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

~OVEMBER 1983

HEGGLI ET AL.

1883

f

cies of liquid water were observed than in the area-
wide conditions. LWC of b.l g m-3 or greater Were
observed 40% of the time ~p to -150C 50 km west
ofthe crest. A secondary maximum is evident 120 km
west of the crest. Fig. 9b shows the ICC stratification
characteristics for the ban4ed cases. A region of low
ICC relative frequencies was found in the higher tem-.-
peratures, though in regimds less than -50C there was
nearly a 50/50 chance of these conditions being met.
The ratio of LWC/ICC, sh~wn in Fig. 9c, showed the
largest relative frequencies I in the high temperatures,
similar to the area-wide caSes, with smaller values at
temperatures less than -Sic.
Within cellular echo types LWC > 0.1 g m-3 was
measured over a broad dpanse from 40 to 90 km
west of the crest (Figs. lOatc). These conditions were
observed 50% of the time from OCC through -150C.
This figure is most useful in! showing where convective
supercooled liquid water clouds generally form on the
mountain barrier, as indicated by the strong horizontal
gradient 90 to 100 km wes~ of the crest. In the general
area where L WC relative frJquencies were greatest, the
relative frequencies of low! ICC appears to be maxi-
mized. The relative frequtjncy of ratios confirm this
coincidence. LWC/ICC ratios> 10""g per crystal were
observed 60% of the time ~t distances 40 to 100 km
west of the Sierra crest, with a secondary maximum
evident directly above the prest.
I
I
I

d. Extent of significant /iqLid water regions
I
I
The extents of regions having significant L WC were
evaluated by determining the distance flown in cloud
while within significant LfC. A threshold of 0.2 g
m-3 was used as a measure of significance, two tem-
perature regions (0 > T ;a.1-lOoC, T < -lOoC) were
considered, and data were ~tratified by PET. If the 0.2
g m-3 criterion was not satisfied within a small pocket
(< 1 km) but was satisfied +thin adjacent regions, the
calculated extent of the region ignored the pocket.
Figures lla and lIb snow the relative frequency
that regions containing significant L WC were en-
countered at 0 > T;a. -IOcC and T < -looC, re-
spectively. At the higher temperatures, regions 32-64
kIn wide occurred in all PET categories, but the relative
frequency of significant IJWC regions, regardless of
extent, was greatest for cellular echoes and least for
area-wide echoes. At the 16wer temperature, cellular
echoes contained the widest regions of significant L WC
(up to 32 km), whereas the I regions of significant LWC
in area-wide echoes were qnly 2-4 km wide. Further-
more, the frequencies ofLFC regions were similar in
the cellular and banded echoes up to 8 km, but no
regions wider than this occttrred in the banded echoes.
A comparison of charabteristics between the two
temperature regimes revclus distinct differences. In
particular, regions contairling significant L WC were-

~

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r 0
a
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e leo
a

DISTANCE WEST OF SIERRA NEVADA CREST (km)

-20

15 150
b

0-

-~

100

50

DISTANCE WEST OF SIERRA NEVADA CREST :II.",:

-20

-lO
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~ ~
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DISTANCE WEST OF SIERRA NEVADA CREST (kmt

FIG. 10. (a) As in Fig. 8a, but for cellular echo types. Cloud
markings indicate an area within which 100 in cloud observations
were made; (b) same as (a) except ICC < 5 L-1 shown; (c) same as
(a) except LWC/ICC > 10 /lg per crystal shown.

wider at the higher temperatures in all PET categories.
Relative frequencies generally were greater at the higher
temperatures, except for banded echoes < 8 km in
width in which case the frequencies at the two tem-
perature regimes were similar.