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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- 85 -

number concentration of ice particles to 100R,-1 lengthens the trajectory~ so

that the particles are carried over the Cascade crest, and decreases the

precipitation rate.
The above results can be readily understood as follows. In the case of
the particles which originate at an altitude of 1. 5 km~ when the concentration
of ice particles is 1!l, -1 the cloud does not convert the available water vapor
into precipitation in the most efficient manner. An increase in the ice
particles to 25t-l does not cause a reduction in the growth rate of an
individual ice particle, therefore, all 25 ice particles per liter grow at a
similar rate to the original one crystal per liter. Consequently, the
trajectory does not change but the precipitation rate goes up by a factor of

25. (This result is~ of course, somewhat unrealistic since the model does

not take into aCCo\U1t the effect which the growing ice particles will have on
the liquid water content of the cloud if the concentration of ice particles
is less than the critical concentration. This omission becomes increasingly

serious as the ice particle concentration approaches the critical

concentration.) When the ice particle concentration is increased to 100R,-l

the particles begin to compete for the available moisture. Roughly.the same

total mass of moisture is available as in the case of 25,t-l but it is now

divided between 100 particles per liter. The growth rates and fall speeds of

the ice particles are therefore reduced so that the precipitation rate

decreases and their trajectories lengthen.

Figure 3.6 shows a set of trajectories corresponding to the upper set

of trajectories in Fig. 3.5 but in this case the air has not been blocked.