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

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The size of the largest particles produced by CO2
seeding was also very consistent. Particles as large as I
mm were seldom observed at the aircraft flight altitude,
300 m below the top of the seeded curtain. The case
studies presented earlier indicate that for these seeding
temperatures compact plates and columns are nu-
cleated and quickly begin to rime and fall. This is sup-
ported by the model calculations ofHeymsfield ( 1986).
For rimed plates and columns fall velocities greater
than the general updraft of 0.3 m S-1 are quickly at-
tained, so they fallout sooner and at a smaller diameter
than branched crystals or crystal aggregates. For ex-
ample, a rimed column 0.5 mm in diameter has a fall
velocity of 0.75 m S-1 (Locatelli and Hobbs 1974),
equivalent to a rimed dendrite nearly 2 mm in diam-
eter. One notable exception to the observation of uni-
formly sized small crystals was the AgI seeded case of
22 December 1986 where crystals in excess of I mm
were found in the plume 42 min after seeding, and 3-
4 mm crystals were observed 81 min after seeding. The
corresponding growth rates, assuming these crystals
nucleated near the time of seeding, were 0.55-0.72}Lm
s -1, which are reasonable for the temperature range
(-6 to -120C) through which the crystals ascended.
Since with CO2 seeding ice nucleation is instanta-
neous, ice crystal growth rates can be determined by
estimates of ice crystal sizes from 2D-C images. These
growth rates are included in Table 6. Similar estimates
were not made for cases of AgI seeding since the time
of ice crystal nucleation is unknown. The estimates of
crystal growth rate were quite variable; however, some
import.:'lnt consistencies were noted. Except for one
penetration on 5 February 1983, all growth rate esti-
mates were less than 1 }Lm s -I. This agrees with the
growth rates (0.25-0.75 }Lm s -1) measured by Ryan et
a1. (1976) for temperatures between -7 and -l3oC.
According to Ryan et aI., growth rates only exceed I
}Lm S-I for temperatures near -60C, or -13.5 to
-180C. Neither of these temperature regions is well
represented by the data in Table 6. One exception is
18 December 1986 when the aircraft sampled at
-6.20C. Here the 2D-C images were rimed particles
(probably columns) rather than sheaths that would ex-
hibit the largest axial growth rates.
It was obvious in some cases that growth rates varied,
according to SL W in the seeding plume. On 5 February
1983 measurements in different curtains of equivalent
ages showed that growth rates were higher when more
SL W was available. Examples from Table 6 are four
curtains sampled on 5 February 1983, two at 28 min
and one each at 16.3 and 17.5 min. In each case the
curtain with higher amounts ofSLW had substantially
larger particles.
In general the growth rates in Table 6 deviated from
laboratory measurements (Ryan et al. 1976) as the time
after seeding increased. The largest observed crystals
from measurements within 10 min of seeding were in
good agreement with laboratory values. Examples are: