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High‐resolution photography of clouds from the surface: Retrieval of optical depth of thin clouds down to centimeter scales
Author(s) -
Schwartz Stephen E.,
Huang Dong,
Vladutescu Daniela Viviana
Publication year - 2017
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/2016jd025384
Subject(s) - radiance , zenith , downwelling , remote sensing , pixel , solar zenith angle , image resolution , environmental science , optics , geology , geography , physics , upwelling , oceanography
This paper describes the approach and presents initial results, for a period of several minutes in north central Oklahoma, of an examination of clouds by high‐resolution digital photography from the surface looking vertically upward. A commercially available camera having 35 mm equivalent focal length up to 1200 mm (nominal resolution as fine as 6 µrad, which corresponds to 9 mm for cloud height 1.5 km) is used to obtain a measure of zenith radiance of a 30 m × 30 m domain as a two‐dimensional image consisting of 3456 × 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 about 2. 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 on a pixel‐by‐pixel basis. COD for thin clouds determined in this way exhibits considerable variation, for example, an order of magnitude within 15 m, a factor of 2 within 4 m, and 25% (0.12 to 0.15) over 14 cm. This 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.