Inferring Domain-Averaged Cloud Properties from the ARM Observations and Testing the PCLOS Models

Ma, Y.(a) and Ellingson, R.G.(b), University of Maryland (a), Florida State University (b)
Fourteenth Atmospheric Radiation Measurement (ARM) Science Team Meeting

Clouds play a key role in regulating the Earth's climate. Real cloud fields are non-uniform in both the morphological and microphysical sense. However, most climate models neglect the 3D structure of real cloud fields and the inhomogeneity of the optical properties within individual clouds. This is equivalent to modeling the clouds as plane-parallel plates with homogeneous optical properties (PPH). One way to parameterize the 3D cloud effect is to relate the various cloud properties to a statistical cloud field parameter called the Probability Of Clear Line Of Sight (PCLOS) and then to a simple integral parameter - the effective-cloud-fraction (Ne). The PCLOS gives a summary description of the geometrical properties of a cloud field. It depends on cloud distributions, sizes and shapes. Aimed at improving the parameterization of longwave radiative transfer through 3D clouds, this study tests various PCLOS models with the ARM cloud observations. First, an evaluation technique was developed to investigate the sampling issue that arises from attempting to obtain domain-averaged information from time series of observations at one location. It answers such questions as: Over how long a time period should the average be taken? How do observation frequencies of the different instruments affect the results? And, how well does the time averaged value represent the domain-averaged value? Based on the evaluated sampling strategy, various cloud properties such as the absolute cloud fraction (N), cloud thickness, cloud spacing, and horizontal size were inferred from the ARM observations. The modeled PCLOS was then tested with the PCLOS inferred from ARM whole sky images. In this poster, the sampling strategy evaluation technique, the method of extracting the various cloud properties and the test results will all be presented.

Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).