Slope Correction for Albedo on Inclining Terrain

Introduction:

Albedo is an important concept in climate studies for maintaining a ground radiation balance and water and heat exchanges within the land–atmosphere system. Albedo is measured parallel to the ground, but this presents a problem when one is trying to measure sloping or rough terrain. In this study, the researchers presented a theory that one can accurately measure albedo on a mountain or hill by applying a slope-correcting calculation.

Set Up:

This study occurred at Jellhaug, a burial mound located on the Vik plain in Norway, in August and September 2022. The researchers selected four locations on the mound in north, east, west, and south-facing areas. Each area had a slope of 15°, 17.5°, 12°, and 16°, for the northern, eastern, southern, and western slope, respectively.

Horizontal and slope-parallel albedo were taken in clear, overcast, and cloudy conditions. Horizontally measured albedo was provided by Apogee’s upward-looking (SP-510-SS)‬‬‬ and downward-looking (SP-610-SS) thermophile pyranometers‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬, which were connected to microCache Bluetooth data loggers (AT-100)‬‬‬‬‬‬‬‬‬‬‬. Slope-parallel albedo was measured by an Apogee net radiometer (SN-500-SS)‬‬‬‬‬‬‬‬‬‬‬.

Results:

In this study they found that the slope correction formula improved the linear correlation coefficient between the slope-parallel and horizontally measured albedo by 0.60, 0.51, and 0.24 for clear, overcast, and cloudy conditions. When it was clear, 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%. Daytime 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 both overestimated and 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%.

Conclusion:

The findings show that slope correction improves 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 and should not be applied when avoidable.

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