WMOD00537

• Title: Remote and In Situ Observations of Sierra Nevada Winter Mountain Clouds: Relationsips between Mesoscale Structure, Precipitation and Liquid Water
• Date: 2/2/1988
• Weather Modification - Doc Type: Report

142

JOURNAL OF APPLIED METEOROLOGY

VOLUME 27

I

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AREA SHOWN ..~ l :
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SI<YWATER
RADAR

39030' -

I? ,? 10 20 , 30

40 50
, . . kilometers

10 0 10
. . ,oJ

~

2? nautical miles

12;1.
I

1984-85 '20'
SCpp Instrumentation Locations

38030' -

4

l'3

~2
::>
I-
~ I SHERIDAN

o
100

50

WEST

(.m)

SIERRA CARSON 50
~~~:~A EAST RANGE

FIG. 1. Layout of SCPP instrumentation for the 1984-85 field season (top). and
elevation transect taken through the American River Basin (bottom) between Shendan,
Kingvale and the Nevada border (see text for details).

a. 7-8 February 1985

The precipitation event of 7-8 February was signif-
icant because it produced the greatest 24-h snowfall at
Blue Canyon, California, in 86 years of record keeping. .
Approximately 1.19 m (47 in) of snow fel~ betwee~
0300 UTC2 on the eighth and 1900 on the mnth. ThiS
event was initiated by the merging of strong split flow
off the California coast, bringing together cold air frorn
the Gulf of Alaska and warm moist subtropical air from
the central Pacific, inducing a strong, moist, zonal
westerly flow into central California.
Figure 2 presents a composite diagram of data frorn
various sensing instruments which describe the pr~cip-
itation event which occurred over the central SIerra
Nevada from 0000 UTC 7 February to 0000 UTC 9
February. Figure 2a, a time-height cross sectio~ of
equivalent potential temperature (~e) was compIle?
from soundings launched at Shendan. From thIS
sounding information, the accumulated surface-5 km,
2500 (barrier perpendicular) condensate supply rate

2 All times reported in this paPer will be UTC.

(CSR) was calculated following the technique of Rhea
et al. (1984) (see Appendix).
As Fig. 2 shows, there were several distinct sta~es
that could be identified within the storm, two of which
were repeated before the storm dissipated. These stages
can be related quite well to Browning's split-front
model as was the case described in Heggli and Reynolds
(1985). The first stage is what Browning describes ~s
forward sloping ascent of the warm conveyor belt. ThIS
is depicted in Fig. 2a between 0000 and 1800 UTC 7
February. Thewell-defined stable layer (SL) acts .m~ch
like a warm front; however, for this case, no dIstmct
warming (increase in ()e due to moistening) was noted
after the stable layer passed. It appears this stable layer
was enhanced by the preexisting stable air trapped be-
tween the coast range and the Sierra Nevada. This stage
was important in that prior to the onset of precipitation
a 6-h period of SLW existed (Fig. 2b) within th~ low
and midlevel overrunning clouds. As the mOIsture
deepened to near the full depth of the troposphe~e a
rather efficient seeder-feeder precipitation mechamsm
developed (inferred from fallstreaks seen on the Sher-
idan radar tirne-height), which effectively rernoved the
SLW from the cloud (Hobbs, 1978). During this period