Modeling thermal infrared radiative transfer within 3D vegetation covers

The radiometric temperature of vegetation surfaces can vary widely depending on many factors (viewing direction, temperature distribution, canopy structure, etc.), which makes it difficult to interpret remotely sensed thermal infrared (TIR) data. The authors present a 3D radiative transfer model that simulates the TIR radiative budget (absorption and emission) and upward radiance of vegetation covers. The model is an extension to the TIR region of the DART (discrete anisotropic radiative transfer) model developed for the short wave domain. Radiative transfer simulation relies on discrete 3D scene representations which include any distribution of trees, understory and soil surfaces, possibly with topography. Propagation of emitted and scattered radiation is tracked with a ray tracing approach and the discrete ordinate method. The model verifies the Kirchhoff law and was successfully tested against a physically based model for homogeneous canopies and a geometric projection model specifically for row crops. Moreover, a partial validation was carried out with directional TIR measurements of a cotton crop. An application of the model is presented in order to illustrate the influence of canopy architecture on directional brightness temperature, in the case of tree covers (maritime pine stand) and row crops