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. Limbacher, Ralph A. Kahn
2017 Atmospheric Measurement Techniques  
<p><strong>Abstract.</strong> As aerosol amount and type are key factors in the <q>atmospheric correction</q> required for remote-sensing chlorophyll <i>a</i> 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)
more » ... evals. 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. <br><br> 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 Chl<sub>in<span class="thinspace"></span>situ</sub><span class="thinspace"></span>&amp;lt;<span class="thinspace"></span>1.5<span class="thinspace"></span>mg<span class="thinspace"></span>m<sup>−3</sup>, 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 (<i>r</i>) 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 (<i>p</i><span class="thinspace"></span>&amp;gt;<span class="thinspace"></span>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 &amp;lt;<span class="thinspace"></span>1.5<span class="thinspace"></span>mg<span class="thinspace"></span>m<sup>−3</sup>, MISR and MODIS show very good agreement: <i>r</i> = 0. 96, MAE<span class="thinspace"></span> = <span class="thinspace"></span>0.09, and RMSE<span class="thinspace"></span> = <span class="thinspace"></span>0.15. <br><br> 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.</p>
doi:10.5194/amt-10-1539-2017 fatcat:len27iwxwzas3cxdnozctux3xu