WMOD00500

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

OCTOBER 1988

HOLROYD, McPARTLAND AND SUPER

1129

acoustic sensor at the base of the chamber. These
sounds were counted and recorded once per second by
appropriate electronics. However, only about one tenth
of the IN were detected in this manner because of losses
to the walls and bottom of the chamber (Langer 1973).
Furthermore, there was a delay time of about half a
minute between the injection of IN and the start of
their detection. A period ofa few minutes was required
to flush the chamber before each new detection of an
AgI plume could be made, This operational charac-
teristic of the unit makes it possible to estimate only
the entry plume edge position on any given pass.
As discussed in detail by Super et al. (1988), the
NCAR acoustical ice nucleus counter had some limi-
tations for precise plume edge determinations. Primary
among these was the variable delay time from the
ingestion of the nucleating agent until the first acous-
tical counts occurred. Because the sampling aircraft
operated at a true air speed of about 95 m S-l, this
could result in edge location errors of >0.5 km when
low concentration plumes were penetrated. A further,
more serious complication could result because of in-
cloud scavenging losses of the AgI. The net result of
the uncertainties in AgI plume edge estimation are il-
lustrated in Super and Boe (1988) and Super and
Heimbach (1988), where a leading AgI plume edge
was often found within the ice particle plume rather
than at or before its leading edge. It is therefore likely
that the real plume widths and spreading rates were
larger than the values quoted below for the AgI plumes.
The presence of a seeding generator on the observation
aircraft did not interfere with the IN measurements
because the generator was mounted outside the rear of
the aircraft. Background IN counts ranged from the
usual zero to only a few per minute even while the
generator was operating. Counts in excess of this back-
ground were found only in air that the trajectories
showed had been previously exposed to AgI smoke.
A Particle Measuring Systems optical array probe
(2D-C) of 0.025 mm resolution was used to detect air-
borne ice particles initiated by AgI seeding. The images
were processed buffer-by-buffer (about 20-40 particles
per buffer) in the manner described by Super et al.
(1988), based on the software of Holroyd (1987), in
order to identify the abrupt changes in ice particle con-
centration (IPC) associated with the microphysical ef-
fects of the seeding. When present, the ice particles
offered a more precise indicator of the AgI plume edges
than was obtainable from the acoustical ice nucleus
counter. However, if the clouds were discontinuous,
the ice particle plumes could be narrower than the IN
plumes because of a lack of opportunity for the IN to
generate ice particles between clouds.

c, Surface instrumentation systems

Supporting data were available from a network of
surface instruments, whose locations are indicated in

I.

Fig. 1. Primary measurements pertinent to the studies
reported here were wind and temperature data. Surface
measurements of these parameters were obtained at
seven sites using automated weather stations (PROBE),
which provided 5-min averaged data. Two of those sta-
tions, both at site A, monitored conditions at the sur-
face and at the 70 m level on a tower. Vertical profiles
of wind speed and direction were also obtained at site
A from an Aero Vironment Model 2000 doppler
acoustic sounder, which was configured to produce
soundings up to 570 m agl in 30 m bin increments.
An Atmospheric Instrumentation Research tether-
sonde was operated both at the seeding sites and at
other locations on numerous occasions to provide ver-
tical profiles of wind and temperature. Tethersonde
operation at levels with winds much over 10 m S-1 was
not possible. Twice daily rawinsonde soundings were
also available from the National Weather Service sta-
tion at Grand Junction (GJT) located approximately
45 km WNW of the mesa center.

4. Conduct of sampling missions

Data were available from four basic mission types.
These were IFR or VFR missions conducted while
tracing either ground- or airborne-released AgI. During
the ground-release experiments, the aircraft usually flew
reciprocal pairs of passes perpendicular to the wind
direction at several altitudes and distances downwind
from the seeding site. During the aircraft-release ex-
periments, the seeding line was generated along a line
or arc generally perpendicular to the wind direction.
Reciprocal pairs of sampling passes, required for AgI
plume edge determination, were then flown parallel to
the wind direction, sometimes at a variety of altitudes.
Paired passes were flown even if there was a reasonable
probability of detecting an ice particle plume that
would yield both edges of the plume on each pass.
Flight patterns and procedures were similar for both
IFR and VFR tracing of seeding materials. Minor vari-
ations in these basic flight patterns were allowed when
conditions warranted. Due to flight restrictions, the
aircraft was precluded from sampling below 12400 ft
(3780 m) during IFR conditions. Measurements were
made within 100 m of the surface during some VFR
missions.
During in-cloud (IFR) conditions in which a seeding
signature was observed, the primary plume dimension
measurements were based on the locations of ice par-
ticle concentrations in obvious excess of background,
with a secondary reliance on the IN data. Plume size
estimates were necessarily based solely on edges deter-
mined from the IN data for ice-free situations.

5. Air flows at the Grand Mesa

The Grand Mesa was selected as the CRADP re-
search area in part because of the apparent simplicity
of the mountain barrier, especially when compared to