Emissivity
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Joint Emissivity Database Initiative (JEDI)

Welcome to the website for  the Joint Emissivity Database Initiative (JEDI). The goal of the project is to create a unified land surface emissivity Earth System Data Record (ESDR). An ESDR is defined as a long-term consistent and calibrated dataset valid across multiple missions and satellite sensors for a given parameter of the Earth system, which are optimized to meet specific requirements in addressing science questions. Emissivity products are produced from NASA sensors in low earth orbit such as MODIS on the Terra and Aqua platform, AIRS on Aqua, ASTER on Terra, and the more recent VIIRS on Suomi NPP, all at different spatial, spectral and temporal resolutions.

The Earth emits energy at thermal wavelengths we can not normally see, and that energy is a function of the temperature and the emissivity of the surface. The surface emissivity is an intrinsic property of the surface and primarily depends on surface composition, and as the surface composition changes through, for example, land cover land use change, so does the surface emissivity. The emissivity is independent of the surface temperature, which varies with solar irradiance and local weather conditions. The emissivity of most natural Earth surfaces for the TIR wavelength range between 8-12 μm is from ~0.65 to close to 0.99. Narrowband emissivities less than 0.85 are typical for most desert and semi-arid areas due to the strong quartz absorption feature (reststrahlen band) between 8-9.5 μm range, whereas the emissivity of vegetation, water and ice cover are generally greater than 0.95 and spectrally flat in the 8-12 μm range.

Of the three NASA satellite sensors, ASTER provides the most detailed emissivtiy images with a pixel spatial resolution of 90m. Currently a mean composite of all ASTER scenes ever acquired is being produced on global 1 degree grids at 100m resolution, termed the ASTER Global Emissivity Database (ASTER-GED). The ASTER-GED for N. America, Africa, Australia, and Eurasia is currently available for download from Reverb, and more detailed information on the products can be found at the LP DAAC.

The image below shows examples of the ASTER-GED, and standard MODIS and AIRS emissivity products for the southwestern USA at 8.6 μm at spatial resolutions of 100m, 5km, and 50km respectively. There is good correlation at the regional scale, but the level of detail is much higher in the ASTER image due to its larger spatial resolution.

Red areas correspond to regions with large amounts of vegetation cover which have a high, spectrally flat emissivity, and regions of blue correspond to areas with low emissivities such as the desert regions of the southwest USA. The lowest emissivities in the 8.6 μm are associated with sand dune areas in the Sonoran desert due to their abundance of quartz. Greens and yellows indicate transition areas as you move from the desert regions to more heavily vegetated regions.