WMOD00277

• Contract/Permit #: 14-06-D-6467
• Title: An Operational Adaptation Program for the Colorado River Basin
• Prepared By: Lewis O. Grant, Chappell, Crow, Mielke Jr., Rasmussen, Shobe, Stockwell, Wykstra
• Date: 10/1/1969
• State: CO
• Country: United States
• Weather Modification - Doc Type: Report

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500 MB TEMPERATURE ("C)

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Figure 1. --Relationship of ice crystal concentration,
ice crystal size, cloud top temperature, and vertical
motion which optimizes the efficiency of cloud water
utilization.

ana the concentration is expressed in particles per
liter. This relation is shown in Figure 2. It is
emphasized that this is an average spectrum of ice
nuclei and actual counts may differ an order of mag-
nitude or more at a given time or place.

Some observations have lndicated the
existence of a "plateau, " or leveling off of the ice
nuclei counts in the temperature range from -20C to
-30C followed by sharp increases at temperatures
below - 30C. This effect has most recently been
found by Veal, et al. (1969) for cap clouds in southern
Wyoming. Since average ice nuclei counts taken at
Climax for temperatures near -20C are in general
agreement the average exponential activity spectrum
will be invoked as representative of conditions in the
Colorado Rockies.

e. Ice crystal imbalance in precipitating cold
orographic clouds

One further bit of information is
needed before estimates can be made of the number of
-artificial ice nuclei required for maximum cloud water
utilization. A vital point to be clarified is the number
of ice crystals that will be generated for each iCE!
nuclei activated within the cloud system. The ratio
of these concentrations is a critical control on
whether a potential for modification exists with the
cloud system. If this ratio of ice crystal

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-AVERAGE EXPONENTIAL Nm_165.-.6T
- -VEAL. .t 01. 1I969l

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TEMPERATURE (oG)

Figure 2. --Concentration of effective ice nuclei as a
function of temperature.

concentration to ice nuclei concentration (R) is
multiplied by the effective ice nuclei concentration
given in (11), and the optimum ice crystal concentra-
tions expressed by (10) are subtracted, an
expression for ice nuclei excess or deficit may be
obtained. This relation is given by (12)

tlN=R(10-5)e-' 6T_7. 7(10-1l)[w/r][(q -q )]/F(T)]
s700 s500
For a given upward speed, crystal
size and ice crystal to ice nuclei ratio, the excess or
deficit of ice nuclei may be depicted as a function of
the 500 mb temperature. Cur ves are shown in
Figure 3 for a one to one correspondence of ice
crystal concentration to ice nuclei concentration, and
for mean upward speeds and crystal sizes relevant
to the Climax and Wolf Creek Pass areas.

It is seen from Figure 3 that the
maximum utilization of cloud water for a mean upward
speed of 50 cps and crystal radii from 300 microns
to 500 microns (estimated to be representative of tre
Wolf Creek Pass area) occurs at 500 mb temperatures
from -23C through -25C. These temperature values
are defined in Figure 3 where the relevant curves
intersect the zero axis (actually the O. 1 excess or
deficit axes since a log scale is utilized). For an
upward speed of 15 cps and crystal radii of 300
microns to 500 microns (a size range frequently observed
at C 1 i m a x ) the natural precipitation process
attains an optimum mode at 500 mb temperatures
from -21C through -23C, The increase in nlean

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