WMOD00430

• Title: A Diagnostic Technique for Targeting During Airborne Seeding Experiments in Wintertime Storms over the Sierra Nevada
• Date: 7/7/1988
• Weather Modification - Doc Type: Report

820

JOURNAL OF APPLIED METEOROLOGY

VOLUME 27

falling seeding material curtain and was the altitude
typically flown by the research aircraft. Referring to
Fig. 6, a first estimate of the particle trajectory (line
AB) was calculated. The error in this trajectory was
calculated by determining the direction and distance
between the impact point and the target (vector BD).
The first estimate of the SLCP (point C) was determined
by moving from the origin the direction and length of
BD. This procedure was repeated again using point C
as the new starting point. More than one iteration was
required because the particle moved through a different
region of the wind field in each iteration. The procedure
continued iteratively until the solution coverged on the
SLCP. Normally, convergence to within I km of the
target occurred after three iterations. Once the SLCP
was calculated, the seedline orientation and length were
determined. The seedline was oriented perpendicular
to the line connecting the SLCP and the target. For
research purposes in SCPP, the length of the seedline
was varied with distance from the target (see Table 2).
To maximize seeding effects over a small target area
at the ground, it was desirable to seed as much of the
volume of air as possible by making the seedlines
shorter and closer together. However, errors in the cal-
culated wind direction have a greater potential to cause
particles created by seeding to miss the target when the
seedlines are short. The length of the seedlines were
calculated by assuming a :t200 error in the calculated
wind direction. As the trajectories of particles became
longer due to higher wind speeds, the length of the
seedlines were increased proportionally. The values
appearing in Table 2 were chosen to satisfy the 200
error criterion and operational requirements of the air-
craft. Once the SLCP and seedline were known, a final
trajectory was calculated for a crystal originating at the
SLCP. The predicted crystal habit, rime characteristics,
fall velocity evolution and total fall time were recorded
for later analysis.
The areal coverage of the seeding effect at the ground
is a function of the depth the seeding material falls in
the atmosphere, the vertical wind shear within that
depth, the variations in fall velocity that develop as
crystals originating near the same location encounter
differences in saturation and liquid water content along
their trajectories, and, in the case of AgI, variation in
the activation time of individual AgI aerosol (see Rhea
and Elliott, 1986). For CO2 and pyrotechnic seeding,
all effects except AgI activation were considered.

TABLE 2. Seedline length versus trajectory length (T).
SCPP fixed target experiments.

Trajectory length (T)
(km)

Seeding line length
[km (nmi)]

T< 10
30>T;;,,1O
45 > T;;" 30
T;;" 45

10 (5)
19 (10)
28 (15)
37 (20)

Stewart and Marwitz (1982a) discuss in detail the
mechanism of spreading of an initially vertical column
of ice crystals due to vertical shear and variation in
particle fallspeed. Unfortunately, the variations in par-
ticle fall velocity that occur in a sample of ice particles
due to variations in particle growth rates have not been
determined quantitatively. Variations in particle ve-
locity were parameterized by allowing five particles to
fall from each level. The fall velocity of each of these
particles was given by

Vtn = O.I(n + 6)Vt, n = 1,2,3,4,5 (7)

where Vtn is the fall velocity of particle n and V; is the
standard terminal velocity calculated from the equa-
tions in Table 1.
The depth of the seeding curtain varied with the type
of flare used or the mean diameter of the CO2 pellets.
In SCPP, extensive tests were conducted prior to each
field experiment to determine the depth that flares fell
while burning and CO2 pellets fell before sublimating.
The flares (TBI-20 g flares)l consistently fell about 1000
m. The size of CO2 pellets varied from year to year
depending on the dispensing system. The correspond-
ing fall distances varied from 600-1000 m. Fig. 7 shows
the predicted SLCP, seedline and fallout area of crystals
for an aircraft flare release at 4000 m into a cloud with
average LWC = 0.10 g m-3 at 1800 UTC 5 February
1986.

3) PROCEDURES FOR AIRBORNE ACETONE GENER-
A TORS

The targeting calculations for experiments which
utilized airborne acetone generators were nearly iden-
tical to those for flare or CO2 releases, but the methods
to calculate the SLCP and fallout area differed. The
primary difference between the two seeding techniques
was that the flares and CO2 produced a deep vertical
curtain while airborne generators produced narrow
continuous plumes. The target crystal for airborne
generator releases was initiated at the altitude of the
seeder aircraft. The method to determine the spread
of the seeding effect took into account the variation in
activation time of individual aerosols.
Various chemical solutions have been developed for
use in airborne aerosol generators. The solution used
in SCPP generators, a 3% AGINH4I acetone mixture
with 30 mole% NH4Cl04, has been studied extensively
in the isothermal cloud chamber at the Colorado State
University Aerosol and Cloud Simulation Laboratory
(DeMott et al. 1983). DeMott et al. found this com-
pound to have a high yield at warm (-60 to -lOoC)
temperatures and function primarily by contact nu-
cleation. They observed 50% of all nucleation events
within 5 min after introduction of the nuclei into the
chamber and 90% within 20 min.
DeMott et al. data concerning the nucleation rate

1 The flares were manufactured by Nuclei Engineering Inc.