Atmospheric Correction

1. Step 1 Cloud Detection SeaWIFS uses the 870 nm band – water leaving radiance is zero in this band Step 2 Ozone attenuation Sun glint and foam Rayleigh path radiances Aeresols LT = tDLw + tDLF + tLG + LR + LA + LRA Figure 6.11 Atmospheric Correction

2. Seasonal Dependence on ozone Spatial and temporal distribution of ozone is determine by Total Ozone Mapping Spectrometer (TOMS) <= 0.035 Ozone Attenuation

3. Sun Glint and Foam • Sun glint is a function of sun angle and wind speed • Examine NIR radiances: if these radiances are above a threshold apply the sun glint mask • Foam depends on wind speed and assumed to be uniform across an image • If LF is too large image is discarded

4. Rayleigh Path Radiances • Radiance from molecular scatter • Largest term in received radiance for shorter wavelengths

5. Aerosol Path Radiance • Step 1: Remove ozone attenuation, sun glint, foam, and Rayleigh path radiances • Step 2: Assume Lw is zero in NIR and measure reflectance at LA(865) and LA(765) • Step 3: Compare measured values of LA(865) and LA(765) to look up table of known aerosol models • Step 4: If you find a match in Step 3, extrapolate Aerosol irradiance in the visible band and subtract

6. Algorithms make use of reflectance behavior for < 550 nm where reflectance increase with decreasing concentrations Algorithms make use of fluorescence peak at 683 nm – determination of fluorescence magnitude requires radiance measurements at 667, 678, and 748 nm. Independent of CDOM Chlorophyll Reflectance and Fluorescence

7. Semi analytic vs. Empirical Algorithms • Both for wavelengths of 400-550 nm • Empirical derived from regression of coincident ship and satellite observations of LW and shipboard observations of [chl]. • Restricted to Case 1 waters because output is only Chl • Use ratios based on wavelength pairs 443/555, 490/555, 510/555 • Semi analytic relate Rrs to backscatter/absorption ratio to determine water constituents

8. A maximum band ratio empirical algorithm Uses whichever Rrs ratio is largest (443/555, 490/555, 510/555) SEAWiFS OC4

9. MODIS OC3M • Also maximum band ratio empirical algorithm • Uses whichever Rrs ratio is largest (443/551, 490/551, 510/551) • Statistics are about the same as those of OC4 • Overestimates chl in case 2 waters • Underestimates chl below 1 mg

10. Semi Analytical Algorithms • Inputs: SST, NDT, and RRS at 412, 443, 488, 531, and 551 • SST and NDT are used to classify the ocean in three regimes • A. warm regime with unpackaged chlorphyll • B. transition regime • C. cold upwelling regime where phytoplankton consist of fast growing diatoms with packaged chlorphyll • Outputs: ARP (absorbed radiation by phytoplankton), aCDOM(400), aP(675) • Formulas exist to extend aCDOM(400), aP(675) across the visible spectrum to yeild aT. Components can then be inverted to yield [chl] and [CDOM].

11. Species Specific Algorithms • Coccolithophores, dinoflagellate Karenia brevis, and phycoerythrin-containing species such as Trichodesmium, have unique absorption curves

12. Satellite Images

13. Satellite Images

14. Modis Retrievals