Cloud Detection over Snow and Ice with Oxygen A- and B-band Observations from the Earth Polychromatic Imaging Camera (EPIC)

Abstract

Satellite cloud detection over snow and ice has been difficult for passive remote sensing instruments due to the lack of contrast between clouds and the bright and cold surfaces; cloud mask algorithms often heavily rely on shortwave IR channels over such surfaces. The Earth Polychromatic Imaging Camera (EPIC) onboard the Deep Space Climate Observatory (DSCOVR) does not have infrared channels, which makes cloud detection over snow/ice even more challenging. This study investigates the methodology of applying EPIC’s two oxygen absorption band pair ratios in A-band (764 nm, 780 nm) and B-band (688 nm, 680 nm) for cloud detection over the snow and ice surfaces. An elevation and zenith angle-dependent threshold scheme has been developed based on radiative transfer model simulations. The new scheme achieves significant improvement over the existing algorithm that imposes fixed thresholds for the A-band and B-band ratios. The positive detection rate nearly doubled from around 36 % to 70 % while the false detection rate dropped from 50 % to 15 % in January 2016 and 2017. The improvement during the summer months is less significant due to relatively better performance in the current algorithm. The new algorithm is applicable for all snow and ice surfaces including Antarctic, sea ice, high-latitude snow, and high-altitude glacier regions. This method is less reliable when clouds are optically thin or below 2.5 km because the sensitivity is low in oxygen band ratios for these cases.