Exploring the potential of Rayleigh-corrected reflectance in coastal and inland water applications: A simple aerosol correction method and its merits

Atmospheric correction methods that are designed for either ocean color or land applications often result in low-quality or even no surface reflectance data for coastal and inland waters. In contrast, Rayleigh-corrected reflectance (Rrc) has been used in water applications, although without removing aerosol scattering. However, a systematic effort has not been made to investigate the uncertainties and applicability of Rrc in ocean color studies. This knowledge gap was filled in this study by using Moderate Resolution Imaging Spectroradiometer (MODIS) data covering a large area (almost the entire middle and lower reaches of the Yangtze River Basin and the Yangtze River Estuary) between 2002 and 2016. We first examined the correlation between a quality-controlled reflectance product that was generated using a shortwave-infrared (SWIR)-based atmospheric correction method (Rrs_swir) and Rrc. Improved relationships between these two products were found for all MODIS bands if a subtraction of the Rayleigh-corrected reflectance at 1240-nm was utilized as the aerosol correction for Rrc (Rrc-1240). The robust correlations between the two products allow for Rrs_swir to be replaced with Rrc-1240-converted reflectance (denoted as Rrs_rrc-1240) in water applications. In situ validations further demonstrated that the accuracy levels between usable Rrs_rrc-1240 and Rrs_swir data are comparable for most MODIS wavelengths. The most striking superiority of Rrs_rrc-1240 over Rrs_swir is the pronounced increase in data coverage (especially for small water bodies), where the percentages of usable observations (PUOs) of the former are several times to more than one order of magnitude higher than those of the latter. The differences in PUOs were mostly due to perturbations related to land adjacency effects (LAEs) in the SWIR-based atmospheric correction. Such effects could also explain the reduced PUOs for smaller water bodies. Ocean color applications in the examined regions could benefit from such a dramatic increase in PUOs, which could enhance the capability of tracking short- to long-term dynamics and could create new possibilities for inland water system monitoring at the basin scale.