Simulation and Analysis of the Topographic Effects on Snow-Free Albedo over Rugged Terrain

Abstract

Topography complicates the modeling and retrieval of land surface albedo due to shadow effects and the redistribution of incident radiation. Neglecting topographic effects may lead to a significant bias when estimating land surface albedo over a single slope. However, for rugged terrain, a comprehensive and systematic investigation of topographic effects on land surface albedo is currently ongoing. Accurately estimating topographic effects on land surface albedo over a rugged terrain presents a challenge in remote sensing modeling and applications. In this paper, we focused on the development of a simplified estimation method for snow-free albedo over a rugged terrain at a 1-km scale based on a 30-m fine-scale digital elevation model (DEM). The proposed method was compared with the radiosity approach based on simulated and real DEMs. The results of the comparison showed that the proposed method provided adequate computational efficiency and satisfactory accuracy simultaneously. Then, the topographic effects on snow-free albedo were quantitatively investigated and interpreted by considering the mean slope, subpixel aspect distribution, solar zenith angle, and solar azimuth angle. The results showed that the more rugged the terrain and the larger the solar illumination angle, the more intense the topographic effects were on black-sky albedo (BSA). The maximum absolute deviation (MAD) and the maximum relative deviation (MRD) of the BSA over a rugged terrain reached 0.28 and 85%, respectively, when the SZA was 60 • for different terrains. Topographic effects varied with the mean slope, subpixel aspect distribution, SZA and SAA, which should not be neglected when modeling albedo.

Sensitivity of Coarse‐Scale Snow‐Free Land Surface Shortwave Albedo to Topography

“…Although the multiscattering of the adjacent terrain can increase the outgoing radiations, multiscattering effects generally have little impact unless the land surface has high reflectance. This is in agreements with the model‐based analysis in Hao et al (), which showed that the more rugged the terrain, the more obvious topographic effects on the albedo and the albedo difference between the flat and rugged terrain could reach 0.28.…”

“…There is an urgent need to better understand the topographic effects on the land surface albedo to increase our knowledge of mountainous role in global climate change. Figures and reveal that the snow‐free MODIS albedo is highly sensitive to the topographic distribution, and thus, the topographic consideration is required in the surface albedo estimations even though the terrain is relatively gentle (10–20°), which is in line with previous studies (Cherubini et al, ; Hao et al, ; Wen et al, ). Figures 3–5 show that as the terrain becomes more and more rugged, MODIS albedo shows an overall decreasing trend, due to the stronger shadowing effects and the terrain obstruction for the hemispherical sky.…”

“…In spite of the gradually increasing understanding of the topographic effects on the surface albedo over rugged terrain, how the relative importance of different explanatory variables (such as C ab , LAI, and SZA) to the surface albedo varies with the topography and the quantitative contributions of the topography to the surface albedo remain unclear. With the purpose of deriving an improved understanding of surface albedo over rugged terrain and its main driving variables, the global sensitivity analysis (GSA) method, as an alternative method to diagnose and quantify the influential and noninfluential input variables governing the model outputs (Saltelli et al, ), was adopted based on a well‐validated snow‐free land surface albedo simulated model (Hao et al, ) over rugged terrain in this study. It is anticipated that an in‐depth quantitative investigation of the GSA on the surface albedo over rugged terrain can deepen our understandings of topographic effects on the surface albedo and further offer new insights to the future model development and parameter retrievals of land surface albedo over rugged terrain.…”

Accelerated RAPID Model Using Heterogeneous Porous Objects

“…By recording the number of self-overlapped pixels of i as N ii (Ω), the inner multiple scattering is included by new inner view factors (F ii ). (10) Assume that the total incident light is defined as 1.0 with a diffuse fraction (k d ). The single scattering of i + and i − from diffuse light will be…”

“…Within a domain with a size larger than 0.5 km (hereafter named the km-scale), the land surface is more likely to be heterogeneous with changing elevations or mixed land covers. Especially in mountainous areas, forest BRDF and albedo are significantly affected by the heterogeneous terrain and complex vegetation composition [9,10]. To simulate the combined effects of heterogeneous terrain and complex vegetation composition on MODIS or MISR BRDF products, RT models are expected to contend with a domain (or scene) size greater than 0.5 km.…”