Atmospheric Correction
DSCOVR-EPIC is contributing to the Surface Reflectance Earth System Data Record. The shortcoming of EPIC's rather coarse spatial resolution is compensated by its high (almost hourly) observation rate, which produces up to 8-12 images of the same area from dawn to dusk, globally. This provides early morning observations, which are unavailable from MODIS or VIIRS, for climatically important tropical regions of the world such as Amazonia where tropical convection generates more clouds in the afternoon. A comparison of statistics between MODIS Terra and Aqua shows about 10% more clouds from MODIS Aqua with equatorial overpass time 13:30pm as compared to MODIS Terra crossing equator at 10:30am.
The surface products include spectral bidirectional reflectance factors (BRF, or surface reflectance) and bidirectional reflectance distribution function (BRDF) represented by 3 parameters of the Ross-Thick Li-Sparse model. The suite also includes cloud mask and aerosol optical depth (AOD) required for atmospheric correction.
The unique backscattering observation geometry of EPIC will allow us to revisit models of BRDF near the hot-spot direction (scattering angle close to 180 degrees). So far, only a limited analysis has been conducted based on POLDER multi-angle observations. Such models have a high importance for vegetation monitoring in tropics when geometric variation from shifting azimuthal plane overlays seasonal vegetation cycle, as well as to more accurate aerosol retrievals near backscattering directions over brighter surfaces.
Because EPIC differs significantly from MODIS or VIIRS in spectral bands and spatial and temporal resolution, a new processing algorithm is being developed based on elements of the NASA Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm originally developed for MODIS. The main idea of MAIAC is to take advantage of differences in the space-time variability of atmosphere (aerosols and clouds) and surface to separate their contributions in measured radiance. Such an approach requires observing the same area over time, therefore EPIC processing starts with gridding observations to a 10 km regular sinusoidal grid. Continuous observations of the same grid cell over time yield multi-angle coverage for spectral BRDF retrievals, which then helps cloud detection and aerosol retrievals. MAIAC also characterizes spatial variability between adjacent grid cells under clear skies that helps cloud detection.
The EPIC MAIAC cloud detection employs a set of tests including absolute brightness test, spatial variability test, oxygen A, B-band test for cirrus detection, and "deviation from expected" test based on our knowledge of spectral surface BRDF which is translated to an expected top-of-atmosphere (TOA) reflectance. An additional filtering takes place during aerosol retrievals and atmospheric correction, which significantly increases an overall data quality.
Aerosol retrieval is based on characterization of the surface spectral reflectance ratios from the time series of EPIC observations using the minimum reflectance method. Following aerosol retrievals, the atmospheric correction stage includes BRDF retrieval and computation of surface reflectance. The BRDF retrieval uses linear inversion to derive three parameters of the RTLS BRDF model from the multi-angle EPIC dataset accumulated from 2-3 days of observations over each 10 km grid cell. A preliminary example of atmospheric correction processing is shown in Fig. A1. It includes the RGB TOA EPIC image for 27 March 2016 at 1312 UTC (left), the atmospherically corrected land surface RGB image (middle), and retrieved AOD0.44 both over land and ocean on the right.
The EPIC MAIAC products are available at the Atmospheric Science Data Center (ASDC) at NASA Langley Research Center: https://search.earthdata.nasa.gov/search/granules?p=C1451664065-LARC_ASDC&tl=1503585107!4!!&q=DSCOVR +MAIAC&ok=DSCOVR+ MAIAC&ac=true
Figure A1. An example of EPIC data processing on 27 March 2016 at 1312 UTC: a) EPIC TOA RGB, b) atmospherically corrected RGB surface reflectance, 3) aerosol optical depth (AOD).
2016 Global daily-average MAIAC AOD from EPIC
The following link shows a movie of the global daily-average MAIAC AOD from EPIC for the entire 2016 (the scale is 0-1).