Cloud statistics measured with the infrared cloud imager (ICI)

The Infrared Cloud Imager (ICI) is a ground-based thermal infrared imaging system that measures spatial cloud statistics with a 320/spl times/240-pixel uncooled microbolometer detector array. Clouds are identified from the residual radiance that remains after water vapor emission is removed from radiometrically calibrated sky images (the water vapor correction relies on measurements of precipitable water vapor and near-surface air temperature). Cloud amount, the percentage of an ICI image containing clouds, is presented for data from Atmospheric Radiation Measurement (ARM) sites at Barrow, AK in February-April 2002, Lamont, OK in February-April 2003, and Barrow, AK in March-April 2004. In Oklahoma, the percent cloud cover determined from full ICI images was slightly higher than that found from a single-pixel time series, suggesting that cloudiness may be under sampled by vertically viewing lidars or radars under highly variable conditions. Full-image and single-pixel statistics agreed more closely for Arctic clouds, which tend to be uniform for long periods of time. Good agreement is found in comparing cloud amount from ICI and active remote sensors during day and night, but much worse agreement is found between ICI and the ARM Whole Sky Imager during nighttime relative to daytime, indicating the importance of the diurnally consistent ICI measurements.