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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into the broad Ellensburg valley. To the north of this valley is a region of

high mountains which is the downwind region for a southwesterly wind.

Fig. 3,5 shows six sample trajectories, all starting 50 km upwind of the

target area. Blocking has been simulated in this case so that the mountain

crest does not appear to be as high as it is in Fig. 3.6 where blockin~ is

ignored, It should be noted that when blocking is simulated the trajectories

predicted by the model terminate at the modified profile of the mountains. The

trajectories of the particles below this level, which are not shown in Fig~ 3,5

(or Fig. 3.7), will depend on the low level winds on the east side of the

mountains, The trajectories start from both 1.5 km and 2.0 km above sea level.

For each height three different concentrations of ice particles are assumed,
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l~ , 25~ , and lOO~ and the resulting trajectories calculated. Although the

variation in the natural concentrations of ice particles is great, our

observations indicate that a concentration of l~-l is typical in natural clouds

over the Cascade Mountains. In the case of the particles which orif.inate at

2,0 km, when the crystal concentration is increased the ice particles are

carried further downwind and instead of reaching the ground to the west of the

Cascade Divide they are carried over the crest and land east of the divide,

However, the precipitation rate at the ground due to particles falling along

this trajectory (which is shown in cm hr-l water equivalent in Fig. 3.5)

decreases as the concentration of ice particles increases.

If we consider the ice particles which originate at an altitude of 1.5 km

it is seen from Fig. 3.5 that an increase in the number concentration of ice

particles from 1 to 25~-1 has no significant effect on the trajectory of the

particles. It does, however increase the precipitation rate. An increase in

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