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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Operation procedures are:
a. If OCT ~ PNS, then LCT OCT
b. If PNS < OCT, then PNS OCT from three hours before to one hour after
passage of front or trough at 800 mb, or if CIT < OCT.
Various difficulties arise in calculating the pressure at the nodal surface
using the minimum energy sub-routine in the area of effect model. Hence, the model
was converted into a multiple regression model and 123 cases were used to derive an
estimator for PNS using stability, winds at two levels, and the height of the barrier.
Wind inputs include speed, shear, and profile curvature. A multiple correlation
coefficient of 0.77 was derived from the 123 cases. The formula is given in Table
1. 2-1.

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1.2.1.2 Modification of model computations based on data from San Juan
analysis. The evaluation of the Upper Colorado River Basin Pilot Project in the
San Juan Mountains generated a data base of observed and calculated seeded and not
seeded three-hourly precipitation blocks. Comparisons were made between the observed
and calculated values and showed the following in regard to validation of th,e model:
1. The model underpredicted the precipitation by a fixed factor in all seeded
cases and in the not seeded cases with cloud top temperatures colder than
-20C. This was probably due to the model's ignoring the frontal lifting
mechanism. In the not seeded case with temperatures warmer than -20C the
model underforecast by a factor that increased with temperature. This
was probably due to the model's ignoring the coalescence mechanism 'which
would take over in the warmest clouds where there is a great dearth of
natural nuclei.
2. The model calculations for precipitation were, relative to observed pre-
cipitation, low on the upwind slope and high on the downwind slope. That
is, the whole calculated precipitation profile was shifted about 5 nautical
miles downwind.
3. Calculated patterns of mean seeded minus mean not-seeded precipitation
for categories of two wind and four temperature stratifications matched
the observed patterns quite well.
In line with the above model validation information, the analysis procedure
outlined below was developed for the Twelve Basin Investigation.
The determination of the P-T curves for a crest precipitation station follows
the procedure illustrated in Figure 1.2-3. In the region A B the calculated not-
seeded curve is adjusted by a fixed factor (1.2 in the example) to be determined

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