Formerly Lecture 12 now Lecture 10: Introduction to Remote Sensing and Atmospheric Correction*

1. Formerly Lecture 12 now Lecture 10: Introduction to Remote Sensing and Atmospheric Correction* Collin Roesler 11 July 2007 *A 30 min summary of the highlights of Howard Gordon’s 9hr Short course Ocean Optics XVI 2002 Do not use any of the figures for any public presentation without Howard’s permission

2. 1970’s Jerlov Kd Classification 0 0.2 0.5 0.9 1.6 inf Type Kd(440) Chl . I 0.017 0.01 … III 0.14 2.00 1 0.20 >2.00 … 9 1.0 >10.00 Variability attributed primarily to chlorophyll. Suggested that the inverse problem to estimate Chlorophyll from Lu(l) should be tractable.

3. Chl <0.1 0.3 1.3 3.0 1970 Clarke, Ewing, Lorenzen • aircraft-based radiometer (305 m) • Sargasso to WHOI • vertically polarized light • 53o (Brewster’s Angle) to avoid skylight Blue to green ratio Decreased with chl

4. Coastal Zone Color Scanner • launched in Nov 1978 • proposed mission was 1 year • proof of concept • degraded over time (mirrors) • lasted until 1986

5. 955 km 825 m pixels res. Coastal Zone Color Scanner

6. measured by sensor atmospheric radiance water-leaving radiance reflected radiance Coastal Zone Color Scanner

7. measured by sensor atmospheric radiance water-leaving radiance reflected radiance The Problem The atmosphere contributes > 90% to the radiance detected by the satellite-based sensor 1% error in atmospheric correction or satellite calibration  10% error in water leaving radiance

8. measured by sensor atmospheric radiance water-leaving radiance reflected radiance The Problem The atmosphere contributes > 90% to the radiance detected by the satellite-based sensor Atmospheric Correction CZCS Gordon and Clark 1981 First order correction, clear water SeaWiFS Wang and Gordon 1994 Gordon 1997

9. What is being measured by the satellite sensor? • Path radiance, L* • molecular scattering • aerosol scattering • molecular-aerosol multiple scattering • white caps, Lwc • sun glint, Lg • water leaving radiance, Lw

10. Remotely sensed radiance equation Lt = L*r + L*a + L*ra + TLg + tLwc + tLw • L*r, Rayleigh molecular scattering, • L*a, Aerosol scattering • L*ra, Rayleigh-Aerosol multiple scattering • Lg, sun glint • Lwc, white caps • Lw, water leaving radiance • T, direct transmittance (~beam attenuation) • t, diffuse transmittance

11. 1. Atmospheric Effects (L*, T, t) • Gaseous absorption (ozone, water vapor, oxygen) • Scattering by air molecules (Rayleigh) • Scattering and absorption by aerosols (haze, dust, pollution) • Polarization (MODIS response varies w/ signal polarization) Rayleigh (80-85% of total signal) • small molecules compared to nm wavelength, scattering efficiency decreases with wavelength as -4 • reason for blue skies and red sunsets • can be accurately approximated for a given atmospheric pressure and geometry(using a radiative transfer code) Aerosols (0-10% of total signal) • particles comparable in size to the wavelength of light, scattering is a complex function of particle size • whitens or yellows the sky • significantly varies and cannot be easily approximated

12. Atmospheric Effects Direct Transmittance T Lsun(top of atmosphere) = e-t , t is the optical depth Lsun(bottom of atm) t %T 0.1 90 0.3 75 1.0 37 2.0 14

13. Ozone O2 Water vapor 1. Atmospheric EffectsA. Absorption (T)

14. Molecular scattering Aerosol scattering Atmospheric EffectsB. scattering (T, L*) t %T 0.1 90 0.3 75 1.0 37 2.0 14

15. volume scattering function b(y) ~ br , (1 + cos2y) where br is the scattering coefficient (~ to air density) Molecular scattering The Rayleigh optical depth is given by: tr = ∫ br(h) dh where h is altitude and the spectral dependence is given by: tr ~ l-4 Atmospheric EffectsB. scattering (T, L*)i. Rayleigh Hansen and Travis 1974

16. 1. Atmospheric EffectsB. scattering (T, L*)ii. aerosol Use Mie theory to compute the volume scattering function

17. Haze v Water, sea salts 1. Atmospheric EffectsB. scattering (T, L*)ii. aerosol Use Mie theory to compute the volume scattering functions

18. ~ ~ y ba(y) ba(y) y 1. Atmospheric EffectsB. scattering (T, L*)ii. aerosol Phase functions

19. t(l)/t(865) l (nm) t(l)/t(865) l (nm) 1. Atmospheric EffectsB. scattering (T, L*)ii. aerosol and the spectral dependence of the optical depth

20. 1. Atmospheric EffectsB. scattering (T, L*)ii. aerosol ta(l) ~ l-a Pacific vs Atlantic aerosol optical depth 0.07 0.1 spectral dependence 0.7 0.9 but generally ta< 0.1 Observations by Smirnov et al. 2002

21. 2. Surface Effects (Lg, Lwc) Sun Glint Corrections based on statistical models (wind & geometry) White Caps

22. 3. Water leaving radiance term • Law(0,q,f) = t(q,f) Lw(z,q,f) = fcn(Lu) • where t(q,f) = diffuse transmittance Note that t(q,f) is a function not only of the atmospheric composition (i.e. phase function) but also of the radiance distribution, and that t(q,f) can be >1.

23. Up to this point we have • L(0,q,f) = L*r(0,q,f) + L*a(0,q,f) + t(q,f) Lw(z,q,f) • where Lr and La are a function of • incident radiance distribution • respective phase functions • respective optical thicknesses • radiance is non-dimensionalized to reflectance r = pL Fomo

24. ~0.2 ~0.022 Ex. from SeaWiFS (D. Clark) 5% accuracy in rw requires <0.5% absolute error in rt

25. Thus far we can do a good job on the Rayleigh contribution, but the aerosols are much more difficult • start by defining e(l,lo) = ra (l) ra (lo) • which is independent of aerosol concentration and nearly independent of position over an image (pathlength)

26. * Solving for rw(l) t(l) rw(l) = rt(l) – rr(l) – ra(l) = rt(l) – rr(l) – e(l,lo) ra(lo) = rt(l) – rr(l) – e(l,lo)(rt(lo)- rr(lo) – t(lo)rw(lo)) and t(l) is a function of attenuation by tr(l) x ta(l) Assumptions 1. aerosol phase functions are strongly peaked 2. aerosols have high single scattering albedo 3. we can find a lo for which rw(lo) = 0, i.e. red ls

27. Solving for rw(l) rw(l) = 1 (rt(l) – rr(l) – e(l,lo)(rt(lo)- rr(lo)) t*(l) so we only need to solve for e(l,lo) where lo =670 nm So we have to recall that rw(l) is directly proportional to the incident radiance distribution the IOPs of the water column e(l,670)= rt(l) – rr(l) – t*(l)*rw(l) rt(670)- rr(670)

28. 0.009 0.0 0.005 What are the clear water reflectance values?

29. Gordon and Clark’s (1981) Clear water radiance concept can solve for e(520,670) and e(550,670) e(520,670) = rt(520) – rr(520) – t*(520)*0.009 rt(670)- rr(670) e(550,670) = rt(550) – rr(550) – t*(550)*0.005 rt(670)- rr(670)

30. e(l,lo) = (l/lo)n Gordon et al. 1983 Then calculate e(440,670) by extrapolation e(l,lo) = ra(l)/ra(lo) ~ ta(l)/ta(lo) ~ (l-a1/lo-a2) ~ (l/lo)n All other terms of e are weakly dependent upon l Further, if the aerosol type remains constant over an image, even if the concentration changes, e will be constant over the image and only one n need be used

31. So the approach is: • from pixel geometry, compute rt-rr for each wave band • find clear water pixel (chl < 0.25 mg/l) • use clear water approximations for rw at 520, 550, 670 nm • calculate e(520,670) and e(550,670) • calculate e(440,670) by extrapolation • hold e(l,lo) constant for whole image

32. Example from CZCS

33. Which lead to the first ocean color pigment climatology

34. Problems with first order algorithm (CZCS) t*(l) rw(l) = rt(l)–rr(l)– e(l,670)(rt(670)- rr(670)–t*(670)rw(670)) Check on Assumptions 1. rw(670) =0 and other assumed clear water values are not valid for even moderate chl~1.0 (no clear water pixel) 2. rr(l) is dependent upon surface atm pressure, ozone 3. e does not really satisfy Angstrom’s Law 4. multiple scattering and polarization in the atmosphere are ignored 5. t* should be replaced by t and is dependent upon aerosol concentration, particularly absorbing aerosols

35. Problems with first order algorithm (CZCS) Check on Assumptions 1. rw(670) =0 and other assumed clear water values are not valid for even moderate chl~1.0 (use better in water model but NIR wavelengths would be better) 2. rr(l) is dependent upon surface atm pressure, ozone (see corrections by Gregg et al. 2002) 3. e does not really satisfy Angstrom’s Law (see figures at beginning of lecture, used to define e(l,lo) model) 4. multiple scattering and polarization in the atmosphere is ignored

36. Multiple scattering • error in Rayleigh scattering term approaches • 5% across a scan line • 20% with increasing latitude • greatest for 440 nm, then 670 nm bands • need to include the Rayleigh-aerosol multiple scattering term

37. Problems with first order algorithm (CZCS) Check on Assumptions 1. rw(670) =0 and other assumed clear water values are not valid for even moderate chl~1.0 (use better in water model but NIR wavelengths would be better) 2. rr(l) is dependent upon surface atm pressure, ozone (see corrections by Gregg et al. 2002) 3. e does not really satisfy Angstrom’s Law (see figures at beginning of lecture, used to define e(l,lo) model) 4. multiple scattering and polarization in the atmosphere are ignored (add multiple scattering, polarization terms) 5. t* should be replaced by t and is dependent upon aerosol concentration, particularly absorbing aerosols (still ignore assume aerosols are weakly absorbing)

38. SeaWiFS

39. Sensor improvements since CZCS NEDr – Noise equivalent reflectance SeaWiFS ~2-4 times more sensitive than CZCS MODIS ~2-4 times more sensitive than SeaWiFS Thus we require significantly improved atm correction to take advantage of added sensitivity e.g. Drw(443)N < 0.0002

40. First order correction for SeaWiFS • Still ignore absorbing aerosols if aerosol is non-abs even up to ta=0.2, it works well when aerosol absorbs, it fails even for low ta

41. First order correction for SeaWiFS • Still ignore absorbing aerosols • use multiple scattering for R/A • develop look up table for a range of aerosol types • e78 = L(765) ~ La+ra(765), L(865) La+ra(865) select aerosol model AE  correction, Km

42. First order correction for SeaWiFS • Still ignore absorbing aerosols • use multiple scattering for Rayleigh • use single scattering for aerosol • use exponential model for e(l,lo)  (,)

43. First order correction for SeaWiFS • Still ignore absorbing aerosols • use multiple scattering for Rayleigh • use single scattering for aerosol • use exponential model for e(l,lo) • use NIR bands 750 and 865 nm for rw=0 approximation

44. water P.J. Werdell, 2007

45. Other issues • “augmented reflectance” of white caps contaminates NIR bands, error improved but still needs work with respect to spectral variations [rwc]N (sr-1) Moore, Voss, Gordon 2001

46. Other issues • Earth’s curvature (Ding and Gordon 1994) still not applied to imagery • band 7 encompasses an O2 abs band requires correction (Ding and Gordon 1995), corrected in MODIS • rw=0 in NIR

47. Siegel et al. 2000, relax dark pixel approximation Use iterative approach chlo rw(NIR) atm corr  rw(l) and chl1  repeat

48. Siegel et al. 2000, relax dark pixel approximation

49. Additional problems in coastal zones • high CDM concentrations yield low rw(blue), and thus requires very high accuracy (issue with negative radiances) • high sediment loads result in amplified rw(NIR) thus impacting aerosol models (see Gordon et al. 2002 site-specific approach for coccolithophore bloom; Ruddick et al. 2000) • absorbing aerosol (e.g. spectral matching algorithm Gordon et al. 1997 for dust storm events)

50. Summary from Howard’s a to y course can get chl or other products with accuracy approaching surface measurements most of the time have methods to deal with some episodic events like dust storms have a reasonable foundation for coastal water algorithms