Abstract
The Qinghai-Tibetan Plateau is characterised by a very strong relief which affects albedo retrieval from satellite data. The objective of this study is to highlight the effects of sub-pixel topography and to account for those effects when retrieving land surface albedo from geostationary satellite FengYun-2D (FY-2D) data with 1.25km spatial resolution using the high spatial resolution (30 m) data of the Digital Elevation Model (DEM) from ASTER. The methodology integrates the effects of sub-pixel topography on the estimation of the total irradiance received at the surface, allowing the computation of the topographically corrected surface reflectance. Furthermore, surface albedo is estimated by applying the parametric BRDF (Bidirectional Reflectance Distribution Function) model called RPV (Rahman-Pinty-Verstraete) to the terrain corrected surface reflectance. The results, evaluated against ground measurements collected over several experimental sites on the Qinghai-Tibetan Plateau, document the advantage of integrating the sub-pixel topography effects in the land surface reflectance at 1km resolution to estimate the land surface albedo. The results obtained after using sub-pixel topographic correction are compared with the ones obtained after using pixel level topographic correction. The preliminary results imply that, in highly rugged terrain, the sub-pixel topography correction method gives more accurate results. The pixel level correction tends to overestimate surface albedo.
Highlights
Land surface albedo, defined as the fraction of incident solar radiation reflected by a surface, is a major parameter controlling the radiative forcing at the land surface and its energy balance
The objective of the present study is to derive sub-pixel topographically corrected surface albedo from 1.25km resolution geostationary satellite FengYun-2D (FY-2D) data combined with a fine resolution Digital Elevation Model (DEM) from ASTER-GDEM2 taking into account the sub-pixel adjacency effect
The improvement brought by integrating the sub-pixel topography effects in the reflectance retrieved at 1.25 km is obvious as depicted in the figure 4, in which the three lower maps show: (1) a surface reflectance map without any topographic correction; (2) a surface reflectance map topographically corrected at pixel level, performed using the slope and aspect values degraded from 30m to 1.25 km pixel size; (3) a surface reflectance map topographically corrected at sub-pixel level
Summary
Land surface albedo, defined as the fraction of incident solar radiation reflected by a surface, is a major parameter controlling the radiative forcing at the land surface and its energy balance. The plateau is characterised by an extreme relief and many studies have recognized the importance of topographic effects on albedo retrieved from satellite data [1]. It is essential to account for topography induced effects to accurately estimate albedo from satellite observations in this area. Based albedo retrieval methods using remote sensing observations have been extensively developed at local and global scale [2]. In the case of global surface albedo product, e.g. MODIS or AVHRR, the effect of topography is quoted as an important source of error if retrieval assumes a horizontal surface [3].
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