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

834

JOURNAL OF APPLIED METEOROLOGY

VOLUME 34

A Comparison of Seeded and Nonseeded Orographic Cloud Simulations
with an Explicit Cloud Model

MICHAEL P. MEYERS, PAUL J. DEMoTT, AND WILLIAM R. COTTON
Department of Atmospheric Science. Colorado State University. Fort Collins. Colorado
(Manuscript received 21 December 1993, in final form 14 September 1994)

ABSTRACT

Ice initiation by specific cloud seeding aerosols, quantified in laboratory studies, has been formulated for use
in mesoscale numerical cloud models. This detailed approach, which explicitly represents artificial ice nuclei
activation, is unique for mesoscale simulations of cloud seeding. This new scheme was applied in the simulation
of an orographic precipitation event seeded with the specific aerosols on 18 December 1986 from the Sierra
Cooperative Pilot Project using the Regional Atmospheric Modeling System (RAMS). Total ice concentrations
formed following seeding agreed well with observations. RAMS's three-dimensional results showed that the
new seeding parameterization impacted the microphysical fields producing increased pristine ice crystal, aggregate,
and graupel mass downstream of the seeded regions. Pristine ice concentration also increased as much as an
order of magnitude in some locations due to seeding. Precipitation augmentation due to the seeding was 0.1-
0.7 mm, similar to values inferred from the observations. Simulated precipitation enhancement occurred due
to increased precipitation efficiency since no large precipitation deficits occurred in the simulation. These maxima
were collocated with regions of supercooled liquid water where nucleation by man-made ice nucleus aerosols
was optimized.

1. Introduction

Precipitation processes in cloud systems in the Sierra
Nevada have been investigated for many years because
of their importance to the water resources of the area.
The emphasis of these programs, such as the Sierra
Cooperative Pilot Project (SCPP) (Reynolds and Den-
nis 1986), is to examine and quantify the microphysical
and dynamical properties of these orographic cloud
systems and to determine if weather modification is a
viable alternative for enhancing precipitation in these
areas.
This study applied an artificial ice formation param-
eterization specific to the ice nucleating aerosols used
during a portion of the SCPP program for use in a
detailed cloud model. The nucleation processes re-
sponsible for ice formation were quantified based on
detailed laboratory studies performed at Colorado State
University (CSU). A preliminary seeding simulation
that employed a microphysical parcel model was used
to demonstrate the details of ice formation imple-
mented for artificial ice nucleus aerosols into the me-
soscale model and to investigate expected dominant
ice formation modes. The artificial nucleation param-
eterization was then added to the Regional Atmo-
spheric Modeling System (RAMS) developed at CSU,

Corresponding author address: Dr. Michael P. Meyers, Department
of Atmospheric Science, Colorado State University, Fort Collins, CO
80523.

@ 1995 American Meteorological Society

which was used to examine the 18 December 1986
seeded orographic cloud system from SCPP. The W orId
Meteorological Organization's (WMO) Third Inter-
national Cloud Modeling Workshop (WMO 1994)
provided the motivation for examining this case study.
Three-dimensional (3D) nested-grid simulations were
employed to fully evaluate the microphysical evolution
of clouds in both seeded and nonseeded numerical
studies and allowed comparison to the observed struc-
ture of the clouds.

2. Background

a. Historical

Numerical cloud models have been used by a num-
ber of researchers to simulate the effects of various
seeding techniques in different types of clouds. Al-
though one must treat the transfer oflaboratory results
into numerical models with some caution, DeMott
( 1994) has summarized evidence from atmospheric
seeding tests that the laboratory results on ice initiation
processes do correlate with observed cloud micro-
physical effects in many circumstances. The schemes
employed by various researchers to initialize ice for-
mation by man-made ice nucleus aerosols have varied
widely. In some cases these schemes have been based
on a combination of theory and laboratory results that
are not necessarily compatible. Also, due to the lack
of information, the schemes are seldom specific to the
nucleant used in a particular operational or research