WMOD00500

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

Reprinted from JOURNAL OF ApPLIED METEOROLOGY, Vol. 27, No, 10, October 1988
American Meteorological Society

Observations of Silver Iodide Plumes over the Grand Mesa of Colorado

EDMOND W. HOLROYD III, JACK T. MCPARTLAND AND ARLIN B. SUPER
Bureau of Reclamation, Denver, Colorado
(Manuscript received I June 1987, in final form 2 November 1987)

ABSTRACf

A series of wintertime airborne tracing experiments was examined to dete~ine some characteristics of the
plumes of silver iodide smoke released either from the gro~nd or from an, lUrcraft ,over the Grand ,Mesa of
Colorado, The plumes were identified in nearly every expen~ent by detectm~ the airborne AgI particles ~nd
often also by observing resulting ice particle plumes in essentJally the same airspace, The. lateral an~ vertical
plume positions of ground-released AgI from eight sites were dete~ined for, several wmd,o c10udmess and
stability conditions. The instantaneous ground-released plume had a median spreading angle of 15 and meande~ed
within a median angle of 380, The median plume height above the crest exc~~ed 500 m, T?e lateral spr~~dmg
rates of aircraft-released AgI were estimated at over 2 m S-I for cloudy conditions and less In clear condltJons,
The implications for future cloud seeding strategies are discussed.

1. Introduction

One of the least-documented and understood and
yet most funtlamental aspects of winter orographic
cloud seeding is the transport and diffusional spread
of the artificial ice nucleant after it is released. It is
obvious that a seeding effect cannot be demonstrated
unless the seeding agent can be reliably delivered to
the intended part of the cloud in the appropriate con-
centration. Yet few programs, research or operational,
have sufficiently emphasized this crucial area. The lit-
erature reveals that several programs utilizing ground
generators may have been seriously flawed due to in-
adequate or untimely delivery of the nucleating agent
under certain atmospheric conditions.
Reid's (1979) airborne silver iodide (AgI) tracing
studies at a valley-bottom generator site in the central
Colorado mountains used during the Climax I and Cli-
max II experiments, indicated that under the stable
conditions experienced, little vertical mixing occurred.
Instead, the generator eftluent was trapped under near-
surface capping inversions, and moved both up-valley
and down-valley in response to terrain channeled syn-
optic flow and countersynoptic drainage flow. One case
indicated the existence ofa localized convergence zone
that provided a sufficient vertical component to allow
the AgI to penetrate the inversion in relatively low
concentrations.
Results of similar investigations conducted in the
San Juan Mountains of Colorado as part of the Col-

Corresponding author address: Dr. Edmond W, Holroyd III, Bu-
reau of Reclamation, Remote Sensing Section, PO Box 25007 D-
3744, Denver CO 80225,

@ 1988 American Meteorological Society

L

orado River Basin Pilot Project were reported by Hobbs
et al. (1975) . Aircraft measurements showed that
transport from valley generators to the clouds was neg-
ligible during stable conditions, and that most of the
AgI particles were trapped or "pooled." This. reservoir
of trapped ice nuclei (IN) could then contammate sub-
sequent supposedly nonseeded periods. Significant IN
concentrations were found over the target as much as
18 hours after the end of a seeding event. Transport
was better during near-neutral conditions, where orog-
raphy provided sufficient lift to transport material to
cloud base. However, this may not have occurred far
enough upwind for resulting ice crystals to grow and
fall in the target area. Unstable conditions produced
excellent vertical mixing over the target, but these con-
vective situations were not included as experimental
units in the project design.
Hill (1982) presented results obtained during an
evaluation of the Utah State operational seeding pro-
gram. Ground and airborne measure~ents were m~de
in the vicinity of the Tushar Mountams to determme
if the AgI seeding material was being effectively trans-
ported from upwind valley generators to clouds over
the mountains. A high incidence of inversions was
noted, in which the AgI was frequently trapped. Sig-
nificant IN concentrations were measured at the surface
in the target area for as long as two days following the
end of a seeding event, as pooled AgI slowly drifted
over the target. Airborne measurements showed that
IN were rarely transported to sufficient height and in
adequate concentrations to achieve significant seeding
effects. It was concluded from the physical evidence
that the ground-based generator network was ineffec-
tive. This finding was reinforced by model studies de-
scribed in Long (1984) that used a Tushar Mountain