A global infrared land surface emissivity database

A global infrared land surface emissivity database with high spectral and high spatial resolution will be introduced. The database is derived from a combination of high spectral resolution laboratory measurements of selected materials, and multiple years of MODIS (MOD11) observed land surface emissivities at 3.7, 3.9, 4.0, 8.5, 11.0 and 12.0 micron wavelengths. For a given month, a spectrum of emissivity from 3.7 to 14.3 microns is available from this database for every latitude/longitude point globally at 0.05 degree resolution.

A two-step process is involved in the derivation of the emissivity. First, baseline emissivity spectra (BES) are developed for each of the non-ocean IGBP ecosystem categories. The BES are based on inspection of laboratory data from the USCB MODIS land team's emissivity library and the JPL ASTER spectral library. Eight wavelength inflection points are used in the BES, with the wavelength values chosen to best characterize the shape of the relevant laboratory emissivity spectra. The laboratory measurements are at high spectral resolution, but the spectral properties of the materials measured are not strictly the same as those of actual land surfaces seen by satellites. For example, the emissivity of a single pine needle or a pile of pine needles may not be representative of the emissivity of an evergreen forest canopy as seen from space. Until further aerial measurements of the emissivity of actual land surfaces are available, we use these laboratory measurements for our baseline.

In the second step of the derivation of the emissivity, we use the BES derived for each IGBP ecosystem category and adjust the magnitude of the emissivity at each of the inflection point wavelengths based on the observed MODIS MOD11 emissivity values. The result is a spectrum of emissivity at eight inflection points for each month at each MOD11 latitude and longitude point (0.05 degree resolution) over land. Because the inflection points of the BES were carefully chosen to capture as much of the shape of the higher resolution spectra as possible, these spectra can be interpolated between inflection points to arrive at an emissivity for any wavelength between 3.7 and 14.3 microns.

Application of this emissivity database to the MODIS (MOD07) atmospheric temperature and moisture profile product will be presented. The global emissivity database is used to characterize the surface below each of the profiles that are used to train the synthetic regression. Validation of the MODIS MOD07 total precipitable water product with the new emissivity will be presented, and compared to those retrievals made using the original NOAA-88 emissivity. In addition, the MOD07 retrievals will be compared with those made using the global emissivity averaged into ecosystem categories or latitude bands.