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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lower than tlH~ typical values were frequently found in rising elements where

the cloud bases were considerably warmer than the temperature at the observa-

tion point.

Most of the crystals collected by the Wyoming Q/A were either unrimed or

lightly rimed" supporting the picture of these clouds as primarily ice clouds

without extensive liquid 'water regions. Exceptions occurred over the mountains

and just do\.mwind of identified liquid water regions, Hhere heavily rimed crystals

Here common. The crystals observed on the ground by Dr. Vardiman Here frequently

rimed, indicating passage through a liquid Hater region.

b. Trajectories

Fig. 3.5 shaHs some hypothetical crystal trajectories which are consis~

tent \vith our observations. The very stable storm periods (Fig. 3.5a) often

do not have a clearly defined upwind cloud edge, and there is no clear origin

for the ice crystals. There seems to be a relatively uniform concentration

throughout the cloud, with increased gro\vth near the mountain because of the

slight vertical deflection of the flow.

Huch of the liquid water region in the neutral case is difficult to seed

because it is so close to the mountain. The trajectories in Fig. 3.Sb illus-

trate this problem. However, the problem of fall against the updrafts is less

serious than in the unstable stage, because the updrafts generally are weaker.

The unstable stage (Fig. 3.5c) has the highest updrafts. It is likely

that most of the precipitation is formed by rather large ice crystals entering

the updraft region, where they can grow by riming and by diffusional growth to

sizes large enough to fall against the updraft and reach the target area.

Convection may playa role in this transport, by providing a dmVl1ward transport

mechanism in portions of the cloud.

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