t- u (x,0) was then transmitted through the interface as described by Austin 

 (1974) yielding the water-leaving spectral radiance L.(x). An example of 



W 



four such L spectra and their associated chlorophyll concentrations is 



W 



presented in Figure B-2. Note that the main effect on the color of the 

 ocean of increasing the chlorophyll concentration is a depression of L (x) 

 in the blue region of the spectrum, i.e., a shift in color from blue to 

 green. The actual enhancement of L in the green at high chlorophyll 

 concentrations is due to scattering by detrital material which covaries 

 with phy top lank ton. 



Finally, L w ( x) was weighted by the spectral responses of the CZCS (spectral 

 resolution of about 20 nm) to provide <L.(X)>, the CZCS weighted water- 

 leaving radiance. This is the component of the upwelling radiance just 

 above the sea surface which carries information concerning subsurface con- 

 stituents. As discussed in the previous section, the goal of the atmo- 

 spheric correction algorithm is the retrieval of <L ..(*)> from the CZCS 



W 



measured radiance l- t (*). 



Since the solar irradiance backscattered out of the ocean may have actually 

 penetrated to significant depths in the ocean, the relationship between C 

 and the water-leaving radiance must depend on the vertical distribution of 

 the phytoplankton. Gordon and Clark (1980b) have shown that the pigment 

 concentration < C > f or an optically homogeneous ocean which would produce 



the same water-leaving radiance as an optically stratified ocean with 

 pigment concentration C(z) is 



' z 90 



C> = y° f(z)C(z)dz / J 90 f(z)dz, (4) 



where 



z 90 



f(z) = exp[-2C 90 K d (z')dz'], (5) 



K d (z') is the attenuation coefficient for downwelling irradiance (given by 

 Eq. 2 with L u replaced by E d ), and z gQ is the 'penetration depth' defined 

 to be the depth at which E d falls to 1/e of its value just beneath the sur- 

 face. It is the depth above which 90 percent of the radiance contributing to 



B-4 



