WMOD00496

• Title: LOREP 1993 Summary Report: Airborne Measurements of Meteorological Variables, Atmospheric Particles and Sulfer Hexafluoride
• Date: 3/1/1996
• Weather Modification - Doc Type: Report

(Vardiman et al., 1971). More recently, liquid propane has been used in converting cloud condensate
to ice crystals in winter storms over the Sierra Nevada (Reynolds, 1988, 1989, 1991). The 1993
LOREP field exercise, led by Reynolds, furthers his recent work.
A tracer material injected into the atmosphere, in known quantity, allows direct study of
transport and dispersion processes and aids in validating atmospheric model calculations. Since
introduction of the concept (Saltzman et aI., 1966), halogenated gases, such as sulfur hexafluoride,
have become established as atmospheric tracers detectable at very low concentrations without pre-
concentration of the sample. Sulfur hexafluoride is particularly suitable as an in-cloud tracer because
it is insoluble in water (Collins, et al., 1965). Sampling and detection of SF 6 at less than 10 parts per
trillion by volume (pptv) is accomplished easily and at low cost by electron capture gas
chromatography (Clemons and Altshuller, 1966). SF6 is an inert gaseous compound with no natural
sources and a calculated atmospheric lifetime exceeding three thousand years (Ravishankara et aI.,
1993). As it is used as an atmospheric tracer, and because it is a byproduct of certain industrial
activities, there remains a worldwide background of less than a few parts per trillion (Lovelock and
Ferber, 1982). When atmospheric diffusion of the tracer plume reduces the SF6 concentration to a
value that is near the global background, the plume will no longer be detectable.
The co-release of SF 6 with propane was used to determine how effectively individual releases
were in dispersing ice crystals (tracer) into regions of supercooled liquid water over the intended
target area. The tracer gas trajectory and the propane-generated ice crystal plume trajectory will
match until the growth of the ice crystals causes them to fall away from the tracer plume. Should the
crystals not grow to precipitation size particles, rates of precipitation at the ground would not
mcrease.
During the period between 5 February and 20 March, 1993, an exploratory seeding program
was accomplished through a series of randomized releases. SF 6 tracer was released, when
meteorological conditions were favorable, through mass-flow controllers at two dispenser sites along
the base of the Grizzly Ridge, on the Sierra Crest, near Graegle, CA. Propane was released from the
same two sites and others along the base of the ridge. There were a total of eleven experiments based
on meteorological conditions that would include winds from the south-southwest and passage of a
front capable of producing precipitation. The random seed/no-seed pattern resulted in six of these
experiments having both propane and SF 6 released, while five had SF 6 only. On one occasion,
February 19, SF6 and propane were released for one hour; however, due to high winds and icing,
airborne operations were suspended. On February 22, though battery failures at both release sites
halted the dispensing of SF 6, the aircraft flew a three hour mission in a propane plume.
The complement of measurements systems in the target area included a high altitude
mountain-top observatory, a network of automated precipitation gauges, mountain-top weather
stations, and a rawinsonde observations linit. A dual frequency (20.6 and 31.54 GHz) microwave
radiometer was deployed at the Jackson Creek high altitude observatory. The radiometer was used
to measure path integrated liquid water in millimeters and precipitable water vapor in centimeters.
The radiometer was used for real-time indications of supercooled liquid water; to develop a
climatology of supercooled liquid water in the area; and to determine the impact of seeding on the
liquid water. This site also housed a ground microphysics laboratory consisting of an aspirated
Particle Measuring Systems (PMS) 2D-C cloud particle probe and microscope with camera for
photomicrographs of snow crystals. A continuous SF 6 analyzer monitored for the presence of SF 6
tracer during seeding operations. A total of twelve precipitation gauges were installed within the
target area. Data were collected at IS-minute intervals and transmitted hourly via the GOES satellite
to the California Data Exchange Center down-link in Sacramento and the USBR down-link in