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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 10, issue 4 | Copyright
Atmos. Meas. Tech., 10, 1539-1555, 2017
https://doi.org/10.5194/amt-10-1539-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 24 Apr 2017

Research article | 24 Apr 2017

Updated MISR dark water research aerosol retrieval algorithm – Part 1: Coupled 1.1 km ocean surface chlorophyll a retrievals with empirical calibration corrections

James A. Limbacher1,2 and Ralph A. Kahn1 James A. Limbacher and Ralph A. Kahn
  • 1Earth Science Division, NASA Goddard Space Flight Center, Greenbelt, 20771, USA
  • 2Science Systems and Applications Inc., Lanham, 20706, USA

Abstract. As aerosol amount and type are key factors in the atmospheric correction required for remote-sensing chlorophyll a concentration (Chl) retrievals, the Multi-angle Imaging SpectroRadiometer (MISR) can contribute to ocean color analysis despite a lack of spectral channels optimized for this application. Conversely, an improved ocean surface constraint should also improve MISR aerosol-type products, especially spectral single-scattering albedo (SSA) retrievals. We introduce a coupled, self-consistent retrieval of Chl together with aerosol over dark water. There are time-varying MISR radiometric calibration errors that significantly affect key spectral reflectance ratios used in the retrievals. Therefore, we also develop and apply new calibration corrections to the MISR top-of-atmosphere (TOA) reflectance data, based on comparisons with coincident MODIS (Moderate Resolution Imaging Spectroradiometer) observations and trend analysis of the MISR TOA bidirectional reflectance factors (BRFs) over three pseudo-invariant desert sites.

We run the MISR research retrieval algorithm (RA) with the corrected MISR reflectances to generate MISR-retrieved Chl and compare the MISR Chl values to a set of 49 coincident SeaBASS (SeaWiFS Bio-optical Archive and Storage System) in situ observations. Where Chlinsitu<1.5mgm−3, the results from our Chl model are expected to be of highest quality, due to algorithmic assumption validity. Comparing MISR RA Chl to the 49 coincident SeaBASS observations, we report a correlation coefficient (r) of 0.86, a root-mean-square error (RMSE) of 0.25, and a median absolute error (MAE) of 0.10. Statistically, a two-sample Kolmogorov–Smirnov test indicates that it is not possible to distinguish between MISR Chl and available SeaBASS in situ Chl values (p>0.1). We also compare MODIS–Terra and MISR RA Chl statistically, over much broader regions. With about 1.5 million MISR–MODIS collocations having MODIS Chl <1.5mgm−3, MISR and MODIS show very good agreement: r = 0. 96, MAE = 0.09, and RMSE = 0.15.

The new dark water aerosol/Chl RA can retrieve Chl in low-Chl, case I waters, independent of other imagers such as MODIS, via a largely physical algorithm, compared to the commonly applied statistical ones. At a minimum, MISR's multi-angle data should help reduce uncertainties in the MODIS–Terra ocean color retrieval where coincident measurements are made, while also allowing for a more robust retrieval of particle properties such as spectral single-scattering albedo.

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Short summary
Aerosol amount and type affect the “atmospheric correction” needed to derive ocean surface chlorophyll a concentration (Chl) from satellite remote sensing and, conversely, the ocean surface representation affects aerosol retrieval products. We introduce a coupled atmosphere-surface retrieval for Multi-angle Imaging SpectroRadiometer observations over dark water aimed at improving both aerosol and Chl results. We also refine the MISR calibration, critical to achieving high-quality retrievals.
Aerosol amount and type affect the “atmospheric correction” needed to derive ocean surface...
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