Algorithms for Calculating Topographic Parameters and Their Uncertainties in Downward Surface Solar Radiation (DSSR) Estimation

Downward surface solar radiation (DSSR) plays an important role in the earth’s surface energy budget. However, it has significant spatial–temporal heterogeneity over the rugged terrain. To accurately capture DSSR, many analytical terrain parameter algorithms based on digital elevation models (DEMs) have been proposed. However, the uncertainties of the DSSR components associated with these algorithms remain unclear. In this letter, we compared three types of terrain parameter algorithms and their respective DSSR component uncertainties at different spatial scales by using 3-D discrete anisotropic radiative model simulations under different atmospheric conditions. The comparison results indicated that differences in slopes, sky view factors, and terrain view factors can be up to 4°, 0.165°, and 0.264°, respectively. For a high atmospheric visibility, the maximum discrepancies of direct solar irradiance and adjacent terrain-reflected irradiance over the high reflective surface (e.g., fresh snow and ice) are 26.7 and
$42.8~\text {W}\cdot \text {m}^{2}$
, respectively. In addition, for a low atmospheric visibility, a maximum difference of 31
$\text {W}\cdot \text {m}^{2}$
is identified for diffuse skylight. These uncertainties are nonnegligible when using a high-resolution DEM (e.g., 30 m), but as the DEM resolution becomes coarser, the uncertainties decrease.