WMOD00403

• Title: Special Regional Weather Modification Conference Augmentation of Winter Orographic Precipitation in the Western US
• Date: 11/11/1975
• Weather Modification - Doc Type: Report

of natural nuclei due to the presence of cold cloud tops (see Figure 10) and
2) the effect of the stronger wind normal to the barrier (compare Figure 7
and Figure 8). We should expect that the deficits might continue during zero
hour, even though the wind factor is reduced. However. excesses are restored
at this time. The small deficits on the upwind slopes that appear after
system passage are of a magnitude which very likely lies within the noise
level of this analysis. Another stratification by 700 mb wind direction has
shown large seeding related excess to be associated with a westerly flow.
Since this type of flow occurs after system passage, one should expect ex-
cesses to appear in the post-passage region. Rather strong excesses do appear
in the -9 hour portion thereof. It should be noted that westerly winds at
700 mb are accompanied by more southerly winds at lower levels. This ensures
entrainment of the nucleant into the westerly flow aloft where it experiences
a long travel time moving parallel to the crest. allowing for more complete
nucleation, growth. and fallout of ice particles. particularly on the crest
and downwind flank.

Finally, it should be mentioned that the San Juan data have been examined
to determine if they exhibit a warm side cutoff of the seeding window. Such
a cutoff has been discussed by Grant & Elliott (1974). It has been suggested
that on the warm side of this cutoff natural ice multiplication is adequate to
bring down the cloud water available. If so, it is quite possible that arti-
ficial nucleation at these warmer temperatures could lead to overseeding and
a reduction in precipitation, at least over the upwind slope. Observations
of adequately iced clouds upwind of the San Juan range under relatively warm
cloud top conditions have been reported by the EGG staff. Analyses of the
6-hourly group precipitation data indicated a seeding related deficit for
clouds whose cloud base mixing ratio was over 3.8 g/kg and whose cloud top
temperature exceeded -20 C. However. ,after removal of convective cases likely
to produce cold tops over the mountains, there were too few cases remaining
upon which to form a judgement, even though the deficit remained. In the
Sierra Nevada, where high mixing ratio cloud bases are the rule and there is
evidence for some coalescence mechanism being operative, this warm side cutoff
may present more of a problem. There is the additional complication of a
greater amount of convective instability in the Sierra Nevada.

5. CONCLUSIONS

During the first 4 years of this 5 year project, there has been no dis-
cernable difference between target area preeipitation on treated and control
experimental days. Stratification by rawinsonde-determined cloud top tempera-
ture has provided evidence of seeding produced increases when cloud top tem-
peratures exceed -27 C. During a considerable fraction of the time clouds
were seeded when colder top clouds existed, and seeded precipitation was less
than not seeded precipitation under these circumstances. This latter result
was predictable on the basis of the Climax experiment results and that class
of clouds had been ruled out of bounds in the project design. The reason for
the failure tm observe this criterion has been analyzed and appears to be al-
most wholly due to the implicit requin~ment that the operator make forecasts
of cloud temperatures up to 24 hours in advance, a task not in the present
state of weather forecasting art. This is not a requirement in a purely re-
search project where all cases are seeded and the stratification done post-hoc;
nor is it a requirement in a purely opl~rational project where frequent opera-
tional updating in response to new information is possible.

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