Surface Emissivity Maps for Use in Retrievals of Longwave Radiation Satellite

Abstract An accurate accounting of the surface emissivity isimportant both in the retrieval of surface temperatures and inthe calculation of the longwave surface energy budgets whichare derived from data collected by remote sensing instrumentsaboard aircraft and satellites. To date, however, high qualitysurface emissivity data have not been readily available forglobal applications. As a result, many remote sensing andclimate modeling efforts have assumed the surface to radiateas a blackbody (surface emissivity of unity).Recent measurements of spectral reflectances of surfacematerials have clearly demonstrated that surface emissivitiesdeviate considerably from unity, both spectrally andintegrated over the broadband. Thus, assuming that a surfaceradiates like a blackbody can lead to potentially significanterrors in surface temperature retrievals in longwave surfaceenergy budgets and in climate studies. Taking intoconsideration some recent spectral reflectance measurements,we have constructed global maps of spectral and broadbandemissivities that are dependent on the scene (or surface) type.To accomplish our goal of creating a surface emissivity map,we divided the Earth's surface into a 10" lat. X 10" lon. grid,and categorized the land surface into 18 scene types. The first17 scene types correspond directly to those defined in theInternational Geosphere Biosphere Programme (IGBP)surface classification system. Scene type 18 has been added torepresent a tundra-like surface which was not included in theIGBP system. Laboratory measurements of the spectralreflectances for different mineral and vegetation types werethen associated, individually or in combination, with each ofthe 18 surface types, and used to estimate the emissivities ofthose surface types. Surface emissivity maps were generatedfrom the band-averaged laboratory data for 12 longwavespectral bands (> 4.5 I-tm) used in a radiative transfer code aswell as for the NASA's Clouds and the Earth's Radiant EnergySystem (CERES) window channel band (8-121_tm). Thespectral emissivities for the 12 spectral bands weresubsequently weighted using the Planck function energydistribution to calculate a broadband longwave (5-1001_tm)emissivity. The resulting broadband emissivities were usedwith a surface longwave model to examine the differencesresulting from the use of the emissivity maps and theblackbody assumption.