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

311

DESHLER, REYNOLDS AND HUGGINS

b

a

c

d

FIG. 16. Photomicrographs of snow crystals collected at KGV at: (a) 1847, (b) 1937, (c) 1947, and (d) 1957.

duced at -60C ICC > 100 L -1 20 to 30 min after
release in a cloud with droplet concentrations of 100
cm -3. The errors in these concentrations would come
primarily from statistical counting errors and thus be
< 10 percent. The droplet size distributions shown in-
dicate clearly that ice particles were not affecting the
FSSP measurements, and the low liquid water contents
and small drop sizes indicate that the 2D-C concen-
trations result from ice particles. This ICC is much
higher than expected from ice nucleation activity mea-
surements at this temperature. Finnegan and Pitter
( 1987) and Blumenstein et al. ( 1987) have speculated
that transient water supersaturations can increase both
the yield and rate of nucleation. Perhaps supersatura-

tions induced by combustion of the acetone in solution
with AgI produced excesses of water vapor within the
treatment volume.
(ii) The seeded plumes dispersed at approximakly
1 m S-I, increasing slightly as the particles descended
(Table 2).
(iii) Seeding produced small compact crystals that
rimed substantially before fallout. Growth rates inferred
from aircraft and surface observations averaged 0.3 ~m
S-I, with a maximum of 0.6 ~m S-I. These agree with
the theoretical results of Heymsfield ( 1986).
(iv) Liquid water decreased in S2 as the ice crystals
grew.
(v) Aggregation appeared to be important in one

FIG. 15. Measurements collected at KGV of precipitation rate and surface snow crystal concentrations, size, habit, riming, and aggregation.
The PPE for PI-P3 and SI-S3 are indicated at the bottom. The bottom two panels display snow particle concentrations from the 2D.C as
a function of size and habit. The top of each shaded region represents the concentration of all particles, ~ to the indicated size, or all habits
up to and including the one indicated. The size (",m) and habit of particles are defined with the grey scale at the right. The habits are
determined from the images using the method of Holroyd ( 1987). The top panel presents observa,tions from the photomicrographs, with
15 min resolution. The symbols are defined as: ( + ) habit observed, (*) habit observed in aggregates. A large symbol ( + or *) indicates the
predominant habit. Riming characteristics for each habit are indicated using: O-zero, L-light, M-moderate, and H-heavy. Precipitation
rate from a high resolution load cell is given by the dashed line in the top panel.