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

Accurate satellite remote sensing of surface temperature has become mo re and more important in the context of calculating the radiation bala nce input to climate models for assessment of global change. Surface t emperatures can be determined from radiance measurements in both the 3 -5 mum and 8-14 mum atmospheric windows, given some estimate of surfac e emissivity. The 3-5 mum region offers a potential advantage in the a ccuracy of temperature determination, because temperature estimates ar e more than two times less sensitive to emissivity errors in this regi on. However, very little is known about the emissivities of land surfa ce 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 hemispherica l spectral reflectance, from which emissivity can be calculated, of a wide variety of earth surface materials in the 3-5 mum region, and com pares their average reflectances in AVHRR Band 3 (3.55-3.93 mum) to th at in Bands 4 (10.3-11.3 mum) and 5 (11.5-12.5 mum). It appears from t hese data that, based on natural variations in emissivity of different surface materials, AVHRR Band 5 is best used for temperature remote s ensing of rocks, soils, and senescent vegetation, while Band 3 is best for green vegetation and water. Band 4 is preferred for thermal remot e sensing of ice, especially rough ice. Emissivity and reflectivity in the 3-5 mum region affect other significant remote sensing measuremen ts of global change besides those of surface temperature alone. The ac curacy of atmospheric sounding of CO near 4.7 mum, for example, depend s in part on knowledge of such surface properties to account for surfa ce upwelling radiation. Because of the multiple uses to which our data can be put, digital records of all spectra used in the figures are av ailable from 2.01 mum to 14 mum upon request.