WMOD00071

• Applicant: Western Weather Consultants
• Project Name: Vail Pass
• Date: 11/1/1984
• Weather Modification - Doc Type: Application

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6. SUMMARY DISCUSSION ~~ RECOMMENDATIO,S

The'_ principal conclusions arrrived at 0:':1. the basis' of the overall evaluation

are:

1) There vas no stgnificant difference in the net target area precipitFtlon "
. .
on s~~ded experimental days ahd that on not-s~eded experioental days. This null
effect vas apparently due to the.m1xinR of positive and negative seeding effects.
Negative effects occurred vhen seeding ~as conducted du~ing out~f-bounds condi~
tions ~nd positive effects occurred vhen seeding vas conducted during in-bounds
.. _ .- conditions. The findings'discussed in it;eI:lS 2 through 8 below are based upon
diagnostic analyses (see Chapter. 5) ~f six-hour block precipitation data rather

'than Oft the 24-hour experimental day data. "~is permitted the sorting out of
the various effects of seeding.
2) During the 147 experimental days, stable orographic clouds prevailed.
about 52% of the time and- accounted for about 33% of the experimental day pre-'
cipitation (at K03) covered by the diagnostic analysis. This stable orographic
cloud category includes eases witb shallow unstable layers no thicker than 75 mb.
Excluding the stable ones, cases' ~ange mostly from neutrally stable stratiform
clouds to ones containing embedded shallow cumuli or stratocumulus. During 37%
of the t1me!..!~~ about 10% of the diagnostic analysis experimental day precipita-
tion, warm l~!} stable clouds occurred. \~en these were seeded, substantial pre-
cipitation increases occurred in the target area and art the downwind flank.
During .15% of the time, or about 23% of the experimental day precipitation,
stable orographic clouds vhose tops ~ere colder than -29C occurred. Uhen these
were seeded, decre~ses in precipitation occurred. The decreases were marked
.'. when the cloud level_wind component nor~~l to the barrier was strong and the

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cloud deep~ If the uind component nornl<nl to the barrier ""'as ueak and tbe_ cloud
shallow, then increases occurred in the cold top category. This latter effect
had not been anticipated from earlier cloud seeding tests.
)) During approximately 48Z of the rloe. and accounting for about 67% of
the experimental day precipitation, the air mass had sufficient thermodynamic
instability to be considered outside of the srable orographic category, i.e..
deep convection uas present. Seeding und~r these conditions may have resulted
in an increase in precipitation on do~nwind slopes. particularly when the normal
vind component \Ins low. It probabl)' resultcc! in decreases on the upvind slopes.

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