ω) inversion used G of bb/b = 1.0%. Rrs(ΩΩ0) compared with Rrs(Ω0) Rrs(Ω)  Rrs(Ω0) Rrs comparison Rrs(Ω0) of bb/b = 0.5%. 15o 90o

  17. Example 2: Incorrect phase function Rrs(Ω) is generated with bb/b = 0.5%; Rrs(Ω) inversion used G of bb/b = 1.0%. Comparison of total absorption atot [m-1] atot [m-1] Comparison of backscattering coefficient bbp [m-1] IOP comparison bbp [m-1] 15o 90o

  18. Phase function selection: Phase function is an input, then how to determine it? 1. Average (default) particle phase function 2. From water-mass classification: assign different bb/b for different water mass 3. Iteration: w/ default bb/b bb/b; Phase function Rrs(Ω) {a,bbp, Y} bb/b = 0.1 bbp(555) + 0.005 Y + 0.05 bbp(555)/a(490); Lee et al (2004) bb/b; Phase function Rrs(Ω0) Rrs(Ω) G(Ω) {a,bbp} & G(Ω0)

  19. Conclusions: a. Wavelength independent model parameters are derived from modeling of Rrs(λ); b. These model parameters are tabulated for 5 phase functions, 6 sun angles, (7x12+1) view angles, and 4 wind speed; phase function is the only required input from below the surface; c. When phase function is well estimated, both IOP retrieval and BRDF correction can be well achieved from Rrs(Ω); d. For incorrectly estimated phase function, less influence to BRDF correction; slightly more influence to derived absorption; much larger impact to particle backscattering coefficient; f. It is not necessary to have a Case1 assumption for the above objectives.

  20. Acknowledgement: The supported from NASA Ocean Biology and Biogeochemistry Program is greatly appreciated.

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