| Description |
Topographic shading involves two components: shaded relief and cast shadowing. Shaded relief occurs from self-shadowing due to the slope and aspect of a given location; cast shadowing involves projecting shade from nearby terrain onto an adjacent surface. The combined effect of topographic shading plays a fundamental role in determining surface energy balance for glacier ice. However, this parameter has been oversimplified or incorrectly incorporated in some past studies. Here we develop a topographic solar radiation model to examine the variability in mean irradiance throughout the melt season due to topographic shading and combined slope and aspect. We utilize the 30-meter resolution ASTER GDEM and multihour solar geometry to simulate topographic shading on two glaciers of differing morphologies in regions of contrasting terrain. We test the sensitivity of shading to valley-aspect and latitude for the same two glaciers, and observe patterns in these parameters for a suite of glaciers across High Mountain Asia (HMA). Our results show that topographic shading significantly alters the potential direct clear-sky solar radiation received at the surface for valley glaciers in HMA. Additionally, contrary to the findings of some previous studies, we find that shading can be extremely impactful in the ablation zone of some valley glaciers, particularly for north- and south-facing valleys. A daily mean change in irradiance of more than -70 Wm-2 due to cast shadowing is found in the ablation zones of some HMA glaciers. Cast shadowing is the dominant mechanism in determining total shading for valley glaciers in parts of HMA, especially at lower elevations. Although shading has some predictable characteristics, it is overall extremely variable between glacial valleys, and therefore very difficult to parameterize. We use a modified temperature-index model that includes potential clear-sky irradiance to calculate melt for one selected glacier. Excluding topographic shading for this glacier results in an overestimation of total summer melt in the ablation zone by up to 10%. This demonstrates that topographic shading is not only an important factor contributing to surface energy balance, but can also influence the mass balance of glaciers throughout HMA. |
