Updated MISR Dark Water Research Aerosol Retrieval Algorithm - Part 1: Coupled 1.1 km Ocean Surface Chlorophyll a Retrievals with Empirical Calibration Corrections

• Published in: Atmospheric measurement techniques Vol. 10; no. 4; pp. 1539 - 1555
• Main Authors: Limbacher, James A., Kahn, Ralph A.
• Format: Journal Article
• Language: English
• Published: Goddard Space Flight Center Copernicus Publications 01.04.2017; Copernicus GmbH
• Subjects: Aerosol research; Aerosols; Albedo; Albedo (solar); Algorithms; Archives & records; Atmosphere; Atmospheric correction; Bidirectional reflectance; Calibration; Cameras; Channels; Chlorophyll; Chlorophyll a; Color; Corrections; Correlation; Correlation coefficient; Correlation coefficients; Data processing; Detection; Earth Resources And Remote Sensing; Empirical analysis; Imaging techniques; Light; Marine fishes; Mathematical models; MODIS; Ocean color; Ocean colour; Ocean surface; Oceanography; Oceans; Ratios; Reflectance; Remote sensing; Retrieval; Root-mean-square errors; Scattering; Spectra; Spectral reflectance; Spectroradiometers; Statistical analysis; Storage; Temperature (air-sea); Trend analysis; Trends
• ISSN: 1867-1381; 1867-8548; 1867-8548
• DOI: 10.5194/amt-10-1539-2017

As aerosol amount and type are key factors in the 'atmospheric correction' required for remote-sensing chlorophyll alpha 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 Chl(sub in situ) less than 1.5 mg m(exp -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 greater than 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 less than 1.5 mg m(exp -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.