Retrieval of diffuse attenuation coefficient in the Chesapeake Bay and turbid ocean regions for satellite ocean color applications

Abstract

There are several empirical and semianalytical models for the satellite‐based estimation of the diffuse attenuation coefficient for the downwelling spectral irradiance at the wavelength 490 nm, Kd(490), or the diffuse attenuation coefficient for the downwelling photosynthetically available radiation (PAR), Kd(PAR). An empirical algorithm has been used to routinely produce NASA standard Kd(490) product from the Moderate Resolution Imaging Spectroradiometer (MODIS). However, these models are generally applicable for clear open ocean waters. Our results show that for the Chesapeake Bay, Kd(490) data from the existing models are significantly underestimated by a factor of ∼2–3 compared with the in situ data. In this paper, new Kd(490) models for the Chesapeake Bay and coastal turbid waters are derived using a relationship relating the backscattering coefficient at the wavelength 490 nm, bb(490), to the irradiance reflectance just beneath the surface at the red wavelengths. For coastal turbid ocean waters, bb(490) can be more accurately correlated to the irradiance reflectance at the red bands. Using the in‐situ‐derived bb(490) relationship in the Chesapeake Bay, Kd(490) models are formulated using the semianalytical approach. Specifically, two Kd(490) models using the MODIS‐derived normalized water‐leaving radiances at wavelengths 488 and 667 nm and 488 and 645 nm are proposed and tested over the Chesapeake Bay and other coastal ocean regions. Match‐up comparisons between the MODIS‐derived and in‐situ‐measured Kd(490) and Kd(PAR) products in the Chesapeake Bay show that the satellite‐derived data using the proposed models are well correlated with the in situ measurements. However, the new models are mostly suitable for turbid waters, whereas existing empirical and semianalytical models provide better results in clear open ocean waters. Therefore, we propose to use a combination of the standard (for clear oceans) and turbid Kd(490) models for more accurate retrieval of Kd(490) (or Kd(PAR)) products for both clear and turbid ocean waters.