Graduate Thesis Or Dissertation
 

Toward the estimation of errors in cloud cover derived by threshold methods

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  • The accurate determination of cloud cover amount is important for characterizing the role of cloud feedbacks in the climate system. Clouds have a large influence on the climate system through their effect on the earth's radiation budget. As indicated by the NASA Earth Radiation Budget Experiment (ERBE), the change in the earth's radiation budget brought about by clouds is ~-15 Wm⁻² on a global scale, which is several times the ~4 Wm⁻² gain in energy to the troposphere-surface system that would arise from a doubling of CO₂ in the atmosphere. Consequently, even a small change in global cloud amount may lead to a major change in the climate system. Threshold methods are commonly used to derive cloud properties from satellite imagery data. Here, in order to quantify errors due to thresholds, cloud cover is obtained using three different values of thresholds. The three thresholds are applied to the 11 μm, (4 km)² NOAA-9 AVHRR GAC satellite imagery data over four oceanic regions. Regional cloud-cover fractions are obtained for two different scales, (60 km)² and (250 km)². The spatial coherence method for obtaining cloud cover from imagery data is applied to coincident data. The differences between cloud cover derived by the spatial coherence method and by the threshold methods depends on the setting of the threshold. Because the spatial coherence method is believed to provide good estimates of cloud cover for opaque, single-layered cloud systems, this study is limited to such systems, and the differences in derived cloud cover are interpreted as errors due to the application of thresholds. The threshold errors are caused by pixels that are partially covered by clouds and the errors have a dependence on the regional scale cloud cover. The errors can be derived from the distribution of pixel-scale cloud cover. Two simple models which assume idealized distributions for pixel-scale cloud cover are constructed and used to estimate the threshold errors. The results show that these models, though simple, perform rather well in estimating the differences between cloud cover derived by the spatial coherence method and those obtained by threshold methods.
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