Impact of sea surface waves and bubbles on water-leaving reflectance using Monte Carlo simulations
Authors: Angara Bhogeswara Rao , Palanisamy Shanmugam , He Xianqiang , Sandhani Chavapati Gouse 
Published: October 2024

Bubbles formed in the upper layers of the sea surface due to wave breaking caused by strong winds significantly alter the radiative properties of seawater. These changes affect the water-leaving radiance recorded by a satellite or airborne sensor and lead to uncertainties in the retrieved water color measurements. This study focuses on how sea surface waves and different bubble-size populations affect the optical properties of seawater (particularly scattering) in upper ocean surface layers and how these changes impact the radiative transfer of solar energy within the sea. In this work, the spectral water-leaving reflectance caused by time-evolving sea surface waves and varying bubble sizes in the upper ocean surface layers is investigated using Monte Carlo simulations. For this study, the minimum bubble size in the distribution is assumed to be 1 µm, generated at a wind speed of 13 m s-1. The air-sea interface is modeled using the ECKV sea surface model. The in-situ inherent optical properties (IOPs) measured from Bay of Bengal (BoB) waters in the absence of bubbles were used to derive bubble population statistics from the Hall-Novarini (HN) model, while the scattering coefficients and scattering phase function resulting from bubbles in seawater were computed using Mie scattering theory and used for Monte Carlo simulations. The results demonstrate that changes in the time-evolving sea surface wave conditions significantly affect the spectral water-leaving reflectance, altering its magnitude by up to two-fold within a minute. Three distinct populations of bubble sizes were examined to calculate water-leaving reflectance. The findings indicate varying magnitudes of water-leaving reflectance among these populations. Specifically, the presence of smaller bubbles in the population results in higher water-leaving reflectance compared to larger bubbles. This increase in magnitude is primarily attributed to the number density and strong backscattering of these smaller bubbles. Additionally, the impact of the scattering phase function due to bubbles was investigated, revealing its significance in accurately calculating the water-leaving reflectance. In-situ measurements in coastal waters of the BoB using RAMSES TriOS radiometric sensors indicated that the magnitude of the water-leaving radiance spectrum changes due to the time-evolving sea surface conditions at a given wind speed. The theoretical outcome from Monte Carlo simulations was also consistent with the in-situ measurements. Additional measurements at different time intervals during wave breaks and bubble generation revealed that varying bubble sizes led to different magnitudes of water-leaving radiance, which also matched the simulation results, confirming the theoretical predictions. These results will significantly impact further studies investigating the spatial effects of waves and bubbles on water-leaving radiance. This will also contribute to enhancing the accuracy of watercolor products retrieved from satellite data.