A forest thermal radiation directionality model with structural and thermal stratification

Abstract:

Land surface temperature (LST) is an essential indicator of vegetation monitoring, forest drought and fire assessment. However, retrieving LSTs from satellite-based thermal infrared (TIR) observations prone to directional anisotropies (DAs) because a pixel-averaging temperature is usually aggregated from different components with different temperatures such as tree crown, trunk, understory vegetation and bottom soil. Currently, existing thermal radiative transfer models can hardly explain varying DAs of LSTs due to complicated structural and thermal heterogeneity of a forest. In this paper, a famous radiative transfer model, FRT, in visible and near-infrared (VNIR) domain was introduced to study thermal radiation directionality, in which thermal emissions of components were added. The thermal FRT model was validated using a measured dataset and a simulated dataset, which indicated a stable performance of the proposed thermal version. DAs of LSTs varied a lot with forest change and forest stratification was found an important factor based on this model. DAs of LSTs were also affected by vertical temperature profile of tree leaves, particularly for cases with dense canopies. The thermal FRT model has capable of acting as not only a tool to link satellite observations and component temperatures, but also a supplement for combined applications using both VNIR and TIR remotely sensed observations.