A simple slope correction of horizontally measured albedo in sloping terrain

Abstract

In sloping terrain, albedo measured in the horizontal plane is typically not representative for the underlying surface. Accordingly, albedo should be measured either parallel (termed slope-parallel albedo) or corrected from horizontal measurements (termed slope-corrected albedo) to represent the actual sloping surface. This study presents the theory and the effect when applying a simple slope correction with the aim to transform albedo measured in the horizontal plane over a slope (termed horizontally measured albedo) to the sloping surface. Simultaneous measurements of horizontal and slope-parallel albedo for three different classes of atmospheric clearness conditions (clear, partly overcast, cloudy) and for four different terrain aspects (north, east, west, south) were collected during the study period. The results show that applying the slope correction improved the linear correlation coefficient between the slope-parallel and horizontally measured albedo by 0.60, 0.51 and 0.24 for clear, partly overcast, and cloudy conditions, respectively. During clear atmospheric conditions, slope-parallel and slope-corrected albedo deviated by 5% in terms of mean absolute error, while the slope correction reduced the deviation between the horizontally measured and slope-parallel albedo by 70%. Diurnal trends revealed a large discrepancy with smaller/larger horizontally measured albedo than slope-parallel albedo for the western/eastern slope during morning (opposite during afternoon). For the southern/northern slope the horizontal orientation of the radiation sensors overestimated/underestimated the albedo for all atmospheric clearness conditions. The horizontally measured reflected radiation had on average a difference of 3 Wm-2 from the slope-parallel reflected radiation, corresponding to a deviation of 3%. The findings show that the simple slope correction considerably improve the reliability of the horizontally measured albedo in sloping terrain when slope-parallel measurements are not possible. However, during partly overcast conditions, the simple slope correction suffers under the complex radiation regime and should preferably not be applied.