Effective thermal parameters for a heterogeneous land surface

In this article we explore the relations between the thermal parameters of a partially vegetated land surface and the radiance observed by a satellite or airborne instrument. The calculations are carried out for four simple approximations of the canopy structure. For each of these we have derived the canopy geometry in the field of view as a function of zenith and azimuth viewing angles. These models can be inverted to allow estimation of the canopy geometry from measurements of the vegetation fractions at given viewing angles. The shape factors, determining the irradiance of each component of the canopy system by the other components, have also been calculated analytically for each of the four geometries. A technique for defining the effective radiative temperature and emissivity for a pixel is given, and the errors due to linearization of Planckʹs function and first-order scattering are evaluated quantitatively. A significant contribution to the effective emissivity in the form of a cavity term is identified, and the implications for reflection of atmospheric radiance are discussed. Finally the sensitivity of the effecive thermal parameters to the choice of vegetation geometry is evaluated by comparing the results from each of four canopy geometries, with similar ground cover proportions and canopy lengths.