Sky imager for the analysis of the optical influence of thin homogeneous clouds

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We manage and scientifically QA data from a Canada-wide network of sunphotometers (AEROCAN) in collaboration with Environment Canada. Ground based sunphotometry has become a standard tool for evaluating satellite based aerosol remote sensing algorithms and a key indicator of aerosol model performance.  In spite of the viewpoint that sunphotometry represents turn-key ground-truth there are instrumental and environmental issues which merit attention. One of the most critical is the impact of thin homogeneous clouds (THCs) on estimates of aerosol optical depth (AOD). The most widely used cloud screening algorithm is limited by its algorithmic definition of clouds in terms of high frequency optical variations. AODs associated with THC events often bypass this criterion.      A hybrid methodology for a spectrally and temporally based form of cloud screening is currently being investigated. This technique has only been tested in a limited fashion using independent indicators of THC. In the next few years we will focus on the issue of THCs at the calibration site of Bratt's Lake, the relatively turbid site at Egbert and our local site at Sherbrooke. This latter site is influenced by a mixture of clear to moderately turbid atmospheres and is often affected by THCs.      Automated systems for the acquisition of sky images can be employed to yield spatially based indicators of THC. Our experience has been that sky images give a contextual support for THC presence. Sherbrooke is a key AEROCAN site for the study of cloud screening procedures since it is the home site of the applicants (and because we know from visual experience that THCs are a frequent occurrence at this site). That the sky imager be located where the investigators and their HQPs are located is important since we have come to realize that human observations are necessary to support automated measurements and that frequent visual observations are an essential component in graduate student involvement and training. Funding is accordingly requested for a total sky imager to complement the AEROCAN sunphotometer at Sherbrooke.