UCAR > Communications > Staff Notes > September 1996 Search

Deluge and drought:
Two summer studies precipitate progress

S-Pol sees through Colorado weather tricks

Early in the evening of 12 July, an estimated 7.6 to 9 centimeters of rain created a 3.6-meter wall of water along Buffalo Creek southwest of Denver. The creek flooded its banks, washed away a bridge and road, and left two people dead. At the same time, hail pelted the Colorado plains as a number of severe storms thundered through the hot afternoon.

Both areas were continuously watched by an experimental radar perched atop four seatainers (seaworthy trailers) at Front Range Airport in Watkins, the first town east of Denver on I-70. Operated from 1 June to 21 August by the Remote Sensing Facility (RSF) of the Atmospheric Technology Division (ATD), the radar's dual-polarimetry technology proved its state-of-the-art precision on 12 July by distinguishing between the large, flat raindrops swelling Buffalo Creek and the round hailstones pounding the eastern prairie. Meanwhile, Denver's Front Range Doppler radar--part of the National Weather Service's nationwide network (WSR-88D, formerly known as NEXRAD)--showed both areas as having similarly heavy rain and/or hail, without distinguishing between the two.

"Measuring heavy rains accurately is important for anticipating flash floods," explains ATD senior scientist Jim Wilson, who headed the S-Pol precipitation experiment. "Hail can fool the WSR-88D into 'thinking' it's raining harder than it actually is, thereby introducing uncertainty into the issuance of flash flood warnings. S-Pol can more accurately measure the size and shape of raindrops. This helps us spot areas of heavy rainfall and predict the resulting runoff."

To quantify the advantages of a dual-polarimetric radar over the WSR-88D's more conventional technology (single polarization), NCAR is planning a series of experiments over the next few years in various seasons and locations around the United States. This summer's deployment of S-Pol at Front Range Airport was the first in the series.

Anticipating promising results from the current refinement of the now 15-year-old technology, scientists at the National Severe Storms Laboratory (NSSL) are already working on how to add it to the existing WSR-88D network installed around the country in the early 1990s. Colorado State University and NSSL use dual-polarimetric radar data and are collaborating with NCAR on the S-Pol tests. Similar studies are taking place in Germany, Italy, and England.

According to Matt Kelsch, NOAA meteorologist and an instructor with the Cooperative Program for Operational Meteorology, Education and Training (COMET), "WSR-88D is set to measure maximum rainfall rates of 2.94 inches per hour [7.5 cm] in Colorado. Above that limit, hail looks like very heavy rainfall. S-Pol, however, has no set threshold."

S-Pol is NCAR's second dual-polarimetric radar. Its predecessor, CP-2, was expensive to set up because it required the construction of a concrete pad at each new site. By contrast, S-Pol can be placed on a base of four seatainers--the same ones it's shipped in--for assembly at any stable, accessible site in the world. The 8.5-m aluminum dish is sturdy in winds up to 80 kph and can be covered with a radome (protective shell) if necessary in more severe weather.

Not only is S-Pol convenient to ship and assemble, but it sports a much-improved antenna, which provides more accurate measurements than CP-2 did. A new data processor using modern digital technology contributed by ATD's Mitch Randall, Eric Loew, and Jon Lutz further supports S-Pol's state-of the-art status. "It costs a lot less to move into the field," says Joe Vinson, one of RSF's six radar operators, "and it's so much easier to to operate. We're pretty happy with it so far." RAP's Ed Brandes and Jotharim Vivekanandan will be conducting data analysis for this summer's test.

The unusually stormy July across northeast Colorado, while providing many study cases for S-Pol, brought several tornadoes to the vicinity of Front Range. One was so close that radar operator Al Phinney took shelter at the Front Range terminal until the threat had passed. On 9 August, two other twisters passed within five kilometers of the radar, giving Rick Parsons some anxious moments.

Dennis Heap, director of aviation at Front Range, donated airport grounds for the experiment. The large white dish of S-Pol scanned the horizon from its seatainer base on the former north-south runway at the airport's west side. Says Heap, "The Front Range Airport Authority provided the land at no cost to NCAR because it saw the long-range benefits to the aviation community, which heavily depends on accurate weather information." About 125 airplanes from the Denver metro area are based at Front Range and many more from around the country stop there for fuel. •Anatta, UCAR Communications

How does dual polarization work?

S-Pol measures the average height and width of raindrops and hailstones in a volume of air about 150 meters long by 1 degree wide. It uses signals that are polarized into alternating horizontal and vertical orientations. Small drops tend to be round, while larger drops--contrary to familiar images of raindrop shape--flatten into hamburger-like shapes as they fall. These large, flat drops will reflect more of the horizontally polarized signals than the vertically polarized ones. In contrast, smaller raindrops, being comparatively round, will tend to reflect the same amounts of signal whether that signal is horizontally or vertically polarized. The same goes for hailstones: even if they are oddly shaped, they will tumble as they fall, resulting in the radar "seeing" them as round, on average. By comparing the radar returns for vertical and horizontal orientations, scientists can deduce how much hail or heavy rain is inside a storm.

RAP tests new rainfall-enhancement technique in drought-ridden Mexico

Field work this summer and fall is bringing new credibility to the science of weather modification and perhaps helping to bring some rain to the drought-ridden Mexican state of Coahuila. Working with scientists from several Mexican universities, researchers from the Research Applications Program (RAP) and the Mesoscale and Microscale Meteorology Division (MMM) conducted the first field trials in North America of new techniques for seeding clouds to enhance rainfall.

The Mexican government, along with agricultural and industrial interests, is sponsoring the research. Field trials will continue from June to October over the next four years in Coahuila. The program will also transfer cloud-seeding technology to Mexico and train Mexican scientists in its use and evaluation.

The new seeding technique uses pyrotechnic flares mounted on aircraft to seed the clouds. While the aircraft flies at the base of the cloud, moisture-attracting (hygroscopic) particles produced by the burning flares rise into the cloud. As the cloud's water vapor is attracted to the particles, droplets are formed, which then fall out as rain. The wide range of droplet sizes produced by the particles encourages and accelerates the precipitation process.

This method of cloud seeding was first tried in 1990 in South Africa. The seeding appeared to increase rainfall by 30% to 60% over what would have occurred without the seeding. Roelof Bruintjes (RAP/MMM) and Al Cooper (MMM senior scientist/Advanced Study Program director) collaborated on the evaluation of the South African results; a forthcoming paper of theirs will describe modeling studies of the effects of hygroscopic seeding. Observations from a multi-institutional wintertime experiment, the Arizona Project--led by the University of Arizona early in 1995 in the region around the Mogollon Rim--appear consistent with those from South Africa.

RAP director Brant Foote is coordinating the overall research program in Mexico over the next four years. Brant cautions that it is too early to tell whether the hygroscopic seeding technique will be broadly applicable to Mexico or any other rain-deficient area. "The results in South Africa and Arizona are striking," he says, "but there is no guarantee of success elsewhere. These efforts are clearly still in the research phase. We anticipate that the disciplined research program in Mexico will help us to better understand the physical processes that could lead to increased rainfall and to clarify whether this technique works as well as it appears."

In late August, Hurricane Dolly brought some welcome relief to the drought-stricken parts of Mexico, but ironically, it hindered the seeding experiments. "There was so much low overcast we weren't able to fly," says Brant. Overall, he adds, "We've had a lot of flight days this summer, but it's much too early to know what the effects of the seeding are. We're looking for 20-30% changes, so you need to have a large enough sample to do valid comparisons."

One of the goals of the Mexican study, says Roelof, is to determine what part of the cloud is the most effective site for seeding: "Does the material stay in one updraft or does it mix through the cloud?" To shed light on this and other questions, pilots have been seeding in double-blind fashion. Radar operators are not told if seeding is taking place for a given storm.

The research agenda in Coahuila includes

• a statistical evaluation of the seeded and nonseeded clouds in the area

• studies of the cloud responses using high-resolution numerical models

• analyses of data collected by the research aircraft and ground-based radar

• strong collaboration with scientists in Mexico in the areas of cloud physics, data analysis, and numerical modeling

Flares mounted on a research aircraft are a main tool in the cloud-seeding project now taking place in Mexico's Coahuila province. The flares emit hygroscopic particles that attract water vapor, helping to accelerate the rain-formation process in Mexican thunderstorms such as the one above, observed this summer. (Photos courtesy Roelof Bruintjes/RAP.)
Mexican collaborators include scientists from the Universidad Autonoma de Coahuila, the Department of Meteorology at the Universidad Autonoma Antonio Narro in Saltillo, Centro Ciencias de la Atmosfera at the Universidad Nacional Autonoma de Mexico, and the Instituto Mexicano de Tecnologia del Agua. Altos Hornos de Mexico, a large steel plant in the region, is leading the private-sector interests. The North Dakota firm Weather Modification, Inc., is providing logistical support, including the research aircraft, pilots, a ground-based weather radar, and computer displays. Dan Breed (RAP/MMM) is serving as on-site field coordinator.

Training and technology transfer are important goals of the project. During the first month, NCAR and Coahuila personnel are jointly developing a training plan to transfer the new cloud-seeding technology. Mexican university and state personnel will be integrated into all aspects of the program, including cloud physics studies, radar data analyses, and numerical modeling. NCAR will also train Coahuila state personnel in how to use and evaluate cloud-seeding methods using hygroscopic flares and other materials.

Key aspects of the training and technology transfer will include

• lectures by NCAR scientists on basic cloud physics and dynamics, the history of cloud seeding, cloud-seeding techniques, and statistical analysis procedures

• pairing of NCAR scientists with Mexican scientists for the entire four-year period to facilitate mentoring

• recommendations for Mexican scientists to attend U.S. universities and specialize in cloud seeding and weather modification studies at the bachelor's or master's degree level

• training of Mexican personnel to launch soundings, operate radar, conduct seeding flights, and coordinate seeding operations

Although cloud seeding has had a checkered past, the Mexican study and the new technique are once again raising hopes. Roelof and Brant have hosted visitors from Korea and Indonesia and received calls from around the world spurred by interest in the project. "We could be interested in doing more such projects," says Brant, "if we found a sponsor who's really interested in learning about the clouds and in scientific evaluation of the seeding. We're not in the weather modification business--we're trying to evaluate what the seeding effects are."

Brant adds that seeding is not an instant fix for ongoing drought, but rather a tool for maximizing rainfall during the wet phases of climates that alternate between wet and dry cycles. "There's absolutely no prospect of changing a drought regime to a rainy regime through weather modification. That's just not in the cards. If this technique proves to be effective, the time to apply it will be in rainy years, when you can make a lot more water."

A very brief history of weather modification

Participants in the National Hail Research Experiment of the mid-1970s crane their necks to watch a radiosonde ascending over far northeast Colorado.
Cloud seeding has been practiced in various places in the world since 1946, but most experiments have produced inconclusive results. The initial optimism that soared in the 1950s and 1960s has given way to a much more cautious approach over the last 20 years. Although there have been many rainfall-enhancement programs around the world, many have lacked the ability to validate the results of the seeding because of the natural day-to-day variability of clouds across geographical regions.

One of the largest seeding experiments was the National Hail Research Experiment, based at NCAR and conducted in northeast Colorado from 1972 to 1976. NHRE used silver iodide seeding techniques to try to reduce the size of hailstones falling from summer storms. Although the experiment failed to confirm any reliable effect from the silver iodide seeding, says RAP director Brant Foote, "It was highly successful in giving us insight into processes taking place in hailstorms."

While organized weather modification research has been at low ebb in the United States in the years since NHRE, smaller seeding efforts for agriculture and research have continued in Texas, Kansas, North Dakota, and other states. Enthusiasm has remained higher outside the United States, says Brant. "There's people all over the world seeding clouds to decrease hail, including many groups in Europe." •BH

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Prepared by Jacque Marshall, jacque@ucar.edu, 303-497-8616