Semianalytic Moderate‐Resolution Imaging Spectrometer algorithms for chlorophyll a and absorption with bio‐optical domains based on nitrate‐depletion temperatures

This paper describes algorithms for retrieval of chlorophyll a concentration and phytoplankton and gelbstoff absorption coefficients for the Moderate‐Resolution Imaging Spectrometer (MODIS) or sensors with similar spectral channels. The algorithms are based on a semianalytical, bio‐optical model of remote sensing reflectance, R rs (λ). The R rs (λ) model has two free variables, the absorption coefficient due to phytoplankton at 675 nm, a ϕ (675), and the absorption coefficient due to gelbstoff at 400 nm, a g (400). The R rs model has several parameters that are fixed or can be specified based on the region and season of the MODIS scene. These control the spectral shapes of the optical constituents of the model. R rs (λ i ) values from the MODIS data processing system are placed into the model, the model is inverted, and a ϕ (675), a g (400), and chlorophyll a are computed. The algorithm also derives the total absorption coefficients a (λ i ) and the phytoplankton absorption coefficients a ϕ (λ i ) at the visible MODIS wavelengths. MODIS algorithms are parameterized for three different bio‐optical domains: (1) high photoprotective pigment to chlorophyll ratio and low self‐shading, which for brevity, we designate as “unpackaged”; (2) low photoprotective pigment to chlorophyll ratio and high self‐shading, which we designate as “packaged”; and (3) a transitional or global‐average type. These domains can be identified from space by comparing sea‐surface temperature to nitrogen‐depletion temperatures for each domain. Algorithm errors of more than 45% are reduced to errors of less than 30% with this approach, with the greatest effect occurring at the eastern and polar boundaries of the basins.