WMOD00413

• Title: Validation of Precipitation Management by Seeding Winter Orographic Clouds in the Colorado River Basin
• Date: 9/1/1993
• Weather Modification - Doc Type: Report

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The specific observations involved will now be discussed in the same order. All the
instrumentation systems discussed are summarized in table 4.1 at the end of this subsection.
The relative locations of the systems are shown on figure 4, which represents an approximate
composite of the two proposed experimental areas.

4.6.1 Existence of excess SLW over the barrier

The existence of SLW over the mountain barrier will be monitored with two microwave
radiometers. One will monitor cloud conditions over the upwind side of the barrier where
orographic production of liquid is maximized (Huggins, 1992; Huggins et al., 1992:). The other
radiometer will be located near the centerline of the barrier top, to measure any excess SLW
just prior to its downward transport within the lee subsidence zone. Both radiometer sites will
be equipped with accurate tower-mounted sensors to measure air temp€!rature and
dewpoint/frostpoint temperature. These observations will indicate when the lower atmosphere
is saturated with respect to water and ice.

Liquid cloud (or at least ice saturation) would have to exist over one or both radiometers for
seeding potential to be present. Existence of SLW over the upwind radiometer but not over the
downwind unit might suggest that nature had converted all condensate to ice prior to the air
crossing the mountain range so that little seeding potential exists. This possibility villI be tested
during the direct detection experiments, by aircraft and surface observations. The downwind
radiometer would not likely detect SLW when none existed over the upwind instrument unless
individual convective cells were being transported across the barrier.

In addition to radiometer observations, SLW will be monitored by icing rate meters maintained
at some of the seeding sites and on towers on top the barriers. These units res.pond to the
buildup of ice fonned by the impact and freezing of supercooled cloud droplets. After ice
thickness reaches a preset limit, the small exposed sensing rod is briefly heated to shed its ice,
after which the unit is ready for another buildup cycle. The rate of cycling is proportional to the
SLW flux past the instrument. Wind will also be measured at the icing rate meter sites, and
accurate sensors will monitor whether the air is saturated with respect to ice so that at least
diffusional growth of ice crystals is possible. Periodic rawinsonde releases will provide further
evidence of saturated layers upwind of the mountain barriers.

Aircraft observations of SL W periodically will be made over the windward slopes, ~md over the
target area, during daytime direct detection seeding experiments. These measurements will be
made from about 300 m above highest nearby terrain to the top of the liquid water cloud.

4.6.2 Reliable production of seeding agents

Each AgI generator and liquid propane dispenser will be automatically monitored to check
proper operation. Flame temperature and solution flow will be measured at the AgI generators,
and the propane dispensers will use both a flow rate sensor and a temperature sensor to verify
vaporization of the liquid propane at the output nozzle. These observations will be transmitted
by radio to a central monitoring station. Frequent manual checking of remote-controlled units
will be accomplished until confidence is gained in their reliable operation. Sample generators
will be tested for ice nucleus effectiveness in a cloud simulation laboratory.

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