Retrieval of properties for semitransparent clouds from multispectral infrared imagery data

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Abstract
A method is presented for extracting the properties of clouds which may be semitransparent in the infrared from imagery data at infrared window wavelengths for 50‐ to 100‐km regions that contain single‐layered cloud systems. The scheme utilizes (1) differences in the optical properties of water and ice at the infrared window wavelengths, (2) the nonlinear relationship between emissivity and transmissivity when clouds are semitransparent, and (3) differences in the sensitivity of blackbody emission to temperature at the infrared window wavelengths. A prescription is given for using the constraints on the emitted radiances imposed by a single‐layered cloud system to retrieve the regional‐scale emission of the cloud layer where it becomes opaque and an index related to the effective droplet/ice particle size. Within the region, the prescription provides pixel‐scale retrievals of fractional cloud cover and 11‐μm emissivity (equivalently, liquid/ice water column amount). The retrieval scheme has obvious limitations. First, the scheme is restricted to single‐layered cloud systems. Such systems are frequently found on the 50‐ to 100‐km regional scale. Second, to perform the retrieval, some of the image pixels in the region must be overcast by semitransparent cloud. Third, the effective radius of the hydrometeors must be sufficiently small that the extinction and absorption cross sections at the different wavelengths are significantly different. For combinations of emission at 11 and 12 μm, the hydrometeors must have effective radii of less than 20 μm; for 3.7‐ and 12‐μm emissions, the hydrometeors must have effective radii of less than 60 μm; and for 8‐ and 12‐μm emissions, the hydrometeors must have radii of less than 200 μm (in the case of ice). Fourth, the retrieved effective radius is taken to be only an index of the hydrometeor size within the region being observed. Owing to the typical variability of hydrometeor size within mesoscale regions, the interpretation of emitted radiances as due to either broken cloud or semitransparent cloud is ambiguous. The retrieval scheme is illustrated through application to samples of 1‐ and 4‐km advanced very high resolution radiometer 3.7‐, 11‐, and 12‐μm observations.