The Dirty Truth of Coastal Ocean Color Remote Sensing Dave Siegel & St é phane Maritorena Institute for Computational Earth System Science University of California, Santa Barbara

Cal/Val Results from Santa Barbara Channel

Atmospheric Correction Radiance budget for satellite radiance Measure L t ( ) Model L r ( ), L f ( ) & L g ( ) Unknowns are L w ( ), L a ( ) & L ar ( )

Atmospheric Correction Differences can be MUCH larger in coastal waters

Aerosol Correction (L a ( )+L ra ( )) =  (,865) L a+ar (865) KEY:  (,865) does not cross for different models

From Menghua Wang [GSFC] Spectral Effects

NIR reflectances are not enough to retrieve absorbing aerosol properties From Menghua Wang [GSFC]

Dust, Smog & Other Crap (1) Coastal aerosol distributions are complex –Smog, dust, pollens, etc. all interact with the marine atmosphere Aerosol mass, quality & vertical distribution change –Aerosol absorption & height variations are not considered Existing atmospheric correction procedures often do not work

Dust, Smog & Other Crap (2) Atmosphere properties… –Aerosol abundance, scattering & absorption properties (N a (D), m r, m i, vert. dist.; as f( )) Ocean properties… –Phytoplankton, CDOM, detritus, sediment (Chl, CDM, BBP, etc.) –Model may be site specific…

What to do (1) Coupled Inversions - Chomko et al. [2003] –Chl, CDM, BBP,  a (865), slope of N a (D), m r & m i –But, SeaWiFS only has 8 wavebands –Even then, aerosols are black & fixed height What’s needed… –Assess aerosol height variations &  o = f( ) –BUT we need more degrees of freedom

What to do (2) Hyperspectral resolution can help –More wavebands add degrees of freedom for inversions (both UV & NIR) –BUT I do not believe this will be sufficient Geostationary viewing can help –Look at same ocean with different illumination through the day

Thank You!!

Path Radiance Contribution Satellite Water-leaving

Atmospheric Correction

Gordon–Wang (1994) Algorithm Measure L t ( ) Model t( ), L r ( ), L f ( ) & L g ( ) Let L w ( NIR ) = 0 -> L a+ar ( NIR ) Choose “aerosol model” to estimate L a+ar ( ) Determine L w ( ) & L wN ( )

Atmospheric Correction L t = L r + (L a + L ra ) + t L wc + T L g + t L w meas mod hardpart mod avoid need 1. Assume L w (865) = 0 (black pixel assumption) L t (865) - L r (865) => (L a+ra (865)) 2. Model (L a ( )+L ra ( )) =  n (,865) (L a+ra (865)) 3. Select n using value of  (765,865) from table of aerosol models 4. Knowing t, solve for L w ( )

How to Model Ocean Color? Water-leaving photons are controlled by ratio of backscattering to absorption – High absorption -> low L wN ( ) – High backscatter ->high L wN ( ) L wN ( ) = f ( _______ ) b b ( ) a( )

Problems –Only first order understanding –Parameterizations are imperfect Garver & Siegel, JGR [1997] UCSB Ocean Color Model UCSB Ocean Color Model L wN ( ) Products (Chl, CDM & b bp ) Parameters (a ph * ( ), S, etc.)

Atmospheric Correction (L a ( )+L ra ( )) =  (,865) L a+ar (865)

Coastal Reflectance Spectra Toole & Siegel, JGR, 2001 Even more complicated