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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was not possible to relate a sounding with a precipitation episode, especially if it
was of short duration. The basic assumption is that the' air mass characteristics
depicted by a sounding from a site upwind of a massif will be applicable to the air
mass characteristics over the massif at a later time. The Northern Hemisphere series
of daily surface and 500 mb charts were used to determine the time lag from the sound-
ing station to the massif. Those charts are available only through 1966, at present,
and subsequent to this, the NWS daily weather map series were used to determine the
time lags.
For the twelve major basins, there were thirty-three massifs identified. These
consisted of independent mountain ranges and in some cases divisions along a mountain
range due to differences in orientation and/or topography. In order to provide the
necessary input data to complete the analysis for the first seven priority basins,
it was necessary to identify precipitation episodes with associa.ted sounding station
and time lags for twenty-five of the thirty-three massifs for the period 1951-01971-
1. 2. 5 ~omparison of Precipita!:jo~B-at~~-L<.>r 59_Q_..!!I1b anE_ Cl~l!d T_o2-J_~pera~~res.
In the Climax investigations mentioned earlier, the 500 mb temperature was consi-
dered representative of cloud top temperature for stratification purposes. Although
this may be a reasonable assumption for the winter cases in the Rockies, it is con-
sidered to be an over-simplification when looking at the period October through
April. In the Twelve Basin Investigation it was considered that a better approxima-
tion of cloud temperatures could be made based upon the temperature dewpoint spread
as outlined by Elliott et al (1967).
Climax 2NW precipitation rate versus cloud top temperature may be seen in Figure
1.2-6. In Figure 1.2-7, Climax 2NW precipitation rate versus 500 mb temperature may
be seen (note different precipitation rate scale). It may be noted that the peak at
-l6C based on 500 mb temperature is shifted to -26C based on cloud top temperature.
A recent study of precipitation rates versus 500 mb temperature in the Sierra
(Rowland et al., 1973) indicated that although precipitation rates increased w~th
warmer 500 mb temperatures, there was not a break in the curve representative of a
critical temperature. Figure 1.2-8 shows precipitation rate versus cloud top tempera-
ture for Hamilton Branch in the' northern Sierra Lassen massif. Figure 1.2.-9 shows
the precipitation rate versus 500 mb temperature for this station. The latter is
similar to the curves found in the Sierra study mentioned above. In Figure 1. 2-8
the high rates at temperatures colder than -35C is felt to be a reflection of cases
with warm 500 mb temperatures, but with cold cloud tops. On this curve a critical

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