Up close with Caribbean cumulus

Cool heads prevail in warm-rain study that brings more downpours than expected

by Nicole Gordon

Studying clouds and warm rain on tropical islands from November through January sounds like an excuse for atmospheric scientists to take a midwinter trip to the beach. But for more than 100 scientists and technicians who went to Antigua and Barbuda, the Rain in Cumulus over the Ocean (RICO) field project was no holiday.

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

Despite the travails, RICO’s organizers say the project was a spectacular success. The topic of interest was trade wind cumulus clouds, with an emphasis on the process by which the cloud droplets coalesce into warm rain (precipitation that lacks an ice phase). These shallow clouds are found over tropical oceans around the world. Yet for more than 50 years, scientists have struggled to explain why theoretical calculations tell them the clouds should take twice as long to form rain as they actually do.

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

NCAR's Charles Knight took photos of developing cumulus each morning from a perch on the highest point of Barbuda. (Photos by Carlye Calvin.)

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. They hope to better represent climate-critical exchanges of radiant energy, moist heat content, momentum, and trace constituents between the atmosphere and ocean.

RICO’s airborne fleet included the NSF/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 Measurement. Equipped with lidars, radars, dropsondes, and other instruments, the three aircraft made a total of 57 flights. A ship from the Woods Hole Oceanographic Institution was also on hand.

“When you’re in an airplane flying around in a regime like that, everything looks like random clouds popping up,” says Donald Lenschow, an NCAR scientist and a 40-year veteran of field projects. The aircraft got assistance from staff in the RICO observing center who tracked cloud structures with radar and communicated what they saw via the Internet. “The radar can discern patterns that the airplane has no clue about. Having that big picture was so useful.”

While the aircraft component of RICO went smoothly, setting up the radar was, literally, a mess. Two NCAR radars, the S-Pol and K-band, were delivered to the relatively undeveloped island of Barbuda. The site was just off the aptly named River Road, soon to be inundated under two feet of water from unseasonably heavy rains.

With the use of a forklift and crane, Ferraro and others from NCAR’s Earth Observing Laboratory managed to get the S-Pol radar set up before the rain began in earnest. The K-band system was shipped separately in five large wooden crates, along with 42 cylinders of helium. It took two days to get the crates and cylinders by truck from the pier to the radar site through mud and water.

Despite the difficulties, the two radars, dubbed S-Polka when operating as a unit, gathered some of the cleanest and most detailed radar measurements to date of trade wind cumuli. “The radar site was in the middle of a swamp and it was just perfectly awful for those guys,” Knight 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.” Data from S-Polka and from other weather observing systems are being archived by UOP’s Joint Office for Science Support.

Another critical instrument during RICO allowed scientists to collect a historic data set on large cloud particles, or nuclei. The instrument, called a Giant Aerosol Impactor, is a small glass slide attached to the end of a rod outside the aircraft’s cabin. 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 NCAR’s Jorgen Jensen, who led the effort on the instrument.

Back in the lab, Jensen and colleagues are building a custom microscope to analyze about 700 slides they brought back from RICO. On each slide they expect to count on the order of 50,000 salt particles. The data should help them figure out how clouds form raindrops around the giant particles of salt.

“The data are amazing in their enormous variety and wealth,” says Bjorn Stevens, an NCAR affiliate scientist at the University of California, Los Angeles, and one of RICO’s principal investigators. “There couldn’t be a more exciting data set to look at. But whether it does what we want it to do—we’ll have to find out.”