PNNL

Abstract

We outline a methodology for estimating fractional sky cover for an effective 160O field-of-view using surface measurements of downwelling total and diffuse shortwave irradiance. The sky cover estimates show a high degree of repeatability given nominally well maintained and operated radiometer systems, and the use of the Long and Ackerman (2000) methodology for estimating the clear-sky total and diffuse SW. Thus, the resultant fractional sky cover estimates appear to be independent of the particular climate regime and model of radiometers used. The sky cover estimates agree to better than 10% RMS sky cover amount with sky imager retrievals and human observations, which is about the level of agreement between sky imaging systems and observers themselves. Analysis shows that the technique also produces realistic frequency distributions. As such, this methodology becomes a powerful tool for satellite and model validations, and climatological analyses including the study of trends in cloud amount.