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

1011

SEPTEMBER 1988

MARK F. HEGGLI AND ROBERT M. RAUBER

100

as high as 6 mm h-I within convective regions. The
duration of offshore storms was as long as 57 h, indic-
ative of the formation of the slow moving, cold, deep
trough.

5. Vertical distribution of supercooled liquid water

An important aspect of the distribution of super-
cooled water in winter storms is the altitude, depth,
and temperature of the supercooled-water-bearing lay-
ers. In order to investigate this relationship, rawinsonde
data were paired with radiometer liquid water data av-
eraged over the first 20 min of balloon ascent.
Initially, the rawinsonde relative humidity had to be
adjusted to account for water-saturated conditions
when such conditions were indicated by the measure-
ment of supercooled water by the radiometer. The
source of this problem is the inability of the rawinsonde
hygristor to report 100% humidity, as discussed by
Brousaides (1975) and Shaffer (1982). The hygristor
problem results in an underestimation of relative hu-
midity; therefore, water-saturated conditions are rarely
measured. The problem was rectified by raising the
maximum measured relative humidity to 100% when
the radiometer detected su.percooled water greater than
0.05 mm. The increase in relative humidity required
to attain saturation was then added to all levels of the
sounding. Usually this increase was not greater than
5%. A saturated layer was then defined if the dewpoint
temperature was within 0.20C of the ambient (free air)
temperature. The height and temperature measure-
ments at saturated levels were then used in the following
analyses. There were 158 soundings launched in clouds

100
80 90
70 60 :g
z
0
60 70z
::>
0
so(f)
>- 50 -I
U <(
z 50b
l.1.I
~ 40 I-
l.1.I 40~
a:
LL.3Q l.1.I
30 C>
;:
20 z
20 lj
a:
10 w
10 a.

2.0 2.4 2,6 3,2 3,6 4.0 44
HEIGHT (km)

FIG. 19, Frequency distribution of the height (MSL) of water- sat-
urated layers above Kingvale. The solid line indicates the percentage
of soundings that were water saturated in a given height interval.
The method used to determine water saturation is discussed in the
text. The altitude of Kingvale is 1856 m.

60

l.1.I
70C>
;:
z
60~
a:
50~

l.1.I
40~
I-
<(
305
:::;:
::>
20u

90

50

60

40

>-
u
~ 30
::>
o
l.1.I
a:
LL.
20

10

10

o 0
o 200 600 1000 1400 1600 2200 2600 3000
DEPTH (m)

FIG. 20. Frequency distribution of the depth of water-saturated
layers above Kingvale. The solid line shows the cumulative percentage
frequem:y of saturated layers of depth... the value on the abscissa.

during periods when radiometric measurements ofliq-
uid water exceeded 0.05 mm. These soundings form
the basis of the following discussion.
Figure 19 shows a frequency distribution histogram
of the height of water saturated layers. The curve in-
dicates the percentage of soundings in which water sat-
uration was measured in a given height interval. The
most frequently saturated layer was found at 3.0 to 3.2
km. This layer was 1.2 to 1.4 km above Kingvale, and
about 0.5 km above the highest peaks in the area. The
aircraft minimum obstruction clearance altitude was
approximately 3.2 km in the vicinity of Kingvale,
which is at the top of this layer.

100
90
60
w
70 C>
;:
z
60 ~
a:
w
50 a.
l.1.I
40 ~
I-
<(
305
:::;:
::>
20 u
10
0
4.4 4.6

40

36

32

26 '

>- 24
U
z
~ 20~
o
l.1.I
a: 16-
LL.

12-

6-

0-
1.6 2,0 2.4 2,6 3.2 3,6 4,0
HEIGHT (km)

FIG. 21. Frequency distribution of the altitude of the first (lowest)
saturated layer encountered above Kingvale. The solid line shows
the cumulative percentage frequency of the lowest saturated layers.