WMOD00500

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

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JOURNAL OF APPLIED METEOROLOGY

VOLUME 27

860214

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FIG. 3, An example of the variability of vertical profiles of potential
and equivalent potential temperature during a mission,

below appears to have been mixing with 60 warmer
air from above as a result of strong speed shear and
flow perturbations caused by the mountain. The
acoustic sounder recorded winds of about 4 m s - I at
3.4 km, increasing linearly to about 15 m S-I at 3.8
km with about 400 of angular shear.
The situation presented in Fig. 3 was selected because
it illustrates the process well. In the less extreme ex-
amples, the AgI was still detectable to near the altitudes
where the warmest potential temperature of the seeding
site equaled the coldest potential temperature at flight
level. Beyond that level only trace amounts were found.
The normal temperature profile of an upwind sounding
never shows variations like those illustrated in Fig. 3
and is therefore not a reliable indicator of the highest
level of plume rise, because the mountain barrier can
sometimes lift stable air if the wind momentum is suf-
ficiently great.
Figure 4 presents vertical cross sections of the plumes
in the plane near their middle positions, as shown in
Fig. 2. The approximate terrain within 1.5 km of that
plume center line is given for reference. The left side
of each cross section in Fig. 4 is shaded (with a dot
pattern) to show the approximate altitudes of stable
layers as indicated by profiles like that in Fig. 3. The
individual dots near the plumes indicate the locations
where a pass initially detected (or failed to detect) an

320

AgI or ice particle plume. The horizontal lines indicate
the positions through which an ice particle plume was
detected on passes along the wind. The vertical extents
of the seeding plumes were determined when the air-
craft passes failed to detect the AgI or ice signatures,
or found only weak ones. When such null data were
available, the boundary line in Fig. 4 is solid. When
the boundary can only be approximated, it is dotted.
The approximate vertical thicknesses of the plumes as
they crossed the crests are listed in Table 1. If the top
was not determined, the highest measured position is
given with a plus sign. In one case, the plume continued
to rise and was at a higher position over the southwest
arm than over the northwest arm of the mesa. The
median plume thickness over the crest exceeded 500
m. The range of thicknesses was about a factor of two
from the median.
The twelve experiments illustrated in Fig. 4 are
identified by the letters a-I. In Fig. 4a, an inversion
limited vertical development. An opposing air flow at
the crest prevented the plume from continuing over
the crest in its original flow direction. The downwind
extent in Fig. 4b was not well determined and the
plume may have been transported downhill by subsi-
dence. The plume rose rapidly to the stable layer in
Fig. 4c, but the vertical extent far downwind was not
determined. The same is true for Fig. 4d, where the
plume position over the southwest arm of the mesa
was only measured near the surface. The plume top in
Fig. 4e was near a weak stable layer over the northwest
arm and continued to rise to a second stable layer near
the tip of the southwest arm. In the experiment shown
in Fig. 4f there were no barriers until 4.4 km, and no
pass was made that was above the plume. The strong
flow that created a lenticular cloud in the plume po-
sition in Fig. 4g was probably responsible for the very
rapid rise of the plume to near cloud base. The plume
was below the aircraft on a few passes near the down-
wind escarpment, being transported down by subsiding
air. Another rapid rise is shown in Fig. 4h, where the
plume rose in cloudy conditions, creating ice particles
in air that had no stable layers. The upwind edge of
the plume was well-determined but not its vertical or
downwind extent. In Fig. 4i the stable layer began at
4.6 km, and the plume actually rose to that altitude.
The plume in Fig. 4j was repeatedly detected but only
at one position and altitude; it was not detected for
passes at 4.5 km, but that did not provide much res-
olution for determining the vertical extent in an at-
mosphere that had a stable layer at about 4.4 km. The
plumes in Figs. 4k and 41 rose to the tops of the stra-
tocumulus clouds and were detected by ice particles as
well as by AgI.
In an extreme example not illustrated here, AgI
smoke was released in an open meadow on top of the
mesa at about sunset on 5 February 1986 and was
trapped in a drainage flow. Sensors at a nearby PROBE
station showed rapid radiational cooling at the surface

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