WMOD00412

• Title: Final Report on Utah Cloud Seeding Experimentation Using Propane During the 2003/04 Winter
• Date: 3/1/2005
• State: UT
• Weather Modification - Doc Type: Report

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The first storm after installation began on 14 December with typical southwest flow at the HAS. A plot
of the frequency shift with time for the period before and during the first EU showed exactly what was
expected (see Fig. 12). The frequency decreased with time in an almost linear fashion from the ice free
value near 40,000 Hz to 39,400 Hz at which point the heater was automatically turned on to melt ice off
the sensing rod assembly. Heating resulted in the frequency rapidly increasing back to the ice free value
over about 3 min. Icing appeared to be very rapid as the frequency quickly decreased again after reaching
40,000 Hz and the decrease continued past the end of the 40 min EU. The sensor appeared to be
functioning just as expected.

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40000

39500

Experimental Unit
starting 1733 MST
on 14 Dee. 2003

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39400
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5

10 15 20 25 30
Minutes Into Experimental Unit

35

40

Fig. 12. Frequency versus time from the Goodrich icing rate sensor during the first EU on 14 December 2003.
The heater automatically removed ice from the sensor rod when the frequency reached 39,400 Hz.

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After the second EU of the first pair, the HAS wind shifted from southwest to west-northwest and
remained there for the remainder of the storm passage of 14-15 December. An additional 20 EUs were
triggered by the icing sensor using the 34 Hz threshold. However, none of them are believed to be valid
EUs in that the SL W cloud droplets were approaching the sensing rod from a rear quarter. That fact,
combined with improperly tilted sensing rod, likely resulted in a very uneven pattern of rime ice buildup
along the rod, evidenced by a quite noisy frequency record with the frequency increasing and decreasing
over a range from few to over 100 Hz. But the decrease was never sufficient to reach the 39,400 Hz
heater trip point before the storm's end. Frequency values were plotted for each 40 min EU and linear
regression equations were fitted to the 1 min observations. Some of the EUs showed gradual decreases in
frequency over time, some gradual increases and some essential no change over the 40 min period.
However, 34 Hz or greater decreases were frequent enough in the noisy data to trigger the many
additional EUs with west-northwest flow. They and similar suspicious EUs from other northwest flow
periods, discussed below, have been kept in the pool of all EUs as required by the randomized design.

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25

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