During the morning of 10 October 2017, NWS Pueblo forecasters noticed a small, very bright (high reflectance) signal in the GOES-16 visible channels in the San Luis Valley of southern Colorado (Fig. 1). Interrogating further, the localized area appeared relatively warm in the 3.9 um channel (Fig. 2) and had relatively high reflectance in the 0.86 um, 1.38 um, 1.6 um and 2.2 um near-IR channels.

Figure 1: 10 October 2017 GOES-16 0.64 um VIS. Arrow points to area of reflectance off solar energy generating facility. Full res

Figure 2: 10 October 2016 GOES-16 3.9 um shortwave IR. Full res

After quickly ruling out a cloud, wildfire, explosion, and alien invasion, it was determined that this phenomenon was very high reflectance off of a smooth surface feature. There was no smoke in the VIS, and a wildfire hotspot would not exhibit such high reflectance, especially so early in the life of the fire. This phenomenon know as “sunglint” most often occurs off of a calm water surface given the high degree of smoothness, but can occur off of smooth land surfaces as well.  Interrogation of google earth revealed this feature was in fact a Solar Energy Generating Facility (Fig. 3)! The angle of the sun during that period of day lined up just right with that of the solar panels to have its sunlight reflected directly into the GOES-16 ABI.

Figure 3: Google Earth satellite view of Solar Energy Generating Facility in the San Luis Valley, CO

After some investigation, two other solar farms were found causing the same phenomenon in GOES-16 imagery. The Ivanpah Solar Power facility is located in southeast California near the Nevada border in the Mojave desert, and is a concentrated solar thermal plant. As of 2014, this was the world’s largest solar thermal power station. The reflectance signatures and hotspots were picked up by GOES-16 during a similar period as was found in Colorado (Fig. 4).

Figure 4: 12 October 2017 GOES-16 0.64 um 5-min VIS over Ivanpah facility southwest of Las Vegas near California/Nevada border. Full res

The Crescent Dunes Solar Energy facility in west central Nevada also had a reflectance and hotspot signature detected by GOES-16 (Fig. 5). This facility is also a concentrating solar power plant. Interestingly, the reflectance and hotspot signatures were detected in GOES-16 imagery at this plant from sunrise through early afternoon, much longer than the other two facilities. Why? Perhaps the angles of the large number of panels must vary considerably to be reflecting energy toward GOES-16 for such a long period of time. Or, maybe the solar power central towers are so bright that they are picked up by GOES-16. Neither explanation, however, explains why the Crescent Dunes signal was picked up for such a long period but the Ivanpah signal was significantly more brief.

Figure 4: 12 October 2017 GOES-16 0.64 um 5-min VIS over Crescent facility near Tonopah in central Nevada. Full res

-Bill Line and Tom Magnuson, NWS

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