MCD18A2 is the short-name for the Level-3 (L3) Terra and Aqua combined MODIS-derived photosynthetically active radiation (PAR) product in the shortwave spectrum (0.4 µm – 0.7 µm). PAR, the primary science dataset (SDS) offered by this product is the solar radiation incident on the surface in the 0.4 µm – 0.7 μm spectral window, which is primarily used by organisms in the photosynthesis process. PAR constitutes an important input in terrestrial models in agriculture and forestry, and is also useful in calculating the euphotic depths (the water depth of the sunlit zone) in lakes and oceans. Until now, satellite-derived global surface radiative flux products have been fairly coarse in their resolution. This Terra and Aqua MODIS combined surface radiation algorithm generates two products at 5 km resolution: MCD18A2, which offers PAR, and MCD18A1 that provides downward shortwave radiation (DSR) over land surfaces. MCD18A2 provides a daily, 3-hour global product at 5 km resolution in Sinusoidal gridded map projection, and fills a long-persisting gap in the requirements of the terrestrial modeling and applications communities.
The algorithm starts by estimating surface reflectance from a set of multi-temporal data inputs, and evaluates the PAR flux for each of them. It determines surface reflectance from observations under the clearest atmospheric conditions for each temporal window. It calculates incident PAR for the derived surface reflectance and Top-of-Atmosphere (TOA) radiance/reflectance using a Look-up Table (LUT) approach. The LUTs account for the different types and loadings of multiple aerosols and clouds at various illumination and viewing geometries. A number of follow-up studies were conducted using inputs from other sources including Advanced Very High Resolution Radiometer (AVHRR), geostationary satellites (NOAA’s Geostationary Operational Environmental Satellite, JAXA’s Geostationary Meteorological Satellite, etc.) to help prototype global PAR and DSR products, each with its own LUTs to handle different cloud types and aerosol loadings.
MCD18A2 comprises two types of science dataset variables: an instantaneous (total, direct, and diffuse variants) PAR array for each individual MODIS overpass, and eight, three-hour total DSR arrays. The image dimensions for this HDF4 format product are 240 rows by 240 pixels. The presence or absence of fill-values should indicate the successful PAR retrievals. No quality-specific flags or information exist