Overcast versus clear-sky remote sensing: comparing surface reflectance estimatesP

Average diurnal cloud cover in Central and Northern Europe is above 60%, with a significant number of days of overcast conditions. Such high cloud cover severely limits the use of satellite remote sensing in the optical wavelength bands. With the unprecedented spread of near-surface platforms such as unmanned air vessels (UAVs) and drones, remote sensing below cloud cover offers high resolution and frequent monitoring of the environment in most diverse applications. Clear-sky remote sensing is regarded by many as optimal for determining surface reflectance. One of its advantages over overcast remote sensing is its lower magnitude of sensing noise (instrument, atmospheric, environmental) relative to the total irradiance (signal-to-noise ratio, or SNR). On the other hand, overcast conditions are advantageous due to their lower directional irradiance heterogeneity and higher isotropic directional reflectance compared with clear skies. Spectral reflectance estimates based on nadir viewing assuming isotropic reflection, obtained on one clear-sky day and two overcast days, are compared for four targets (asphalt, pavement, grass, and Grey sol foam sheet). Correlation coefficients (rho) between clear-sky and overcast reflectance estimates exceeded 0.9 in the majority of measurements. Low spectral distances, as measured by a spectral angle mapper (SAM), were found between estimates obtained for the overcast day of lowest irradiance and those acquired under clear-sky conditions: similar to 3 degrees, 3.17 degrees, 5 degrees, and 6 degrees, for asphalt, pavement, grass, and grey sol foam sheet, respectively. Overcast remote sensing thus provides results which are highly similar with clear sky remote sensing when considering nadir viewing. The higher homogeneity of incoming and upwelling directional reflectance fields during overcast conditions may lead to an advantage of overcast remote sensing over clear sky remote sensing when utilising low-altitude scanners and cameras mounted with off-nadir viewing geometries.