2NFOVCLOUD

MASRAD: Cloud Study from the 2NFOV at Pt. Reyes Field Campaign

2 June 2005 - 30 September 2005

Lead Scientist: Warren Wiscombe

Observatory: AMF, PYE

Cloud optical depth is one of the most important cloud optical properties, and vital for any cloud-radiation parameterization. Our ARM Science Team project has pioneered an algorithm to retrieve cloud optical depth in a fully three-dimensional cloud situation using zenith radiances from the ARM ground-based passive two-channel (673 and 870nm) Narrow Field-Of-View radiometer, called the 2NFOV. The underlying principle of this algorithm is that clouds have nearly identical optical properties in these two channels while there is a strong spectral contrast in vegetated surface reflectance between the two channels. Based on the resulting very different surface-cloud reflection interactions in these two channels, this algorithm retrieves not only cloud optical depth, but also a “radiation-effective cloud fraction” that is valuable for the climate modeling community. Due to the one-second time resolution of 2NFOV measurements, for the first time we are able to capture local, rapid changes in 3D cloud structures at the natural time scale of clouds. We have applied this algorithm to one month of 2NFOV measurements at the ARM Southern Great Plains site, and compared retrievals with those estimated from other instruments. This algorithm shows an excellent ability to retrieve cloud properties in various cloud situations, while retrievals based on solar flux and on microwaves have serious limitations. We expect to have extensive comparisons with a larger data set from the Pt. Reyes field campaign to further validate this algorithm, and to advance our understanding of cloud property statistics. Pt. Reyes was ideal for our work because of the high abundance of clouds and because competing retrieval algorithms work best for low stratocumulus clouds and thus are most able to furnish comparison data for our method.