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March 2005

RICO field project:
Cool heads prevail during a complicated study of warm rain

Studying clouds and warm rain on tropical islands from November through January sounds like an excuse for atmospheric scientists to take a mid-winter trip to the beach. But the nearly 80 staffers who went to Antigua and Barbuda for the Rain in Cumulus over the Ocean (RICO) field project were thoroughly occupied flying aircraft through clouds, setting up radars, monitoring instruments, and, in some cases, observing some of the more down-to-earth aspects of tropical rain.

“Being on Barbuda in the early stages was like being on an extended camping trip in a soggy barnyard,” says EOL’s Don Ferraro, who dodged donkeys, goats, cows, and chickens on waterlogged roads to set up a radar in a swarm of mosquitoes.

During RICO’s educational outreach component, local schoolchildren on the island of Barbuda watch EOL’s Steve Onckley demonstrate a balloon launch.

Though hardly a beach vacation, RICO’s organizers say it was a spectacular success. NCAR staffers and their collaborators from universities and research centers around the world pulled off a complicated, two-month field project with three aircraft, a ship, and dozens of airborne and ground instruments that all converged on two small Caribbean islands.

Local schoolchildren react to a balloon launch demonstrated by EOL staffers during RICO’s educational outreach component.

The main objective of the scientists who planned RICO was to study the properties of trade wind cumulus clouds, with an emphasis on the process by which the clouds generate warm rain. Trade wind cumuli are shallow maritime clouds found over the tropical oceans around the world. The scientists specifically wanted to examine the rapid onset of rain in these clouds because, for more than 50 years, they haven’t been able to explain why theoretical calculations tell them that it should take twice as much time for droplets to coalesce into rain than it actually does.

“Either the observations or the theories are wrong because they don’t match each other,” explains MMM’s Charlie Knight, one of RICO’s principal investigators. “Our goal was to find out how fast rain formation happens in the real world.”

On a more general level, scientists hope that data from RICO will show them what controls the structure and coverage of shallow tropical cloud systems and help them better represent the exchange of radiant energy, moist heat content, momentum, and trace constituents between the atmosphere and ocean. Because shallow tropical clouds are one of the most prevalent cloud types on the planet, characterizing their properties is important to understanding global climate and energy balance.

Observations from the sky

To study the clouds around Antigua and Barbuda, the research team deployed the NCAR C-130 along with two other aircraft, one from the University of Wyoming and the other from the United Kingdom’s Facility for Airborne Atmospheric Measurements. Equipped with lidars, radars, dropsondes, and other instruments, the three aircraft made a total of 57 flights.

“From the airplane perspective, things went remarkably well,” says MMM’s Don Lenschow. With more than 35 years of NCAR field project experience under his belt, Don says that he’s seen enormous improvement over time in all aspects of doing aircraft research. “The programs are more professional and everything goes better than it did 30 years ago,” he says.

He says that one of the high points of RICO was that someone in the operations center would observe cloud structures with radar and then communicate via the Internet with people on the aircraft.

“When you’re in an airplane flying around in a regime like that, everything looks like random clouds popping up,” Don says. “The radar can discern patterns that the airplane has no clue about. Having that big picture was so useful because the people watching the radar could tell us about a particular event.”

The dirt on setting up radars

While the aircraft component of RICO went smoothly, setting up the radar was, literally, a mess.

Unusually high amounts of rain turned the RICO radar site on the island of Barbuda into a muddy mess. Despite the difficulties of setup and operation, the site allowed the S-Pol radar to provide some of the cleanest and most detailed radar measurements to date of trade wind cumuli clouds.

Two radars, the S-Pol and K-band, were delivered to the relatively undeveloped island of Barbuda on a sand barge from Antigua, after which they still needed to be transported to the setup site. The site was located on the lowest part of the island just off the aptly named River Road, which was soon inundated by two feet of water after it began to rain. According to locals, it was the most rain they’d had in 30 years and was occurring unusually late in the season.

With the use of a forklift and crane, Don Ferraro and other EOL staffers managed to get the S-Pol radar set up before the rain began in earnest. The K-band system was shipped separately and arrived in five large wooden crates that weighed between 350 and 750 pounds each, along with 42 cylinders of helium for filling weather balloons to launch radiosondes. The team spent two days moving the crates and cylinders by truck from the pier to the radar site through mud and water.

“We had to ford River Road every day to get to the radar site, and with all the entailing mud and vehicles getting stuck, it was not a pleasant situation,” Don says. “You felt soggy all the time and there were clouds of mosquitoes.”

Despite the difficulties of setup and operation, the site allowed the S-Pol radar to provide some of the cleanest and most detailed radar measurements to date of trade wind cumuli clouds. The radar looks at the atmosphere and records all the returned echoes, so that a history of cloud development and life cycle is recorded for later analysis.

“The radar site was in the middle of a swamp and it was just perfectly awful for those guys,” Charlie says. “But the job they did was truly heroic, and it was worth it because the site turned out to be perfect for the radar.”

Giant splatting aerosols

Another critical instrument during RICO allowed scientists to collect an historic data set on large cloud particles, or nuclei, in and around clouds. The instrument, called a Giant Aerosol Impactor, is a small glass slide that scientists attach to the end of a rod and place outside the aircraft’s cabin in the airstream that rushes past. Small aerosol particles follow air trajectories around the glass, but giant ones, which over the ocean are mostly salt particles created from the spray of waves, have enough momentum that they “splat” on the glass.

“It’s a bit like bugs hitting the windshield,” explains EOL’s Jorgen Jensen, who led the effort on the instrument.

Back in the lab, Jorgen and colleagues are building a custom microscope to analyze the glass slides they brought back from RICO. They’ll humidify the salt particles to make droplets, and then digitally photograph them. Using an image analysis software program, they’ll determine the size of the droplets. During RICO they exposed about 700 slides, and for each slide they expect to count 50,000 salt particles. The data should help them figure out how clouds form raindrops out of the giant salt aerosol particles.

In addition to ground and airborne instruments, a ship from Woods Hole Oceanographic Institution was on hand during RICO with Doppler radar, a wind profiler, and other meteorological instruments to observe clouds upwind of the islands.

Now that the field component is complete, scientists will spend the next five years or so analyzing data. RICO should provide the most comprehensive data set of trade wind cumuli that has ever been assembled. Although the initial data looked very promising, it’s too early to say what the information will ultimately reveal.

“The data are amazing in their variety and wealth,” says Bjorn Stevens, an NCAR affiliate scientist who is an associate professor at UCLA and one of RICO’s principal investigators. “We couldn’t have hoped for a more exciting data set to explore, but whether it does what we want, only time will tell.” • Nicole Gordon

The student side of RICO

In addition to its unique location and logistics, RICO stands out for its array of educational activities. A total of 32 students were in the field during the field project, spread out between the air, ground, and water. They included graduates and undergraduates from 15 UCAR member and affiliate universities in the United States and elsewhere.

The students had an exceptional opportunity during RICO: they operated their own research flight. Under the direction of RICO’s primary investigators, they organized and developed a flight plan to carry out three scientific missions: one targeted at aerosol processing by cloud lines, a second at understanding the dynamics and microphysics of a recurrent tail cloud produced by the island of Barbuda, and a third to compare aerosol measurements between the C-130 and the Antigua ground site.

The students were required to submit a science plan prior to the flight, and during the flight they took all of the leadership positions and as many science positions as possible on the aircraft and ground. The flight was successful, with all the targeted cloud types sampled in the manner prescribed in the student operation plan.

In addition to running their own flight, all students had the opportunity to fly on the C-130 during regular missions, as well as to spend at least two days on Barbuda working with scientists at the radar site and launching radiosondes. With guidance from principal investigators, students also took the lead in producing daily forecasts.

And even though they were in the field, the students still had some classroom time, as RICO scientists discussed their work in seminars throughout the field project. Audio and visuals from each seminar are now available on line (RICO).

Students gather around Robert Rauber from the University of Illinois in the RICO operations center.

On the Web

RICO (including seminars and other links)

Also in this issue...

It’s playtime: Parents give high marks to
UCAR Child Care Center

Rotating scientists recall time at NSF

Short Takes

Greg Holland, MMM’s new director

Python interface to NCL’s graphics library now available

Steve Schneider’s 60th

Delphi Questions

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