WMOD00002

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

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6. SUMMARY DISCUSSION Ahn RECO}~1E~UATIO~S

The principal conclusions art'rived at ~:'J the basis of the overall evaluation

are:

1) There was no significant difference in the net target area precipitation
on seeded experimental days and rhat on nor-s.~~rlcd experimental days_ This null
effect was apparently due to the. mixing of positive and negative seeding effects.
Negative effects occurred when seeding was conducted during out-of-bounds condi-
tions and positive effects occurred when seeding was conducted during in-bounds
conditions. The findings discussed in items 2 through 8 below are ba$ed upon
diagnostic analyses (see Chapter 5) of six-hour block precipitation data rather
-than on the 24-hour experimental day data. ~1s permitted the sorting out of
the various effects of seeding.
2) During the 147 experimental days, stable orographic clouds prevai~~.
about 52% of the time and accounted for ahout 33% of the experimental day pre-
cipitation (at K03) covered by the diagnostic analysis. This stable orographic
cloud category includes cases witn shallow unstable layers no thicker than 75 mh.
Excluding the stable ones, cases range mostly from neutrally stable stratiform
clouds to ones containing embedded shallow cumuli or stratocumulus. During 37%
of the time, or about 10% of the diagnostic analysis experimental day precipita-
tion, warm top stable clouds occurred. Uhen these were seeded, substantial pre-
cipitation increases occurred in the target area and Od the downwind flank.
During ~5% of the time, or about 23% of the experimental day precipitation,
stable orographic clouds whose tops were colder than -29C occurred. Yhen these
were seeded, decre~ses in precipitation occurred. The decreases were marked
when the cloud level wind component normal to the barrier was strong and the
c.loud deep. If the vind compOnent normal to the barrier was .....eak and the cloud
shallow, then increases occurred in the cold top category. This latter effect
had not been anticipated from earlier cloud seeding tests.
J) During approximately 48Z of the ti~e, nnd nccounting for about 67% of
the experimental day precipitation, the air mass had sufficient thermodynamic
1nstabiliry to be considered Outside of the stable orographic category, I.e..
deep convection was present. Seeding und~r ~hese conditions may have resulted
in an increase in precipitation on downwind slopes, particularly when the normal
wind component 1.1,'5 10\01. It probably rcsllltcc. in decreases on the upwind slopes.

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