INFERRING THE THERMAL-INFRARED HEMISPHERIC EMISSION FROM A SPARSELY-VEGETATED SURFACE BY DIRECTIONAL MEASUREMENTS

The thermal-infrared (longwave) emission from a vegetated terrain is g enerally anisotropic, i.e., the emission temperature varies with the v iew direction. If a directional measurement of temperature is consider ed to be equal to the effective temperature of the hemispheric emissio n, then the estimate of the latter can be significantly in error. The view-direction (zenith angle theta(eq)) at which the emission equivale nce does hold is determined in our modeling study. In a two-temperatur e field-of-view (soil and plants), theta(eq) falls in a narrow range d epending on plant density and canopy architecture. theta(eq) does not depend on soil and (uniform) plant temperatures nor on their ratio, ev en though the pattern of emission vs, the view direction depends cruci ally on this ratio. For a sparse canopy represented as thin, vertical cylindrical stalks (or vertical blades uniformly distributed in azimut h) with horizontal facets, theta(eq) ranges from 48 to 53 circle depen ding on the optical density of the vertical elements alone. When plant elements are modeled as small spheres, theta(eq) lies between 53 to 5 7 degrees (for the same values of the canopy optical density). Only fo r horizontal leaves (a truly planophile canopy) is the temperature mea sured from any direction equal to the temperature of the hemispheric e mission. When the emission temperature changes with optical depth with in the canopy at a specified rate, theta(eq) depends to some extent on that rate. For practically any sparsely vegetated surface, a directio nal measurement at the zenith angle of 50 degrees offers an appropriat e evaluation of the hemispheric emission, since the error in the estim ate will, at most, only slightly exceed 1% (around 4 W m(-2). Estimate s of the hemispheric emission through a nadir measurement, on the othe r hand, can be in error in some cases by about 10%, i.e., on the order of 40 W m(-2).