WMOD00490

• Title: The Effects of Mountain Lee Waves on the Transport of Liquid Propane-Generated Ice Crystals
• Date: 9/9/1996
• Weather Modification - Doc Type: Report

JOURNAL OF APPLIED METEOROLOGY

VOLUME 35

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AL1Tl'UDE (M)

FIG. 10. Average SF6 concentrations plotted with respect to altitude
for all SF6 plume penetrations from site 7 along the valley track.

can see that it is sampling a different portion of the
wave at different altitudes. If the waves are vertically
in phase (stacked vertically), the assumption is made
that the wave has the same vertical velocities and the
same position directly below the altitude from which it
was sampled. This is much like what is shown in Fig.
4 for the first two streamlines above the mountain, be-
tween 2 and 4 kIn. At higher altitudes the waves are no
longer in phase but tilt upwind with height. However,
it should be remembered that for this study we are only
interested in the lowest 1000 m above the crest and can
disregard anything above 3.2 kIn.

2700

It is necessary, however, to determine the temporal
stability of these lee-wave characteristics if we are to
determine their effects on particle trajectories from
storm to storm or within individual storms. Lee waves
are known to change their wavelength and amplitude
based on changing winds and atmospheric stability. To
determine the magnitude of these changes, soundings
were analyzed from the last several winter field seasons
using the technique shown in Fig. 5. In analyzing these
plots, two recurring patterns appeared. Pattern 1 is rep-
resented in Fig. 7. The two cases included from 1992
were days when SF6 was observed to descend rapidly
into the valley, as observed using an SF6 detector
mounted in a van (Reynolds 1992). Several hundred
parts per trillion of SF6 were observed on 5 March in
the valley. This was one of the largest concentrations
observed during the several winter seasons in which
tracer releases were made. In these situations, the
strong downdraft is followed by a strong updraft as the
balloon begins to pass over the downwind ridge.
The wave velocity pattern shown in Fig. 7 appears
to occur most frequently prior to the passage of the
surface cold front in moderate to strong southerly to
southwesterly winds, as determined from the three
mountaintop weather stations. Bruintjes et al. (1994)
observed a similar relationship over the Mogollon Rim
of Arizona. In fact, the stronger the winds near the ridge
top (inferred from the rapid downwind displacement of
the balloon just after launch), the more intense the
downdraft, as was shown for 17 February 1994.

TABLE 4. Site 9 SF6 plume characteristics observed by aircraft for the valley track.
Plume
width
Dist. site 9 Max SF6 Avg SF6 Alt Temp Avg LWC Wind speed Wind direction
Date Pass e) (Ian) (Ian) (ppt) (ppt) (m) (oC) (g m-3) (m S-I) ("1')
17 February 11 15 5 19.5 57 19 2472 -6.6 0.07 18 184
17 February 13 14 4.8 19.2 89 36 2479 -6.5 0.14 15 181
17 February 15 15 5.1 19.4 86 26 2478 -6.2 0.12 17 184
. 2 March 8 13 4.1 18.0 68 29 2273 2.9 0.00 8 228
2 March 9 22 6.6 17.2 133 58 2267 2.8 0.00 na na
2 March 10 32 11 18.9 135 46 2145 4.0 0.00 8 217
2 March 11 38 11 16.0 140 44 2145 3.8 0.00 7 198
9 March 5 10 3.1 17.7 100 43 2163 6.0 0.00 10 200
9 March 6 19 5.6 17.1 271 70 2229 5.4 0.00 8 205
9 March 7 17 5.3 17.4 182 80 2285 5.0 0.00 11 212
9 March 8 19 6 17.7 130 64 2375 4.2 0.00 10 218
9 March 9 28 9.1 18.0 124 64 2346 4.5 0.00 9 217
9 March 11 19 6 17.5 92 40 2400 4.3 0.00 6 227
9 March 13 12 3.5 17.6 15 6 2484 3.1 0.00 8 222
16 March 5 13 3.9 17.4 33 12 2533 0.7 0.16 16 218
19 March 7 7 2.2 17.4 38 20 2480 3.7 0.00 13 209
19 March 11 11 3.4 17.4 50 28 2178 4.7 0.00 11 229
19 March 12 9 2.6 17.2 24 11 2317 3.6 0.00 10 204
19 March 13 5 1.4 17.3 24 11 2476 3.3 0.00 15 215
Average 17 5.2 17.6 93 36 2352 2.2 0.03 11 208