Environmental & Climate Sciences Department Seminar
"High-Resolution Photography of Clouds from the Surface: Retrieval of Cloud Optical Depth down to Centimeter Scales"
Presented by Stephen Schwartz, Environmental and Climate Sciences Department
Thursday, June 16, 2016, 11:00 am — Conference Room, Bldg 815E
Initial results are presented of a analysis of high resolution photographs of clouds at the ARM SGP site in July, 2015. A commercially available camera having 35-mm equivalent focal length up to 1200 mm (nominal resolution as fine as 6 µrad, which corresponds to 12 mm for cloud height 2 km) is used to obtain a measure of zenith radiance of a 40 m x 40 m domain as a two-dimensional image consisting of 3456 x 3456 pixels (12 million pixels). Downwelling zenith radiance varies substantially within single images and between successive images obtained at 4-s intervals. Variation in zenith radiance found on scales down to about 10 cm is attributed to variation in cloud optical depth (COD). Attention here is directed primarily to optically thin clouds, COD less than roughly 3. A radiation transfer model used to relate downwelling zenith radiance to COD and to relate the counts in the camera image to zenith radiance, permits determination of COD and cloud albedo on a pixel-by-pixel basis. COD for thin clouds determined in this way exhibits considerable variation, for example, an order of magnitude within the 40 m domain examined here and 50% over a distance of 1 m. An alternative to the widely used areal or temporal cloud fraction, denoted radiative cloud fraction, also evaluated on a pixel-by-pixel basis, is introduced. This highly data-intensive approach, which examines cloud structure on scales 3 to 5 orders of magnitude finer than satellite products, opens new avenues for examination of cloud structure and evolution.
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