Effect of Ship Wake on Glint Ocean Reflectance

During the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS), the NASA P-3B aircraft flew over the Pacific Ocean off the coast of California to characterize primarily the emissions from cargo ships, which are largely unknown and are thought to contribute to air quality problems in California. This experiment provides a golden opportunity to study ocean glint, particularly the effect of ship wake on glint ocean reflectance using measurements made by the NASA’s Cloud Absorption Radiometer (CAR) instrument aboard the P-3B. The Cloud Absorption Radiometer (CAR) has been used to acquire spectral BRDF of the ocean, sea ice, snow, tundra, savanna, smoke, vegetation, desert, salt pans, and clouds, and played a key role in the ARCTAS deployment in spring and summer of 2008. This airborne sensor has a wide aperture of 190°, an instantaneous Field of View of 1°, and can capture the full BRDF, including the hotspot under low sun angle conditions commonly found in the Arctic. The instrument was developed for low- to medium-altitude aircraft and can be used to obtain data with varying spatial resolutions that are important for addressing upscaling needs for satellite validation. The instrument has a unique ability to measure almost simultaneously, both downwelling and upwelling radiance at 14 narrow spectral bands located in the atmospheric window regions of the ultraviolet, visible and near-infrared. When combined with simultaneous airborne measurements of sun/sky radiance, the CAR sky radiance measurements provide information on aerosol (size distribution, single scattering albedo, refractive index) both above and below the aircraft. We show from the CAR measurements that glint reflectance increases by as much as 50% in some bands due to the presence of the ship wake. Since glint contributes a significant fraction of the ocean albedo (>70%), this effect, especially in an area with large concentration of ships, could lead to large uncertainty in the ocean albedo.