This study presents a new instrument called a low-cost albedometer (LCA) composed of two illuminance sensors that are used to measure in situ incident and reflected illuminance values on a daily timescale. The ratio between reflected vs. incident illuminances is called the albedo index and can be compared with actual albedo values. Due to the shape of the sensor, the direct radiation for zenith angles ranging from 55 to 90° is not measured. The spectral response of the LCA varies with the solar irradiance wavelengths within the range 0.26 to 1.195 μm, and the LCA detects 85 % of the total spectral solar irradiance for clear sky conditions. We first consider the theoretical results obtained for 10 different ice and snow surfaces with clear sky and cloudy sky incident solar irradiance that show that the LCA spectral response may be responsible for an overestimation of the theoretical albedo values by roughly 9 % at most. Then, the LCA values are compared with two <q>traditional</q> albedometers, which are CM3 pyranometers (Kipp &Zonen), in the shortwave domain from 0.305 to 2.800 μm over a 1-year measurement period (2013) for two sites in a tropical mountainous catchment in Bolivia. One site is located on the Zongo Glacier (i.e., snow and ice surfaces) and the second one is found on the crest of the lateral moraine (bare soil and snow surfaces), which present a horizontal surface and a sky view factor of 0.98. The results, at daily time steps (256 days), given by the LCA are in good agreement with the classic albedo measurements taken with pyranometers with R2 Combining double low line 0.83 (RMSD Combining double low line 0.10) and R2 Combining double low line 0.92 (RMSD Combining double low line 0.08) for the Zongo Glacier and the right-hand side lateral moraine, respectively. This demonstrates that our system performs well and thus provides relevant opportunities to document spatiotemporal changes in the surface albedo from direct observations at the scale of an entire catchment at a low cost. Finally, during the period from September 2015 to June 2016, direct observations were collected with 15 LCAs on the Zongo Glacier and successfully compared with LANDSAT images showing the surface conditions of the glacier (i.e., snow or ice). This comparison illustrates the efficiency of this system to monitor the daily time step changes in the snow and ice coverage distributed on the glacier. Despite the limits imposed by the angle view restrictions, the LCA can be used between 45° N and 45° S during the ablation season (spring and summer) when the melt rate related to the albedo is the most important.
|Number of pages||10|
|Journal||Geoscientific Instrumentation, Methods and Data Systems|
|State||Published - 18 Jun 2018|
Bibliographical noteFunding Information:
Acknowledgements. This study was funded by the French Institut de Recherche pour le Développement (IRD) through the Andean part of the French glacier observatory service, GLACIOCLIM (http: //www-lgge.ujf-grenoble.fr/ServiceObs/SiteWebAndes/index.htm, last access: 2 May 2018), and was carried out within the framework of the International Joint Laboratory GREAT-ICE, a joint initiative of the IRD as well as universities and institutions in Bolivia, Peru, Ecuador, and Colombia. All of the contributing authors acknowledge the contribution of LABEX OSUG@2020, ANR grant no. ANR-10-LABX-56. The Pléiades satellite image used for Fig. 1 was obtained from the CNES-ISIS FC18473 program funded by the BIOTHAW project (AAP-SCEN-2011-II). The authors would like to thank everyone who participated in the field campaigns: Patrick Ginot, Maxime Harter, and Pierre Vincent. We thank Sara Mullin and Lance Brooks for the correction of the English text in a previous version of the paper.
© 2018 Author(s).