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<br />ATMOSPHERIC TESTS OF AN ORGANIC NUCLEANT IN A SUPERCOOLED FOG
<br />
<br />William L Woodley
<br />Woodley Weather Consultants
<br />Boulder, Colorado
<br />
<br />and
<br />
<br />Thomas J. Henderson
<br />Atmospherics Incorporated
<br />Fresno, California
<br />
<br />Abstract: An organic artificial ice nucleant (Pseudomonas syringae) was
<br />dispersed as a fine powder from an aircraft within a supercooled fog over
<br />Mono Lake, California on 2 December 1989. The dispersal took place about
<br />30 m below fog top at a temperature of -80e. Two test runs about 3 to 4 km
<br />in length were made; 10 gm and 100 gm of nucleant were distributed over the
<br />test track during the first and second runs, respectively. Following each test
<br />run, a highly instrumented cloud physics aircraft flew tracks within the fog
<br />that were orthogonal to the original test track. Particle measuring probes
<br />(FSSP, 1D-C and 2D-C) were used to determine fog characteristics and to
<br />quantify a seeding signature.
<br />
<br />The light seeding rate (10 gm over the test track) of P. syringae produced no
<br />detectable seeding signature, probably because the monitoring aircraft missed
<br />the treated plume. The heavier rate (100 gm over the test track), however,
<br />produced an obvious seeding signature that was first detected about 5 minutes
<br />after release of the nucleant. The signature began with high concentrations
<br />of relatively small (50 to 100 Jlm) particles, and the ice crystal concentration
<br />decreased with time as their size increased. The mean, median and modal
<br />particle sizes at a particular time are virtually equal in the plume, suggesting
<br />a common origin for the ice crystals.
<br />
<br />P. syringae clearly produced glaciation of a supercooled fog at temperatures
<br />of about _80C in agreement with laboratory test results. Additional
<br />atmospheric tests are planned at warmer temperatures.
<br />
<br />1. INTRODUCTION
<br />
<br />For the past 40 years, modern cloud seeding
<br />technology has been largely focused on the ice nucleation
<br />processes which are central to the natural production of
<br />snow and rain over much of the earth's surface. Within
<br />these atmospheric processes, the ration of ice particles to
<br />supercooled liquid water often establishes the efficiency of
<br />clouds and storm systems to produce precipitation at
<br />ground level. In many cases, the concentration of naturally
<br />occurring ice nuclei (IN) is lower than required for the
<br />most efficient precipitation process, even at temperatures
<br />as cold as -20.e. (Pruppacher and Kleth, 1980). For this
<br />reason, a number of substances such as silver iodide
<br />(Vonnegut, 1947) have been selected to act as
<br />supplementary IN within a broad range of cloud seeding
<br />programs throughout the world.
<br />
<br />Because most naturally occurring IN found in clouds
<br />are not particularly active at tempfratures warmer than
<br />about -10.C, there has been a continuing search for a
<br />source of IN, either natural or artificial, which have the
<br />ability to convert supercooled cloud droplets to ice crystals
<br />at the warmest possible temperature. Solid carbon dioxide
<br />(dry ice) is an excellent candidate and, since its early
<br />demonstrated use in 1946 (Schaefer, 1948), has been used
<br />
<br />in many cloud seeding programs. However, dry ice suffers
<br />from availability in remote areas, storage problems,
<br />quantities required for airborne dispersal, and its effective
<br />dispersal is restricted to altitude where the temperature is
<br />colder than We.
<br />
<br />The presence of microorganisms in precipitation has
<br />received a number of investigations dating back more than
<br />20 years (Gregory, 1967; Schnell and Vali, 1972, 1973;
<br />Maki, et ai, 1974; Schnell and Vali, 1976; Vali, et aI, 1976;
<br />Lindow, et aI, 1978; Maki, et aI, 1978; Yankofsky, et aI,
<br />1981; Lindow, et aI, 1982; and Levin, et aI, 1987, 1988).
<br />The value of these investigations is noteworthy because of
<br />the ice nucleation properties of some naturally occurring
<br />microorganisms at temperatures near We. P. syringae was
<br />one of the major candidates for investigation within many
<br />of these research programs.
<br />
<br />In more recent years, the Eastman Kodak Company
<br />developed the expertise and production facilities necessary
<br />to produce large quantities of beneficial bacteria,
<br />principally P. syringae. The material has found its way to
<br />SnowmaxTM Snow Inducer, a product of Kodak's Bio-,
<br />Products Division useful in snowguns installed at many ski
<br />
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