EMISSIVITY OF TERRESTRIAL MATERIALS IN THE 3-5-MU-M ATMOSPHERIC WINDOW

Accurate satellite remote sensing of surface temperature has become more and more important in the context of calculating the radiation balance input to climate models for assessment of global change. Surface temperatures can be determined from radiance measurements in both the 3-5 mum and 8-14 mum atmospheric windows, given some estimate of surface emissivity. The 3-5 mum region offers a potential advantage in the accuracy of temperature determination, because temperature estimates are more than two times less sensitive to emissivity errors in this region. However, very little is known about the emissivities of land surface materials in the 3-5 mum region, If emissivities were more variable in the 3-5 mum region than in the 8-14 mum region, this might cancel the advantage of lower sensitivity to emissivity error in the shorter wavelength region. This article documents the directional hemispherical spectral reflectance, from which emissivity can be calculated, of a wide variety of earth surface materials in the 3-5 mum region, and compares their average reflectances in AVHRR Band 3 (3.55-3.93 mum) to that in Bands 4 (10.3-11.3 mum) and 5 (11.5-12.5 mum). It appears from these data that, based on natural variations in emissivity of different surface materials, AVHRR Band 5 is best used for temperature remote sensing of rocks, soils, and senescent vegetation, while Band 3 is best for green vegetation and water. Band 4 is preferred for thermal remote sensing of ice, especially rough ice. Emissivity and reflectivity in the 3-5 mum region affect other significant remote sensing measurements of global change besides those of surface temperature alone. The accuracy of atmospheric sounding of CO near 4.7 mum, for example, depends in part on knowledge of such surface properties to account for surface upwelling radiation. Because of the multiple uses to which our data can be put, digital records of all spectra used in the figures are available from 2.01 mum to 14 mum upon request.