The role of the critical angle in brightness reversals on sunglint images of the sea surface

Abstract

A wide variety of oceanic and atmospheric phenomena are often observed in and around the sunglint region on optical images of the sea surface. The appearance of these phenomena depends strongly on the viewing geometry with areas on the sea surface that are rougher (or smoother) than the background appearing as either brighter or darker than the background depending on their position relative to the specular point. To understand these sea surface signature variations, this paper introduces the concept of a critical sensor viewing angle, defined as the sensor zenith angle at which different sea surface roughness variances produce identical sunglint radiance. It is when the imaging geometry transitions through the critical angle that a surface feature goes through a brightness reversal. Knowledge of where this transition takes place is important for properly interpreting the characteristics of the sea surface signature of these phenomena. The theory behind the concept of the critical angle is presented and then applied to sunglint imagery acquired over the ocean from space by the Moderate Resolution Imaging Spectroradiometer onboard NASA's Aqua and Terra satellites.