IEEE Access (Jan 2024)
Influence of the Bubbles on the Hyperspectral Reflectance and Water Color Products
Abstract
Bubble clouds produced by wind-induced breaking surface waves are highly reflective features that significantly influence the spectral shape and magnitude of remote sensing reflectance in sea surface waters and introduce uncertainties in the water colour products derived from airborne and satellite data. Under windy conditions, the uncertainty can extend over several orders of magnitude due to the spectral enhancement effects of bubble clouds on remote sensing reflectance ( $R_{rs}$ ). In this study, the effects of bubbles on the spectral properties of $R_{rs}$ and water colour products are investigated using radiative transfer simulations and field measurements of bubble clouds. Radiative transfer (RT) simulations with HydroLight were performed with the inputs of the inherent optical properties (IOPs) of different waters and the scattering coefficients of bubbles to generate $R_{rs}$ , which in turn became the input for the retrieval algorithms of chlorophyll (Chl), suspended sediments (SS) and coloured dissolved organic matter (CDOM), and for planning of our field experiments. The experiment data were obtained from the Chennai harbour on 15 February 2020 (from 11 am to 3 pm, local time, IST) using a set of RAMSES TriOS radiometric sensors. These measurements were made over the time period less than two minutes to capture the wave formation, breaking and dissipation conditions. HydroLight simulations and field measurement data showed that the $R_{rs}$ spectra in the visible and near-infrared (NIR) wavelengths are significantly enhanced in the presence of bubble clouds. The effect of bubble clouds ( $b^{bub}$ ) on the water-leaving reflectance was well pronounced in clear waters than in turbid waters, particularly in the green-NIR wavelengths due to the strong backscattering of bubbles and weak backscattering of water molecules. The $R_{rs}$ bubble cloud ratio of different water types showed more variation with the increasing effect toward the longer wavelengths. In clear ocean waters, when Chl $\lt =1$ mg m3, the $R_{rs}$ bubble cloud ratio was increased from 1.5 to 2.5 (across the visible-NIR spectrum) in the case of less bubble clouds ( $b^{bub} =0.1$ m1) and 4 to 15 in the case of more bubble clouds ( $b^{bub} \gt 0.9$ m1). More than 50% changes were observed at higher bubble populations as confirmed by our field experiments and earlier studies. In turbid coastal waters, the effect of bubbles on the $R_{rs}$ was less pronounced due to the strong influence of water IOPs and the weak effect of bubbles. The magnitude of the $R_{rs}$ spectra obtained from the field experiments also increased with increasing bubble fraction/bubble density. Consequently, the error in the water colour products retrieved from the $R_{rs}$ data was magnified due to the overestimation of Chl and SS and underestimation of CDOM in the presence of bubbles. The results presented will have significant implications for further studies on investigating the spatial effects of bubble clouds on $R_{rs}$ data and improving the accuracy of the water colour products retrieved from satellite data.
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