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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The extension of the cloud top P-T curves towards colder cloud top temperatures
will be determined by the extent of the data within the sample. The limit of exten-
sion on the warm side will generally be considered to be -SC (activation threshold
of AgI).
The upwind and downwind limits of seeding effects is determined by judgement
on the basis of the average topographic profile of the massif, physical and theo-
retical considerations, and input data from operational and research programs as
applicable.
Although SC temperature blocks have been used in the illustration, one degree
blocks are used for the historical data.
The above approach provides an estimate of increased precipitation based solely
upon microphysical effects in orographic cloud and is considered a conservative esti-
mate. There is some experimental evidence of additional increases which are believed
to be the results of positive dynamic effects of seedling upon cumuliform clouds that
are often embedded within the stratiform orographic cloud deck. It is possible to
form another estimate that includes this additional seeding effect based upon the
fraction of precipitation normally occurring in storms with warm cloud top tempera-
tures (temperatures warmer than the critical temperature) and t~he fraction of these
cases which could be expected to be unstable. Evidence suggests that increases in
these cases are on the order of 100% and for our computations, it has been assumed
that the precipitation is doubled in the fraction of cases whic:h are unstable. The
estimate of increased precipitation which includes both microphysical and convective
effects should therefore be considered as a liberal estimate.
1. 2.2 Identification of Massifs. In the Five Basin Study (NAWC 1971),
seedabi1ity considerations were oriented toward individual natural cloud seeding
units. Such units were mountain massifs, or portions thereof, having a distinct
orientation, and physically separated from other such seeding units. Due to the
limitations of raingage and rawinsonde data, a breakdown of the twelve basins into
seeding units of the limited size of those appearing in the Five Basins Study was
not justified. The approach in the Twelve Basin Study identifies the major mountain
massifs contributing runoff to each major basin. An individual massif may be related
to several study watershed seeding units within a major basin, and also may be re-
lated to more than one major basin. Figure 1.2-4 shows the massifs identified for
the study and the basic orientation of each massif. Table 1.2-2 lists the massifs
for each major basin and the streamgages used in the analysis.
1.2.3 Identification of yrecipitation Episodes. In order to make optimum
use of the methods available for analyzing seeding potential, it would be desirable

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