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

1008

1.5

JOURNAL OF APPLIED METEOROLOGY

VOLUME 27

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FIG. 15. As in Fig, 6, but for 0000 UTe 7 January 1985 to 0900 UTe 8 January 1985.

3 to 30 h, depending on the trajectory of the cutoff
low. Storms in which the center passed from west to
east over the project area had longer durations of su-
percooled water (> 10 h). Cutoff storms that passed
south of the project area had shorter durations of su-
percooled water (<6 h).
After the frontal passage, supercooled water was
produced primarily by convection. This convection
period lasted until the passage of the core of the low,
typically 8 to 17 h, depending on the distance between
the front and the trough. Supercooled water was gen-
erally 0.05 to 0.20 mm with instantaneous values as
high as 0.50 mm in convection. The largest amounts
of precipitation were associated with the frontal pas-
sage, with rates ranging from 2 to 9 mm h-I. Precipi-
tation was generally less than 1 mm h -I after the front.
Precipitation and supercooled water usually dissipated
with the passage of the trough.

e, Developing storm in northerly flow: 1-2 February
1985

A large amplitude ridge situated in the eastern Pa-
cific, near l40oW, was present during the 1-2 February
1985 storm. A strong short wave had moved over the
ridge and dug down the east side of the ridge over the
project area. The storm brought cold temperatures and
snow to very low elevations.
The satellite picture and 500 mb height and tem-
perature contours shown in Fig. 16 depict the storm
flow and cloud cover as the storm was about to impact
the study area. A time section of the storm is shown
on Fig. 17. Radiometer liquid water and precipitation
datasets are shown in Fig. 18.
Winds observed near the onset of the storm were
northerly to northwesterly above 2 km. As the system
approached, winds shifted to a more westerly direction,
which favored orographic lift. Supercooled water typ-
ically measured 0.20 mm to as high as 0.70 mm during
this period. The prefrontal liquid water lasted about 5
h. There was no precipitation recorded during this time.
The front shown in Fig. 17 was confined to the upper
levels (>3 km). The front preceded the trough by 3 h.

The frontal band passed through the project area be-
tween 0000 and 0300. Precipitation started with the
passage of the cold front and ended with the passage
of the trough. Precipitation varied from 0.2 to 0.4 mm
h-I in the precipitation band associated with the front.
Supercooled water was typically 0.05 mm with peaks
from 0.20 to 0.40 mm in convec~ion during the frontal
passage.
The associated trough passed between 0300 and
0600, as evidenced by the dip in the isotherms on time-
height section. Both precipitation and supercooled wa-
ter dissipated as wind shifted in response to the passing
trough. Winds shifted to become northeast below 2
km, and generally out of the north above this altitude.
The storm was through the project area in about 13 h,
which indicates its extent and speed.
Discussion. This group represents the coldest and
fastest moving storms of the total population. Eleven
out of 63 storms were grouped in this category. Seven
out of these 11 occurred during the 1984/85 winter
field season, which was evidence of the persistence of
flow fields and storm types experienced in one given
winter. Nine storms had complete radiometer datasets.
The variations in this category revolved around the
actual trajectory of the storm. In onshore systems, sim-
ilar to the case discussed, the center of the system
moved directly over the project area. Typical values of
supercooled water ahead of the front were 0.05 to 0.50
mm with peak measurements to 1.00 mm. Typical
prefrontal supercooled water episodes ranged from 6
to 8 h. Precipitation rarely occurred in the prefrontal
region. Supercooled water generally decreased to 0.05-
0.20 mm within the frontal band and after frontal pas-
sage. Precipitation was greatest with the frontal passage.
Precipitation decreased after the frontal passage. Pre-
cipitation and supercooled water ended with the trough
passage.
Storms with inland trajectories provided continuous
liquid water with little or no precipitation. Supercooled
water values were typically 0.10 to 0.40 mm through
the entire storm. The storm duration was 15 to 21 h.
No distinct frontal structure was observed.