WMOD00465

• Contract/Permit #: 14-06-D-7368
• Title: Twelve Basin Investigation: Analysis of Potential Increases in Precipitation and Streamflow Resulting from Modification of Cold Orographic Clouds in Selected River Basins of the Western United States
• Date: 8/15/1972
• Weather Modification - Doc Type: Report

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Any layer with a top below a height: of D' above the valley floor is assumed not to
participate in the precipitation process. Excluding such layers, the base of the
lowest .layer encountered is taken to be the base of the effective cloud layer. If
there is a layer above this first layer, a check is made to determine whethl~r the
two layers are separated in the vertical by more than a distance D. If they are,
the top of the lower layer is taken as the top of the effective cloud layer. If
not, the process is continued upward until a layer top is found which has clear air
extending for a distance of at least D above it. Both D and D' are set to 500 m in
this study. It was determined that the values of D could be changed by a factor of
two without noticeably affecting the analysis. Once the effective cloud top is thus
determined, it is lifted along a streamline (using the nodal surface concept) to a
point over the crest. The temperature is determined assuming a moist adiabatic
ascent and ~ermed the lifted cloud top temperature.
It should ]Je noted that this method produces an over-abundance of cloud tops
in the vicinity of -40C. By convention, it is at this temperature that the report-
ing of the relative humidity is discontinued due to instrument unreliability~ For
temperatures colder than -4OC, the relative humidity is arbitrarily set to 5% in
this study. The dewpoint depression criteria would always return a clear air indi-
cation for such humidities. So no matter how high the cloud tops actually 1;..Tere,
they could not be recorded with temperatures colder than -40C. The fact that some
lifted cloud top temperatures have values colder than -40C is due to the lifting
of effective cloud tops near -40C to colder temperatures. This aspect of the
method should not hinder the analysis since these cloud top temperatures are at the
edge of or out of the region of interest.
In computing the average normal wind component in the effective cloud layer,
the method is to average the components for the standard levels that lie within
the layer. No interpolation is performed in attempts to produce resolution finer
than the standard 50 mb intervals.
1.2.7.2 Analysis of master files. Three separate 'programs were used to
analyze the data stored in the master file. The first produc,ed a plot of precipi-
tation rate versus lifted cloud top temperature and corresponding tabulated data.
The second produced scatter diagrams of the various parameters against lifted cloud
top temperature. The third produced for each parameter its average, standard devia-
tion, frequency, relative frequency, maximum and minimum stratified by SC intervals
of lifted cloud top temperature.

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