Angular dependence of radiometric surface temperature for sparse vegetation

Surface heat fluxes can be determined from the difference between surface and air kinetic temperatures using the aerodynamic resistance for the transfer of heat. To obtain representative fluxes on a regional scale remotely sensed radiometric surface temperatures have to be used in place of the kinetic surface temperature required in such models. The Along Track Scanning Radiometer (ATSR) provides improved measurements of thermal radiance's from approximately 1 km squares on the earth's surface at frequent intervals and has the unique ability to view the scene at two angles. which enables determination of the atmospheric correction over oceans and hence enhances retrieval accuracy. There is far greater spatial heterogeneity over the land, so that it is uncertain whether the dual look capability could determine acceptable atmospheric corrections. To investigate the utility of the dual look angle data, radiometric temperatures of a semi-arid sparse grassland site were measured using a number of ground based radiometers set up at different view angles. On a typically cloudless day differences of up to 3.5 K were observed between the nadir view (0 degrees from zenith) and the forward view (55 degrees from zenith) of the same surface and a linear relationship between the two temperatures was inferred. Studies of possible mechanisms causing this difference showed that it is much greater than previously observed angular variations in the emissivities of soils.