Abstract
Downward surface shortwave radiation (DSSR) is the main energy source for most glacial melting, and Moderate Resolution Imaging Spectroradiometer (MODIS) and Landsat Thematic Mapper (TM) data have been used extensively in the inversion of input parameters for estimating DSSR. However, for valley glaciers under complex climatic conditions, the values of MODIS atmospheric products, especially aerosol products, are often invalid, and TM images are always saturated with snow. Furthermore, an estimation model based on optical satellite images must simultaneously consider terrain and atmospheric effects and the transient nature of ice/snow albedo. Based on a high-resolution (12 m) digital elevation model (DEM), the newly launched Sentinel-2 satellites, rather than MODIS and TM, were used to provide input data for our published mountain radiation scheme in a valley glacier. Considering Laohugou Glacier No. 12 as the study area, 62 typical Sentinel-2 scenes were selected and spatiotemporal DSSR variations on the glacier surface were obtained with a 10 m spatial resolution during a mass-balance year from September 2017 to August 2018. Ground-based measurements on 52 clear-sky days were used for validation and the mean bias error (MBE = −16.0 W/m2) and root-mean-square difference (RMSD = 73.6 W/m2) were relatively low. The results confirm that DSSR is affected mainly by the solar zenith angle and atmospheric attenuation in flat areas of valley glaciers, while in areas with complex terrain, the DSSR received by the glacier surface is affected primarily by the terrain and ice/snow albedo, which exhibits very high spatial heterogeneity.
Highlights
Glaciers provide valuable aid in the monitoring of climate change and downward surface shortwave radiation (DSSR) is a major driving factor in the melting of glaciers [1,2,3,4,5,6]
In addition to the well-known sun-surface geometrical relationship, the following aspects are important components in the complexity of estimating the solar radiation received by valley glaciers: (1) local topographic factors, such as the slope and aspect, the obstruction coefficient, the sky view factor, and the topographic configuration [11,12,13,14,15]; (2) the fact that radiation in the atmosphere is attenuated more by aerosols and precipitable water (PW) than by Rayleigh scattering and absorption achieved through the ozone column and uniformly mixed gases [16]; and (3) the transient nature of ice/snow cover and the high-reflection characteristics of the snow surface [17,18]
Many researchers [20,21,22,23] have developed a variety of complex topographic factors to finely describe the solar radiation energy received by the slope surface: the cosine of the solar illumination angle on a sloped grid affected by the terrain slope and aspect, solar zenith, and azimuth angles; the obstruction coefficient (Vs), which indicates whether the surface is obscured by the surrounding terrain; the sky view factor (Viso), which describes the area ratio of the sky dome seen by the slope surface; and the topographic configuration factor (Fij), which describes the energy ratio of the surrounding visible pixels to the object pixel
Summary
Glaciers provide valuable aid in the monitoring of climate change and downward surface shortwave radiation (DSSR) is a major driving factor in the melting of glaciers [1,2,3,4,5,6]. In addition to the well-known sun-surface geometrical relationship, the following aspects are important components in the complexity of estimating the solar radiation received by valley glaciers: (1) local topographic factors, such as the slope and aspect, the obstruction coefficient, the sky view factor, and the topographic configuration [11,12,13,14,15]; (2) the fact that radiation in the atmosphere is attenuated more by aerosols and precipitable water (PW) than by Rayleigh scattering and absorption achieved through the ozone column and uniformly mixed gases [16]; and (3) the transient nature of ice/snow cover and the high-reflection characteristics of the snow surface [17,18]. The global radiation received by a rugged terrain surface consists of three components: direct, diffuse, and surrounding-reflected radiation. Many researchers [20,21,22,23] have developed a variety of complex topographic factors to finely describe the solar radiation energy received by the slope surface: the cosine of the solar illumination angle (cos is) on a sloped grid affected by the terrain slope and aspect, solar zenith, and azimuth angles; the obstruction coefficient (Vs), which indicates whether the surface is obscured by the surrounding terrain; the sky view factor (Viso), which describes the area ratio of the sky dome seen by the slope surface; and the topographic configuration factor (Fij), which describes the energy ratio of the surrounding visible pixels to the object pixel
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