Investigating the Surrounding Topographic Effects on Target Reflected Radiance by Extending the BOST Model

Topography impacts the fraction of radiation that reaches a target surface from the sun, sky, and surrounding terrains, which leads to distortions in the radiance levels that are observed by sensors. Although several mountain radiative transfer (RT) models have been developed, the surrounding topographic effects are rarely considered, which can cause non-negligible uncertainty in forward radiance modeling. In this study, we first extended the canopy reflectance model to be suitable for both continuous and discontinuous canopies over sloping terrain (BOST) to rugged terrain, and used it to investigate the impact of the surrounding topography on the reflected radiance and evaluate the contributions of diffuse irradiance from the sky and adjacent terrains. Discrete anisotropic RT (DART) simulations and remote sensing (RS) observations in a real mountain environment were used to evaluate the physical mechanism and model performance. The results suggested that the extended BOST model can capture terrain-induced variations in direct and diffuse radiation, which can be successfully used to simulate the reflected radiance in a real mountainous region. The model shows significant improvement compared with that before extension (i.e., root-mean-square error (RMSE) decreases (
$R^{2}$
increases) of 1.252 (0.025) and 2.035 (0.054) in the red and near-infrared (NIR) bands, respectively). In addition, we discovered that the surrounding topographic effects on the diffuse sky and terrain irradiance are significantly influenced by the wavelength, solar direction, and visible area of the sky and terrains. The extended BOST model serves as an effective tool for improving simulations of the observed radiance and facilitates the development of RT models for rugged terrains.