The possibility of global climate change is a major threat to human society, therefore climate models are needed to predict these changes. To improve model prediction, a better description of the interaction between earth surfaces and the atmosphere is needed. Climate models mainly regards vegetation as an important sink for greenhouse gases, but vegetation influences the climate in more ways, such as the exchange of long wave radiation (LWR) fluxes with the surrounding. Limited information exists on LWR (4.0-100mm) interaction with canopies. Over a 24 hours, LWR exchange with the atmosphere can account for more then 20 percent of all net radiation exchanged. Long wave optical properties of vegetation and its radiation sources differ from those of the short wave.
An analytical and mathematical model that describe long wave radiation regime inside vegetation canopy was developed. The model assumptions are: leaves that present vegetation elements are arranged in infinite layers; they are described as a small flat elements evenly spread; leaves have random azimuthal direction and defined zenithal direction; and temperature of the leaf layer changes with height. Leaves absorb all thermal radiation and emit isotropic radiation. ‘View factor’ is used to describe the radiation exchange between surfaces.
First, the model calculates the amount of irradiance impinges on a single tilted leaf which comes from a flat surface area (like the ground). It was found that if the emitted surface was isothermal and infinite, and both sides of the leaf had absorbed the same amount, then the leaf’s angle tilt did not influence the amount of irradiance which impinged on it. Next, calculation of the amount of radiation exchanged by two leaves in nearby layers is carried out, followed by calculation of the irradiance that reaches the leaf from a nearby leaf layer. The model consideres that leaves have a finite area so they disturb some of the emitted radiation from leaving the layer and reaching leaves above. That calculation leads to the ‘layer attenuation coefficient’, which depends on the parameters of the emitting and the receiving leaf layer, and on the distance between the two. The ‘emitted coefficient of a leaf layer’ is obtained by considering that nearby leaves prevent part of the emitted radiation from leaving the leaf layer. In the last stage, the plane parallel radiation transfer equation of thermal fluxes radiance in vegetation is developed.
Since experimental results dealing with LWR as a function of canopy parameters and incoming radiation are limited, model validation was not possible. It was concluded that for an infinite, homogeneous and isothermal erectophile leaf canopy, outside irradiance penetrates deeper and releases less thermal radiation to the surroundings, than for the planophile leaf canopy. From this point of view erectophile leaf canopy is better suited to cold climates.