Abstract
With the ability to observe the entire sunlit side of the Earth, EPIC data have become an important resource for studying cloud daily variability. Inaccurate cloud masking is a great source of uncertainty. One main region that is prone to error in cloud masking is the sunglint area over ocean surfaces. Cloud detection over these regions is challenging for the EPIC instrument because of its limited spectral channels. Clear sky ocean surface reflectance from visible channels over sunglint is much larger than that over the non-glint areas and can exceed reflectance from thin clouds. This paper presents an improved EPIC ocean cloud masking algorithm (Version 3). Over sunglint regions (glint angle ≤25°), the algorithm utilizes EPIC’s oxygen (O2) A-band ratio (764/780 nm) in addition to the 780 nm reflectance observations in masking tests. Outside the sunglint regions, a dynamic reflectance threshold for the Rayleigh corrected 780 nm reflectance is applied. The thresholds are derived as a function of glint angle. When compared with co-located data from the geosynchronous Earth orbit (GEO) and the low Earth orbit (LEO) observations, the consistency of the new ocean cloud mask algorithm has increased by 4∼10% and 4∼6% in the glint center and granule edges respectively. The false positive rate is reduced by 10∼17%. Overall global ocean cloud detection consistency increases by 2%. This algorithm, along with other improvements to the EPIC cloud masks, has been implemented in the EPIC cloud products Version 3. This algorithm will improve the cloud daily variability analysis by removing the artificial peak at local noon time in the glint center latitudes and reducing biases in the early morning and late afternoon cloud fraction over ocean surfaces.
Highlights
When the geometric configuration of Sun, surface, and viewing angles form a mirroring path, the specular reflection, or sunglint, creates a bright spot on the remote sensing imagery
This study aims to improve the Earth Polychromatic Imaging Camera (EPIC) cloud mask over the sunglint region and granule edges over ocean based on the radiative transfer simulations and collocated observations from geosynchronous Earth orbit (GEO)/low Earth orbit (LEO) platforms
Because of EPIC’s large pixel size, one EPIC pixel corresponds to many GEO/LEO pixels each with its own cloud mask and optical properties retrievals; a composite pixel reports a cloud fraction based on cloud masks of the GEO/LEO pixels within it
Summary
When the geometric configuration of Sun, surface, and viewing angles form a mirroring path, the specular reflection, or sunglint, creates a bright spot on the remote sensing imagery. Ocean surface reflectance in the visible spectrum over sunglint is much larger than that from other areas. If the ocean surface were perfectly smooth, sunglint would appear in remote sensing images as the mirror image of the Sun, occupying a relatively small portion of the images. Because of the wave and ocean currents, the ocean surface is tilting toward different directions, causing the sunlight to scatter and resulting in a large area of glint zone. The size of the glint zone in the satellite imagery depends on the ocean surface roughness, which in turn can be parameterized in terms of the vector
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