The Relationship between the Directional Brightness Temperatures and Hemispherical Thermal Emission over Three Scenes

Upwelling long wave radiation (UPLW) is one of the key input parameters for many land surface models, and it plays an important role in estimating the energy state of the Earth´s surface. Hemispherical LWUP can be determined indirectly by integrating the surface directional brightness temperatures (BT) over the hemisphere. A physical-based model of radiation angular anisotropy over discontinues scene, the Modified Geometric Model (MGP), has been used to estimate hemispherical LWUP from directional BT. The MGP model is developed to simulate directional thermal radiance and it is a highly parameterized model over homogeneous and discontinuous canopy. However, the MGP model has its limitations since it can not describe the reality accurately. At the same time, the directional BT over other scenes should also be simulated to estimate the hemispherical emission using other models because of the complexities of land surface covers. In this study, three physical-based models, MGP, Bi-directional Gaps Model and Therm5a, have been used to investigate the relationship between the directional BT and the hemispherical thermal emission over forest, corn canopy and grass scene, respectively. We firstly introduce the single scattering into the MGP model to describe scene thermal radiance more accurately. Then, extensive thermal emission simulations are conducted under a wide variety of solar illumination and view conditions, component temperatures, vegetation fraction covers and canopy leaf angle distributions. The best substituted angles of hemispherical thermal emission estimates are given over the three scenes. The sensitivity of study is also conducted to determine which surface parameters most strongly affect the relationship over all scenes. Finally, the UPLW are calculated over a forest scene using MGP (after considering the single scattering) and field component temperatures measurements collected during the SAFARI 2001. We compared the results with tower-based pyrgeomete- - r data.