WMOD00591

• Title: Investigations of Winter Storms over the Wasatch Plateau during the 1994 NOAA-Utah Field Research Program - Final Report
• Date: 7/1/1996
• State: UT
• Weather Modification - Doc Type: Report

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

C. 19 February 1994 Case Study
Preliminary results from this case appear in Huggins ( 1996), with the preprint attached to
this report as an Appendix. In addition to several other studies describing seeding effects from Utah
\-\inter storms (. e.g.. Holroyd et aI., 1995: Super, 1995; and Super and Holroyd, 1994), this case adds
to the de\'eloping conceptual model concerning ground-based seeding with silver iodide. This
particular case is the first in the ~OA.A.-Ctah project to observe the effects of seeding in the radar
data. and therefure adds siQIlificantlv to our knowledge of the three-dimensionalitv of seeding
- - - - --

plumes, and the area which they affect at the surface.

1. Atmospheric Conditions during the AgI Seeding Experiment
The general weather conditions on 19 February were described in Sec. IIID. Much of the
additional discussion of the atmospheric conditions over the Wasatch Plateau appears in Huggins
(1996; and will only be discussed briefly here. Conditions at RRS were presented in Fig. 23, and
showed good potential for AgI seeding with temperatures below -IO~ persistent SL\V, and generally
light winds which were directionally favorable for targeting TAR Sounding plots from 0900 and
1034 are sho\-\TI in Figs. 53 and 54, and reveal a shallow moist layer existed through the seeding
period (0800-1017). The latter sounding indicated the moist layer was convectively unstable. The
Doppler sounder at mid-plateau detected winds from 220-230e in the lowest 200 m, with a shift to
200-210" after I 000. The Doppler wind speeds were relatively constant at about 6 m S.I.
Conditions at the HA.S seeding location are shovm in Fig. 55. The third panel documents the
SF6 release period, which ended about 15 min prior to the end of AgI seeding. The temperature at
the seeding site varied from about -lOe at the start of seeding to -7.Y at the end. The vvind
fluctuated between 220" and 240", \-vith the speed being quite constant at 3 m s.;. HAS may have
been sli!Zhth- below cloud base. but RRS was consistentlv in cloud as indicated bv the icing rate
~ r I ., .;_

meter counts in Fig. 23. The research aircraft flew from 0800 to 1130. The cloud tops were 'toted
to be near the highest flight altitude of 3750 m at a temperature of about -18". The aircraft King
probe detected liquid water contents of 0.3-0.5 g m-3 in the altitude layer from 3200 m to 3750 m.
These aircraft measurements indicated the cloud depth over the plateau was generally less than one
kilometer. This minimal cloud depth and a lack of strong synoptic forcing reduced the potential for

72