WMOD00318 ‎(2)‎

• Project Name: Sierra Cooperative Pilot Project
• Title: A Comparison of Seeded and Nonseeded Orographic Cloud Simulations with and Explicit Cloud Model
• Prepared By: Michael P. Meyers, Paul J. DeMott and William R. Cotton
• Date: 4/4/1995
• Weather Modification - Doc Type: Report

836

JOURNAL OF APPLIED METEOROLOGY

VOLUME 34

1730 UTC 18 December 1986 Microphysical Conceptual Model

5

4

! 3
i
'Ol
..Cl

2

o

-20

o

20 40 60
horizontal distance (kIn)

100

80

120

FIG, 1. Cross section of the microphysical structure over the barrier for 1730 UTC 18 December
1986 adapted from Deshler et al. (I 990). Microphysical characteristics from aircraft measurements
are highlighted by stippled areas, Cloud boundary and temperature structure is also included. Sheridan
is located near the O-km distance, and Kingvale is located near the 80-km distance.

oratory and in the field generators lead to some amount
of uncertainty in specifying seeding for numerical cal-
culations (DeMott 1991; Deshler and Reynolds 1991).
This sensitivity was also examined in this paper.

3. Parameterization of artificial ice nucleation
processes

a. Development of parameterization

An important goal of the study presented in this
paper was to explicitly specify ice formation by the
actual artificial ice nuclei (IN) used in the SCPP pro-
gram for the sim ulation that follows in section 4c. This
detailed approach has not previously been used for
mesoscale simulations of case studies of clouds seeded
for precipitation enhancement. Ice formation is quan-
tified as a fraction of available IN (based on conser-
vative mixing ratios of seeding material released in
known quantities) determined for deposition, conden-
sation-freezing, immersion-freezing, and contact-
freezing nucleation processes based on the laboratory
studies of DeMott (1990, 1994). Deposition nucleation
is used to describe ice formation ascribed only to a
response to ice supersaturation. Condensation freezing
is the instantaneous ice nucleation response of aerosols
to water supersaturation, while immersion freezing de-
scribes the freezing of cloud droplets containing pre-

viously scavenged (by condensation or collection) ice
nucleus aerosols. Contact freezing describes the freezing
that instantaneously follows collisions between ice nu-
cleus aerosols and cloud droplets caused by various
transport mechanisms that are not instantaneous. The
experimental results represented in the equations pre-
sented by DeMott ( 1994) go far beyond the results of
DeMott et al. ( 1983), which quantified primarily the
contact-freezing activity of AgI-AgCl aerosols. Depen-
dencies on temperature, humidity, and particle size
were included as appropriate for each process.
For implementing explicit seeding into RAMS, the
size dependence was first removed from the equations
given by DeMott (1994) by integrating them over a
particle size distribution considered to be representative
of that produced operationally. Unfortunately, the size
distribution of AgI-AgCl aerosols produced by the air-
borne solution-combustion generator used in the SCPP
case study was not measured. Two possibilities were
considered. DeMott et al. (1983) presented an AgI-
AgCl size distribution characteristic of a ground-based
solution-combustion generator. A size distribution
more characteristic of airborne solution-combustion
generators was also estimated for AgI-AgCI aerosols.
This distribution was based on years oflaboratory data
collected for airborne generators burning in a wind
tunnel flow simulating flight speeds at the CSU Cloud
Simulation and Aerosol Laboratory. Both distributions