To Ignore or Not: Understanding the Influence of Hillshade

Hillshade poses an important challenge in remote sensing research at mountainous regions. In order to understand the influence of hillshade on the solar direct radiation (SDR) transfer process, this study performed the calculation of hillshade (including both self-shadow and cast-shadow) and the evaluation of its radiative effects over the Tibetan Plateau. When utilizing the ray tracing method to calculate hillshade, a search radius of 6 km (equivalent to 200 NASADEM pixels) efficiently captures about 99% of the hillshade area at a solar zenith angle (SZA) of 80°. And the necessary search radius decreases rapidly as the SZA decreases. By calculating the hillshade on the Tibetan Plateau, it was found that the surface experiences hillshade for approximately 10% of daylight hours, with more than 30% in the most severely impacted areas. The hillshade dynamics over the Tibetan Plateau reveal minimal seasonal variation but significant diurnal differences, moreover, the impact of hillshade varies throughout the day. It is worth noting that one hour after sunrise and one hour before sunset, the impact of hillshade reaches its maximum, with over 30% of the area covered by hillshade. In particular, ignoring the cast-shadow can lead to overestimating the average SDR of the entire plateau by 40 W/m². This impact is most severe during the winter solstice. In contrast, with a smaller SZA, the influence of hillshade on SDR is minimal at noon.