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

The evidence that artificial ice nuclei routinely reach the supercooled water
in a cloud is not impressive, except in the case of direct injection of the ice
nuclei. Four types of indirect evidence have been offered in support of success-
ful ice nuclei delivery in the case of the San Juan and Climax Project. They
are as follows:
1. Ground released AgI has been detected at reasonably high altitudes with
airborne ice nuclei measurements on clear sky days and below convective cloud
bases. However, no indication of the relative magnitudes of mechanical mixing!.
convective mixing and clear air turbulent mixing was presented (Orgill, 1971).
2. Diffusion experiments in a wind tunnel simulation of the San Juan and
Climax area have been made (Orgill, 1971). Marwitz (1974) pointed out, based on
a conversation with Dr. Cermak, that the mechanical turbulence in the wind tunnel
is not similar to the mechanical turbulence (and hence, mixing) in the atmosphE~re.
In fact, in the atmosphere free convection serves the same purpose as mechanical
turbulence in the wind tunnel. This is conststent with Orgill's field case study
which best agreed with the wind tunnel experiments (Grant et al., 1971).
3. High concentrations of ice nuclei have been observed in-the target area
on top of Chalk Mountain following seeding in the upwind valley (Grant and Mielke,
1967; Reinking and Grant, 1968). These data say nothing about the vertical
distribution of ice nuclei in the cloud nor why" or how consistently the nuclei
arrived at the mountain. It is conceivable that they came up the mountain in the
shallow planetary boundary layer (PBL) or were delivered by free convection processes.
4. The most impressive set of data from Climax and the Colorado San Juan
Project* which clearly indicates that the nuclei often got to supercooled water is
the impecable statistical analysis which has been published (Grant and Mielke,
1967; Chappell, 1971, etc.). The disconcerting aspect of these analyses is that
the stratifications based on 700 mb wind direction and 700 mb ee also repeatedly
gave consistent and highly significant statistical results. Double and triple .
stratifications of this large data sample would surely help clarify these impres-
sive but somewhat confusing results.
Sufficient residence time is definitely a limiting problem in the case of
high wind speeds over a small mountain. Elk Mountain is a prime example. Over
Elk Mountain it is a rather simple procedure to increase the ice crystal concentra-
tion to over 1000/t without appreciably depleting the liquid water, nor causing
any additional snowfall. The reason is simply that there is insufficient resi-
dence time in order for the various cloud physics processes to operate. Such
things as contact nucleation, ice crystal growth and fallout all require more time
than is typically available over Elk Mountain. Elliot et al. (1974) have presented
a preliminary analysis of the first three years from the San Juan Project which
indicates that very strong wind speeds at 700 mb are associated with a decrease
in precipitation.

III. Available Case Study Data

The 1973 flight with the NCAR Queen Air over the San Juan Mountains was
published by Marwitz (1974). These data quite vividly pointed out the importance
of convective clouds. The upwind sounding indicated a conditional instability of
1 to 2C of positive buoyancy. In 1974, a more concerted effort was made to

*From 1964 to 1969 Colorado State University conducted a randomized cloud
seeding project over the San Juan Mountains for the State of Colorado. The results
were reported by Grant et al. (1971), and were similar to the Climax Project.

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