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

818 JOURN AL OF APPLIED METEOROLOGY VOLUME 27
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CRYSTAL MAJOR AXIS Imm)

FIG. 5. Terminal velocity function for an (a) unrimed and rimed dendrite, (b) unrimed plate
and lump graupel, (c) unrimed and rimed column, and (d) unrimed and rimed needle from
Locatelli and Hobbs (1974), Davis (1974), Brown (1970) and Nakaya (1954).

The final growth stage extended from the time when
heavy rime coverage first occurred to impact with the
ground. The particle growth along the major axis con-
tinued at a rate specified by the diffusional growth rate.
The particle fell as a rimed particle. The terminal ve-
locity parameterizations for each of the four habit clas-
sifications are shown in Fig. 5.
The simple approach, discussed above, neglects im-
portant aspects of crystal growth and fallout in the in-
terest of operational utility. First, the assumption of
water saturated conditions throughout the cloud system
can be incorrect. Evidence from aircraft measurements
show that most cloud systems are inhomogeneous with
saturated and subsaturated regions somewhat ran-
domly distributed within the cloud. In addition, over-
seeding water saturated clouds has the potential to re-
duce the saturation level to below water saturation be-
cause of competitive growth between large numbers of
ice particles. In general, the assumption of water sat-
urated conditions throughout the cloud system will re-
sult in an overall overestimation of the ice particle
growth rates. A second factor not accounted for in the
parameterization is the pressure dependence of the dif-
fusivity of water vapor in air. Alone, this would result
in an underestimation of ice particle growth rates, par-
ticularly at high altitudes. The dependence of the fall
velocity on the density and dynamic viscosity of air
and on the Reynolds number (R) is also not accounted
for in the parameterization. Typical particles observed
by aircraft in seeded regions had R < 50. Underesti-
mations in fall velocity from 2%-13% were possible
due to this simplification for atmospheric conditions

characterizing the seeding experiments. Aggregation
and ice multiplication were not considered in the tra-
jectory calculations. Aggregation was not considered
because the process was seldom observed in any aircraft
or ground based observations of seeding effects in fixed
target type cloud systems (e.g., Martner 1986; Deshler
and Reynolds 1987). However, it should be noted that
aggregation was important in purely convective clouds
(Huggins and Rodi 1985). Ice multiplication was not
included because the process would have little effect
on the trajectory of the target crystal although it could
affect the final distribution of precipitation.

c. Seeding techniques

Aircraft generally employed one of three seeding
methods; dry ice (C02), droppable pyrotechnics (AgI),
or airborne acetone generators (AgI). During the SCPP,
all of the methods were utilized and tested to the extent
possible. Choosing the appropriate location in the cloud
to seed for each method, and the appropriate time to
sample precipitation to identify seeding effects was
fundamental to the experiment's success. The proce-
dures incorporated into targeting computations to ad-
dress each of these seeding strategies are discussed in
this section.

I) LOGISTICAL PROCEDURES

Aircraft seeding involved a highly interactive process
between the ground based forecaster, who initiated and
interpreted targeting guidance and other meteorological
data, and the seeder and research aircraft crews, who