WMOD00500

• Title: Observations of Silver Iodide Plumes over the Grand Mesa of Colorado
• Date: 10/10/1988
• State: CO
• Weather Modification - Doc Type: Report

1126

JOURNAL OF APPLIED METEOROLOGY

VOLUME 27

dataset. Application of the GUIDE model (Elliott et
al. 1983), with local upwind soundings as input, re-
sulted in the conclusion that "only 36% ofthe sound-
ings considered showed seeding material was delivered
to the clouds." In the majority of cases, intended trans-
port was apparently not achieved, due to easterly
downslope flow, trapping inversions, or air flow parallel
to the mountains.
A common element in all ofthese programs was the
use of ground-based seeding generators located at low
altitudes. In contrast, seeding sites for the Bridger Range
Experiment (BRE) were selected to avoid trapping
problems by placing ground generators more than
midway up the windward slope of the barrier (Super
and Heimbach 1983). Tracing experiments performed
during this program showed that the AgI seeding ma-
terial was routinely transported up and over the oro-
graphic barrier (Super 1974). Recent physical studies
conducted over the BRE target area have confirmed
that the artificial IN can be routinely transported over
the target during storms and in sufficient quantities to
induce observable microphysical effects (Super and
Heimbach 1988). The positive targeting results from
this program strongly emphasize the need for careful
investigation of ground generator siting prior to initi-
ation of any seeding program using this mode of IN
delivery.
Obviously, alternatives to ground generators exist
for seeding. Principal among them is the use of aircraft
to dispense AgI smoke at an appropriate altitude and
upwind distance from the target clouds, although use
of this technique involves other logistic considerations
and higher costs. In addition, the potential problem of
low diffusion rates in the free atmosphere has recently
been documented. Karacostas (1981) reported on the
results of two experiments during light turbulence in
stratiform clouds. A Particle Measuring Systems
2D-C optical array probe was used to determine the
along-the-wind horizontal spreading rate of ice particles
resulting from both aerial AgI and dry ice seeding. The
median value of the 11 observational points given was
1.7 m S-I, with a range from 1.4 to 2.1 m S-I, consid-
ering both upwind and downwind plume edges. Times
from seeding to observation ranged from about 230 to
3500 s. Hill (1980) used a National Center for At-
mospheric Research (NCAR) acoustical ice nucleus
counter (Langer 1973) to monitor along-the-wind
spreading of AgI lines released by aircraft. The 11 mea-
surements made in winter orographic clouds had a
median value of 0.8 m S-1 over a range of 0 to 4.2 m
s -1 . Times ,from release ranged from 1200 to 6600 s.
The relatively low median value and wide range of
spreading rates may be partially due to scavenging
losses in the clouds and to additional problems asso-
ciated with using an NCAR acoustical ice nucleus
counter to determine AgI plume edges in cloud. These
problems are discussed later in this paper (sec. 3.b) and
in Super et al. (1988).

Ideally, airborne seeding should be conducted suf-
ficiently far upwind so that individual lines of material
will merge prior to reaching the target clouds. However,
at the low rates cited, frequent repetitions along a fixed
upwind line are necessary to achieve plume mergers,
which necessitates the use of multiple aircraft operating
simultaneously to treat anything other than a small
target area effectively. Additionally, Hill (1980) ob-
served a vertical diffusion rate of less than 0.1 m s -I ,
indicating that only a small vertical extent of the cloud
is likely to be treated in the absence of convection.
The transport and dispersion of cloud seeding ma-
terials in winter orographic cloud systems is an example
of dispersion in complex terrain. A considerable
amount of work on the latter topic is currently being
done, as summarized by Egan and Schiermeier (1986).
One of the goals of this research is to identify a critical
height above which the air will flow over a terrain ob-
stacle rather than around it. The critical height for any
given topography will vary with the vertical profiles of
stability and wind speed and direction; it can therefore
change throughout a day. This has direct bearing on
the altitude selected for ground-based generators. First-
order effects of terrain on altering flows are achievable
with current models and can serve as a guide to the
behavior of seeding plumes over a numerical simpli-
fication of complex terrain. But it appears that there
may be a limit to the guidance coming from the im-
proved models of the future. If the models become
good enough to describe flows over real terrain accu-
rately, it is likely that they may not be able to be prop-
erly initialized with real three-dimensional observations
of the wind and temperature fields at a sufficient num-
ber of points. In view of the turbulent nature of the
atmosphere on many scales, the end result of the re-
search on transport and dispersion in complex terrain
may only be generalized flow fields with a statistical
description of the plume characteristics. The use of
scatter diagrams in the field results described below
may therefore be an appropriate presentation.

2. The Grand Mesa experiments

One of the major objectives of the 1985-86 Colo-
rado River Augmentation Demonstration Program
(CRADP) field effort was transport and dispersion in-
vestigations. A series of experiments was conducted
around the Grand Mesa area of west-central Colorado
during February and March 1986.
Investigations were made using both ground and
airborne AgI releases, while tracing was accomplished
using an instrumented aircraft. Flight measurements
were made during both Visual Flight Rules (VFR) and
Instrument Flight Rules (IFR) conditions. The primary
objective of the experiments was to improve knowledge
in the following areas:

1. The minimum elevation (if any) at which upwind
ground generators can be placed for AgI smoke to be