Abstract
Abstract. Coastal waters serve as transport pathways to the ocean for all agricultural and other runoff from terrestrial sources, and many are the sites for upwelling of nutrient-rich, deep water; they are also some of the most biologically productive on Earth. Estimating the impact coastal waters have on the global carbon budget requires relating satellite-based remote-sensing retrievals of biological productivity (e.g., chlorophyll a concentration) to in situ measurements taken in near-surface waters. The Multi-angle Imaging SpectroRadiometer (MISR) can uniquely constrain the “atmospheric correction” needed to derive ocean color from remote-sensing imagers. Here, we retrieve aerosol amount and type from MISR over all types of water. The primary limitation is an upper bound on aerosol optical depth (AOD), as the algorithm must be able to distinguish the surface. This updated MISR research aerosol retrieval algorithm (RA) also assumes that light reflection by the underlying ocean surface is Lambertian. The RA computes the ocean surface reflectance (Rrs) analytically for a given AOD, aerosol optical model, and wind speed. We provide retrieval examples over shallow, turbid, and eutrophic waters and introduce a productivity and turbidity index (PTI), calculated from retrieved spectral Rrs, that distinguished water types (similar to the the normalized difference vegetation index, NDVI, over land). We also validate the new algorithm by comparing spectral AOD and Ångström exponent (ANG) results with 2419 collocated AErosol RObotic NETwork (AERONET) observations. For AERONET 558 nm interpolated AOD < 1.0, the root-mean-square error (RMSE) is 0.04 and linear correlation coefficient is 0.95. For the 502 cloud-free MISR and AERONET collocations with an AERONET AOD > 0.20, the ANG RMSE is 0.25 and r is 0.89. Although MISR RA AOD retrieval quality does not appear to be substantially impacted by the presence of turbid water, the MISR-RA-retrieved Ångström exponent seems to suffer from increased uncertainty under such conditions. MISR supplements current ocean color sources in regions where sunglint precludes retrievals from single-view-angle instruments. MISR atmospheric correction should also be more robust than that derived from single-view instruments such as the Moderate Resolution Imaging Spectroradiometer (MODIS). This is especially true in regions of shallow, turbid, and eutrophic waters, locations where biological productivity can be high, and single-view-angle retrieval algorithms struggle to separate atmospheric from oceanic features.
Highlights
Coastal and generally turbid waters represent some of the most biologically productive waters in the world, constraining their impacts on the global carbon budget remains challenging (Bauer et al, 2013; Behrenfeld et al, 2005)
R is the Pearson correlation coefficient, MAE is the median absolute error, RMSE is the root-mean-square error between the satellite retrieval and AErosol RObotic NETwork (AERONET), bias is the mean Multi-angle Imaging SpectroRadiometer (MISR)–AERONET value, 0.03 or 10 % represents the fraction of MISR retrievals falling within the maximum of 0.03 or 10 % of the AERONET aerosol optical depth (AOD), and number is the number of validation cases included
Results indicate that the MISR retrieval algorithm (RA) has sensitivity to both AOD and ANG, even over bright waters, and typically outperforms the Moderate Resolution Imaging Spectroradiometer (MODIS) ocean color (OC) and MISR standard aerosol product (SA) in coverage, or in accuracy compared to expectation
Summary
Coastal and generally turbid waters represent some of the most biologically productive waters in the world, constraining their impacts on the global carbon budget remains challenging (Bauer et al, 2013; Behrenfeld et al, 2005). The instrument samples upwelling radiance over a ∼ 400 km wide swath in four spectral bands centered at 446 nm (blue), 558 nm (green), 672 nm (red), and 866 nm (near-infrared, or NIR) (Diner et al, 1998) These measurements are taken for each of the nine MISR cameras, viewing in both the forward and aft directions along the satellite’s orbit (±70.5, 60.0, 45.6, 26.1, and 0.0◦ or nadir), resulting in optical path lengths that vary from one to three. With nine cameras taking measurements at four wavelengths, MISR has the angular information content needed to constrain aerosol properties, even over bright turbid water, but lacks the specific spectral bands optimal for ocean color applications, though it can fill in glint-contaminated regions of single-view instruments such as MODIS (Limbacher and Kahn, 2017).
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