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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. |
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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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