Abstract
Advancements in remote sensing, along with greater access to high spatial and temporal resolution imagery, have improved our ability to model glacier surface energy and mass balance in remote regions of complex terrain, such as High-mountain Asia (HMA). In general, net shortwave (SW) radiation accounts for the majority of energy available on a glacier surface during the summer months, suggesting that SW modeling errors can critically impact surface energy balance estimates. In this study, we model the clear-sky SW irradiance for a group of glaciers in the Everest region of HMA using a high-resolution (8-meter) digital elevation model (DEM) composite derived from commercial stereo satellite imagery. We then systematically downsample this DEM and considered the effect on incoming SW irradiance, with a sensitivity analysis for standard terrain attributes. The slope and aspect (combined) and topographic shading have the greatest impact on daily SW irradiance and also introduce a larger SW bias when DEM resolution is downsampled. Our results show that modeled incident SW is overestimated as resolution becomes coarser. For 10 selected glaciers in the Everest region, decreasing spatial resolution from 8 to 30 meters results in a range of average daily biases between +20 to +60 Wm-2 (or ~7 to 20%) at some high and low elevations, and an average bias of more than +100 Wm-2 (~33%) as resolution is coarsened to 500 meters. In order to determine the bearing these results have on surface melt, we explore the diurnal variability of this bias. Additionally, we compare our results with modeled incident SW using several global DEM products (ASTER, SRTM, and ALOS) to evaluate error introduced by lower resolution. Models using the 30-meter products show an overall average daily SW bias of +24 Wm-2 (or ~8%) across elevation with some elevations showing a bias up to +60 Wm-2 (~20%) on multiple glaciers. Taken together, our results demonstrate the value of high-resolution data to correct biases in modeled SW radiation and constrain uncertainties for glacier energy balance modeling in regions of complex terrain.
Highlights
More than 1.4 billion people in Asia rely on freshwater that originates from remote High-mountain Asia (HMA) glaciers (Immerzeel et al, 2010)
Coarser digital elevation model (DEM) resolution causes an overestimation in incident SW radiation, and a similar daily SW bias is observed when using common 30-m resolution DEM products
During March, which represents the average annual impact of terrain on incident SW radiation, the East Rongbuk Glacier has a significant decrease in irradiance from combined terrain attributes, largely due to its north-facing orientation (Figure 3A)
Summary
More than 1.4 billion people in Asia rely on freshwater that originates from remote High-mountain Asia (HMA) glaciers (Immerzeel et al, 2010). In many remote regions, where meteorological station data are not available, such as much of High Mountain Asia, energy fluxes are often derived from coarse resolution reanalysis products or climate models, and must be distributed across the surface of a glacier. While previous studies imply the importance of DEM spatial resolution on modeled surface energy balance and melt, they are often limited to expensive airborne imagery over few glaciers or utilize DEM resolutions of 30 m or lower Results from these previous studies suggest the influence of topography on glacier surface energy balance will be amplified in regions of complex terrain, such as HMA, requiring higher resolution DEMs to adequately model SW radiation at glacier surfaces.
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