Atmospheric Correction Algorithms for Remote Sensing of Open and Coastal Waters

1. Atmospheric Correction Algorithms for Remote Sensing of Open and Coastal Waters Zia Ahmad Ocean Biology Processing Group (OBPG) NASA- Goddard Space Flight Center GEO-CAPE Workshop, August 18-20, 2008

2. Acknowledgements Charles R. McClain and Ocean Biology Processing Group (OBGP), NASA-GSFC

3. Overview • Background • Atmospheric Correction (General) • Overview of the Operational Method • Recent Enhancements • Summary and Conclusions

4. Ltot (λ) = Latm (λ) + t1Lg (λ)+ t2Lwc (λ)+t3LWL (λ) Goal: LWL(λ) Ig IWC IWL LWL carries valuable information about organic matter, phytoplankton, particulate matters, and other constituents of the upper ocean

5. Remote Sensing Reflectance Rrs Rrs(λ)=LWL (λ)/Ed (0+) (λ) Effect of CDOM Effect of Phytoplankton

6. Scattering and Absorption Coefficients of Water and Chlorophyll Water Chlorophyll absorption scattering Abs. coeff. for 765nm band is ~400 times higher than the abs. coeff for 412 nm band. Scattering coeff. for 765 nm band is ~16 times lower than scatt. coeff at 412 nm Chlo. specific abs. coeff (a*) for 443 nm band is ~3.2 times higher than the chlo. specific abs. coeff for 555 nm band.

7. Ltot (top-of-atmosphere) and ILw • Contribution from water-leaving radiance (t3LWL) to the TOA radiance (Ltot) @ 412 nm is ~ 12% for open ocean and ~5% for C. Bay

8. Atmospheric Correction Top-of-Atmosphere Radiance: Ltot (λ) = [Latm (λ) +t1Lg (λ)+ t2LWC (λ)]+ t3LWL (λ) Goal: Determine [Latm (λ) +t1Lg (λ)+ t2LWC (λ)] as accurately as possible • Latm(λ) = f (scattering by air molecules and aerosols in the atmosphere, and absorption by aerosols and trace gases like O3, H2O and NO2. Also, Fresnel reflection and sea state characterized by wind speed) • Lg(λ) = f (Fresnel reflection, and sea state - characterized by wind speed and direction over the ocean) • LWC(λ) = f (sea state - characterized by wind speed)

9. Aerosol Models for Atmospheric Correction • Howard Gordon’s (HG) Aerosol Models are Based on Shettle and Fenn’s Models for Tropospheric and Oceanic Aerosols • Twelve (12) aerosol models are used in operational processing • Oceanic • O99 • Maritime (1% oceanic and 99% tropospheric) • M99, M90, M70, M50 • Coastal (0.5% oceanic and 99.5% tropospheric) • C99, C90, C70, C50 • Tropospheric • T99, T90, T50

10. Some Properties of Operational Aerosol Models Size Distribution Phase Function c50 c50 c90 c90 - Effective radius varies from 0.14 to 4.74 μm - Single Scattering Albedo (SSA) varies from 0.98 (T50) to 1.0 (O99)

11. Atmospheric Correction Methodology • Gordon and Wang’s algorithm uses measurements in NIR bands to select aerosol model ε765, 865 = ρ765/ρ865 ρWL (λ)=0 ρWL (λ)=0 ε765, 865 = ρ765/ρ865 ελ, 865 = ρλ/ρ865 - Select two models that bracket the observed ε765, 865 - In operational processing, ratio of single-scattering-reflectance values are used to compute ε765, 865

12. Atmospheric Correction Methodology (cont.) • NIR Correction • For higher concentration of chlorophyll (chlo > 0.7 mg/m3 ) the assumption that water-leaving radiance in the NIR bands is zero is no longer valid -The correction is based on a bio-optical model that relates the Rrs in the NIR as: Rrs (λ) = Rrs(λ0)*[atot(λ0)/atot(λ)]*[bb(λ)/bb(λ0)]η atot(λ)=aw(λ)+aph(λ)+adg(λ) bb(λ)=mλ+c Here, λ0=670-nm, and λ=765- and 865- nm

13. Vicarious Calibration • The objective of vicarious calibration is to normalize the observed TOA radiances to RT computed radiances. • The method uses in situ data from MOBY site to calibrate the visible bands, and data from South Pacific Gayer and South Indian Ocean sites to calibrate NIR band. • LwL is assumed to be zero for NIR bands (765 and 865 bands),

14. Vicarious Calibration (cont.) Time series of Gain Coefficients Locations of in situ Data - The gain coefficient for IR channel (765 nm) is determined from match-up data collected over the South Pacific Gayer (SPG) and the South Indian Ocean (SIO) sites - The gain coefficients for all VIS Channels are determined from match-up data over MOBY site

15. Vicarious Calibration (Validation) Results for Deep Water (d >1000 m) Locations of in situ Data - Results for deep water show very good agreement between in situ data and satellite retrievals.

16. Recent Enhancements • Use of SWIR Bands in Retrieving ILw Over Bay Area • Wang and Shi’s Algorithm for Coastal Areas - Uses SWIR Bands to select aerosol models RGB Image Chlo. using NIR Bands Chlo. using SWIR Bands

17. A Comparison of NIR and SWIR Based Retrieval over the Bay Area

18. An Example of Negative nLw(412 nm) over the Eastern Coast of US • Possible Reasons - Operational aerosol size dist. are not representative of Bay area aerosols - Perhaps Bay area aerosols are more absorbing than what is assumed in the operational processing April 7, 1998

19. Chesapeake Bay (AERONET Stations) SERC SERC: 38o, 53/ N, 76o, 30/ W COVE: 36o, 54/ N, 75o, 42.5/ W Wallops Island Wallops Island: 37o, 56.5/ N, 75o, 28.5/ W COVE

20. Physical and Optical Properties of Aerosols over Bay Area Cove Wallops Island SERC Monthly Mean Modal Radius Monthly Mean Modal Std. Dev. Effective Radius Spectral Dep. Of SSA Single Scattering Albedo Monthly Mean SSA

21. Aerosol Opt. Thickness (MODIS vs. AERONET) AOT Based on New Models AOT Based on Operational Models COVE:: Red Wallops Island:: Blue SERC: Black Ewa Kwiatkowska

22. Summary and Conclusions • Gordon and Wang (GW) algorithm for atmospheric correction works reasonably well over open ocean (Case 1 waters). The retrieved values of water-leaving radiances and chlorophyll amount compare favorably with in situ data. • Absorbing aerosols are problems. Also, over coastal areas, GW algorithm sometimes gives negative water-leaving radiance in 412 nm band. Work is progress to address this problem. • With new aerosol models, the retrieved AOT in the four bands of the SeaWiFS sensor are in fairly good agreement with the AERONET data over the Chesapeake Area.