The Angular Behavior of Emitted Thermal Infrared Radiation (8–12 μm) at a Semiarid Site

Abstract

Radiometric measurements of the angular distribution of infrared (8?12 ?m) radiation from a field site in a semiarid sandy soil location are reported. The aim of this study is to investigate the variation of emitted infrared radiation with zenith view angle. The motivation arises through the need to correct wide-field-of-view satellite measurements of land surface temperature for surface-induced zenith-angle effects. A new self-calibrating, scanning radiometer is used to acquire near-continuous measurements for zenith view angles up to 70°. The results suggest that, during the day, the measured brightness temperature is evidently affected by a relation between solar illumination and viewing angle through differential heating and shading. During the nighttime, a significant angular variation (up to 8%) in surface emissivity is discerned. The angular behavior of surface emission at night stemming from this variation is accurately modeled using the δ-Eddington approximation. Implications for satellite measurements of land surface temperature from the Advanced Very High Resolution Radiometer (AVHRR)-2, Geostationary Meteorological Satellite (GMS)-5, Moderate-Resolution Imaging Spectroradiometer (MODIS), and Along-Track Scanning Radiometer (ATSR)-2/Advanced Along-Track Scanning Radiometer (AATSR) are discussed.