WMOD00510

• Title: The Practicability of Dry Ice for On-Top Seeding of Convective Clouds
• Date: 1/1/1978
• Weather Modification - Doc Type: Report

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HOLROYD, SUPER AND SILVERMAN

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JANUARY 1978

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. . PHOTO BOUNDARIES OF CLOUD
---""- AIRCRtlFT PENETRATIONS
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FIG. 6. An altitude-time diagram of the seeded cloud of 8 July 1976. The photographic cloud boundaries, the four aircraft tracks
and penetration times of cloud or precipitation are shown by the various solid lines. The calculated trajectory of the dry ice pellets
is indicated by the shading. The segmented contours ip.dicate ice or large particle concentrations. The major part of the resulting pre-
cipitation came through the base about 20 min after seeding.

unseeded east bubble, it: was excluded from the
calculations.
The heavy line in Fig. 4 shows the boundary of the
liquid cloud j the shading shows the liquid water content
contours. The other lines are contours of the ice crystal
concentration. The decelerator slides that suffered no
melting showed both short and long columns and some
indications of aggregation. An aggregate, though it is
composed of many crystals, receives only one count from
the optical counters.
The three bubbles encountered are seen in passes
3 and 4 (Fig, 4) as the aircraft descends. The largest
liquid water contents, as determined by a PMS-ASSP
(axially scattering spectrometer probe) and generally
confirmed by a Johnson-Williams type liquid water
instrument, occurred at the time of the seeding run.
It appears that only the center and western bubbles
were seeded. A few ice crystal counts of S t-1 or less
were observed in the center and western bubbles on
the seeding run and in the eastern bubble later on.

This represents either the natural background or
crystals produced by the previous presence of the
aircraft, possibly through tip vortices. Large ice particle
concentrations were observed on the first pass after
seeding. Concentrations remained high at the sampling
level until 30 min after seeding. A pulse of high concen-
trations 3S-40 min after seeding may have resulted from
a weak. updraft bringing them back up from below and
generating the small liquid cloud at that time. Other
liquid water peaks after seeding also seem to represent
small bubbles stirring the crystals, sometimes bringing
them back up from below, other times pushing the
crystals aside. The two main ice cores seem associated
with the high liquid water regions encountered on the
seeding pass.
Fig. 6 shows the ice concentrations in the vertical.
The cloud-top and cloud-base locations, shown by the
thick solid lines and data points, were determined by
time-lapse photography of the cloud from a camera at
the surface and from occasional hand-held camera