WMOD00486

• Title: Microphysical Effects of Wintertime Cloud Seeding with Silver Iodide Over the Rocky Mountains - Part III
• Date: 10/10/1988
• Weather Modification - Doc Type: Report

OcrOBER 1988

ARLlN B. SUPER AND BRUCE A. BOE

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Fla. 5. Mean IPCs and estimated precipitation rates recorded by
aircraft at 3.8 km msl in the seeded and nonseeded zones during the
ground-based seeding experiment of 19 Mar 1986. The particle size I
habit shadings apply to both top and bottom panels. See text for
zone identifiers.

the mesa top at 3.8 km, while speeds near the mesa
surface were 6-7 m s -1. Assuming a mean wind speed
in the layer below the aircraft sampling level of 10m
S-l and a mean in-cloud distance of the plume upwind
ofthe sampling line of 5 km, cloud passing the sampling
line would have been subjected to the effects of the AgI
for about 500 s. Since contact nucleation was presum-
ably the ice-forming mechanism for the AgI-NH4I
smoke (DeMott et al. 1983), crystal formation would
have been a continuous process, which could explain
the range of sizes found in the seeded zone. Observa-
tions by Holroyd (1986) showed that crystal growth
rates near 0.1 mm min-I were appropriate at -140C
in shallow orographic clouds over the mesa. Therefore,
crystals of about I mm would be the largest expected
from the AgI seeding, which is consistent with Fig. 5.
When visually examined, many of the small 2D-C
images classified as spherical, hexagonal or irregular
appeared to be embryonic dendrites with limited
branching. Given more growth time these crystals
probably would have been classified as dendrites by
Holroyd's software. In addition, some small crystals
appeared to be joined with larger (presumably natural)
dendrites, suggesting the beginnings of aggregation.
Precipitation rates estimated from 2D-C images us-
ing Holroyd's (1987) scheme are also shown in Fig. 5

according to crystal habits and sizes. All precipitation
(or snowfall) rates, whether at aircraft levels or on the
surface, are given as melted water equivalents. These
estimated precipitation rates should be interpreted only
in a qualitative sense due the limitations discussed in
Part I. Within the seeded zone, the precipitation rate
approximately tripled, largely because of increases in
the number of crystals smaller than I mm and mostly
classified as irregular or aggregates, with lesser increases
in dendritic and hexagonal crystals.

b. Experiment 0120 Mar 1986

Northerly flow persisted for the third consecutive
day. A shallow stratocumulus deck layover the Grand
Mesa during the morning hours, capped by a dry, stable
layer (Fig. 2d). Cloud conditions initially seemed un-
suitable for physical experiments, so an AgI plume
tracing mission was undertaken.
The seeding generator was ignited at Site I on the
north slope at 1025. At 1055 highest cloud tops over
the mesa were measured by the aircraft at 4.3 km and
-12.50C, but most of the ragged, broken deck lay below
4.1 km and contained little SLW. Cloud bases were
near 3.6 lan, high enough to permit Visual Flight Rules
(VFR) sampling of the AgI plume below cloud base.
Visible satellite imagery at 1030 indicated that the oro-
graphic cloud was about 18 km wide.
Several crosswind passes were made at differing al-
titudes, defining the vertical and horizontal extents of
the AgI plume 5.5 km downwind of the seeding gen-
erator. The generator was subsequently turned off at
1225, moved to Site 2, also on the northern slope but
220 m higher (Fig. 1), and reignited at 1251 to deter-
mine what modifications in the plume might result
from the higher site.
While the seeding generator was being moved, the
aircraft crew realized that the position of the seeding
plume coincided with that of a weak snow shower first
observed south of the seeding line at 1148. No snowfall
had been visible below cloud base at 1131 when bases
were near 3.65 km. Visibility over the mesa top was at
least 60 km for the duration of the mission, yet no
other snowfall was observed except downwind of the
generator and south of the initial sampling line.
Four IFR passes were made at the 3.8 km level be-
tween 1251 and 1319, three crosswind and one along-
the-wind. All encountered IPCs exceeding 50 L -I. Total
counts from the acoustical counter ranged from 132
to 262 per pass. Estimated AgI plume edges were in
close agreement with the ice particle plumes, which
were several kilometers wide on each crosswind pass.
The natural (nonseeded) IPC was observed to be almost
zero. During these passes clouds were less than 500 m
thick, and tops were no colder than -120e.
Winds recorded by the doppler acoustic sounder be-
tween 1245 and 1500 ranged from 345 to 0100 and
3.0 to 5.5 m S-I near the surface, to 350 to 0350 and