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

-14-\8 -22 -26 -30
500 Me TEMPERATURE l"C)

Figure 39. --Solid Lines: Mean Daily Snowfall at Wolf
Creek Pass as a function of cloud top temperatures
(500 mb). Dashed Lines: Adiabatic condensate
realized from lifting a parcel upward through a satu-
rated 700-500 mb layer.
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(a)

(b)

[POTENTIAL alNDENSATE
_ MEAN DAILY SNOWFALL
---

( b)

r------,
1 I
I ... PERCHITAGE OF TOTAL
I I SNOWFAll
I 1

FREQUENCY CF OCCURRENCE

,-----
1

o -10 -14 -18 -22 -26 -38
500 MB TEMPERATURE lOCI
Figure 40. --(a) Mean Daily Snowfall at Wolf Creek
Pass as a function of cloud top temperature (500 mb).
(b} Distributions of total snowfall and total occur-
rences as a function of cloud top temperature (500 mb).

rather steadily to the limit of the data around
_340 C. It is interesting that if this trend is
extrapolated the mean daily snowfall reaches
zero at about _370 C. This is thetemperai:ure where
observations by Smith and Heffernan (1954)
indicate concentrations of effective natural ice
nuclei are approximately equal to cloud droplet
concentrations. As a consequence, immediate
icing of the cloud would result in cessation of

I
i

the precipitation process. :

The decrease in mJan snowfall as
cloud top temperatures beconie colder than -230 C
is largely explained by the detrease of potential
condensate in the cloud syste~. This is illus-
trated in Figure 39 by the das;hed lines which
reflect the amount of adiabati~ condensate pro-
duced by a parcel moving upV;ard through a 700-
500 mb saturated layer as a function of the 500
mb temperature. I

The peak in mean cj.aily snowfall at
-230 C appears to reflect a c~oud top temperature
mode where the condensation, supply and available
effective ice nuclei maximiz~ the precipitation
process. !

-38

The decrease of mean daily snowfall
as cloud top temperatures become warmer is
quite intriguing. The mean cj.aily snowfall
decreases steadily from _230, C to a minimum
about _180 C. This occurs irt spite of an increase
in potential condensate for these cloud systems.
This decrease cannot be explained qy a lessening
of any orographic influence siince an increase in
southwesterly flow events and increasing wind
speeds occur within this temperature range. It
appears the natural precipita'tion process is
becoming increasingly ineffi~ient as temperatures
decrease from _230 C to -18~ C probably due to
the accompanying exponentiail. decrease of
available effective ice nuclei'.

The marked incre~se of mean daily
snowfall at Wolf Creek Pass for 500 mb tempera-
tures around _140 C to _~50 C is striking.
However, since this occurs p.ear the tail of the
distribution, sample sizes are relatively small.
Possible explanations for th~s peak do exist.
One possibility is that this is the range of
temperature where dendritic crystal habits might
be expected to form in the upper portion of the
~ cloud system. This could result in fracturing
of the dendritic crystals and: an ice crystal
multiplication process in th~ cloud system. This
should cause an increase in ithe natural efficiency
of cloud water removal and therefore in observed
precipitation. Whatever the mechanism for this
increase in natural snowfall! efficiency at Wolf
Creek Pass, it apparently occurs less frequently
at Climax. Figure 37 indicates only a minor
peak in mean daily snowfall at Climax at these
temperatures.

It is interesting 1tnt the mean daily
snowfall appears to decrease again at the warmest
cloud top temperatures contained in the data
(_110 C to _130 C). However, with the limited
sample size::; involved this rs only conjecturc.

Figure 40 shows ];1ow the total snowfall
and the total occurrences are distribvted with
respect to the 500 mb temperature. It is seen
that 46% of the total snowfall and 37% of the
total occurrences are contained in the class
interval from -20.50 C to -24.50 C. The 9%
excess of tre relative snowfall percentage over
the relative frequency in this class reflects the

54