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 HUGQINS

309

generally dendrites or dendritic arms. During periods region from the bottom on plot of ICC as a function
oflower ICC, such as prior to passage of PI, sheaths, of habit) was diminished. The greatest contribution to
needles, and rimed columns predominated. A photo- increased ICe during the PPE of S 1-S3 came from
micrograph of ice particles collected at KGV at 1847, particles between 0.3 and 0.5 mm, and particles clas-
during the PPE:for PI, is shown in Ff~< 16a. sifj.ect as hexagonal. On the plot of ICC as a function
For tl1e seeded experiment an increase in ICC was of habit compare the measurements at 1935 and 1945.
observed during the PPE for S2 and S3. S l(arrived The concentration of all habits from graupel up to tiny
within the natural precipitation region discuss~d earlier, are identical. The increased ICC at 1945 is solely a
and the increased ICC associated with this feature can result of an increase in images classified as hexagonal,
be seen. Based on photomicrographs at 1937 (Fig. 16b) a category into which small rimed particles fall. The
the predominant snow crystals were dendrites, singly same can be seen comparing the measurements at 1958
and in aggregates, and light to moderately rimM. Based and 2005. Similar features were not observed at other
on the radar and aircraft data, S2 was isolated from times during the period shown, nor were they observed
surrounding natural echo, and thus provides some in- during the PPE of PI-P3. Thus these measurements
sight into the microphysical effects seeding may have are consistent with the photomicrographic observa-
on precipitation at the ground. Based on the beginning tions, and with the calculations of Heymsfield ( 1986),
and end times of increased ICC at KGV, S2 took: 7 suggesting that seeding produces small heavily rimed
min to pass. Thts'is slightly longer than expected based particles. Data from the photomicrographs, shown at
on aIm S-I dispersion rate and a speed of 10 ms-I the top of Fig. 15, are consistent with the measurements
across the site. The maximum concentration within by the 2D-C. Also at the top of Fig. 15 is precipitation
S2 was 13 L - I. Particle sizes from the ground 2D-C rate, from the high resolution gauge located within 10
indicated most particles were between 0.2 and 0.5 mm m o(,the ground 2D-C. Note that increases in precip-
with approximately 2 L -[ of particles between 0.5 and itatibn rates of 0.5 to 1.5 mm h -I above background
1.0 mm. This was comparable to the concentration of were recorded during the PPE for Sl-S3. Given that
particles> 0.4 mm measured with the 2D-P on the these increases occurred over a 7 min period the ad-
aircraft at the 39 min pass in S2. Photomicrographs ditional precipitation received from seeding was 0.06
taken at 1947~t the trailing edge of S2 (Fig. 16c) to 0.18 mm.
showed that the snowfall was composed of heavily It is also interesting to note that the integrated liquid
rimed irregular particles and graupel, with some par- water measured by the vertically pointing radiometer
ticles indicative of heavily rimed columns. The particle~', (Fig. 4) showed a marked decrease just prior to the
were predominantly 0.5 to 1.0 mm. These particle size's PPE of Sl and S2, with a small recovery before S3
and shapes conform well with calculations by Heyms- arrived. It can only be speculated if seeding contributed
field ( 1986 rand Rodi et al. ( 1985). Aggregation was to this decrease in liquid water.
not obvious during the passage of S2. This is diffe,rent
than the aircraft observations of S2; however, 'the pre-
dominant particles observed by the aircraft were small
rimed particles and this agrees with the observations
at KGV. .
A distinct region of higher ICC was observed at KGV
at the PPE for S3; although, radar data showed that
precipitation from the echo that was observed to be
trailing S3 was beginning to affect KGV at this t~me,
The first half of this period of high ICC is simjlatin
width to S2, indicating a duration of nearly 7 rpin.
Photomicrographs taken at 1957 (Fig. 164) inJhe
leading edge of S3 showed the snowfall to be; again,
heavily rimed, almost spherical, particles of 0.5 to 1.0
mm. Dendrites and dendritic aggregates were observed
to be the predominant crystal habits again at 2005.
Although not as clear cut as S2, there is an indication
that a seeding effect and natural precipitation were oc-
cumng III sequence.
Considering the bottom two time series plots of ICC
in Fig. 15, several interesting characteristics of the
seeded plumes can be observed. During the PPE ofS 1-
S3 the small but steady concentration of particles be-
tween 1 and 10 mm (topmost shaded region on plot
ofICC as function of size) and needles (third shaded

'.

4) SUMMARY OF RESULTS

The cloud structure on the 18th was not a simple
orographic cloud but more like what Rangno ( 1986)
has described as an orographic cloud composed of
nimbostratus. The natural precipitation process was
dominated by primary nucleation between -150 and
-20oC, followed by aggregation and riming as den-
drites fell into the lower level liquid water or "feeder"
cloud. Within the shallower liquid-cloud the coales-
cence, process could be identified, but in discontinuous
regions of the cloud, most notably near convection
(Mossop 1985). These regions could also be associated
with columns and needles possibly formed by primary
nucleation in regions of enhanced water supersatura-
tions (DeMott et al. 1983) or by the rime-splintering
mechanism of Hallett and Mossop (1974). These phe-
nomena were documented during experiment I, the
placebo case. Although these processes were occurring,
they did not glaciate the cloud. Liquid water within
these regions remained at 0.1 to 0.3 g m -3.
For the seeded regions of this cloud the major results
were:

(i) An AgI NH4I NH4Cl04 seeding solution pro-