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

2

project was the clearing of supercooled stratus by means of

seeding from an airplane with dry ice or siver iodide. The

history of early weather modification projects is described by

Byers (1974).

Over the years, a need existed for increasing precipitation in

the American River Basin of the Sierra Nevada in California. The

Sierra Cooperative Pilot Project (SCPP) was initiated in the early

seventies to investigate the potential for precipitation

augmentation by seeding agents such as dry ice and silver iodide.

The SCPP is a part of the winter cloud seeding research program

conducted by the Bureau of Reclamation (Burec), u.S. Department of

Interior. One objective in SCPP is to better understand

precipi tation development by studying the physical processes

involved, and thus to identify the important cohdi tions that

prov ide good potential to improve water resources in the Sierra

Nevada.

During the winter months of 1979 through 1986 (except the 1981

season), convective and stratiform-orographic clouds have been

treated with dry ice or silver iodide, and the evolutlon of ice in

the treated clouds was documented with aircraft observations.

To better understand the particle growth process, calculations

were done for this thesis using two models: First, the "Particle

Growth Model" (PGM) of Heymsfield (1982) embedded in a kinematic

flow field representative of the Sierra barrier was used, which

considers crystal growth by diffusion, accretion and the

subsequent fall trajectories. Second, the "Aggr~ation Model"

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