Operational weather modification projects have benefitted from the results of numerous research experiments that have been conducted since the 1950's and 1960's. Many of the early experiments relied on statistical evaluation of precipitation data to determine if cloud seeding was having a positive impact. As techniques and instrumentation evolved, the impacts of cloud seeding began to be documented from the initiation of ice in clouds to the measurement of precipitation at the surface.
Wintertime cloud seeding for snowpack augmentation has historically involved a variety of techniques, seeding materials and dispensing methods. The research has been conducted in numerous mountainous areas of the western U.S., including the Rocky Mountains of Colorado and Montana, the Cascade Mountains of Washington, and the Sierra Nevada of California. Research results can be found in the references listed below:
More recent research has dealt with trace chemistry techniques for detecting seeding effects in the snowpacks of mountainous areas, the use of microwave radiometers for evaluating cloud seeding potential, and the use of numerical models for simulating the dispersion of cloud seeding material. Examples are as follows:
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Atmospheric and Dispersion Modeling |
The following case study of a ground-based silver iodide (AgI) seeding experiment was developed from data collected during a field research project conducted by the NOAA-Utah Atmospheric Modification Program (a cooperative venture between the State of Utah and the National Oceanic and Atmospheric Administration). The experiment took place on the Wasatch Plateau of central Utah. The State of Utah Department of Water Resources, the U. S. Bureau of Reclamation, the Desert Research Institute, the NOAA Air Resources Laboratory, the University of North Carolina - Asheville, and North American Weather Consultants participated in the project. The results shown here were taken from two sources (Huggins, 1996a and 1996b)*, as part of the Desert Research Institute's contributions to program.
Figure 1. Shown here is a southwest to northeast cross-section of Utah's
Wasatch Plateau. The locations of the main instrument sites and the cloud
seeding generator site are noted. The NOAA research aircraft flew tracks
through this cross-section at AC1, AC2 and AC3, and one track along the
cross-section (AC4).
Figure 2. Data from the RRS site at the top of the Wasatch Plateau show
the atmospheric conditions in which the cloud seeding experiment was
conducted. The temperature was relatively steady at about -12° C (top
panel), the wind was relatively light and blowing from the southwest
(along the cross-section in Figure 1), and supercooled cloud liquid
detected by a microwave radiometer was present in small amounts (4th
panel). These conditions indicated that the potential for producing a
seeding effect was very good. The seeding material being used (AgI) can
produce ice crystals in the presence of supercooled cloud water, provided
the temperature is colder than about -5° C. The relatively slow wind
speed suggested there would be adequate time for ice crystals created by
seeding to grow and fall out over the top of the plateau.
The third data panel shows ice nucleus counts and the occurrence of icing
at RRS. The low
ice nucleus counts indicate that the AgI seeding material did not pass
across RRS (see Figure 3). The indication of
icing shows that
the RRS site was frequently in the cloud that formed over the plateau.
Figure 3.
The transport and dispersion of an aerosol plume across the
Wasatch Plateau from a single ground seeding source at HAS is depicted.
The seeding experiment consisted of the simultaneous release of AgI and a
tracer gas, sulfur hexaflouride (SF6). An instrumented van was operated
on roads on top of the plateau and a NOAA research aircraft was flown 300
to 600 m above the plateau (see Figure 1). The
van and aircraft
were equipped with sensors to detect both ice nuclei (AgI) and SF6, and
documented the seeding plume at the locations shown here. Seeding
occurred from 0800 until 1020 PST. Although both ice nuclei and SF6 were
detected, the figure shows only the SF6 plume interceptions. The top of
the seeding plume determined from all aircraft passes through the plume
is shown by the dashed green line in Figure 1. Note that four of the
project precipitation gages were within the area covered by the seeding
plume.
Figure
4. An example of a seeding effect detected by the research
aircraft is shown here at a location about 38 minutes downwind of the
seeding generator. The left panel shows the aircraft flight track with
the box indicating where the aerosol seeding plume was detected. The
panels on the right show the aircraft height and air temperature (top);
the liquid water content in the cloud (2nd); the concentration of ice
particles greater than 0.1 mm in diameter (3rd) as measured by an optical
array probe (OAP); and the SF6 and ice nucleus concentrations (bottom).
The ice nuclei counter takes about 1 minute to process an air sample,
resulting in the time difference in detection of the two plumes. The
dashed red lines in the right panels show the seeding aerosol plume
locations. The seeding effect is clearly seen as an enhancement in the
ice crystal concentration (about 5-20 crystals per liter of air) within
the plume, as compared to cloud regions on either side of the plume.
Figure 5.
Once ice crystals grow to sufficient size they can be detected
by radar. The radar used for this Utah project was the DRI Ka-band radar,
a special short wavelength (8.6 mm) radar used primarily for cloud
physics studies, and capable of detecting cloud particles even before
they reach precipitation size. This image shows the radar echo plume that
evolved as a result of ice crystals created by the AgI seeding from HAS.
The plume-like radar echo was found to coincide with both the aerosol
plume (Figure 3) and the aircraft ice crystal
plume locations (Figure 4).
The aircraft SF6 plume location matching this radar image time is shown
by the line segment near the TAR site. This low level scan detected ice
particles within a few hundred meters of the surface, and therefore also
gives an indication of the areal coverage of precipitation that was being
produced by the seeding.
Figure 6.
This final figure compares precipitation from gages beneath
the seeding plume (as determined by the data in Figures 3 and 5) to
precipitation from gages to the north and south of the seeding plume. The
seeding during this experiment accounted for a precipitation rate
increase of about 1-1.5 mm h-1. Using 1.25 mm h-1 as the average
increase, the two hour experiment would have produced 2.5 additional
millimeters of snow water over the area affected by the plume. The radar
plume areal dimension was about 50 km2. The volumetric amount of snow
water produced by seeding can then be estimated as being 125,000 m3, or
about 100 acre-ft.
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