WMOD00310 ‎(2)‎

• Project Name: Sierra Cooperative Pilot Project
• Title: The Characteristics of Evolution of Supercooled Water in Wintertime Storms Over the Sierra Nevada
• Date: 9/9/1988
• Weather Modification - Doc Type: Report

1012

VOLUME 27

JOURNAL OF APPLIED METEOROLOGY

100

The depth of the water-saturated layers is shown in
Fig. 20. The curve denotes a cumulative percentage in
this figure. The data indicate that approximately 35%
of the saturated layers were less than 0.2 km deep.
About 90% of the time, water-saturated layers were
less than 1.5 km in depth. The depth of the largest
saturated layer was 2.4 to 2.6 km.
Figure 21 shows the altitude of the lowest detectable
saturated layer. The first saturated layer was detected
below 3.2 km, 90% of the time. The layers detected
below this height may well extend above 3.2 km (see
Fig. 19). The distribution of the first detectable satu-
rated layers suggests the importance of orographic lift
in initiating water-saturated levels.
Figure 22 shows the distribution of the temperatures
of the saturated layers of cloud along with the pecentage
of soundings that indicated saturation at given tem-
perature ranges. Approximately 40% of the soundings
indicated saturation at -go to -100C. Saturation con-
ditions were never encountered at temperatures colder
than -240C.

6. Summary

This study has provided a comprehensive exami-
nation of the evolution and vertical distribution of su-
percooled water in Sierra Nevada winter storms from
the 1983/84 through the 1986/87 winter field seasons.
Measurements of supercooled water were made with
a dual-channel microwave radiometer located near the
Sierra Nevada crest line. Sixty-three storm systems oc-
curred during the study period. The analyses were car-
ried out in two parts. In the first part, winter storms
were grouped into two general categories based on the
prevailing flow and resultant storm trajectory. The two
groups consisted of storms with predominant zonal
flow and storms with predominant meridional flow.
In the second part, rawinsonde and radiometer data
were used to infer the vertical distribution of super-
cooled water over the radiometer. The following are
the conclusions developed from these two studies.

a. The relationship of liquid water to general storm
patterns

Table 2 summarizes the measurements of super-
cooled liquid water for all storm patterns that occurred
from 1984 through 1987 during SCPP field seasons.

1) ZONAL STORMS

Storms originating in zonal flow over the eastern
Pacific were either developing, occluding, or dissipating
during the time period when they affected the Sierra
Nevada. The stage of evolution of the storm was found
to dictate the characteristics of supercooled water ob-
served during the storms passage over the Sierra.
Developing storms were often observed to have three
distinct regions in which supercooled water was present
in clouds. The largest and most sustained regions of

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FIG. 22. As in Fig, 19 except for temperature.

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supercooled water were associated with the immediate
postfrontal region of the storm. The duration of su-
percooled water in this portion of the storm varied from
24 to 45 h with amounts ranging from 0.05 to 1.10
mm and peaks to 1.30 mm in embedded convection.
Supercooled water was also observed in the warm front
region (0.05-0.60 mm) and in isolated convective cells
near the end of the storm.
Occluded storms in the northeast Pacific typically
produced a cold front at the project latitude. In these
storms, the greatest supercooled water was measured
in the postfrontal period, immediately following the
passage of the cold front. Values from 0.05 to 0.60 mm
were observed for 11 to 32 h. Lower quantities of su-
percooled water (0.05-0.20 mm) were sometimes ob-
served in the prefrontal period and during the passage
of the cold front. Infrequent peaks of supercooled water
were also associated with isolated convection late in
the postfrontal period. These convective conditions
sometimes persisted for over 24 h.
Dissipating storms accounted for the remaining
storms within the zonal storm category. Supercooled
water was generally present in clouds throughout the
lifetime of the storm. The greatest values ranged from
0.05 to 0.60 mm and were associated with the pre-
frontal altostratus. Within the weak frontal band and
in the postfrontal environment, measurements ranged
from 0.10 to 0.40 mm. The continual presence of su-
percooled water along with low amounts of precipi-
tation in these cloud systems suggested conditions fa-
vorable to ice crystal growth.

2) MERIDIONAL STORMS

The presence of supercooled water in meridional
storms was influenced mainly by the trajectory of the
storm. The meridional category comprised two storm
types: cutoff circulations near 400N latitude and rapidly
digging systems in northerly flow.