WMOD00508

• Title: Physical Response of Winter Orographic Clouds over the Sierra Nevada to Airborne Seeding Using Dry Ice or Silver Iodide
• Date: 4/4/1990
• Weather Modification - Doc Type: Report

APRIL 1990 DESHLER, REYNOLDS AND HUGGINS 303
TABLE 2. Seedline penetrations on 18 December 1986 by the cloud physics aircraft.
Penetration Age of Mean Mean Mean
Advection time seedline Dispersion 2D.C 2D-P FSSP LWC
Seedline (degJm S-l) (min:sec) (min) (m S-l) (L-1) (L-1) (cm-3) (g m-3)
P3 196/17.3 1803:00 0 2 <I <I 0.01
1807:20 5 <I 0 15 0.0
1810:55 8 5.6 8 1 99 0.04
1815:12 12 7.5 16 3 62 0.08
1819:20 16 6.6 16 3 51 0.12
1822:55 20 5.3 17 2 47 0.10
1826:05 23 6.5 19 3 58 0.14
1831:10 28 5.4 20 2 168 0.28
1833:50 31 4.4 23 2 61 0,12
1836:30 34 <I <I 14 0.0
SI 208/17,3 1923:55 43 1.4 42 9 4 0.03
1927:00 46 2.0 47 11 71 0,05
S2 208/15.5 1859:44 9.5 1.3 13 0 47 0.03
1900:25 9 1.7 5 <I 59 0.08
1909:24 19 0.9 32 I 26 0.01
1913:28 22 1.0 45 <I 38 0.02
1921:25 30 0.8 49 4 15 0.01
1929:45 39 0.8 37 6 42 0.03
S3 208/15,5 1856: 10 0 4 <I 87 0.20
1902:05 4 3 <I 67 0.14
1907:54 12 1.5 15 <I 67 0.07
1914:47 19 0.4 15 <I 118 0,20
1919:50 24 0.8 57 4 37 0.06
1931:25 35 0.9 30 7 16 0,01

tion, measurements by the CSIRO and JW (not shown)
liquid water probes are comparable indicating that
there were no cloud drops> 40 JLm, assuming the large
drops are the major cause of differences between the
probes.
The growth of ice crystals in the seedline can be seen
both in the images and in the gray scale plots of ICC,
Fig. 8. The images observed at S2 + 30 and S2 + 39
suggest that aggregation was taking place, accounting

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DIAME TER (mm)

FIG. 10. Particle spectra from the 2D.C and 2D-P during
penetrations ofS2 at 9,5,23,31, and 40 min.

for some of the larger particles indicated on Fig. 8;
however, the predominant particles were small rimed
crystals. Notice that cloud droplet concentrations di-
minished with time in S2; although at 39 min when
the aircraft had descended 300 m below the treatment
level there was substantial liquid water on the edges of
S2, suggesting that particles were falling into higher
liquid-water regions. Composite particle spectra from
the 10 to 39 min penetrations of S2 are shown in Fig.
10. The spectra indicate that small particles continued
to nucleate for 23 min before they diminished, while
the concentration oflarger particles was still increasing
at 39 min, 300 m below the seedline.

10.

(ii) Radar measurements

To search for seeding effects two techniques were
used to analyze the radar data. The simplest approach
was to advect seedlines across PPI elevation scans. To
do this an advection incorporating a wind field chang-
ing with height had to be used. The OTM provided
this capability; although it advects crystals to the surface
whereas echo advection would be faster since it is not
affected by the slower winds at the surface. To account
for the intermediate advection, between that measured
by aircraft and that calculated by the OTM, an average
of the two was used. The result, 14.6 m S-I from 2020,
was used to advect seedlines on the PPI plots. Radar
reflectivity patterns orienting themselves with seedlines
could then be considered evidence of seeding effects.