Gerald Moore, A.C. Workshop, Wimereaux,2012 [ 1 ]

1. The MERIS Bright Pixel Atmospheric Correction (BPAC). Gerald Moore (Bio-Optika) Sam Lavender (SpatiallyScientific/UoP), Jean-Paul Huot (ESA) Constant Mazaran, Francois Montagner, Ludovic Bourg & others– at ACRI Suzanne Kratzer (Stockholm), John Icely and Sonia Cristina (Portugal). Gerald Moore, A.C. Workshop, Wimereaux,2012 [1]

2. Introduction CASI – False Colour DP BPAC Based on the simple assumption that ρ(λ) <> 0 at any wavelength – it only approximates to zero. Compared with dark pixel assumption: at some (NIR) wavelength ρ(λ) = 0Historically developed from the LOICZ programusing CASI over the Humber estuary (MERIS simulation). Versions for CZCS and SeaWiFS. Part of MERIS processor from launch. There have been 3 versions to date: Original – using 709,778,865 2nd reprocessing – simplified - using 709,778,865 3rd reprocessing – improved RT model using {709,778,865}, {778,865,885} [4th reprocessing – under construction…] Gerald Moore, A.C. Workshop, Wimereaux,2012 [2]

3. Model Outline Atmosphere based on simple single scattering model - α, τa. - further processing Antoine & Morel (Gordon & Wang – SeaWiFS) Hydrological optical model - currently use Hydrolight (via LUTs) - Large range of turbidity's: Just detectable … Mud - Classification for band optimisation Low - Medium Turbidity – band 709+ V High Turbidity – bands 775+ (potentially 1020) Differentiate white vs.. yellow particulates – ‘coccolith flag’ - [Correction for Chlorophyll fluorescence at 709] Single State variable – bb (TSM) -- i.e. Three parameters -- assume that aTSM co-varies with bb Gerald Moore, A.C. Workshop, Wimereaux,2012 [3]

4. For initial estimates – TSM=0.01 For Low Band - α = .5 - TSM =1.0 for High Band - α = .5 - determine sediment type - Determine w(705, 775,865) from model - Use Difference Equation to get as(705), as(775) Test 705 not ‘Saturated’ Test Sufficient Radiance 885 ρf(685) ‘Chl’ Correct 705 for Fluorescence Iterate High Band Iterate Low Band Standard AC Combine Valid Estimates Gerald Moore, A.C. Workshop, Wimereaux,2012 [4]

5. RT Model - 1 F’ • Formulation has advantages over ρw=I(f/Q)(bb/a) • Defined Limitreduces to F’ at high bbor F’/(a:bb+1) for absorbing sediments • Derivative reduces as bb →∞ Gerald Moore, A.C. Workshop, Wimereaux,2012 [5]

6. RT Model - 2 F’ model Determine F’ for a log grid of values:a: aw*0.8→60, ω: 0→0.999 Fit values to prove a smooth functionfor any θs,θv,Δϕ F’ at low bb – function of η = bbw/(bbw+bbp) F’ at high bb – function of bb/(a+bb) F’=A0+ C.η+∑ai{bb/(a+bb)}i Typically order 4 polynomial Not dependent on bb:b, but dependent on the shape of the phase function Gerald Moore, A.C. Workshop, Wimereaux,2012 [6]

7. RT Model Issues ‘Saturation of reflectance’ Limit to inversion Non-Linear effects Raman, chlorophyll fluorescence Assumption of heterogeneity. Vertical Distribution Shear – Concentration / PSD Diurnal Thermocline Surface Slicks – Cyanobacteria Surface Properties Whitecaps Bubbles Wave-field %change in reflectance for 25% change in TSM λ=709 Gerald Moore, A.C. Workshop, Wimereaux,2012 [7]

8. aw – various authors IOPs Water aw(λ) - Not Certain aw(T°) bw(λ) TSM bb(λ) λ-0.4from RMD a(775) = f(bb(775)) a(λ)=e-0.07(λ-775) β(θ) – Petzold Are these models adequate? Mixed population: λ-0.4equivalent to α valid λ 400→1020? 600 900 .12 -Δρw/ΔT ρw 0 400 1050 λ Gerald Moore, A.C. Workshop, Wimereaux,2012 [8]

9. TSM - absorption ρw(709) Take simple transect overrange of turbidity's for ‘clear’ image. Get ‘saturation’ ρw(705) comparedwith ρw(865) - less effect with ρw(778). ρw(778) ρw(865) Gerald Moore, A.C. Workshop, Wimereaux,2012 [9]

10. BPAC-RMD WR-Project Severn Amazon .02 TSM Specific abs. ρw(709) 900 700 ρw(865) Water Radiance Data: Rudiger Rottgers Simple atmospheric correction of toa Rayleigh – extrapolate to 0 rhow. Not corrected for transmission (~= 0.98). Similar results for Amazon and Severn estuary. Estimate ρw(709) max from Gompertz curve. Determine a:bb from ρw(max)=F’/(a:bb+1) Gerald Moore, A.C. Workshop, Wimereaux,2012 [10]

11. Rio de la Plata- Full dynamicrange of BPAC – both models. TSM range x0.1 i.e. 0.1 … 500 g.m-3 Gerald Moore, A.C. Workshop, Wimereaux,2012 [11]

12. Sensor Specifics • Calibration may not be optimalover Turbid waters. • Vicarious calibration normallycarried out on oligotrophic watersor land. • Need to account for bandwidthand ‘smile’. • Digitisation will limit algorithm’sapplicability. • Noise. • Simulated data should haveforward sensor model. Gerald Moore, A.C. Workshop, Wimereaux,2012 [12]

13. Interaction of vicarious calibration with BPAC. After application of vicarious calibration – range of pixels where BPACwas considerably reduced. Adjustment of aw – partial success. Vicarious Adjustment Image Convergence of BPAC Gerald Moore, A.C. Workshop, Wimereaux,2012 [13]

14. Top –vicarious. Backgroundbb(775). Green BPACconvergence. Bottom –non-vicarious RMD aw from Kou et al., Tank aw derived from ρw Gerald Moore, A.C. Workshop, Wimereaux,2012 [14]

15. Conclusion: Current BPAC (3rd reprocessing) is not robust to: - minor calibration errors - IOP parameter uncertainty. In the ATBD – it works perfectly with all simulated data. 4th reprocessing – upgrade algorithm. Current BPAC – 3 wavelengths - parameters bb(775), τa, α Any suitable algorithm needs to be over-specified Revised uses 4 wavelengths 709,753,775,865 Assumes the following: F’ - well known aw – similar slope, but absolute value uncertain ap - similar slope, but absolute value uncertain bbw – well known bbp(λ) – poorly known. Also should be robust to adjacency effects. Gerald Moore, A.C. Workshop, Wimereaux,2012 [15]

16. Revised BPAC Amazon Plume NASA Colour Palette αρw(775)ln[ρw(775)] ρrc(775) Black no-convergence Gerald Moore, A.C. Workshop, Wimereaux,2012 [16]

17. Other Issues: Adjacency Not all important coastal waters are Turbid. in-situ Measurements Gerald Moore, A.C. Workshop, Wimereaux,2012 [17]

18. Adjacency-a Reflected Solar beam Direct Solar beam Rayleigh Scattering Aerosol Scattering 20km Gerald Moore, A.C. Workshop, Wimereaux,2012 [18]

19. Adjacency-b Reflected Solar beam Direct Solar beam Rayleigh Scattering Aerosol Scattering N.B. Diffuse Reflection 20km Gerald Moore, A.C. Workshop, Wimereaux,2012 [19]

20. Validation site – Sagres, Portugal – Case 1 water. Simple Transect 1 hour duration Near Shore Off Shore Important problem – major EU expansion of aquaculture Gerald Moore, A.C. Workshop, Wimereaux,2012 [20]

21. Validation Issues Temporal Variability Tides Currents Spatial Variability Vertical Structure Measurement Issues Self Shading Depth of Sensor red / NIR Bandwidth Sensor Issues Stray Light (especially Trios – NIR) Noise >850 nm Gerald Moore, A.C. Workshop, Wimereaux,2012 [21]

22. Thanks – The End Gerald Moore, A.C. Workshop, Wimereaux,2012 [22]