Impacts of topographic shading on direct solar radiation for valley glaciers in complex topography

Abstract

Topographic shading, including both shaded relief and cast shadowing, plays a fundamental role in determining direct solar radiation on glacier ice. However, shading has been oversimplified or incorrectly incorporated in surface energy balance models in some past studies. In addition, no systematic studies have been conducted to evaluate relationships between shading and other topographic characteristics. Here we develop a topographic solar radiation model to examine the variability in irradiance throughout the glacier melt season due to topographic shading and combined slope and aspect. We apply the model to multiple glaciers in high-mountain Asia (HMA) and test the sensitivity of shading to valley aspect and latitude. Our results show that topographic shading significantly alters the potential direct clear-sky solar radiation received at the surface for valley glaciers in HMA, particularly for north- and south-facing glaciers. Additionally, we find that shading can be extremely impactful in the ablation zone. Cast shadowing is the dominant mechanism in determining total shading for valley glaciers in parts of HMA, especially at lower elevations. Although shading can be predictable, it is overall extremely variable between glacial valleys. Our results suggest that topographic shading not only is an important factor contributing to surface energy balance but could also influence glacier response and mass balance estimates throughout HMA.