PNNL

Abstract

Cloud optical depth is the most important of all cloud optical properties, and vital for any cloud-radiation parameterization. To estimate cloud optical depth, the atmospheric science community has widely used ground-based flux measurements from either broadband or narrowband radiometers in the past decade. However, this type of technique is limited to overcast conditions and, at best, gives us an "effective" cloud optical depth instead of its "local" value. Unlike flux observations, monochromatic narrow-field-of-view (NFOV) radiance measurements contain information of local cloud properties, but unfortunately, the use of radiance to interpret optical depth suffers from retrieval ambiguity. We have pioneered an algorithm to retrieve cloud optical depth in a fully three-dimensional cloud situation using new Atmospheric Radiation Measurement (ARM) ground-based passive two-channel (673 and 870 nm) NFOV measurements. The underlying principle of the algorithm is that these two channels have similar cloud properties but strong spectral contrast in surface reflectance. This algorthm offers the first opportunity to illustrate cloud evolution with high temporal resolution retrievals. A combination of two-channel NFOV radiances with multi-filter rotating shadowband radiometer (MFRSR) fluxes for the retrieval of cloud optical properties is also discussed.