WMOD00528

• Title: The Feasibility of Enhancing Streamflow in the Silver Iodide in the Sevier River Basin of Utah bt Seeding Winter Mountain Clouds
• Date: 12/1/1991
• Weather Modification - Doc Type: Report

Only limited analysis of the recent Wasatch Plateau measurements has been accomplished. However,.
some initial impressions will be noted throughout the following discussion by the lead author of this report
who was involved in the Wasatch Plateau field efforts.

A final report by Hill (1982b) discusses the first Tushar Mountains field effort and resulting findings.
Final reports edited by Long (1984, 1986) and Huggins (199Oa), plus a final report authored by Huggins
(199Ob), all present results from more recent field projects in the Tushars. Several conference papers and
joumal articles have dealt with specific findings in detail (e.g., Huggins et al., 1989; Long et al., 1990;
Sassen et al., 1990; Uttal et al., 1990).

Since results from the Tushars are well documented in fmal reports and publications, only the major
highlights will be given here. The main emphasis of most Tushar field programs was to describe the
spatial and temporal distributions of SL W in orographic clouds and to understand the physical processes
that produced and depleted the SL W. It was appropriate to first consider the availability of SL W since
it is needed for cloud seeding to be effective.

3.2.1 Supercooled liquid water observations. . A rather consistent portrayal of SL W distributions
has emerged from the Tushar Mountain obselVations. This portrayal is in good agreement with
impressions gained on the Wasatch Plateau during early 1991, and with published results from locations
near Utah. Examples of other recent studies of SL W in the intennountain West include results from the
Parle Range of Colorado (Rauber et al., 1986), the Grand Mesa of Colorado (Holroyd and Super, 1984;
Boo and Super, 1986; Thompson and Super, 1987), and the Mogollon Rim of Arizona (Super and Boo,
1988a; Super and Holroyd, 1989). Supercooled liquid water investigations from as far away as California
(Heggli and Rauber, 1988) are in general agreement with the Utah portrayal discussed below,

Most of the Utah measurements were made by microwave radiometers. However, lidar obselVations of
the base of the liquid cloud, aircraft measurements of cloud water content, "cloudsonde" data on the
vertical distribution of SL W and surface icing probe obseIVations all have added to the overall
rel!resentations of SL W episodes. The first point is that SL W was obselVed for many hours each winter.
Most stonns had at least some SL W present during portions of their passage.

Vertically integrated values of SLW were below 0.1 nun more than half the time that cloud liquid was
obseIVed. A value of 0.1 mm is equivalent to only 0.1 g m-3 if the water is unifonnly distributed through
1 kIn depth of cloud. Thus, liquid water amounts often are limited. Nevertheless, the high frequency of
occurrence of low SL W values results in these periods producing a large fraction of the seasonal SL W
flux, Occasionally, 15-min means of SLW exceeded 0.5 mm, and rarely 1.0 mm. These less frequent
periods with abundant SL W usually resulted from convection. The wetter periods also produce a large
fraction of the seasonal flux, so neither the many hours with limited SL W nor the infrequent hours with
abundant SL W should be ignored by a seeding project.

The base of the liquid cloud was most often at or below the 3500-m m.s.l. crest of the Tushar Mountains
(Sassen, 1985) with a corresponding temperature generally between -5 to -10 oC. Liquid bases were
rarely more than 1000 m above the crestline. Analysis of saturated layers from rawinsonde data, presumed
to contain most of the SL W, indicated such layers seldom extended more than 2000 m above the crestline.
ObselVations of AgI plumes from high altitude sites have indicated most of the seeding material is found
within 700 m of the barrier in the absence of convection (Holroyd et al. 1988). Assuming the situation
is similar for the Tushars, the SL W zone sometimes would be too wann for effective nucleation, expected
to occur at cloud temperatures colder than about -8 oC. The exact temperature at which an adequate
concentration of ice nuclei will exist is a function of several variables including generator output and the

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