Monday, 23 January 2012
Evaluation of Infrared Sky Imagers for the Atmospheric Radiation Measurement Program
Hall E (New Orleans Convention Center )
Uncertainty in the characterization of clouds in general circulation models (GCMs) is one of the major causes of the broad spread of future climate change predictions. Cloud fraction, which is closely related to cloud coverage, has been an integral part of the observational dataset that feed these GCMs. However, at the U.S. DOE's Atmospheric Radiation Measurement (ARM) Program Climate Research Facility, cloud fraction has only been reliably determined during daytime hours utilizing the Total Sky Imager (TSI). Nighttime cloud fraction has been and remains a critical programmatic gap in ARM's observational dataset. It has long been recognized that infrared sky imaging technology has held great promise in closing this gap. In addition, this technology has the distinct advantage that its ability to characterize clouds is identical for day or night conditions. Therefore, instrument demonstrations were conducted at the ARM Southern Great Plains site in 2005, 2007, and 2009 to evaluate measurements of cloud fraction from different types of infrared sky imagers and to compare the daytime values with an operational TSI. The most promising of the instruments tested is Solmirus Corporation's All Sky Infrared Visible Analyzer (ASIVA). Preliminary analysis of the hemispheric infrared sky images captured by the ASIVA, using simple techniques to provide cloud fraction, show very good agreement with the TSI results during daylight hours. A comprehensive data analysis of observations made during the 2009 campaign has been performed using more sophisticated algorithms to attain an even better correlation with the TSI instrument. Also in development is a robust nighttime cloud fraction data product utilizing the radiometrically calibrated data from ASIVA's two infrared channels.