Abstract
Abstract. The calibrated ground-based sky imager developed in the Marine Physical Laboratory, the Whole Sky Imager (WSI), has been tested with data from the Atmospheric Radiation Measurement Program (ARM) at the Southern Great Plain site (SGP) to determine optical properties of the atmospheric aerosol. Different neural network-based models calculate the aerosol optical depth (AOD) for three wavelengths using the radiance extracted from the principal plane of sky images from the WSI as input parameters. The models use data from a CIMEL CE318 photometer for training and validation and the wavelengths used correspond to the closest wavelengths in both instruments. The spectral dependency of the AOD, characterized by the Ångström exponent α in the interval 440–870 nm, is also derived using the standard AERONET procedure and also with a neural network-based model using the values obtained with a CIMEL CE318. The deviations between the WSI derived AOD and the AOD retrieved by AERONET are within the nominal uncertainty assigned to the AERONET AOD calculation (±0.01), in 80% of the cases. The explanation of data variance by the model is over 92% in all cases. In the case of α, the deviation is within the uncertainty assigned to the AERONET α (±0.1) in 50% of the cases for the standard method and 84% for the neural network-based model. The explanation of data variance by the model is 63% for the standard method and 77% for the neural network-based model.
Highlights
Atmospheric aerosol interacts both directly and indirectly with the Earth’s radiation budget and influences the climate
By developing techniques to extract aerosol characteristics from sky imagers, we propose a value-added to the instrument and a complement to the existing aerosol data bases
Different neural network-based models estimate the aerosol optical depth for three wavelengths using the radiance extracted from the principal plane of sky images from the Whole Sky Imager (WSI) as input parameters
Summary
Atmospheric aerosol interacts both directly and indirectly with the Earth’s radiation budget and influences the climate. IPCC (IPCC, 2007) reported that the anthropogenic contributions to aerosol (primarily sulphate, organic carbon, black carbon, nitrate and dust) together produce a cooling effect, with a total direct radiative forcing of −0.5 [−0.9 to −0.1] W m−2 and an indirect cloud albedo forcing of −0.7 [−1.8 to −0.3] W m−2 (Foster et al, 2007). This is comparable in magnitude to the forcing induced by the increase of the greenhouse effect gases concentration during the last century (Foster et al, 2007). Radiative forcing induced by aerosol has a large uncertainty, and may have much more importance in the overall energy balance
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