Impacts of Sun Glint Off Ice Clouds on DSCOVR EPIC Cloud Products

The Earth Polychromatic Camera (EPIC) onboard the Deep Space Climate Observatory (DSCOVR) spacecraft observes the sunlit face of the Earth from a distance of about one-and-a-half million kilometers. Several studies demonstrated that EPIC images often feature sun glint from water surfaces and from horizontally oriented ice crystals occurring inside clouds. This study presents a statistical analysis of a yearlong EPIC dataset to gain insights into sun glints and their impacts on satellite measurements of cloudiness and cloud properties. The first results discussed demonstrate that over land, the observed glints, indeed, come mainly from ice clouds and not from small water bodies. Subsequent results reveal that sun glints affecting EPIC observations (especially at 388 nm) greatly increase the likelihood and sensitivity of cloud detection, particularly of the elusive thin and small ice clouds. Finally, the results indicate that sun glints often cause spurious increases in the cloud optical thickness (COT) and altitude values in the operational EPIC cloud product. Insights into the frequency, magnitude, and causes of glint effects and suggestions for future work are also provided.

I. Introduction

The Earth Polychromatic Camera (EPIC) onboard the Deep Space Climate Observatory (DSCOVR) spacecraft observes the Earth from a distance of about one-and-a-half million kilometers—that is, from a location about four times farther than the Moon [1]. EPIC always observes the sunlit face of our planet because DSCOVR orbits the Sun at the L1 Lagrangian point, which is near the straight line connecting the Sun and Earth. Typically, EPIC takes 22 images a day from late April to early September, and 13 images a day during the rest of the year [when DSCOVR can transmit less data as the ground communication station located at NASA’s Wallops Flight Facility (Virginia), USA is visible from DSCOVR for shorter times during the shorter days]. EPIC uses a filter wheel to take images at ten wavelengths ranging from 317 to 780 nm. The spatial resolution is around 8 km at the image center and decreases toward the edge of the Earth disk.