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

also made along the seedline prior to seeding. The
MOCA in the Sierra region is approximately 300 m
above the terrain. The radiometric and aircraft mea-
surements were both considered when estimating the
average cloud liquid water content. The value ofliquid
water content used in the targeting calculations was
chosen subjectively based on the values measured by
the radiometer and aircraft, and on the upward or
downward trend of the radiometric measurements.
Operationally, the goal was to reasonably anticipate
the cloud liquid water content during the seeding ex-
periment. The liquid water content assigned as an input
variable in the targeting calculation was assumed to
characterize the entire cloud system along the particle
trajectories.
Several studies have considered the riming process
with particular application to the Sierra Range (Reink-
ing 1974,1979; Rodi et al. 1985; Prasad 1986). Reink-
ing's data provide information concerning the mini-
mum crystal size necessary for onset of riming, but
provide no information concerning the effect of vari-
ations in average liquid water content of the cloud.
Rodi et al. (1985) and Prasad (1986) used a micro-
physical model developed by Heymsfield (1982) and
Heymsfield and Pflaum (1985) to estimate the onset
of riming in Sierra fixed-target type cloud systems.
Their studies specified the time of onset of riming as
a function of liquid water content and cloud temper-
ature. Figure 4 shows the onset time of riming as a
function ofliquid water content for three temperatures
specified by Rodi et al. (1985). The lines are visual fits
to these data points for extrapolation to other liquid
water contents. In the parameterization used in SCPP,
these curves were assumed to apply for dendritic habits
(-I50C, squares), plates (-12 oC, triangles), and col-
umns or needles (-80C, circles). This parameterization
allowed an estimate of the time of riming onset on the
basis of two measurable parameters, cloud liquid water
content and temperature.
Once the onset of riming occurred, the particle was
assumed to encounter supercooled water throughout
the remainder of its trajectory. A uniform distribution
ofliquid along the trajectory was assumed. The rate at
which a particle becomes rimed is a function of liquid
water content. The time after onset of riming that a
particle became heavily rimed was assumed to be a
function of the cloud liquid water content. The param-
eterization used is shown on Fig. 4. The rate of con-
version was relatively slow (40 min) at minimal liquid
water content (<0.10 g m -3), but proceeded quickly at
high liquid water content (0.30-0.50 g m-3). Typically,
in fixed target experiments, the measured liquid water
content was 0.05-0.20 g m-3. Based on past data col-
lected on the frequency of supercooled water, target
crystals in most fixed-target conditions would become
heavily rimed within 15-25 min after onset, although
in very wet conditions, heavy riming could occur in as
short as 2 min (see section 4).

816

JOURNAL OF APPLIED METEOROLOGY

VOLUME 27

Column or Needle
.6 .J

-
~
I
E .5
01

-8e Rodi et
- .4 -12 A a!. (1985)
c
CI) -15.
-
c
0 .3
u
...
CI)
-
0 .2
:=
"0
::I .I
2'
....J
.0
0 10 20 30 40 50
Time after Release (min)
(a )

.6

-
~
IE .5
01

_ .4
c
CI)
-
c
8 .3
...
CI)
~ .2
"0
g. .I
....J

.0
o 10 20 30 40
Time after onset of Riming

50
(min)

( b)

FIG. 4. (a) Onset time of riming as a function ofIiquid water content.
(b) Time after riming onset that an ice particle is assumed to be
heavily rimed.

3) PARTICLE FALL VELOCITIES

Fall velocities of ice particles are a strong function
of particle shape, mass and degree of rime. Studies of
particle fall velocity can be broadly divided into two
types: I) theoretical studies using relationships between
the Best and Reynolds numbers (Cornford 1965; List
and Schemenauer 1971; Kajikawa 1971); and 2) direct
measurements of ice particle fall velocities using stro-
boscopic methods (Brown 1970; Davis 1974; Locatelli
and Hobbs 1974). Our approach was to use the direct
measurements of Brown (1970), Davis (1974), and Lo-