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. / Graduation date: 1992
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28781 |
| Date | 01 July 1991 |
| Creators | Chang, Fu-Lung |
| Contributors | Coakley, James A. Jr |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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