WMOD00567

• Title: A Report on Winter Snowpack-Augmentation
• Date: 11/11/1988
• Weather Modification - Doc Type: Report

Bulletin American Meteorological Society

1291

TABLE L Recent or ongoing winter orographic research programs with a strong in situ and remote-sensing observational base.

Project

Agency 1 Funding

COSE (Colorado
Orographic Seeding
Experiment)
CRADP (Colorado River
Augmentation
Demonstration Program)
SCPP (Sierra Cooperative
Pilot Project)

Colorado State
Univ./NSF

Bureau of
Reclamation

Bureau of
Reclamation

Utah

Multiple
Agencies/NOAA

Bridger Range

NSF/Bureau

for the additional ice particles produced by seeding to grow
and reach the surface.
To test item (b) and to design a physically sound method for
utilizing SLW in augmenting precipitation it is critical that the
horizontal and vertical distributions of SLW in clouds be mea-
sured.
Various means exist for determining the presence of SLW in
clouds. The existence of SLW can be inferred from the obser-
vation of water-saturated layers on a rawinsonde sounding (Heg-
gli and Rauber 1988), or from the existence of rime on snow-
flakes falling on the upper elevations of a mountain (Deshler
et aI. 1986). More direct observations are available, in situ,
from aircraft platforms with on board SLW sensors (Heggli et
aI. 1983; Rauber and Grant 1986), from icing-rate meters lo-
cated at mountain top level (Henderson and Solak 1983), or
through the use of dual-channel microwave radiometers (Hogg
et aI. 1983). Measurements of SLW using combinations of these
observing techniques within the physically based programs dis-
cussed in the introduction have led to the development of con-
ceptual models of SLW production over a mountain. I
Recent studies on the west coast of the United States, spe-
cifically those conducted in the Sierra Nevada of California by
Heggli and Rauber (1988) and Reynolds and Kuciauskas (1988),
have provided detailed observations of SLW in storms there.
The northwest-southeast (3300-1500) orientation of the Sierra
Nevada in the vicinity of Lake Tahoe has a direct influence on
SLW production, based on the local zonal or meridional char-
acter of the large-scale flow.
For storms impacting the Sierra Nevada in a strong west-to-
east zonal flow, liquid water tends to occur postfrontally both
in response to the veering of low-level winds (more perpen-
dicular to the mountains) and the removal of an upper cold

1 For the presence of SLW in mountain clouds in winter, Hill (1986)
has noted the importance of flow normal to a given mountain barrier,
the necessity for warm cloud tops, and weak, embedded convection.
It is generally required that the passage of a transient synoptic-scale
weather system occur in order to provide the large-scale setting for
moist low-level flow into the mountain. It has been observed that SLW
can occur during any portion of a storm. However, there appear to be
certain periods within a storm that produce the highest amounts of
SLW, depending on storm trajectories and the stage of development of
the storm.

Period of
operation

Primary
study area

Winter 1981182
Winter 1984/85

Park Range Colorado

Winter 1982/83
through
Winter 1985/86
Winter 1976/77
through
Winter 1986/87
Winter 1980/81
Winter 1982/83
Winter 1984/85
Winter 1986/87
Winter 1984/85

Grand Mesa, Colorado

Central Sierra Nevada

Tushar Mountains, Utah

Bridger Range, Montana

SEEDING PROGRAMS

o caSE
o . Climax
CRADP

o PHYSICAL
. STATISTICAL

FIG. I. Schematic depiction of project site locations for both
physical (open circles) and statistical (dots) projects referred to in
this paper.

cloud shield that might act as a natural seeding source. Pre-
frontally, the combination of cold, deep clouds and winds blow-
ing along the barrier, sometimes as a strong southerly jet (Parish
1982), minimizes excesses in SLW (see figure 2a).
The zonal-type storms are usually in the dissipating stage as
they impact the Sierra Nevada. Browning and Monk (1982)
have developed a conceptual picture of the frontal structure for
this type of storm that provides a framework for identifying
potentially seedable locations (see figure 3). As mentioned, in
this postfrontal region several factors contribute to excesses in
SLW. First, the warming of cloud tops limits primary-ice pro-
duction. Second, the dry air overriding the moist low-level air
increases potential instability, an instability that manifests itself
in the form of weak embedded convection as the moist low-
level air is forced over the mountain. This instability contributes
substantially to SLW production. Third, the winds are more
barrier-perpendicular postfrontally, increasing the orographic
lift. Reynolds and Kuciauskas (1988) show that even the more
vigorous storms with a more classic cold-frontal structure have
the same tendency for SLW production postfrontally. Figure 4
shows an example of such a storm.. Note the general increase