WMOD00507

• Title: The Persistence of Seeding Effects in a Winter Orographic Cloud Seeded with Silver Iodidde Burned in Acetone
• Date: 6/6/1990
• Weather Modification - Doc Type: Report

486

JOURNAL OF APPLIED METEOROLOGY

VOLUME 29

861222
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30 40 50 60 70 80 90 100 llO 120 t30 140
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Distance from Sheridan (km)

FIG. 5. Cross section of the cloud over the barrier showing temperature structure, seeding location (S), X -Z positions
of aircraft penetrations of the seedline (circles containing the time in min after seeding), predicted envelope of ice
nuclei assuming a vertical dispersion of 0.1 m s -I (hatched area), and predictions of the trajectory model for ice crystals
nucleated at 5, 15,30 and 45 min after seeding. The numbers along the trajectories indicate time (min) after seeding.
The barrier cross section shown is a generalized cross section and does not represent specifically the terrain along the
seedline trajectory. Ice nuclei were measured by the aircraft during penetrations at 5, 12, and 64-92 min,

12 m s -I, indicating that ice crystals would begin
reaching the surface after 60 min. With this advection
the southern end of the seedline crossed three precip-
itation gauges, but passed to the north of the instru-
mented surface station, KGV. The duration of effects
at the ground from this one seedline was estimated to
be < 10 min. For all three gauges intermittent precip-
itation of I mm h-I, the minimum detectable signal,
was observed during the time of seedline passage. Thus
there was no indication in the surface data of seeding
effects; however, the resolution of these gauges, 0.1 mm
in 5 min, is likely too coarse to be sensitive to most
seeding effects. From a survey of wintertime orographic
seeding experiments Reynolds ( 1988) found precipi-
tation increases expected from seeding to be on the
order of 0.25 mm h-I.

4. Summary and conclusions

One line of AgI NH4I NH4Cl04 was released at -60C
in a stable post frontal stratiform cloud with natural
particle concentrations in excess of 50 L -Ion the 20-
C. The research aircraft was able to track this seedline

to the downwind edge of'cloud, some 100 km and 92
rriin after seeding. During the first two penetrations of
the seedline high concentrations of ice nuclei were
measured, but increases in particle concentration on
the 2D-C or 2D-P were difficult to separate from the
background. This may be due in part to a slow nucle-
ation rate. The seedline then apparently ascended
above the research aircraft. No ice nuclei or increases
in ice crystal concentration were detected on penetra-
tions between 19 and 47 min, but based on the ice
nucleus measurements the aircraft was not sampling
the seedline. When the aircraft ascended, seeding ma-
terial was again detected with the ice nucleus counter
during penetrations between 64 and 92 min after seed-
ing. The ascent of the seedline matched the slope of
the barrier. Increases in p~rticle concentrations at the
seedline were clear at the higher altitudes where back-
ground concentrations had decreased to 10-20 L -Ion
the 2D-C, and 2 L -( on the 2D-P. Then increases to
30-40 L -Ion the 2D-C, and 6-9 L -Ion the 2D-P were
observed at the seedline from 55 to 92 min after seeding
in a cloud with droplet concentrations of <40 cm -3.
If CO2 had been used instead of AgI, the effects of seed-