An autonomous multi-band albedometer for long-term monitoring and calibration of optical satellites

<p>Monitoring the surface albedo of snow is crucial for assessing the Earth energy budget in polar and high-mountain areas. Multiple algorithms are thus proposed to compute surface albedo from satellite observations. Still, satellite-retrieved information is limited by scarce calibration/validation sites in remote areas. Ground-based observations guarantee the continuity of measurements under cloudy weather and serve as a cal/val tool, but deployment of autonomous instruments is limited in remote cold areas. In this context, we introduce the Multiband albedometer: a new instrument that performs autonomous albedo measurements. It is battery-operated with low energy consumption, hence suited for long-term monitoring in polar regions. Measured snow spectral albedo and Specific Surface Area (SSA) time series are here used to validate the Pre-operational Sentinel-3 Snow and Ice products (SICE), derived from Copernicus Sentinel-3 Ocean and Land Color Instrument (OLCI) and yet not fully validated on snow. The Multiband albedometer includes two radiometric cosine detectors facing the sky and snow surface respectively. Each channel measures the incoming radiation in 7 selected bands in the visible to short-wave infrared spectrum. Processing includes corrections for the sensor dark current, the imperfect cosine response and cross-calibration of the two channels, the effect of surface slope and sensor tilt. Two multiband albedometers were installed close to the French station of Dumont d&#8217;Urville, in Ad&#233;lie Land in East Antarctica, near the snow equilibrium line and in the upper accumulation area. Albedo and SSA time series were extracted from data collected during austral summers between January 2017 and January 2021. A new cloud mask developed from Sentinel-3 SLSTR products was used to validate the day-to-day weather and validate SICE at the same location. The good agreement between Sentinel-3 derived data and in-situ measured SSA highlights the potential of the Multiband albedometer for calibration and validation of satellite-retrieved snow albedo and SSA in remote regions.</p>