WMOD00490

• Title: The Effects of Mountain Lee Waves on the Transport of Liquid Propane-Generated Ice Crystals
• Date: 9/9/1996
• Weather Modification - Doc Type: Report

Reprilllted from JOURNAL OF ApPLIED METEOROLOGY, Vol. 35, No.9, September 1996
American MeteorolORical Society

The Effects of Mountain Lee Waves on the Transport
of Liquid Propane-Gener~llted Ice Crystals

DAVID W. REYNOLDS

U.S. Department of the Interior Bureau of Reclamation, Sacramento, California

(Manuscript received 16 September 1994, in final form 7 September 1995)

ABSTRACT

A combination of rawinsonde balloon ascent rates, low-elevation aircraft, and ground-based tracer sampling
measurements are presented. These data indicate that mountain-induced gravity waves have a significant impact
on the transport of ice crystals produced by the release of liquid propane from high-altitude dispensers along
the crest of the northern Sierra Nevada in California. Special rawinsonde launches were made just downwind of
the main Sierra Nevada crest. Balloon ascent rates show a very well defined mountain lee wave present during
most precipitation events. Strong descent to the lee ofthe Sierra will thus have a detrimental effect on the growth
of particles generated on the crest. The tracer SF" (sulfur hexaflouride) is used to simulate the transport and
dispersion of propane-generated ice crystals. Sulfur hexaflouride was released from two propane dispenser sites
as a proxy for seeded ice crystals. Aircraft measurements of SF" indicated that at the normal flight altitudes of
2500 m over the downwind valley and 2800 m over the downwind ridge the aircraft was flying near the top of
the plumes. When the aircraft was able to fly below cloud base, near the release altitude of 2200 m, substantial
SF" was observed, The lower portion of the plume was also observed to descend into the valley some 700 m
below the release altitude. A simple two-dimensional model is used to determine the impact that these gravity
waves have on particle trajectories. Model output is presented for one well-documented seeding case to determine
how well such models might be used operationally to predict particle trajectories downwind of the Sierra.

1. Introduction

The Lake Oroville Runoff Enhancement Project
(LOREP) was a three-year randomized seeding exper-
iment conducted by the California Department of Wa-
ter Resources (DWR) for the winters of 1991/92
through 1993/94. The long-term goal of the project
was to increase runoff to Oroville Reservoir, the main
reservoir of the State Water Project, located in northern
California. Physical studies were conducted in the hope
of documenting the magnitude of the increases possible
in the seasonal snowpack, obtained by seeding winter
storms using ground-based liquid propane. Statistical
analyses were also considered, but the sample size ob-
tained during this three-year program proved to be too
small. Reynolds (1989, 1991, 1992, 1993, 1994) de-
scribes the original project design, the development,
and the testing of a remote. propane dispenser and pre-
liminary field studies emphasizing transport and dis-
persion from several propane dispenser sites. These
preliminary studies indicated that the tracer SF6, re-
leased from two different propane dispenser sites, was

Corresponding author address: David W. Reynolds, Stop 5, Na-
tional Weather Service, 21 Grace Hopper Blvd., Monterey, CA
93953.

~~ i

transported rather quickly down the lee slope of the
Sierra due to the presence of a well-defined mountain
lee wave.
As has been emphasized repeatedly in Reynolds
( 1988) and Super (1990), for example, it is imperative
that transport and dispersion studies be conducted
within any intended cloud seeding project area. These
studies are needed to determine whether or not ground-
based seeding can place a sufficient number of ice em-
bryos into supercooled liquid cloud regions for long
enough to produce substantial crystal growth for fallout
within the intended target area.
The LOREP area is located in the northern Sierra Ne-
vada, as shown in Fig. 1. Ten liquid propane dispensers
were positioned along the crest of the Sierra, as shown in
Fig. 2 (dl-dl 0). Liquid propane was chosen because the
temperature of the liquid water, as deterinined from
mountaintop icing rate meters, was wanner than -40C
80% of the time. Conventional silver iodide solutions re-
quire t1emperatures at or below -60C to be activated as
ice nuclei, while propane has been shown to be an effec-
tive seeding agent at temperatures close to OOC (Hicks
and Vali 1973). Figure 3 shows icing results from two
of the three project mountaintop icing stations. Liquid
water concentration from the icing rate meter is derived
using the technique of Hindman (1986) from averages
taken over individual storms.