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

conditions of very high wind speeds where many
unrimed crystals might be carried over the mountain
barrier without deposition.

Another case where the accretion
process might be utilized is for a very warm cloud
system (-5C to -10C cloud top) where the optimum.
number of ice crystals required would be so large,
and present generators so inefficient, that the
economic cost of the operation becomes dispropor-
tionately large.

In the majority of cases it appears
wise to convert all the cloud water through the dif-
fusional process whenever possible. Details of the
accretional process are not well understood at this
time, and the diffusion process is to be preferred
whenever applicable.

h. The ballistics problem
The deposition of artificially produced
ice crystals onto the mountain barrier depends upon
the integrated time required for several complex
processes to transpire. Initially, there is the trans-
port time for artificial ice nuclei to travel from the
generator site to the point of ,activation in the cloud
system. There is the additional residence time that
the ice crystal grows in the cloud system and lastly,
the final settling time of the crystal from cloud base
to the mountain.

The transport time for artificial ice
nuclei to travel from the generator site to the point of
activation in the cloud system is extremely difficult to
determine. The problem is, of course, somewhat
alleviated by airborne seeding. The vertical spread
of the seeding material may be due to diffusion and
convective transport aided by the dynamic and oro-
graphic components of the vertical motion field. Thus,
stability and vertical wind shear considerations over
the site play an important role.

The Pas quill-Gifford diffusion equation
has been found to apply reasonably well in plume
tests conducted in the Park Range (Bollay Assoc., 1968) for
near ncutro.l stability conditions, It is expp.dp.d that
solutions of the diffusion equations, plume studies
from the given areas, and some wind tunnel test
results will supply the information needed to evaluate
the dispersal of seeding material to the cloud system
and the transport time from generator site to
activation.

The residence time of the ice crystal
in the cloud system is a functiCXl of its activation level,
growth rate and settling speed. The growth rate of
ice crystals as a function of temperature is still not:'';.,
well established. Two of the more recent and com- ;
prehensive studies (Todd, 1964,and Fukuta and Wang;
1968) iPustrate the need for further investigation
into the complex subject.

Todd gathered information from tables,
scatter diagrams, and micrographs and extracted
crystal growth information. Ice crystal dimensions
were then related to time of growth and it was found
that the growth of the crystal axes (a and c) could be

fitted to equations of the type:

a = k t (). and c = k t S where (). and S
are discret~ functions of~emperature, and k and k
are continuous functions of temperature. Fi~re 6 c
shows the crystal mass 60 seconds after nucleation
according to Todd. Also in Figure 6 is shown the
, crystal mass after 50 seconds according to the
observations of Fukuta and Wang, and that computed
from the diffusional growth equation. The ventilation
and vapor factors are considered to equal one in the
growth equation. It is seen that observed crystal
growth rates are not well explained by the diffusional
growth equation, especially those Of Todd. All three
curves, however, point to a maxima in the rate of
growth in the temperature range from -15C to -18C.

_ . ht
\

20r

CR"fSTAL MASS AFTER 60
SEes. (TOllD,1964)

15

Co
..::I..
'Q

'en 10
en
c:{
'"
...J
f5f
en
)0-
cr
u

5

co

Figure 6. --Mass of a crystal after 50 to 60 seconds
of growth as a function of environmental temperature.

Crystals growing and settTing with
respect to a saturated orographic stream will
experience a varying temperature environment.
Thus, the growth rate must be integrated with
, respect to temperature which further complicates
its determination.

The settling speed of ice crystals is a
function of their habit, amount of riming, and in some
cases a function of their size. Figure 7 shows the
observed terminal velocities of various crystal habits
as determined by several investigators. Clearly,
the agreement between investigators is not good and
more experim,ental work is needed for better defini-
tion.

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