Long wave radiation regime in vegetation-parameterisations for climate research

An analytical and mathematical model that describes the long wave radiation regime inside a vegetation canopy was developed. The model assumptions are: leaves are the only vegetation elements and are arranged in horizontally infinite layers: they are evenly spread in the layers; having random azimuthal direction and defined zenithal direction. Leaves absorb all thermal radiation, do not scatter any and emit isotropic radiation. An adaptation of the ‘View factorʹ concept describes radiation exchanges between leaves and leaf layers. The model calculated the amount of irradiance incident on a single tilted leaf from a flat surface (like the ground). It was found that for an isothermal and infinite emitting surface, with both sides of the leaf absorbing identically, the leafʹs tilt angle does not influence the amount of irradiance impinging on it. Then the amount of radiation exchanged by two leaves in nearby layers and the irradiance reaching the leaf from a nearby leaf layer are evaluated. Next, the ‘layer attenuation coefficientʹ, and the ‘emission coefficient of a leaf layerʹ are calculated. Finally, the plane parallel radiation transfer equation of thermal fluxes radiance in vegetation canopy is presented. It was concluded that for an infinite, homogeneous and isothermal leaf canopy with erect leaves, outside irradiance penetrates into deeper layers and less thermal radiation is released to the surroundings, than for the plane leaf canopy case. From this point of view an erect leaf canopy is better suited to cold climates.