Use of Cloud Observations and Mesoscale Meteorology Models to Evaluate and Improve Cloud Parameterizations. Technical Progress Report, 1 October 1992--30 September 1993 Page: 4 of 7
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Progress Report: Walcek page 3 of 5
2. ' Project Progress: 1 Oct. 1992 - 30 Sept. 1993
During the second year of this research program, we have continued our analysis of cloud cover
and its relationship with related meteorological parameters using the U. S. Air Force cloud cover
archive. We have developed resolution-dependent algorithms that can calculate cloud cover at
any level of the troposphere using the averaged relative humidity of the layer that agrees with
cloud cover reported by surface observers, aircraft, and satellite observations archived in the U.
S. Air Force 3DNEPH archive. We are currently evaluating these cloud cover algorithms using
the incoming ARM CART shortwave radiation measurements, and are attempting to ascertain
the general applicability of our previous studies. We find that cloud cover algorithms used by
current climate models underpredict cloud cover under dry conditions, and our analysis of cloud
cover measurements shows that cloud amount increases with increasing humidity at all relative
humidities. We see no clear evidence that cloud cover vanishes at a "critical humidity". Based on
these initial comparisons, we feel that current climate models cannot adequately assess the
changes in cloud cover that may results from small changes in relative humidity in a modified
climate. Thus, a potentially important feedback between climate change and changes in cloud
cover are probably not adequately simulated by current models of global warming.
We find that a state-of-the-science mesoscale meteorology model has a tendency to overpredict
relative humidity in the upper troposphere, and this overprediction will lead to overprediction of
cloud cover. These areas of excessive relative humidity tend to occur in conditionally unstable
areas where mean upward motion is occurring. Our initial hypothesis of the cause of this
discrepancy centered around the convective mixing algorithm used to vertically redistribute
moisture and heat within unstable atmospheric columns. However, subsequent testing of this
hypothesis by not allowing any convective-scale mixing within the model did not significantly
remove the tendency of the model to saturate the upper troposphere under unstable conditions.
We are currently investigating other possible sources of this error which would lead to excessive
cloud cover within climate models.
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Walcek, C. J. Use of Cloud Observations and Mesoscale Meteorology Models to Evaluate and Improve Cloud Parameterizations. Technical Progress Report, 1 October 1992--30 September 1993, report, September 30, 1993; Albany, New York. (https://digital.library.unt.edu/ark:/67531/metadc1277236/m1/4/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.