WMOD00318 ‎(2)‎

• Project Name: Sierra Cooperative Pilot Project
• Title: A Comparison of Seeded and Nonseeded Orographic Cloud Simulations with and Explicit Cloud Model
• Prepared By: Michael P. Meyers, Paul J. DeMott and William R. Cotton
• Date: 4/4/1995
• Weather Modification - Doc Type: Report

842
Grid 3
90
85
80
75
E
.:,t;
70
>-
65
60
55

JOURNAL OF APPLIED METEOROLOGY

100 105 110 115 120 125 130 135 140
X ( km)

FIa. 6. The areal extent of the seedline as viewed from above (de-
scribed in text) on grid 3 at t = 1900 UTC (at seeding), t = 1930
UTC (0.5 h after seeding), and at t = 2000 UTC (1.0 h after seeding).

experiment. Initialization of seeded aerosol concentra-
tions was given in section 3b. The location of the sim-
ulated seedline was determined from the elevation and
temperatures of the initial seedline (Deshler et al.
1990). The simulated seedline was nearly 37 km long
and placed at the appropriate grid locations between
the -60C and -70C isotherms.

c. Model results

It was important that the model simulated the nat-
ural precipitation processes properly before seeding was
initiated. Figure 5 shows x-z cross sections of a few
microphysical fields from the nonseeded simulation
that can be compared to the observed structure shown
in Fig. 1. The simulated pristine ice mixing ratio on
grid 2 shows the ice cloud extending from 2 to 5 km
MSL with peak amounts up to 0.2 g kg-I (Fig. 5a).
Predicted peak concentrations of pristine ice crystals
are 10-25 L -I (not shown), which were close to ob-
served values of 10-30 L -I. In Fig. 1 the observations
showed an upper and lower cloud layer with precipi-
tation occurring between the two layers at 4 km MSL.
This feature would be quite difficult to predict in the
model, however, since any precipitation falling out of
the upper cloud would have appeared as one of the
precipitating categories. Therefore the simulated struc-
ture predicted the pristine ice mixing cloud to extend
from 2 to 5 km MSL. Aggregation was observed across
most of the lower cloud but mostly confined below 3
km MSL with peak values near 0.07 g kg-I. Simulated
values of aggregate mixing ratio shown in Fig. 5b in-

VOLUME 34

dicate that the aggregates were confined to below 3.5
km MSL, but peak values were 0.01 g kg -I, which was
less than the observed values. The graupel and rain
mixing ratio fields are shown in Figs. 5c and 5d. These
fields show melting precipitation below 1.5 km MSL
in the form of graupel (partially melted pristine ice
crystals are converted to graupel in the model) and
rain occurring over the lower portion of the barrier.
Peak values were 0.2 g kg-I for graupel mixing ratio
and 0.05 g kg-I for rain mixing ratio. These simulated
fields were close to the location of the higher reflectiv-
ities associated with an apparent bright band, resulting
from melting ice particles, observed in this region (Fig.
1 ). Another area of graupel was predicted over the bar-
rier crest. However, this region was located lower in
elevation and was smaller in magnitude than observed.
After demonstrating the reasonableness of the sim-
ulation, the effects of releasing ice nucleus aerosols in
the simulation can be examined. The initial seedline
is shown in Fig. 6 at 1900 UTe. This figure depicts the
areal extent of the artificial ice nuclei plume viewed
from above at different times to show the relative ad-
vection and dispersion of the seeding material. The
plume-edge concentration value was arbitrarily set to
10 L -I. The relative advection speed of the plume in
the simulation was 10 m S-I, which was comparable
to the advection speed ofthe plumes in the observations
( 10-15 m s -I ). By 2000 UTC the seed started to exit
the northern border of grid 3.
The simulated pristine ice crystal concentrations
from grid 3 at 3000 m MSL are shown in Fig. 7. This
height was used since it was in proximity to the location
of the seed release. At 1920 UTC, 20 min after the seed
was introduced, the pristine ice crystal concentration
field exhibited a similar structure as the seedline (Fig.
6), with a broad area of greater than 20 L -I. Peak
values were 120 L -I (Fig. 7a), which is somewhat
higher than the peak pristine ice crystal concentrations
observed (greater than 100 L -I ). The location of the
pristine ice crystal maxima was just downstream of a
broad region of SL W seen in Fig. 8, which enhanced
artificial ice nucleation. The overall structure of the
pristine concentration field advected to the northeast
by 1940 UTC (Fig. 7b), with a broad area of greater
than 20 L -I , a western peak of 60 L -I , and an eastern
peak of 110 L -I . Peak concentrations decreased to 60
L -) by 2000 UTC (Fig. 7c). Simulated pristine ice
crystal concentration maxima from the nonseeded run
during this period was 10 L -I (Fig. 7d).
The effect of cloud seeding on other microphysical
quantities was also quite significant. Figure 9 shows
the predicted pristine ice mixing ratio fields from the
seeded and nonseeded runs at 2000 UTe. In the seeded
run the pristine ice mixing ratio field extended over a
broader area than the nonseeded run, and peak values
of the seeded run (0.35 g kg -I) were nearly twice as
much as the nonseeded run. There was more graupel
mass (not shown) being produced in the seeded run

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