WMOD00401

• Contract/Permit #: 2-07-81-V0256
• Title: Precipitation Development in Seeded Clouds Over the Sierra Nevada
• Date: 8/1/1986
• Weather Modification - Doc Type: Report

.

.

Prasad, N.

Precipitation lDeve10pment in Seeded Clouds over
the Sierra Nevada
M.S., Department of Atmospheric Science,
August, 1986.

-

.

The Sierra Cooperative pilot Project is a weather modification

experiment in the Sierra Nevada of Central California. During the

winter months 1977-1986 (except 1981) orographica11y-induced

.

convective clouds and stratifonn clouds with isolated and anbedded

convection, respectively, were seeded with dry ice and silver

iodide. Their response to seeding was investigated.

.e

The particle growth model of Heymsfie1d (1982) anbedded in a

kinematic flow field repl:esentative of the Sierra barrier was used

to study the ice particle growth by diffusion, accretion and

.

subsequent fall trajecJcories.

The particles observed by the

aircraft were classified into habits.

The growth of observed

particles were compared with the model predicted evolution. Using
"-~

.e

an aggregation model of Heymsfie1d (1986), the observation of

formation of aggregates in < 10 minutes was verified.

The key

findings of this research were:

(1) Diffusional growth is

i.

extremely important for temperatures near -lSoC in these low

liquid water content clouds.

The particles grow to "'2 mn when

e

released from just colder than -lSoC, and to "'1 TIm when falling

from warmer than -lSoC. (2) Riming is important close to the

barrier in a stratiform cloud when large cloud droplets and liguid

~ater up to 0.3 g m-3 cl~e present. (3) Aggregates (> 1 rrm) form in
4-8 minutes after seeding a convective cloud.

.

.

.
I