WMOD00507

• Title: The Persistence of Seeding Effects in a Winter Orographic Cloud Seeded with Silver Iodidde Burned in Acetone
• Date: 6/6/1990
• Weather Modification - Doc Type: Report

478

JOURNAL OF APPLIED METEOROLOGY

VOLUME 29

trometer Probe (FSSP) in the 3-45 ~m range with 3
~m resolution, and the following PMS Optical Array
Probes: ID-C operated in the range 12.5-187.5 ~m
with 12.5 ~m resolution, 2D-C in the 25-800 ~m range
with 25 ~m resolution, and 2D-P in the 200-6400 ~m
range with 200 ~m resolution (Knollenberg 1981). The
research aircraft was capable of measuring the evolu-
tion of a packet of ice crystals as it drifted downwind.
By integrating true airspeed and heading, the computer
on the aircraft could direct the pilot to make repeated
penetrations of a point drifting with the wind (Cooper
and Lawson 1984; Huggins and Rodi 1985).
The ice nucleus counter on the aircraft was devel-
oped by the National Center for Atmospheric Research
(Langer 1973) and is essentially a small cloud chamber
where ice nuclei (IN) can activate and the resultant
ice crystals can grow to a detectable size. For the SCPP
the temperature of the cloud chamber was held at
- 200C and air was sampled at a rate of 10.0 L min -I.
Although the instrument easily senses high concentra-
tions of IN there are delays in the response and relax-
ation time of the instrument. Empirical tests of the
counter indicated a 25-40 s delay between intake of
IN and registration of the particles, while it frequently
took up to 3 min for the chamber to be purged of IN
sampled in a seedline. The 25-40 s delay measured
here is_consistevt with tests reported by Super~et-al.
(1988). Ice nucleIConcentrations are also difficult to
quantify because of a loss of nucleants and ice particles
to the inside walls of the chamber. Langer ( 1973) es-
timates a 10% counting efficiency, but Sackiw et al.
( 1984) reports only a I % counting efficiency. These
counting efficiencies are not incorporated into the IN
concentrations presented here. Thus the IN concen-
trations may be low by a factor of 10 to 100; however,
the IN measurements represent activity at the chamber
temperature, - 20oC, and will be less at warmer tem-
peratures, perhaps up to a factor of 100 at temperatures
as warm as -60C.
During the SCPP a variety of remote sensing and
surface measurements were also available. A 5-cm
weather radar was located at Sheridan (SHR) near the
base of the Sierra Nevada. Rawinsonde stations were
located at SHR and Kingvale (KGV), near the crest
of the barrier. Telemetered precipitation gauges with
a resolution of 0.1 mm in 5 min (Price and Rilling
1987) were distributed throughout the target area. In-
tensive surface and remote sensing observations were
collected at KGV with a microwave radiometer (Heggli
and Rauber 1988), vertically pointing Ka band radar,
a 2D-C particle sensor, and a high resolution precipi-
tation gauge.
The seeder aircraft, a Cessna Aero Commander,
burned the AgI NH4I NH4Cl04 in acetone in a pres-
surized stainless steel container (Carly-type generator)
mounted on the fuselage. The AgI was released at a
rate of 0.4 g km -I by burning a 3 percent by weight
solution of AgI NH41 NH4Cl04 with the ammonium

perchlorate at a 30 mole ratio. This solution was chosen
since tests indicated it would produce approximately
1013 nuclei g-I at -60C (DeMott et al. 1983). With
this activity a rough calculation of the ice crystal con-
centration (ICC) resulting from seeding can be made.
Assuming that the AgI disperses at 1.0 m S-I horizon-
tally and 0.1 m s -I vertically for winter orographic
clouds (Hill 1980), and that 80 percent ofthe IN have
activated (DeMott et al. 1983), the ICC would be ap-
proximately 40 L -1 after 10 min. However, if as
DeMott et al. ( 1983) suggest, contact nucleation is the
primary nucleation mechanism for AgI ~I, then this
estimate is too high. If the rate at which IN are scav-
enged by cloud droplets is included (Slinn 1971), and
100 cm -3 of 5 ~m radius cloud drops are assumed,
instead of the 2000 to 4000 cm -3 in the cloud chamber
where the material was tested, the expected ICC would
be 1.0 L -I after 10 min. If this is the case, increases in
ICC due to seeding would be difficult to separate from
the background. On the other hand, Finnegan and Pit-
ter ( 1987) presented evidence indicating that AgI may
cause nucleation by condensation freezing when the
AgI acetone solution is burned, due to the large amount
of water vapor produced. If this occurs then the earlier
estimate of 40 L -I is more appropriate. Finnegan and
Pitter point out that oxidation of 1 g of acetone pro-
_ Quces 0.93 g of water. With this estimate the seeding
generator on the aircraft was producing 1.2 g s -I of
water, similar to the water production rate during the
ground seeding experiments described by Finnegan and
Pitter.

3. Case Study

a. Cloud structure and organization

On 22 December 1986 at 1200 (all times expressed
in UTC) a short-wave trough moved rapidly from the
northwest into a split in the upper-level flow over Utah.
In the next 12 hours the wave weakened and moved
onshore with the main energy to the north of California.
The surface front associated with this wave was in the
northwest comer of California at 1200. This weak front
was observed to pass SHR after 1500 and KGV at 1700
and preceded most of the precipitation associated with
it. Liquid water above 0.5 mm, with peaks to over 1.0
mm, were measured with the radiometer at KGV from
1430-1630, but the liquid water diminished to 0.1 mm
as the front passed and precipitation began. In the post-
frontal air mass a well-defined orographic cloud de-
veloped with tops near 5.0 km, -150C, and cloud base
near 1.4 km, ooc. Precipitation began at KGV at 1700,
just after frontal passage and continued at over 4 mm
h -( until 1900. Light and intermittent precipitation,
0.5 to 1.0 mm h -I, and liquid water near 0.1 mm were
then observed until 0400 on 23 December 1986, with
the liquid water persisting until 0700.