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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34

Hmvever, aggregates generally have a terminal velocity similal- to that of

their unrimed parts (Nagono and Nakamura, 1965), so it is unlikely that

aggregation plays a significant role in the formation of precipitation in

these storms.

We conclude that diffusional growth is the primary precipitation

mechanism in stable or neutral storm periods, and that both

diffusional grmvth and accretional growth are important during unstable

periods.

d. An effect of the upwind topography
Frequently the cloud first formed approximately 60 km upwind of the

mountains, in response to the rise in the terrain dovn.1,Jind from Navaj 0

Reservoir. Crystals that formed near the upwind edge of the cloud at high

altitudes and cold temperatures had sufficient time to grow and fall to low

altitudes ahead of the mountain range. They provided high ice crystal con-

centrations in competition with the seeding material, and affected the

seedability of the storms. The raIl of these crystals into the seedable

region of the clouds depends on the formation of cloud ,Jell upwind of the

target area, and would not occur if there were no rise in the terrain up-

wind of the San Juan Mountains.

e. Cirrus seeding

No examples were found in Hhich crystals from a separate cirrus layer

fell into these storms. On one occasion, crystals Here observed to fall

several hundred meters from a cirrus layer into a dry layer before they

evaporated (V.C.2.d). llowever, the concentrations even in this case were

bnly about lO/liter, and the crystal.s evaporated long before reaching the

lower cloud layer.