ARM Cloudiness Intercomparison IOP 2003 Analysis: Sky Cover and Cloud Fraction
Kassianov, E., Long, C., and Ovtchinnikov, M., Pacific Northwest National Laboratory
Fourteenth Atmospheric Radiation Measurement (ARM) Science Team Meeting
Previously (Kassianov et al., 2002), we examined the relationship between hemispherical sky cover and nadir-view cloud fraction by using model simulations. These simulations of ground-based hemispherical measurements were based on four-dimensional cloud fields produced by a large eddy simulation model. In particular, we demonstrated that 15-minute averages of frequently sampled (30-second) sky cover relates much better to nadir-view cloud fractions, especially for some limited field-of-view (FOV) simulated sky images. To test the model results with real cloudiness, we conducted a field experiment using a Total Sky Imager (TSI) during the ARM Cloudiness Intercomparison IOP (CIC IOP) held at the ARM site in Oklahoma. This field experiment ran from February 20 through May 6, 2003, providing samples of many variations and types of cloudiness, including multi-layer occurrences. We selected a subset of 36 days of the experiment that exhibit a wide range of cloudiness other than clear and overcast skies. Analysis of results from the ARM CIC IOP supports the findings obtained from our model simulations. Specifically, we demonstrated that sky cover of “thin” clouds is almost independent on FOV. Since “thin” clouds have typically large horizontal extension and small vertical thickness, their cloud aspect ratio is very low, and this expected invariant behavior of the sky cover is in harmony with our model results. Kassianov, E., C. Long and M. Ovtchinnikov, 2002: ARM Total Sky Imager: Monte Carlo Simulations, Twelfth ARM Science Meeting Proceedings, St. Petersburg, Florida, April 8-12, 2002.
Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).
