WMOD00580

• Project Name: Sierra Cooperative Pilot Project
• Title: Supercooled Liquid Water and Ice Crystal Distributions Within Sierra Nevada Winter Storms
• Date: 11/11/1983
• Weather Modification - Doc Type: Report

1884

100

JOURNAL OF CLIMATE AND APPLIED METEOROLOGY

VOLUME 22

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FIG. 11. (a) Distribution of the extent of regions of cloud where LWC > 0.2 g m-3 at temperatures from 0 to - lOoC shown for the
area-wide, banded and cellular echo combinations; (b) same as in (a) except temperatures less than - lOoC shown,

"

e. Stratification by 700 mb trough passage

This section examines the relationship between the
relative frequency of LWC/ICC ratios> 10 ""g per
crystal and the 700 mb trough environment. Both
Hobbs (l975a,b) and Lamb et al. (1976) have alluded
to a relationship between the occurrence of high LWC
and the postfrontal air mass. The results of this analysis
not only reveal this relationship, but also a certain
tim~ dependency. The 700 mb trough position was
obtained from time section analyses of rawinsonde
data.
Figure 12 shows the relative frequency that L WC
exceeded 10 ""g per crystal by observation temperature
in the post-trough environment. Maximum relative
frequencies of birge L WC/ICC ratios usually did not
exceed 0.2 in the pretrough observations. The relative
frequency that LWC/ICC ratios exceeding lO""g per
crystal were observed did not exceed 0.2 until 7 h after
700 mb trough passage. Recall that the occurrence of
the C 1 PET peaked at this same time. The relative
frequency maximum is sloped from a temperature of
approximately -150C at 7 hafter passage, to OOC,
9 h after trough passage. This may indicate deterio-
rating, convective conditions with time, resulting in
shallower clouds with warmer tops. Beyond 10 h after

trough passage relative frequencies dropped below 0.2.
The optimum time that large relative frequencies were
encountered ranged from 7 to 10 h after 700 mb trough
passage.

6. Conclusions

The results have provided a close look at the su-
percooled L WC and ICC in winter storms of the central
Sierra Nevada for the two successive years, 1978-80.
It should be mentioned that minimum observed clear-
ance altitude restrictions prevented the aircraft from
flying within - 1 km of the Sierra terrain,' and it is
possible that large LWC exist in this layer.
The cellular echo types clearly exhibit the most de-
sirable characteristics of significant liquid water and
relatively little ice. The extent of this region is bounded
in the vertical by the 0 to -150C isotherms and its
core 40 to 100 km upwind ofthe crest. The horizontal
extent of significant L WC was also greatest within the
cellular PET, with segments 32 to 64 km wide fre-
quently measured.
The area-wide and banded echo types provide fewer
instances of significant supercooled water and absence
of ice. L WC were strictly confined to the higher tem-