Clear‐sky albedo measurements on a sloping glacier surface: A case study in the Bolivian Andes

An important potential source of error in snow albedo measurements under clear sky is the tilt of the surface when the sensors are placed parallel to the horizon. The error depends on the surface slope and aspect. A hemispherical radiation sensor receives its signal from within a surface area of several square meters, which generally is not a plane. Here we examined the influence of slope and aspect combinations related to surface irregularities on albedo measurements at two locations on the Zongo Glacier, Bolivia. The slope and aspect distributions determined through topographic measurements were used to correct the albedo measurements. The corrections were different between the two sites but resulted in similar albedo changes: the substantial albedo reductions observed from morning until evening were measurement artifacts. Even for slight slopes, an error of a few degrees on the slope estimation or an error of roughly 20° on the aspect estimation had an appreciable influence on the corrections. If the topography around the measurement site is not precisely known, the most reliable method for determining the daily albedo is to observe the measurements around solar noon. Corrected albedo diurnal variations were low and symmetrical, centered on a minimum at noon. During the dry season (the Southern Hemisphere winter), the diurnal fluctuations of the snow albedo on the Zongo Glacier seem to be controlled by the incidence angle cycle of solar radiation.