WMOD00487

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

between the two stations is about 10 km. This
marked increase in precipitation may be somewhat
exaggerated due to the Blue Canyon gage location,
but the orographic component in this storm was
very pronounced and can be seen in the other
gage data. It should be noted that Castle
Valley (2400 m) also received over 10.16 cm of
rain during this storm. Gages located in the
southern portions of the project area received
somewhat lesser amounts of precipitation but
nevertheless, amounts in excess of 7.6 cm were
not uncommon. If one looks at a north-south
transect near the Sierra Nevada crest, it
appears that the northern portion of the project
area is favored for total precipitation amount.
This conclusion is of course, based on only a
few precipitation measurements.

Precipitation generally began at all stations
between 1800 GMT and 2200 GMT 14 December. It
remained steady though increasing steadily in
intensity throughout the remainder of 14 December.
Ground microphysics measurements commenced at
2030 GMT on 14 December and lasted until about
0100 GMT, 15 December. Temperatures were warm
and rain fell at all elevations to 7200 feet.
The rain changed to snow above 6800 feet at
2050 GMT and temperatures remained near freezing
throughout the afternoon. The first observations
of crystals included large flakes consisting of
some dendrites and columns. Heavy riming and
some melting was evident as warmer air near the
surface was encountered. Some plate growth was
evident on the falling columns. As the after-
noon progressed, crystals that fell were moder-
ately rimed. Columns and dendrites were the
predominate crystal types. There was continuing
evidence of a region of the atmosphere where
plate growth was occurring. The percentage of
dendrites increased later in the day. An
excess of liquid water was apparent both by
riming of crystals at the surface and in Univer-
sity of Wyoming aircraft observations. Univer-
sity of Wyoming observations (Marwitz, 1978)
also showed the habit of the crystals at 3450 m
to be mostly dendritic. This indicates that
convective atmospheric instability associated
with the band passage earlier in the day was
diminished and a more stable atmosphere allowing
for long crystal growth times was present.

Gage observations of precipitation also indicated
stable orographic conditions were present and
precipitation intensity was enhanced from about
0300 GMT to 1100 GMT, 15 December. During that
period maximum precipitation rates for this
storm occurred on almost all the gages. It was
not uncommon to see precipitation rates in
excess of 1.27 cm per hour in the gage records.
This event corresponds very well with maximum
low level wind speeds (see Figure 9) which
enhanced the orographic input to precipitation
because of the large component normal to the
barrier and the maximum moisture present as
seen from the relative humidity cross section
(Figure 10). These very efficient conditions
decreased in presence very close to the frontal
passage. A conclusion from this series of
events is that the approach of the cold front
diminished the orographic nature of the 14-15
December storm. This combination of maximum

r

winds normal to the barrier and high moisture
resulted in from 50 percent to as high as 75
percent of the total precipitation received at
individual gages during the 14-15 December
storm.

Ground microphysics observations were again
taken on 15 December after the frontal passage
in the mountain locations at and above the
freezing level. The morning was characterized
by fair amounts of graupel averageing 1.5 to 2
mm across. Some were round and some were cone-
shaped. The precipitation was very showery
during the period before noon. The gage records
also indicated a marked change to shower activity
after 1500 GMT in the higher mountain locations.
All precipitation had stopped at gages below
1300 m by 1500 GMT. Columns, a few capped
columns and dendrites became more common as the
sampling elevation was increased. Sampling at
2160 m occurred in heavy snow showers. At that
time there was a complete absence of graupel.
eloud top temperatures from aircraft and satel-
lite observations were measured between -150C
and -200C. It was apparent that the convection
was being suppressed by early afternoon.
However, riming was evident on the columns and
other crystals. Figure 12 is an example of the
rimed columns seen during this storm. Few
pristine columns and needles were seen and they
only occurred in the lull between snow showers.
The precipitation continued in the form of
showers for the remainder of the afternoon.
The cessation of precipitation occurred around
dark with the passage of the upper level trough.

5. OTHER DATA

In addition to the data sources utilized for
information on this storm, radar observations
were taken during the storm from the National
Weather Service WSR-57 radar at Sacramento,
California and the USBR 5 cm SWR-75 digital
radar located at Sheridan, California. Although
detailed analysis of the radar data has yet to
be completed, several statements about the
storm can be made. Radar echo motion observed
during the period from 0300 GMT to 1100 GMT 15
December was 200 to 300 to the right of the 70
kPa mean wind direction. The speed of the
observed echos increased markedly after 0200
GMT 15 December to greater than 30 m/s during
the strongest part of the orographic precipitation.
Echo speedS were very close to the mean 70 kPa
wind speeds obtained from sounding data. A
marked decrease in echo speed, intensity and
coverage occurred after 1200 GMT 15 December.
This corresponds to the frontal passage through
the area as is verified by satellite observations.
The maximum dB~ values recorded by the radars
occurred when the heaviest precipitation was
falling into the gages. Other information
obtained from radar observations are discussed
in Sutherland (1978). This storm was observed
from the GOES-WEST satellite and data was
recorded and analyzed by Colorado State University.
Analysis of that information as to cloud-top
storm structure agreed quite well with other
data and detailed discussion of the satellite
study is reported in Reynolds (1978).

157