The incidence and magnitude of the hot-spot bidirectional reflectance distribution function (BRDF) signature in GOES-16 Advanced Baseline Imager (ABI) 10 and 15 minute reflectance over north America

Abstract The Geostationary Operational Environmental Satellite (GOES) Advanced Baseline Imager (ABI) is the most recent U.S. geostationary satellite and has improved resolution and potential for land surface monitoring over previous GOES sensors. The ABI reflectance has diurnal variation due to surface reflectance anisotropy that can be significant, particularly when ABI observations are sensed under hot-spot sensing conditions when the solar and viewing directions coincide. The incidence and magnitude of the hot-spot signature in geostationary satellite data has not been well documented. In this study all the available GOES-16 ABI observations acquired every 15 min in 2018, and every 10 min in 2020, over the Conterminous United States, the southern states of Canada, and the northern states of Mexico, were examined. First, hot-spot sensing conditions were identified by finding ABI 1 km observations with scattering angles (Θ) ≥ 175°, 177°, 178°, 179°, 179.9°, and 179.99°. ABI observations sensed near or in the hot-spot (Θ ≥175°) were found to occur across the study area but only over a 36 day period in the spring (days 46–81 in 2018, and 47–82 in 2020) and over a 36 or 37 day period in the autumn (days 263–299 in 2018, and 264–299 in 2020). Considering either season in each year, about 3.4%, 1.2%, 0.5%, 0.14%, 0.0015%, and 0.000015% of the daytime observations were acquired with Θ ≥175°, 177°, 178°, 179°, 179.9°, and 179.99°, respectively. For certain days up to 5.85%, 3.32%, 2.04%, 0.65%, 0.01%, and 0.0001% of the daytime ABI observations across the study area were sensed with Θ ≥ 175°, 177°, 178°, 179°, 179.9°, and 179.99°, respectively. These incidence percentages are not negligible. Even with the strictest hot-spot scattering angle definition (Θ ≥179.99°) there were 3985 and 4484 hot-spot observations sensed in the spring and autumn of 2018, and 4753 and 5252 hot-spot observations sensed in the spring and autumn of 2020. The dates, times, and locations of the 2018 and 2020 hot-spots acquired with Θ ≥179.99° are tabulated so that readers may obtain hot-spot ABI data for their own investigations. Second, the magnitude of the hot-spot reflectance in the ABI data was quantified by the difference between the ABI reflectance acquired in the hot-spot (Θ ≥179.99°) and in the ABI observations sensed immediately before and after. A total of 1833 hot-spot observations in ten geographically elliptical regions (six in 2020 and four in 2018) over land each composed of >120 ABI observations acquired where the ABI reflectance appeared to be unambiguously cloud-free, cloud shadow-free, and uncontaminated by haze or smoke, were considered. For the 2020 ABI data, the hot-spot peak was characterized by an up to 36%, 38%, 42%, and 43% increase in red (0.64 μm), near-infrared (NIR, 0.86 μm), short-wave infrared 1 (SWIR1, 1.61 μm), and SWIR2 (2.2 μm) reflectance, respectively, for a 2.48° scattering angle difference in the 10 min between successive ABI observations. For the 2018 ABI data, the hot-spot peak was characterized by an up to 43%, 46%, 47%, and 45% increase in red, NIR, SWIR1, and SWIR2 reflectance, respectively, for a 3.73° change in scattering angle in the 15 min between successive ABI observations. The magnitude of the GOES-16 ABI hot-spot is significant and given its non-negligible incidence in the spring and autumn over north America should be taken into consideration for terrestrial applications.