MCD18A1 is the short-name for the Level-3 (L3) Terra and Aqua combined MODIS-derived surface radiation product in the shortwave spectrum (0.3 µm – 4 µm). Downward Shortwave Radiation (DSR) is the primary science dataset (SDS) variable offered by this product. DSR as a surface radiative flux component is a vital input in terrestrial models (for instance, carbon cycle, hydrology and energy balance, weather and climate prediction). 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: MCD18A1, which offers DSR, and MCD18A2 that provides photosynthetically active radiation (PAR) over land surfaces. MCD18A1 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 DSR and PAR products, each with its own LUTs to handle different cloud types and aerosol loadings.
MCD18A1 comprises two types of science dataset variables: an instantaneous (total, direct, and diffuse variants) DSR 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 DSR retrievals. No quality-specific flags or information exist.