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

FEBRUARY 1988

DA V1D W. REYNOLDS AND ARUNAS P. KUCIAUSKAS

l4l

various storm stages. He, too, categorized regions of
storms as well as frontal types which, as Browning
points out, fit into the split-front model since both the
United States Pacific Northwest and the British Isles
are frequently affected by systems in decaying or oc-
cluding stages. Hobbs also developed relationships be-
tween mesoscale organization, frontal structure, and
clouds containing SL W.
Ryan et al. (1985) performed similar studies for
storms affecting southern Australia, specifically noting
the microphysical characteristics of clouds ahead of
and behind the frontal transition zone, and attempted
to document cloud seeding opportunities. According
to the WMO (1982), one of the prerequisites for a cloud
to be considered seedable is that the rate of formation
of supercooled condensate exceeds or is comparable to
the rate of depletion of SL W. Determining when this
condition is rnet within a midlatitude precipitation
system is the focus of question 3.
The Sierra Cooperative Pilot Project (SCPP) was
initiated by the Bureau of Reclarnation in the rnid
1970s as an investigation of cloud seeding to increasing
winter precipitation on the central Sierra Nevada of
California. It is a concerted effort in the development
ofa physically sound cloud seeding technology (Reyn-
olds and Dennis, 1986). The SCPP has provided a
unique set of remote and in situ observations for
studying midwinter precipitation systerns. Attempts to
answer the above three questions will be given in this
paper utilizing SCPP data from a satellite, radar, 3-
hourly serial soundings, and a dual-channel microwave
radiometer situated near the crest of the Sierra Nevada,
which continually monitored the amount ofSLW.

2. Observational facilities and data collection proce-
dures

Figure 1 displays a map of the SCPP project area
with a corresponding terrain profile. The region where
most observations were made is in the American River
Basin (ARB) and is outlined in solid black. The project
field office in Auburn (AUB) receives satellite imagery
and a variety of forecast products during the field sea-
son. In addition, the field office has communication
and computer facilities to access and process in near
real time, data from sensing devices positioned
throughout the project area. Table 1 outlines the pri-
mary equipment and data frequencies available for this
study.
Specifications and data collection procedures for the
radiometer are documented in recent articles by Hogg
et al. (1983), Rauber et al. (1986a), and Heggli et al.
(1987). Since the surface temperature at Kingvale
(KGV), as well as the rnountaintop temperature at
Cisco (CIS) was less than OOC during the periods de-
scribed in this paper, it was assumed that all of the
measured liquid water was supercooled. Heggli and
Reynolds (1984) describe possible problerns in inter-

preting radiometer data if temperatures are greater than
DOC during precipitation periods.
The threshold of SL W detection by the radiometer
was determined to be 0.1 mm. In -SCPP, integrated
SL W values which measured ~O.l rnm indicated a po-
tential for seeding over the upper elevations of the ARB,
since it was assumed that the SL W measured by the
radiometer near the crest was going to pass over the
barrier, evaporate, and be lost to the precipitation pro-
cess (Hill, 1987).
A 5-cm radar, operated near Sheridan (SHR), Cal-
ifornia, within the Sacrarnento Valley just west of the
foothills (Fig. 1), was used to observe and document
mesoscale features, such as rainbands, during precip-
itation episodes. Radar data were processed as de-
scribed by Schroeder and Klazura (1978). Further in-
formation about radar specifications and data collec-
tion can be found in Huggins and Rodi (1985). The
NCAR CP-4 Doppler radar data has not gone through
rigorous postprocessing to date, and only the real time
information is used in this paper.
During storms, rawinsonde facilities at Sheridan,
Blue Canyon, and Kingvale provided upper air data
(Fig. 1); however, only Sheridan soundings were se-
lected for temporal analysis because only its 3-h datasets
provided full 24-h coverage. In these case study anal-
yses, the Sheridan soundings were constructed in time
vs height fashion, which, if one assumes near steady
state conditions (a reasonable assurnption when diag-
nosing frontal type), can be interpreted as a horizonta~
cross section.
Images from the GOES-W satellite were collected in
hard copy at Auburn. Communications with the Na-
tional Environmental Satellite Service's (NESS) Sat-
ellite Field Service Station in Redwood City, California,
allowed selection of either visible light or infrared im-
agery at various spatial resolutions. The infrared data
could be obtained with a gray-scale enhancement to
provide estimates of cloud top temperature (CTT).
Also, an animation device provided motion estimates
of specific cloud features and facilitated prediction of
significant precipitation events within the project area.
A network of 30 Belfort weighing type precipitation
gauges was operated within the ARB (Fig. 1), providing
precipitation rates at 15 min intervals. These data pro-
vided information on the onset and termination of a
given storm.

3. Case studies

The case studies described here are a small subset
from only one field season out of the ten field seasons
conducted in SCPP. These cases were selected because
of nearly complete datasets, variations in frontal struc-
ture, and uncontarninated liquid water observations
from the dual-channel radiometer. These results follow
from the earlier work of Heggli and Reynolds (1985)
and Marwitz (1986).