The Subtleties of Sunglint

For decadesscientists have been intrigued by darkish patches discovered inside in any other case shiny, gleaming areas seen in some astronaut pictures and satellite tv for pc photos of water. On Could 6, 2023, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite tv for pc captured this picture of a darkish space off the Delmarva Peninsula of the U.S. East Coast. The blue tendrils of a phytoplankton bloom, current since Aprilhad been additionally seen.

The brilliant, silvery look of a lot of the water is because of sunglint—an optical phenomenon that happens when daylight displays off the floor of the water on the identical angle {that a} satellite tv for pc sensor views it. The result’s mirror-like specular reflection that reveals up as long, bright areas in MODIS information. A number of areas of sunglint are obvious close to the middle of every swath of MODIS information proven beneath, acquired on July 3, 2014.

The “roughness” of the water floor can affect how sunglint areas seem in MODIS photos—typically creating distinctive patterns inside them. More often than not, winds create sufficient waves and choppiness within the water that gentle will get mirrored in quite a lot of instructions (diffuse reflection) because it hits the ocean floor. The complicated geometry of wind-driven waves signifies that sufficient of that gentle occurs to be directed towards the satellite tv for pc sensor, giving sunglint areas their shiny silvery look.

However an space with weak winds, as was the case east of the Delmarva Peninsula on Could 6, 2023, the ocean floor was easy and glassy, inflicting a lot of the gentle to be mirrored in the identical route. Whether it is mirrored straight towards MODIS, particularly shiny round areas known as specular highlights seem within the imagery. (The very shiny space close to the coast of Surt, Libya, on June 9, 2023is an instance of a specular spotlight.) But when the sunshine is mirrored away from MODIS as a substitute, darkish patches can seem.

“Inside a area of sunshine winds, the looks of the quite flat underlying water floor—both darker than adjoining water, or extraordinarily shiny—is dependent upon the sun-satellite geometry,” defined Scott Bachmeiera analysis meteorologist with the Cooperative Institute for Meteorological Satellite tv for pc Research on the College of Wisconsin-Madison.

“On this case, the darkish options had been doubtless areas with very weak winds,” stated Chuanmin Hua professor of optical oceanography on the College of South Florida. ”However different issues that change the ‘roughness’ of the water can result in adjustments in sunglint patterns as nicely. These embody oil slicksfreshwater slicks, ship wakesocean circulation, internal wavesand ocean circulation.”

Hu added that freshwater slicks—probably these containing colored dissolved organic matter (CDOM), phytoplankton, oil, or quite a lot of biogenic surfactants—may very well be contributing to the darkish coloration in sure areas. “A few of the slim darkish options inside Delaware Bay and Chesapeake Bay could also be partly because of freshwater, however freshwater slicks will not be typically as massive as the primary darkish patch seen east of the Delmarva Peninsula,” Hu stated.

Whereas sunglint usually produces visually gorgeous photos which might be helpful for inferring winds and detecting oil spillsthe phenomenon can even create issues for distant sensing scientists as a result of it could possibly obscure options of curiosity. That is significantly true for oceanographers who use satellites to review phytoplankton and ocean color. Because of this, researchers have developed several methods to correct for sunglint or display screen sunglint-contaminated imagery out of ocean coloration information archives.

Brian Barnesan oceanographer on the College of South Florida, added: “Engineers have additionally developed a number of sensor designs—together with the Ocean Color Instrument (OCI) on the upcoming Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission— that decrease sunglint by tilting.”

NASA Earth Observatory picture by Wanmei Liang, utilizing MODIS information from NASA EOSDIS LANCE and GIBS/Worldview. Story by Adam Voiland, with enter and fact-checking by Ralph Kahn (NASA GSFC), Scott Bachmeier (College of Wisconsin-Madison/CIMSS), Scott Lindstrom (College of Wisconsin-Madison/CIMSS), Chuanmin Hu (College of South Florida), and Brian Barnes (College of South Florida).