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A simple parameterization to predict population characteristics of boundary-layer cumulus clouds Berg, Larry Keith


Estimates of fair-weather cumulus size distributions are made from the joint frequency distribution (JFD) of virtual potential temperature (6V) vs height of the lifting condensation level (zLCL) collected from a single surface weather station. Conceptually, the JFD represents the likelihood that a parcel will rise and the likelihood that condensation will occur. The 6V and the zLCL for each point of the JFD can be compared to the mean 6V of the mixed layer. If the parcel has a 6V less than that of the mixed layer the parcel will not rise. If the parcel has a larger 6V the parcel will rise dry adiabatically. A subset of the these rising parcels will reach their zLCL and form clouds. These parcels will continue to rise, moist adiabatically, until they reach the stable layer above the convective mixed layer. Other rising parcels will not condense but will continue to rise, dry adiabatically, as clear air parcels until they reach the stable layer. The cloud model was designed to use a JFD measured near the top of the surface layer using fast-response instruments mounted on a research aircraft flying over a large area. It is very expensive to obtain surface-layer data using an aircraft. It would be desirable if a JFD based on inexpensive surface measurements could be used. These experiments will not only investigate the CuP model results, but will also determine if a less expensive JFD computed from a single surface weather station can be used instead. Using a JFD of 6V vs zLCL calculated from a single surface station, cloud ensemble estimates are compared to cloud measurements made at the Atmospheric Radiation Experiment (ARM) site in central Oklahoma during the spring and summer 1994 and 1995 intensive operations periods. There was some skill predicting the cloud-base height — in most cases the model estimates were bracketed by observations. No observations of cloud-thickness are made at the ARM site. However, model estimates of cloud-thickness are nearly log-normally distributed, consistent with observations by Lopez (1977) and Stull (1988). Using a JFD constructed from a single-surface station there is little skill predicting cloud cover.

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