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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and on into the post frontal airmass. A jet
maximum occurs near 80 kPa in excess of 15 m/:;
and a direction of about 2380 on the 0600 GMT,
15 December sounding. This jet occurred about
6 to 8 hours before the frontal passage i n thl~
area. This jet presented an enhancement to the
orographic effect because of its perpendicular
component to the barrier. The horizontal
placement of the jet was over the foothills of
the Sierra Nevada as seen from the University
of Wyoming aircraft data. On the 1800 GMT, l!;
December sounding, the wind direction shows a
discontinuity centered near 70 kPa and an
associated jet in the same region. Since the
frontal passage occurred approximately at 1200
GMT at the rawinsonde launch point this probably
represents the cold frontal surface in that
region as it reaches up to the vicinity of thE!
70 kPa jet maximum.

Subsequent to 1800 GMT, soundings no longer
showed a low level jet. The wind speeds con-
tinued to increase with height and the main
upper level jet stream maximum seen in Figure 8
appeared as the dominant feature in the last
sounding. A rather sharp backing in the winds
was evident between the 2100 GMT, 15 December
and the 0000 GMT, 16 December soundi ngs in thE!
low levels. This is further indication that
the cold air intrusion was continuing into thEl
area. By this time, most precipitation had
ceased and clearing skies were the rule for alll
locations except higher mountainous areas whel'e
post frontal convective shower activity continued
for several more hours.

Several other cross sections were analyzed, and
some interesting features are worth mentionin~1
at this time. The temperature cross section of
thi s storm showed a marked tendency for upper.,
1 eve 1 temperature fa 11 s 9 to 12 hours in advance
of the frontal passage at surface stations.
The 50 kPa level between 0300 GMT, on 15 December
and 1200 GMT, 15 December experienced 50C
cooling from -150 to -200C. The OOC temperature
height dropped from about 71 kPa to 80 kPa
during that same 9 hour period. The effect of
the band passage on 14 December on temperature!
was not nearly so evident but falls of 10-20e
did occur and could be attributed to the conVE!C-
tive band passage. The wet bulb potential
temperature analyses indicated moderate insta..
bilities associated with the band and high
instabilities associated with the front. The
stable period of the storm is reflected in the:
low potential temperature gradients between
2100 GMT, 14 December and 0900 GMT, 15 Decembe:r.

The presence of moisture in this or any storm
can be represented in several ways. Analysis
of the relative humidity time cross section of
this storm, figure 11, showed several characte:r-
istics which could be common to many Sierra
storms. First, in advance of the actual preci-
pitation on the ground, a layering of moisture
is seen on the soundings. Two, three or even
more moisture layers are not uncommon in pre-
storm as well as in storm soundings. This
feature is visibly evident in the multiple
cloud layers that are often seen in storms.
Photographs taken by the University of Wyoming

verify this multi-layering of clouds on 14
December. The cross section shows another
feature of interest. In general, between the
main convective band on 14 December and the
cold frontal passage on 15 December, the moisture
remained reasonably uniform and high. Relative
humidities in excess of 70 percent were seen as
high as 44 kPa with increasing amounts toward
the surface. It appeared that the passage of
the band caused some lowering of moisture
content for about nine hours after passage.
The frontal passage was preceded by dropping
moisture levels until 1200 GMT, 15 December,
where 70 percent relative humidities existed
only below 72 kPa. Cloud top temperatures
observed from the GOES-WEST satellite were very
cold during the period from 0300 to 1100 GMT,
15 December. Temperatures below -400e were
common until just before frontal passage and
were associated with a high cirrus deck. Cloud
top temperatures warmed to near -200e as the
front moved through the area and the cirrus
deck dissipated. The cold frontal passage is
marked by an increase in low level moisture
centered around 80 kPa to near saturation.
This is understandable because of the polar
maritime nature of the airmass.

A look at the wet bulb potential temperature
cross section shows instability just prior to
both the band and the front. The atmosphere
was still stable in the sounding preceding the
band. The jet core was located above the
maximum region of instability near the band.
However, as the band passed over the area, the
region of instability and jet maximum occurred
in the same general region. The instability
ahead of the front, is indicated to be about in
the same location as the jet maximum. There
was little vertical displacement.

It does appear that low level jet cores act as
a transport mechanism for low level moisture
(Hobbs, 1976). Further case study analyses
will be necessary to ascertain whether this is
coincidence or is a feature of many storms in
this area.

4. MICROPHYSICS AND PRECIPITATION ANALYSIS

The signature of a storm as determined by
precipitation on the ground as measured in the
gage and the analysis of snow crystals on the
ground can reveal properties of the storm that
are important.

Precipitation received on the ground was reported
by about 40 recording tipping bucket gages in
and near the project area. The analysis of the
gage data available yielded some clues as to
the effects of Sierra Nevada topography on
precipitation amounts. It was noted that the
lowest precipitation total for the storm occurred
at Auburn, ealifornia (350 m) and the highest
total was at Blue Canyon, California (1600
meters). Precipitation increased gradually in
a west-to-east direction across the barrier
from Auburn (4.67 cm) to Baxter (7.1 cm).
However, from Baxter to Blue Canyon the precipi-
tation amount rose from 7.11 cm to 11.77 cm in
about 400 m of elevation increase. The distance

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