WMOD00400

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

35

In most instances, the clouds during the unstable storm stage were capped

by a subsidence inversion, and the very dry layer above the clouds assured

that no crystals would be present. . 29 December 1974, flight 1 is an example.

On that occasion, we found no crystals in the dry air above cloud although we

flew a long flight segment to look for them.

Crystals frequently fell from high parts of a cloud layer into lower

parts of the same layer, such as discussed in (d). This is apparently a

common source for high crystal concentrations at 1varm temperatures in these

clouds.

f. Ice crystal origins

Because conventional (and our own) nucleus measurements indicate nucleus

concentrations far below the observed ice crystal concentrations, the origin

of these high crystal concentrations is important. Clues regarding the origin

of the ice were obtained from many of the flights, and are described in the

case studies. They seem to point to an origin for the ice that is associated

with the initial formation of the water cloud. Some of the more i~mportant

observations are the following:

(1) Formation of ice at the upwind cloud edge. In cases of smooth flow

into a stratiform cloud edge, high ice crystal concentrations developed quite

early in the cloud. An example is disucssed in section V.A.3.d. In that case,

the ice crystal concentration rose rapidly to SO/liter suggesting, that the ice

formed in association \vith the initial condensation process.

(2) Temperature dependence. Fig. 3.4b indicates a good correlation

between temperature and ice crystal concentration for regions where the ice

origin could be determined. (Generally, such regions are limited to regions

near cloud edge or cloud base.) However, penetrations high in convective

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