WMOD00438

• Title: HIPLEX: A Cooperative Program on Rain Augmentation in the High Plains
• Date: 6/1/1984
• Weather Modification - Doc Type: Report

3.5 Microphysical Processes

Modern cloud seeding always involves manipulation
of the microphysical properties of a cloud. Changes
in cloud dynamics. even when sought deliberately.
are brought about as the result of microphysical
effects. A proper understanding of the modification
potential of any convective cloud system must
involve examinations of the microphysical structures
and processes of both natural and seeded cloud tur-
rets and their relationships to precipitation
mechanisms.

Microphysical studies of Texas HIPLEX clouds were
concentrated on precipitation processes. particu-
larly on the relative importance of the coalescence
and ice-phase processes (Takeuchi. 1980: Long.
1980). Takeuchi and Long were in general agree-
ment that. while the coalescence process sometimes
initiates rain in Texas clouds. nearly all of the rain falls
from clouds that contain ice. and that ice processes
dominate most clouds during the mature and late
cloud stages. Furthermore. the clouds undergoing
the ice processes were of longer duration and larger
spatial extent than clouds with only coalescence.
These results support the earlier findings of Smith
et al. (1974) from convective clouds in San Angelo.
Texas. A transition from coalescence rain formation
to ice processes was also noted in a case study by
Humbert et al. (1979).

The major precipitation development by ice proc-
esses during the mid- and later stages of each cloud's
life cycle involved graupel formation. The graupel
was believed to result from the freezing of raindrops.
which presumably formed by coalescence. and not
from the Bergeron ice crystal process. Supercooled
cloud droplets with diameters greater than 24 ,...,m
were observed in concentrations of about 10 cm-3.
and ice particle concentrations were between 10
and 100 L -1. These observations strongly suggest
that the Hallet-Mossop (1974) ice multiplication
mechanism could be operative in westTexas clouds.
They also indicate that the ice processes in the
mature stage of Texas thunderstorms are highly
efficient.

Takeuchi (1980) analyzed plots of vertical velocity.
liquid water content. ice particle concentrations. re-
flectivity values. and turbulence as functions of time
for nine clouds during the 1978 field season and for
three clouds during the 1979 season. Typical char-
acteristics of a cumulus congestus cloud included
the development of downdrafts in the precipi-
tation regions and the concentration of large cloud
droplets in the buoyant updraft regions.

An example of downdraft initiation of new convec-
tion that produced heavy precipitation is shown in

figure 3.5. This radar analysis of echoes on May 27.
1979. shows the outflow region from the old echo
which collided with the easterly low-level flow pro-
ducing convergence and lifting which triggered the
new echoes north of Big Spring. This storm pro-
duced heavy precipitation within the rain gage
network shown in figure 3.2. This thunderstorm
downdraft triggering of new convective cloud sys-
tems appears to have potential for si9nificant precip-
itation enhancement in weather modification
programs. Many opportunities for enhancement of
the downdraft outflow occur throu!~hout the High
Plains where dry environmental air at midtropo-
spheric levels produces natural conditions favorable
to downdrafts. The remaining problem is one of
developing complex hypotheses which clearly estab-
lish the links between seeding and new downdraft
initiation. enhancement and incrElased precipi-
tation efficiency of the newly triggered clouds. Such
hypotheses would amplify that proposed by Simpson
(1980) and Woodley et al. (1982).

There is still a relative scarcity of microphysical data
from west Texas clouds. It will be necessary to col-
lect and analyze additional data to quantify the gen-
eral microphysical properties in both convective
cells and clusters. This information is needed to
determine natural efficiency of the precipitation
process in convective cells and clusters. and to quan-
tify expected microphysical seeding effects on
rainfall.

3.6 Seeding Hypotheses

Seeding hypotheses for Texas convl3ctive clouds
must be based on observations of events in both
seeded and unseeded clouds. As notl3d above, the
data base on microphysical processes in west Texas
clouds is still being built up. However, a number of
suggestions have been offered on ways to seed them
to increase rainfall.

In a case study of one seeded cloud, Humbert et al.
(1979) found that the coalescence process initiated
precipitation development. and the ice process was
just beginning to dominate when the cloud was
seeded. Large drops (~ 750 ,...,m) were present at the
seeding level and liquid water content was relatively
large. reaching a maximum value of :2 gm-3. The
cloud was seeded with Agl shortly after a second
surge in growth. It was noted that the third and most
intense surge recorded had a secondar'y surge asso-
ciated with it. The timing of the secondary surge
coincided with the arrival of seeded air parcels at
cloud top. which would be consistent with a dynamic
seeding effect. The study concluded that hygro-
scopic seeding of the cloud would have been ineffec-
tive due to the 1- to 10 L-1 concentrations of
naturally recycled large drops. However. the

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