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

30g:.

. JOURNAL OF APPLIED METEOROLOGY

VOLUME 29

5.0

86l 18

KBAND TIME -HT

KGV

I
I . III
i 3.O~1~~

4.0

:~

2.0

t t t
PI P2 P3

1.0

.MDS TO -60 dBM Ill-50 TO -54 dBM

Ell-55 TO -59 dBM ~-45 TO -49 dBM

0.0
\800

1830

1900

I
"IIM"
I.~"J
I~....fi;i

.\~

TERRAIN LEVEL - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
t t t
SI S2 S3

1930

2000

2030

2100

TIME (UTe)

FIG. 14. Time-height profile of measurements from the vertically pointing Ka.band radar at KGV, annotated with
the PPE for PI-P3 and SI-S3, Note that the black shading corresponds to the minimum signal detected by the radar.

. with S I, and which followed S3, can be seen affecting
Westville along with most of the other gauges.

(iv) Surface measurements at the fixed target

One of the primary purposes of the fixed target ex-
periment was to target seeding effects to a fixed spot
on the ground. This served two purposes: first to doc-
ument with detailed surface observations the charac-
teristics and magnitude of seeding effects impacting
the barrier, and second to develop a targeting model
that, may be transportal?le to other watersheds.
At the fixed target, KGY, the Ka band (8.6 mm)
radar was operated to provide a direct link between
seeding effects within the cloud and precipitation falling
at the surface (Hobbs et al. 1981). A time-height profile
ofKa radar data collected at KGY is shown in Fig. 14.
For the first experiment there were indications of pre-
cipitation particles falling from near 4 km where den-
drites were observed by aircraft. Dendrites were also
observed at the surface. The lower cloud had an echo
top near the 3-km treatment altitude that produced
columns, needles, and sheaths. During the second ex-
periment, the natural precipitation features that pre-
ceded S I and followed S3 are quite obvious. Also ap-
parent is the region of no echo surrounding S2; and
there is the suggestion of a radar return emanating from

3 km and impacting the surface at the PPE for S2.
Recall that on the SHR radar a distinct line echo was
observed to advect with S2 from 31 min after treatment
until after it had passed KGY. Thus, the timing of S2
to overhead at KGY is very good.
Information about ice crystal size, concentration,
habit, and degree of rime or aggregation was also col-
lected at KGY. The results of these observations are
shown in Fig. IS. The bottom two panels display ICC
as cumulative size and habit distributions. The 2D-C
size and concentration measurements have been cor-
rected for sampling errors due to aspiration (Deshler
1988). The 2D-C images were classified by habit using
a method developed by Holroyd (1987). With Hol-
royd's method three habits-irregular, tiny, and hex-
agonal-have dominated the surface 2D-C measure-
ments collected during the SCPP while two habits rarely
appear-aggregates and dendrites. This fact is evident
on this day, although a relatively high concentration
of needles was observed as were some periods of
graupel.
For the placebo experiment there was a relative in-
crease in particle concentration to about 10 L -1, above
a background of 5 L -1, only with the arrival of PI. No
dominant habit was identified, although there were in-
creases in the concentration of branched planar crystals,