11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Wednesday, 5 June 2002: 4:44 PM
The retrieval of cloud properties for partially covered fields of view
James A. Coakley Jr., Oregon State University, Corvallis, OR; and M. A. Friedman and W. R. Tahnk
Poster PDF (346.7 kB)
Current schemes for retrieving cloud properties from satellite imagery data typically assume that each cloud contaminated field of view is overcast by clouds from a single layer. Even at the 1-km resolution currently available with MODIS, the majority of fields of view are neither overcast nor cloud-free but are partially cloud covered. Assuming that these fields of view are overcast leads to biases in the retrieved properties: cloud cover, visible optical depth, droplet effective radius, cloud emission temperature, etc. For systems of single-layered clouds, these properties may be derived for fields of view that are only partially cloud covered by assuming that the clouds in these fields of view are at the same altitude and thus have the same equivalent blackbody emission temperature as nearby clouds for which the coverage is sufficiently extensive that the fields of view are overcast. This approach was suggested nearly 20 years ago by Arking and Childs, J. Climate Appl. Meteorol., 24, 322 (1985), but was never thoroughly investigated. The approach is taken here to quantify the potential magnitudes of the biases in cloud cover, visible optical depth, and droplet effective radius that arise from assuming that all cloud contaminated fields of view are overcast when, in fact, many are only partially cloud covered. Preliminary results indicate that the visible optical depths of the clouds in the partly cloudy pixels are, of course, larger than those obtained assuming that the fields of view are overcast, but smaller than those of the nearby clouds that completely fill the imager fields of view. Furthermore, the optical depths of the clouds in the partly cloudy pixels are nearly constant for cloud cover fractions from 0.2 0.8. Unlike the optical depths, the droplet radii retrieved for clouds in the partly cloudy fields of view increase with increasing pixel-scale cloud fraction. The retrieved radii are larger if the partly cloudy pixels are assumed to be overcast, but even these larger values are smaller than the radii for nearby clouds with extensive coverage. In addition to exploring these potential biases, the retrieval scheme for partly cloudy pixels is subjected to tests in which the spatial resolution of the radiances are degraded in order to determine the extent to which the retrieved products depend on the resolution of the sensor.

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