The 4SAILT Model: An Improved 4SAIL Canopy Radiative Transfer Model for Sloping Terrain

The scattering by arbitrary inclined leaves (SAIL)-series models are some of the most well-known and widely used canopy radiative transfer models in the remote-sensing community. The latest version of 4SAIL simulates directional radiance from the optical to thermal spectrum range, but it is not suitable for sloping terrain. This limits its use in the currently ever-expanding development of applications for high-spatial-resolution observations. This study extends the 4SAIL model to 4SAILT, which considers the topographical effects on direct solar radiation and the obstruction of the surrounding topography for hemispherical radiation and the gravitropic influences on leaf angle distribution (LAD). The proposed 4SAILT model was evaluated by the 3-D discrete anisotropic radiative transfer (DART) ray-tracing model for various sky radiation conditions, soil and leaf temperatures, observational geometries, leaf area index values, and six typical LAD functions. The simulated results of directional radiance demonstrated that 4SAILT was consistent with DART, having RMSE values less than 2.0 and 0.1 W/ $\text{m}^{2}/\mu \text{m}$ /sr over the 0.35–2.5 and 2.5– $15~\mu \text{m}$ spectra, respectively. As an accurate, efficient, and ready-to-use model, 4SAILT benefits those who intend to use SAIL for modeling terrain areas. The 4SAILT model simulates canopy directional radiance, reflectance, emissivity, and brightness temperature over terrain surfaces, through the optical to thermal ranges. It can also be used as a surface model when estimating shortwave, longwave, and net radiation.