Radiometric and equivalent isothermal surface temperatures

The analytical solution for the heat flux from an anisothermal canopy developed from K theory by Brutsaert and Sugita [1996] (hereinafter referred to as B&S) has been extended to provide a parameterization of the difference between the radiometric and the equivalent isothermal surface temperature. The latter is the isothermal temperature at which a canopy would give the correct sensible heat flux if the sensible heat roughness length z(0h) was given its theoretical, isothermal value derived by B&S. A parameterization of the canopy temperature seen by a radiometer is developed which incorporates the same foliage temperature profile as that used by B&S. From this the weighting fraction (i.e., the fraction of canopy top versus canopy bottom temperature "seen" by the radiometer) can be derived, as can an equation for the view zenith angle at which the radiometric and equivalent isothermal surface temperatures are the same. Field data indicate that multiangle radiometric surface temperatures alone cannot determine the three unknowns describing the foliage temperature profile. Several strategies to overcome this problem are outlined. The parameterization may eventually help to interpret remotely sensed infrared surface temperatures and to improve estimates of sensible heat flux and evapotranspiration, but comprehensive field testing is still needed.