WMOD00403

• Title: Special Regional Weather Modification Conference Augmentation of Winter Orographic Precipitation in the Western US
• Date: 11/11/1975
• Weather Modification - Doc Type: Report

Ground based AgI-NaI generators located at elevations ranging from 6
thousand to near 10 thousand feet MSL were employed for the seeding. They
were generally operated at a 20 gram AgI/hr output but on occasion the out-
put was as high as 140 g/hr. The experimental day extended from 1100 to 1100
MST, but the requirement of a 2-hour purge time between days meant that all
generators were off at 0900 MST if the next day was called seedable and was
to be a control day (not seeded). Days called seedable (called in advance of
the statistical decision whether or not to' seed) had to be predicted to meet
certain "within bounds" cloud top temperature and wind flow conditions during
the course of the experimental day. The O'perational phases of this project
were carried out by EGG Inc. During the first 4 years there have been a total
of 119 experimental day.s of which 59 were seeded and 60 not seeded.

The design of the project was prepared at Colorado State University
(Grant et. al., 1969) and contains a detailed plan for the project.

2. EVALUATION BY EXPERIMENTAL DAY

A comparison of treated (seeded) day to control (not seeded) day preClpl-
tation for the 4 years showed that over the target area where was no signifi-
cant difference between the two sets of precipitation observations. This
pattern was consistent through each of the four years, and in each season;
Fall, Winter, and Spring. It fell to the remainder of the evaluation to
examine the available data to determine why an excess of treated over control
target area precipitation, as anticipated in the design, had not occurred.
The abundant physical data, as well as the recording gage precipitation data
reduced by WSSI to hourly amounts, was used along with the rawinsonde data in
making this determination. The net result was to show that on many 3-hour
periods (approximate time between soundings), the cloud tops, as determined
by sounding data, were colder than the cloud top temperature criterion allowed.
This appears to have resulted in overseeding 'at these times so that there were
some reductions in precipitation superimposed upon the increases that appeared
to have occurred when clouds with warm tops were seeded. The complexities
of forecasting cloud tops up to 24 hours in advance, a wholly new type of
weather prediction task, had proven to be overhwelming. This problem was not
encountered in the prototype Climax experiment where a predetermined stratifi-
cation was carried out on a post-hoc basis; nor would it be a significant
problem in a purely ol'erational project where frequent synoptic and radar
data makes forecast and operational updating feasible.

The difference between the 4 year mean precipitation (inches per 24 hours)
for experimental treated and experim(~ntal control days having an average cloud
top temperature warmer than -27 C is shown in Figure 2. The figure indicates
precipitation excesses .::'.ssociated with the seeding which are most marked in
the western portion of the target area, and to the west thereof, and on the
downuind slopes. Even the dml7l1wind valley (San Luis Valley) contains small
excesses that percentage-wise, are quite large.

Figure 3 displays the seeding-associated 4 year mean precipitation excess
for the experimental days w'hen the average cloud top temperature was out of
bounds on the cold side (T ~ -27 C). There ar:e uniform deficits except in
the downwind valley, indicating a rather severe overseeding effect. However,
further analysis involving the use of covariates, to be discussed below, show
that there was some randomization bias in this sample so that the deficit,
while remaining intact after corrections for the bias, is of lesser magnitude
than that indicated in this figure.

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