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

MEYERS ET AL.

841

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FIG. 5. (a) The simulated pristine ice mixing ratio on grid 2 [x-z cross section along the line of y = 28 of grid 2 (see Fig. 4)] at 1730 UTC
(nonseeded case), contour interval is 0.1 g kg-I. (b) The simulated aggregate mixing ratio on grid 2 at 1730 UTC, contour interval is
0.01 g kg-I. (c) The simulated graupel mixing ratio on grid 2 at 1730 UTC, contour interval is 0.1 g kg-I. (d) The simulated rain mixing
ratio on grid 2 at 1730 UTC, contour interval is 0.05 g kg-I. The abscissa is the x direction (km), the left ordinate is the z direction (km),
and the right ordinate is temperatures (oC). The x and y labels on this and subsequent figures denote horizontal distance (km) from the
center of the coarsest grid.

phoresis, and diffusiophoresis. In the version of RAMS
used in this study, cloud droplet concentration was
specified, and average diameter was diagnosed. Collec-
tion rates for contact freezing were computed based
on average cloud droplet diameter and mean aerosol
diameter following Cotton et al. ( 1986). The diameter
of natural contact-freezing aerosols was assumed to be
1 X 10-5 cm. Secondary ice formation by the Hallett-
Mossop mechanisms was quantified as described by
Cotton et al. ( 1986).
Since cloud water was diagnosed in the mesoscale
simulations, water supersaturation would always be
zero. However, since artificial nucleation due to con-
densation freezing depends on water supersaturation,
a diagnostic scheme was used to calculate water satu-
ration. It was assumed that the supersaturation results
from a balance between the dominant removal mech-
anism (the diffusional growth of cloud droplets and
ice crystals) and the dominant production mechanism
(the moist-adiabatic cooling due to vertical motion)
(Cotton et al. 1986).

b. Model setup

The setup of the 3D nested-grid domain is shown
in Fig. 4 and includes 48 X 38 X 48 grid points on the
two coarsest grids (grid 1 and grid 2), with 50 X 50
X 49 on the fine grid (grid 3). The horizontal grid
spacing is 16 km on grid 1, 4 km on grid 2, and 1 km
on grid 3. The vertical grid spacing was stretched from
100 ill in the seeded region to 800 m near model top.
A 30-s topography dataset was used. A long time step
of 30 s was used on the coarsest grid with a 5-s time
step on the fine grid. An inhomogeneous initialization
from NMC gridded data from 1200 UTC 18 December
and 0000 UTC 19 December 1986 was used. Both the
seeded and nonseeded simulations were run out to 2200
UTC and include full microphysics. The seeded run
was restarted at 1900 UTC from the nonseeded sim-
ulation, at which time ice nucleus aerosols were intro-
duced into the model. Only one release of ice nucleus
aerosols was done in the seeded simulation, which is
different from the 3 releases that occurred in the field