WMOD00293

• Title: Summary of the NOAA/Utah Atmospheric Modification Program: 1990-1996
• Date: 9/1/1998
• State: UT
• Weather Modification - Doc Type: Report

was trapped near the valley for extended periods. However, storm periods with relatively
abundant SL W over the Plateau and embedded convection present usually also had valley AgI
transport to the Plateau top. But effective ice nuclei concentrations from valley-released AgI were
u~~ally quite small at prevailing cloud temperatures.

1. On some occasions, the gravity wave mechanism transported valley-released AgI over the Plateau
in spite of valley-based inversions. The timing and frequency of gravity waves, and the specific
surface locations affected by them, are all uncertain, but this mechanism is sometimes important
in vertical transport pfthe AgI aerosol.

J. High altitude AgI generators have at least two advantages over valley-released generators in
addition to their ability to routinely target the intended cloud zones. The concentrations of AgI
and resulting ice particles from high altitude generators were usually much greater, as monitored
along the Plateau top and above the Plateau by aircraft. The results of model simulations were in
agreement with these observations. In addition, high altitude. generators were usually located
within cloud or just below cloud base. The AgI generators produce a large water by-product from
combustion of propane and acetone. The resulting high supersaturation very near the generators
allows for instantaneous activation by the condensation-freezing mechanism (Finnegan and Pitter
1988, DeMott et a1. 1995). Thus, under favorable conditions, embryonic ice crystals may be
formed immediately downwind of the generators, providing important additional time for growth
to snowflake sizes. The condensation-freezing 'mechanism is unlikely to occur with valley-
releases of AgI, and if ice crystals are occasionally formed because valley fog is present, they will
not survive to orographic cloud altitudes. .

k. Disadvantages of high altitude AgI generators include the practical difficulties of installing and
maintaining them at remote locations, and limited horizontal plume dispersion. While aerosol
from high altitude generators will routinely be transported over the mountain barrier, cross-wind.
spacing of such generators should not exceed perhaps 5 kIn if most of the SL W condensate zone is
to be affected. However, it may be more important to routinely seed a portion of the SLW cloud
than to sometimes seed more of it, but only with weak AgI concentrations~, Vertical dispersion
from the high altitude generators appeared similar to that from the valley-based generators, but
this impression may be partially based on the much greater INC found during aircraft sampling
within high altitude released plumes. Valley-released plumes generally had weak INC at aircraft
altitudes.

1. A seeding solution different from that used in the operational seeding program was tested in the
CSU laboratory. This solution produces an AgICI-0.125NaCI rather than AgI aerosol. It can
nucleate ice crystals by the condensation-freezing mechanism rather than the contact nucleation
mechanism by which operationally used AgI aerosol operates (in the absence of supersaturation).
This fast-acting aerosol was shown to increase the number of effective seeded ice nuclei by over
an order of magnitude in the limited time available for transport through orographic ~L W cloud.
It is strongly recommended that the operational seeding program <use this improved solution. This
is one of a number of actions which could increase concentrations of effective ice nuclei over
Utah's mountain barriers.

m. Numerous physical seeding experiments demonstrated that sufficiently great AgI concentrations
exposed to sufficiently cold SL W cloud will produce abundant ice particles. Ice particle
concentrations were similar to those expected, based on earlier laboratory results, tending to
verify laboratory findings in actual orographic cloud. When obvious seeding-caused snowfall

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