AN EXPERIMENTAL-STUDY OF ANGULAR EFFECTS ON SURFACE-TEMPERATURE FOR VARIOUS PLANT CANOPIES AND BARE SOILS

Surface temperature is a key parameter for assessing fluxes at the sur face-atmosphere interface. Proper estimation of radiative surface temp erature requires corrections for perturbating factors such as atmosphe ric contributions and angular effects. Several models have been derive d to address angular effects, but relevant data for validating such mo dels is still scarce. This paper describes a field experiment dedicate d to collecting angular measurements of brightness surface temperature over several types of surfaces (bare soils with different roughnesses , corn, grass, alfalfa), using a unique measurement protocol with simu ltaneous temperature readings at two angles. For each surface zenithal and azimuthal angular effects are quantified. In some cases (unstress ed, fully-covering alfalfa) the difference between oblique and vertica l brightness temperatures is-within +/-0.5 K. Over stressed corn the t emperature measured at angles of +/-60 degrees is about 4 K less than the nadir looking temperature, but it is 3.5 K higher over a ploughed bare soil, when the inclined radiometer faces the sunlit side of the f urrows. Over a bare smooth soil the observed angular variations are sh own to be compatible with those due to possible angular variations in emissivity. All the results are discussed in terms of surface geometry and microclimatic conditions, and compared to previous studies, Impli cations are deduced for the interpretation of satellite measurements o f surface temperature.