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

APRIL 1995

839

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MEYERS ET AL.

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FIG. 3. Parcel model simulation temporal profiles of (a) temperature, (b) percent water supersaturation, (c) droplet concentration, (d)
average droplet radius, and (e)-(h) cumulative ice crystal concentrations nucleated by different ice-formation modes. Dashed curves are for
the presumed aircraft generator particle size distribution. The dash-dot curves presume the size distribution given by DeMott et al. (1983)
for a ground-based generator. The solid curves are from the parameterized equations (aircraft generator PSD).

The detailed equations from DeMott ( 1994) were used
in these simulations. It is seen that the size distribution
can be quite important to the realized effect of seeding
on ice crystal concentrations. A factor of 3 decrease in
concentrations was noted when the size distribution
measured from a ground generator was used instead
of the typical airborne generator distribution. The
somewhat lower ice nucleus activity of the smaller
aerosols produced qy aircraft units was more than
compensated for by the increased numbers of smaller
particles. The airborne generator size distribution was
deemed most appropriate for use in the mesoscale sim-
ulation. Figure 3 also demonstrates the excellent per-
formance of the parameterized descriptions of ice-for-
mation processes. The simulation results using the pa-
rameterized equations developed for use in the
mesoscale model are indicated by the solid curves in
Fig. 3. These results are directly comparable to the
dashed curves (detailed equations, aircraft generator
PSD). Only a very slight tendency for the parameter-
ization to overestimate ice formation was found.

Figure 3 also demonstrates that the initial concen-
trations of ice nucleated when AgI-AgCI aerosols are
released directly into a wintertime orographic cloud
will come primarily from deposition and condensation-
freezing events and will therefore strongly depend on
the ambient humidity or any higher supersaturation
potentially generated during combustion (Finnegan
and Pitter 1988). For the initial conditions specified,
deposition and condensation freezing contribute nearly
30 L -I, representing approximately 0.02% of the total
aerosol almost instantly for the aircraft generator PSD.
Condensation freezing was less important at later times
despite the decrease in temperature because water su-
persaturation decreased to near zero. Deposition con-
tinued to generate significant numbers of ice crystals
as the temperature decreased and ice supersaturation
increased. Contact freezing was predicted to be a minor
player for the first several minutes after seeding because
collection rates are slow in the orographic cloud and
nucleation efficiencies are lower at warmer tempera-
tures. This mechanism began to dominate ice forma-