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

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SEPTEMBER 1988

MARK F. HEGGLI AND ROBERT M, RAUBER

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FIa. 5, 0900 UTC 26 March 1985 to 0900 UTC 28 March 1985: (Top) Time/height cross section of equivalent potential temperature (Oe) (K);
shading denotes regions where Oe/Z < O. (Bottom) Time/height cross section of temperature, relative humidity with respect to water, and winds, The
shading corresponds to regions where relative humidity is between 60-79% (lightest), 80%-89% (middle) and 90%-100% (darkest). A short wind barb
is 5 m S-I, long barb is 10 m S-I and a flag is 50 m S-I. An arrow indicates winds less than 2.5 m S-I. Frontal positions are indicated in the figure.

characteristics of an anafront, consistent with models
of frontal structure presented by Hobbs (1978),
Browning and Monk (1982), and Browning (1985). The
passage of the front corresponded approximately in
time with the passage of the upper-level jet core. Strong
upper-level subsidence occurred west of the jet with
clearing over the valley behind the front and cloud
tops lowering over the mountain. An orographic cloud
containing embedded convective cells remained over
the Sierra Nevada. Convection within the cloud system
developed in response to the lifting of a layer of po-
tential instability that developed behind the front
(Fig. 5).
The passage of the front at Kingvale was accompa-
nied by upper-level subsidence, a decrease in cloud
depth, a reduction in precipitation rate from 4 to 1

mm h-1 and an increase in supercooled water from
0.10 to 0.50 mm. During the next 9 h, radiometer
measurements of liquid were generally near 0.30 mm
with spikes to 1.30 mm. This type of signature was
often associated with an orographic cloud system con-
taining embedded convection (Heggli and Reynolds
1984).
A second region of moisture moved into central
California at approximately 1200 UTC 27 March. This
moisture was contained entirely within the cold air
mass. The leading edge of this moist air moved over
the Sierra Nevada crest at 1345. With its arrival, pre-
cipitation rates increased and supercooled water values
reduced to 0.05-0.10 mm, similar to what was observed
in the warm frontal cloud. This observation is consis-
tent ~vith observation of frontal clouds that have a