WMOD00508

• Title: Physical Response of Winter Orographic Clouds over the Sierra Nevada to Airborne Seeding Using Dry Ice or Silver Iodide
• Date: 4/4/1990
• Weather Modification - Doc Type: Report

APRIL 1990

DESHLER, REYNOLDS AND HUGGINS

313

seedline, providing a distinct radar seeding signature
after 30 min.
(vi) Observations from KGV indicated that: 1)
seeding effects occurred approximately 55 min after
treatment in agreement with the OTM prediction; 2)
for S2 the Ka radar showed a fall streak with its origin
at the seeding altitude; 3) the snow crystals within
seeded plumes were 0.5 to 1.0 mm rimed columns or
graupel and were consistent from line to line and with
theoretical calculations; 4) precipitation rates measured
at KGV indicated increases between 0.5 to 1.5 mm
h -I during the PPE for S2 and S3, respectively. For S2
the increased precipitation is a measure of seeding ef-
fects because of its isolated nature. For S3 the results
are confused by the natural precipitation trailing S3.
These precipitation rates are ten times greater than
might have been expected from the radar measure-
. ments.

This case study represents the most complete picture
of the chain of physical events initiated by seeding that
the SCPP was able to document. Seeding effects mea-
sured by the aircraft are quite definite. Although less
certainty can be ascribed to the radar and surface mea-
surements they provide an internally consistent picture
that supports the aircraft observations and agrees with
expected results and model calculations. It appears that
the OTM was successful in selecting a treatment lo-
cation for the fixed target.

I'

b. 5 February 1986

1) CLOUD STRUCTURE AND ORGANIZATION

In the morning of this day a minor short wave was
moving down the back side of a ridge building off the
West Coast. At 1200 the 500 mb short-wave trough
was analyzed from a low over Vancouver Island south-
ward to northern California. The 850 mb trough ex-
tended from northern Idaho to southeastern Oregon.
The satellite image showed the system to be over eastern
Washington and Oregon, and moving southward along
the east side of the Sierra Nevada. By 1800 the surface
low was in Idaho with frontal clouds trailing through
northern Nevada into northeastern California. A jet
maximum driving the short wave southeastward in-
duced a surface low pressure center in central Nevada.
This, coupled with the surface high pressure off the
coast, provided the flow necessary to produce shallow,
lightly precipitating orographic clouds over the central
Sierra Nevada. Substantial orographic clouds were ev-
ident by 2100, becoming more convective by 2200.
The surface front had passed through the project area
by 2100 and was dissipating by 2400. At 2400 the 500
mb low was over southeast Oregon with troughing into
central California and strong northwesterly flow mov-
ing into northern California. Clouds lingered over the
barrier through about 0700 on the 6th.
Project soundings began at 1800 and terminated at

2400. There was shallow moisture reaching to 1.8 km
at SHR and 3.0 km at KGV. The winds had compo-
nents normal to the barrier (2500) of 5-10 m s -1 in
the lowest 2-3 km layer, providing lift for the low-level
moisture. Potential instability was also present inducing
further cloud development. The air above cloud re-
mained dry so that embedded convective elements
eroded quickly. The mixing occurring at cloud top was
evident from reports of strong turbulence by both pro-
ject aircraft. Light precipitation over the barrier began
around 1900 and continued through 0500 on the 6th
with a total accumulation at Blue Canyon ofless than
1.Omm.
The cloud physics aircraft made its ascent sounding
and MOCA descent along the barrier at 2100. The JW
sensor was the only liquid water probe available on the
aircraft this day. Measurements in a wind tunnel in-
dicate that at values < 0.5 g m -3 the JW underestimates
liquid water content by 15 to 30 percent. This system-
atic error is accounted for in the data analysis (Sand
et al. 1984). Sampling errors for the JW are judged to
be < 10 percent for droplets < 40 J-Lm. Measurements
from the aircraft instruments indicated that the cloud
contained liquid water of 0.1 g m -3 and average ICC
of 10-20 L -I with> 100 L -1 observed in a few 1 km
wide regions. The ice crystals were between -70C and
cloud top at -1O~e. Based on the 2D-C images they
consisted of small graupel, needles, and columns, with
a few aggregates. Below -70C on the MOCA descent
ice disappeared and liquid water increased to 0.2 to
0.3 g m -3. Streakers were observed on the 2D-P and
water drops 200 to 400 J-Lm were imaged by the 2D-e.
Evidently these drops grew by coalescence. At this time
the 5-cm radar at SHR was measuring 5 to 10 dBZ
echoes below the aircraft altitude, indicating that echoes
were forming at temperatures close to -50e. A Hallett-
Mossop ice multiplication mechanism initiating nee-
dles which then aggregated would explain the rather
warm temperature for echo formation. Aggregates of
needles were observed at KGV later. Also, since these
echoes were small they could have formed at higher
altitudes and yet be unobservable due to beam filling
problems. From the aircraft descent, the 5-cm radar,
and soundings at SHR and KGV, a cross section of
the cloud was constructed showing temperature struc-
ture, reflectivity, and regions of ice and water (Fig.
17). The radiometer at KGV was measuring a fairly
steady 0.1 mm of liquid, equivalent to a liquid water
content of 0.08 g m -3 for a cloud depth of 1.2 km.
Based on liquid water measurements from the ra-
diometer a deliberate seed experiment was conducted.
A seed point to target KGV was calculated using the
OTM and the 1800 soundings from SHR and KGV
as input. Seeding was conducted at 3 km approximately
20 km upwind of KGV. After the sixth seedline the
center of the seedline was moved south 5 km based on
winds measured by the cloud physics aircraft and on
estimated radar-echo motions. The seeder aircraft