Three-dimensional radiative transfer modeling of the terrain-reflected radiation in high altitude mountains.

As a result of the high albedo of snow or ice and the rugged surface, high altitude mountainsget more reflection between the slopes, which increases the incoming radiation at theground surface. Because of the absence of measurements in snow-covered highmountains, the accurate surface solar radiation (SSR) in this area is not easy toderive. A large-scale remote sensing data and image simulation framework (LESS),which is a ray-tracing based three-dimensional (3-D) radiative transfer model, wasused in this study to have an accurate simulation of SSR in a mountainous area inthe Tibetan Plateau. The direct and diffuse fluxes without topographic effect weresimulated with MODTRAN5 for clear sky, used as the input for the LESS simulation.The simulation result indicates that the radiation increases in mountainous area,even if there is no snow cover. The increased energy origins from two parts: directand diffuse radiation and terrain-reflected radiation. The anomalies of the directand diffuse flux with reference to a horizontal surface is related to the slope andaspect distribution, and varies in a day. The terrain-reflected radiation is positivelycorrelated with the surface albedo, and it can be as large as 360 W/m2 in the areacovered with snow. The domain daily energy increase because of the terrain reflectionis generally less than 40W/m2, which is around 8% of downward SSR on a flatsurface. Three terrain configuration factors using in simplified approaches wereevaluated by the ray tracing simulation. Only one terrain configuration factor has verysimilar result with the LESS in domain-averaged (12km*12km) terrain-reflected flux,the other two factors both underestimate the radiation. However, the simulationwith terrain configuration factor cannot derive a reliable result on local position. [ABSTRACT FROM AUTHOR]