WMOD00433

• Title: Generalized Criteria for Seeding Winter Orographic Clouds
• Date: 12/12/1978
• Weather Modification - Doc Type: Report

1776

JOURNAL OF APPLIED METEOROLOGY

VOLUME 17

water content and a cloud temperature between -10
and -300C. Only 16% of all the cases fit this definition,
but when they are seeded they show an 18% increase
in precipitation.
Table 8 shows the statistics for unstable clouds with
a crest trajectory, moderate to high water content and
cloud-top temperatures between -10 and -300C.
This type of cloud constituted 20% of all cases and
provides a 22% increase in precipitation when seeded.
Interestingly, the average 6 h precipitation rate for the
unstable cases is less than that for the stable cases at
the crest. This may be due to the greater fluctuation in
precipitation during unstable conditions or to a change
in location of the precipitation on the mountain.
The unstable cases were divided into moderately
unstable and highly unstable cases to investigate
further the effects of stability. The other variables were
held the same. Table 9 shows the moderately un-
stable condition. Nine percent of all cases fall into
this category and when seeded produce a 52% increase
in precipitation.
Table 10 shows the highly unstable cases.. These
clouds constitute 11% of all cases and produce a 3%
increase in precipitation when seeded, although sta-
statistical significance was lost.
Table 11 is an example where precipitation at the
crest is expected to be decreased by seeding. The clouds
are unstable with a blow-over trajectory, low water
content and cloud-top temperatures colder than -300C.
These clouds constitute 5% of all cases. A 54% reduc-
tion in precipitation occurs when they are seeded. The
mean no-seed precipitation at the crest for this type
of cloud is greater than any other types of clouds
mentioned so far. Perhaps this is due to the location
of precipitation on the barrier or the conversion rate of
moisture as the air is lifted rapidly over the barrier.
It should be noted that both BTI and CBWS bounds
overlap for some seeding stratifications. The blow-over
stratification shown in Table 11 was, in fact, first tested
with the bounds of BTI < -3600 sand CBWS.
<3 g kg-I. Within these bounds, there were too few
cases to achieve statistical significance, although the
seed/no-seed ratio was much less than 1. It was decided
to expand the bounds of BTI to BTI < -1800 s to in-
clude more cases in order to achieve significance.
Adjustments to moisture bounds were made for the
same reason. As Table 6 shows, even by expanding the
trajectory and moisture bounds, the cloud-top tem-
perature remains an overriding factor in determining
a seeding effect. This conclusion is supported by analysis
of the San Juan Project (Elliott et at., 1976).
The results of the crest analysis without CENSARE
indicated that when the cloud-top temperature is
between -10 and -300C, sufficient moisture is avail-
able and precipitation has a trajectory on or near the
crest, increases in precipitation at the crest of about
20% are produced by seeding, irrespective of the
stability, Increases to near 50% occur under moderate
)

instability conditions, assuming the other variables
are in the same range. Decreases of more than 50%
occur under conditions of cold cloud-top temperatures,
low moisture content, blow-over trajectory and in-
stability. These results apply primarily to the Rocky
Mountain region. Increases may be greater in the
Sierra Nevada because of different meteorological and
topographical conditions, but the reason for this
difference, if it truly exists, has not yet been resolved.
These results, which apply to the crest, raise questions
about the water balance over the entire barrier. If the
precipitation is increased at the crest, does this imply
that the precipitation is being reduced somewhere else
on the barrier or is the additional precipitation coming
from an increased efficiency in the cloud process? It
was expected that ,some indication of transbarrier
effects should be evident using several gage groupings
across the barrier.
Preliminary transbarrier analyses showed that under
conditions specified in Table 7, increases in the crest
region and downwind were evident. For more unstable
clouds as specified in Table 8, indications were that
increases across the entire barrier are possible. Under
the blow-over conditions shown in Table 11, seeding
may cause decreases across the entire barrier.

6. Conclusions

Data from several winter orographic cloud-seeding
projects have been successfully combined and general-
ized criteria for seeding winter orographic clouds de-
veloped. Four meteorological variables were investi-
gated to determine the response of seeding on precipita-
tion. The composite data appear to support the use of
these variables. Very strong positive and negative
seeding effects were found. Three of the more important
results are as follows:

1) Stable orographic clouds with a crest trajectory,
moderate water contents and cloud-top temperatures
between -10 and -300C showed an 18% increase in
precipitation on the crest by seeding.
2) Moderately unstable orographic clouds with a
crest trajectory, moderate to high water content and
cloud-top temperatures between _100 and -300C
showed a 52% increase in precipitation on the crest by
seeding.
3) Unstable orographic clouds with a blow-over
trajectory, low water content and cloud-top temperature
colder than -300C showed a 54% decrease in precipita-
tioy of the crest by seeding,

These results give confidence that the seedability
of winter orographic clouds is dependent upon rather
general meteorological conditions. The cloud conditions
(i.e., stability, cloud-top temperature, moisture, etc,)
are the predominant features which determine if seeding
will be effective. The interaction of these synoptic-scale
conditions with a mountain barrier determine how and
when seeding should be conducted. These generalized