WMOD00317

• Contract/Permit #: 14-06-D-6999
• Project Name: CAWRP
• Title: Cascades Atmosphere Water Resources Program - Volume I
• Prepared For: US Department of the Interior Bureau of Reclaimation
• Date: 7/1/1970
• Weather Modification - Doc Type: Report

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- ii -

and accretion, the trajectories of solid precipitation

particles can be predicted from an upwind sounding of wind,

temperature and humidity. The effects of changing the con-

centrations of ice particles on the trajectories of preci-

pitation particles and the distribution of precipitation on

the ground are considered.

The microstructure of the clouds and the types of cloud

and precipitation particles observed in aircraft flights

over the Cascade Mountains are summarized in Section 4. In

pre-frontal conditions the winds near the surface over the

western slopes of the Cascade Mountains are easterly and

produce drying conditions, but from about 6000 to 12000 feet

the air is moist and from the southwest. Over the eastern

slopes, the low level easterly winds produce orographic

clouds. Ice particles dominate over water droplets in the

pre-frontal clouds and above the -lOoC level riming is rare.

However, the crystals often become rimed in falling from

about 5,000 ft. to the ground. In post-frontal conditions

the air is more unstable, the winds are westerly at all

levels, and the cloud tops decrease in height. Riming is

common and graupel particles are observed. In the absence

of frontal activity, but in strong westerly airstreams, ex-

tensive orographic clouds form over the Cascade Mountains.

On average, the liquid water content in clouds over the

Cascades reaches a peak value about 13 nautical miles west

of the divide. Details on the types of ice particles