WMOD00400

• Contract/Permit #: 14-06-D-6801
• Title: Structure and Seedability of San Juan Storms
• Date: 11/1/1976
• Weather Modification - Doc Type: Report

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too many unaffected cases.

There is no direct conflict between these results and the earlier

successful experiments at Climax (Grant et al. 1967; Chappell, 19.71)

and in the San Juans (Chappell, 1971), be~ause in both of these

experiments all storm stages were seeded. It is curious that a positive

correlation between warm 500 mb temperature and positive seeding effects

occurred in the Climax experiment and was not duplicated in this experiment.

\~e can only state that, whatever was the physical significance of the 500 mb

temperature in the previous experiments, it was not a useful predictor of

the cloud top temperature or ice crystal concentration in the San

Juans.

3. Recommendation for post-hoc analysis

Based on our results, a number of restratifications of the statistical

experiment based on physical arguments seem appropriate. Perhaps the simplest

criterion for seeding to be effective appears to be the presence of a conver-

gence zone ahead of the mountain, and the data needed to detect this conver-

gence zone are available from the E.G. & G. surface wind network. The presence

of a convergence zone means that AgI can be transported to cloud level, and

that significant vertical displacement of the flow will occur with associated

development of a liquid water cloud. This convergence zone should be present

on about half of the experimental days, and its absence almost certainly

means that no strong seeding effect is likely. Use of this criterion should

e~clude many extraneous cases from the statistical experiment.

This is a simple way of excluding the stable cases from consideration,

but it would also be possible to accomplish the same goal through other .

criteria, for example by using the presence of a blocked flow pattern or

by reference to the Durango sounding.

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