WMOD00539

• Title: Reply
• Date: 6/6/1991
• Weather Modification - Doc Type: Report

Reprinted from JOURNAL OF ApPLIED METEOROLOGY, Vol. 30. No.6. June 1991
American Meteorological Society

Reply

TERRY DESHLER .

Depanment of Physics and Astronomy, University of Wyoming, Laramie, Wyoming

DAVID W. REYNOLDS
United States Bureau of Reclamation, Sacramento, California
23 November 1990

Finnegan and Pitter ( 1991 ), hereafter referred to as
FP, correctly point out some errors we made in de-
scribing the seeding material used in an orographic
cloud (Deshler and Reynolds 1990, hereafter referred
to as DR). We were careless in describing both the
combustion process and the nucleating aerosol pro-
duced in the combustion of the AgI NH4I NH4Cl04
acetone solution used for seeding. This is true both in
DR and in Deshler et a1. (1990) where seeding effects
from an experiment on ,i8 December 1986 are de-
scribed, four days prior to the experirnent described by
DR, which used the same seeding material and pro-
cedure. In keeping with the results of DeMott et a1.
( 1983) the nucleating agent used in DR and in Deshler
et a1. should have been stated as AgI-AgCl rather than
as AgI NH4I NH4Cl04. It is important to reiterate that
it is this AgI-AgCl aerosol that provides the irnproved
ice nucleating properties at the warmer temperatures
as FP point out.
We do, however, take issue with FP concerning the
mechanism for production of the regions of enhanced
ice crystal concentration (ICC), which were observed
60-90 rnin after seeding. Based on the width of the
regions of increased ICC, DR calculate a dispersion
rate of 1.3 m s -I for the seeding material, directly corn-
parable with other similar measurements (Hill 1980).
To suggest that these regions of high ICC have occurred
naturally requires an explanation of why the effect
would be limited to such a local region. In particular,
there is no reason to expect natural ice nuclei to be
confined to such a small region. Ice multiplication, on
the other hand, may occur in a small region of cloud
where the conditions are appropriate. There were many
instances during the Sierra Cooperative Pilot Project
(SCPP) when ice multiplication processes were mea-
sured (an example is given by Deshler et a1. 1990);

Corresponding author address: Dr. Terry Deshler, Department of
Physics and Astronomy, University of Wyoming, P. O. Box 3905,
Laramie, WY 82071.

however, in all cases the width of the region of high
ICC implied dispersion rates of more than 4 m S-I,
assuming the ice crystals originated at the location of
seeding. This dispersion rate is much higher than has
been measured during this or other seeding experi-
ments. In addition at 60 min after seeding on 22 De-
cember 1986 the ice crystal habits, cloud-droplet sizes,
and temperatures were not appropriate for a Hallett
and Mossop (l974) ice multiplication mechanism, the
type most commonly observed during the SCPP. Based
on the 2D-C images collected 60-90 min after seeding
on 22 December 1986, the regions of increased ICC
were composed of small plates and rimed particles,
whereas the surrounding ice crystal regions were dom-
inated by larger dendritic crystals, singly and in aggre-
gates. Needles, which may be expected from a Hallett-
Mossop ice multiplication mechanism, wc~re not ob-
served. Given the coexistence of the regions ofincreased
ice nuclei and ice crystal concentration, and that the
widths of the regions of increased ICC are comparable
to other measurements of seeded plurnes (Deshler et
al. 1990; Hill 1980), the most straightforward conclu-
sion is that the increases in ICC are a direct result of
seeding.
It is a more difficult rnatter to identify the primary
or for that matter the multiple nucleating mechanisms
that may have occurred during the experiment de-
scribed by DR. We agree with FP that during the first
5 to 12 min after seeding there is no indication of seed-
ing effects. These observations are consistent with con-
tact nucleation as the nucleating mechanism, since only
1 l-I of ice crystals should be observed after 10 min
frorn seeding in a cloud of 100 cm.-3 of 5-Mm cloud
drops (Stinn 1971), assuming dispersion rates for the
seeding material of I m s -[ horizontally and 0.1 m S-1
vertically. (The 40 l-I of ice crystals calculated by DR
was based on these same di~persion rates, but assuming
that 80% of the ice nuclei had activated without regard
to the details or timing of the mechanism.) After 60
min, however, a seeding effect is apparent and the