WMOD00487

• Title: A Microphysics and Synoptic Study of the 14-15 December 1977 Sierra Storm
• Date: 6/19/1978
• Weather Modification - Doc Type: Report

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Figure 12. Rimed columns on 15 December.

6. SEEDING eONSIDERATIONS

The previous discussions of the properties of
this storm lead to several possible alternatives
for precipitation enhancement. This storm
would require the use of both ground based and
airborne seeding systems. The analysis indicated
that excessive amounts of liquid water were
indeed present during the storm. This was
especially true near the band passage early in
the storm and during the strong orographic and
post frontal convection phases. Initially,
these areas offer immediate potential for
enhancement. The use of aircraft would have
been required to accurately target seeding
material during the band and post frontal
convection because of the presence of a low
level stable layer. The presence of the low
level jet near bands and front do provide foy' a
transport mechanism but its mere presence and
strength make the targeting of material very
difficult. Ground seeding systems could be
considered close to bands or front if accurate
timing of the features and associated jets
could be established and if no stable layers
were present. The orographic component of the
low level jet and the presence of low level
moisture permited the very large amounts of
precipitation that fell during the period from
0300 to 1100 GMT, 15 December. In this case, a
very efficient process was underway and it is
unc 1 ea r whether seed i ng mi ght ha ve added mOrE!
precipitation. If it can be shown that this
strong orographic situation could produce
positive seeding effects, then ground based
generators located above valley inversions
could be used. The placement of these genera-
tors must not be so close to the barrier crest
as to prevent sufficient time for transport a.nd
nucleation of seeding material and the fallout
of precipitation. The enhancement of the post
frontal convection seems possible because of
the excessive liquid water contents present.
The treatment of these showers almost certainly
would be by aircraft because of their spacin9
on the mountain as well as the fact that they
are fairly weak and it is likely that material

released on the ground would not be drawn up
into the cloud.

7. SUMMARY

The Sierra storm of 14-15 December exhibited
several unique features during its passage
across the SCPP project area. A low pressure
center located at fairly high latitudes produced
a short-wave trough at 50 kPa and below which
moved towards the west coast of the United
States. Support from a 150 kt jet at 30 kPa
was given to the short wave as it moved onshore.
Rain began in the project area between 1800 and
2200 GMT, 14 December, as the main precipitation
area shifted southeastward and a pre-frontal
band past just north of the project area. The
storm: was fairly warm with snow levels higher
than 2100 m during the first several hours.
Winds at 50 and 70 kPa increased as the storm
moved closer and low level jet developed in
advance of the pre-frontal band as well as the
main precipitation event. Ground observations
indicated the storm was initially convective
but rapidly became stable with a strong oro-
graphic component. Excesses in liquid water
were evident in the riming of crystals as well
as moisture levels shown in time cross sections.
Fifty to 75 percent of all precipitation from
this storm occurred during the period between
0300 and 1100 GMT, 15 December. A strong low
level jet developed with a strong orographic
component. Steady precipitation rates up to
1,27 cmlhr were common during this period. As
large amounts of low level moisture were fed
into the system these high rates of precipitation
occurred when cloud top temperatures were very
cold, reaching below -400C during this period
of the storm. Cold frontal passage occurred
between about 1200 to 1400 GMT, 15 December in
the project area. Temperatures fell at the 50
and 70 kPa levels several hours in advance of
the front and for some time after frontal
passage. The cirrus deck associated with this
system diminished with the frontal passage.
Ground observations showed that instability
behind the front developed quickly with the
mountain stations reporting convective shower
activity through most of the remainder of 15
December. Liquid water contents continued at
high levels as seen from crystal photographs
and aircraft observations. Trough passage
occurred at about 2100 GMT, 15 December and all
precipitation had stopped in the project area
by that time. This storm had the potential for
seeding during the band passage, strong oro-
graphic and post frontal convection phases.
Seeding from both ground and airborne platforms
would have been required to accurately target
material.

8. REFERENCES

Hobbs, P. V. et al., 1976. Dynamical and
Microphysical structures of Cyclonic
Storms in the Pacific Northwest. Research
Report XI. University of Washington,
Seattle, Washington.

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