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Summer 2001

An EPIC tale: Probing the eastern Pacific tropics

Much of the observing in EPIC-2001 will take place along longitude 95°W, transecting the equatorial cold tongue in the eastern Pacific Ocean. The Woods Hole Oceanographic Institution is providing the IMET (improved meteorology) buoy at the far south end. (Illustration courtesy EPIC and Michael Shibao.)

by Bob Henson

Almost a decade ago, scientists cast a huge observational net across the tropical warm pool at the western end of the Pacific Ocean. This year, the spotlight turns to the east. Overlaid with a shallow, warm surface layer and packed with strong north-south temperature contrasts, the eastern tropical Pacific is a fertile region for exploring the ocean-atmosphere interactions that still confound even the best computer models.

About 50 scientists and technicians are gathering in September and October for EPIC-2001, the East Pacific Investigation of Climate Processes in the Coupled Ocean-Atmosphere System. The lead scientist is David Raymond (New Mexico Institute of Mining and Technology). Also participating are several U.S. universities (see sidebar) and the National Autonomous University of Mexico (UNAM).

According to Raymond, "Global coupled ocean-atmosphere models have great difficulty reproducing the conditions in the East Pacific, including in particular the seasonal cycle." The hope for EPIC, he says, is "to use the insights we obtain to improve physical parameterizations in global models. There's significant interest in this work among a number of modelers."

Just the facts

Who: More than 20 institutions and roughly 50 scientists from the United States, Mexico, and Ecuador. The lead scientist is David Raymond (New Mexico Institute of Mining and Technology). Other principals include Chris Fairall (NOAA Environmental Technology Laboratory) and Robert Weller (Woods Hole Oceanographic Institution), managing the R/V Ron Brown observations; Steven Esbensen (Oregon State University), coordinating aerosonde deployment; and Chidong Zhang and Nick Shay (University of Miami), aboard the NSF/NCAR C-130.

With what: Two U.S. aircraft (the NSF/NCAR C-130 and, if hurricane duties permit, a NOAA P-3), research vessels from the United States (the NOAA Ronald H. Brown and the NSF/Scripps Institution of Oceanography New Horizon) and Mexico (UNAM El Puma), aerosondes (from the Australian firm Aerosonde), the Tropical Atmosphere Ocean array of moored buoys, and aircraft- deployed dropsondes and oceanic probes.

When: September to mid-October 2001

Where: Across the eastern tropical Pacific, with observations focused at longitude 95°W. The main operations base is at Huatulco, Mexico, with a secondary base on San Cristobal Island made possible by Ecuador's meteorological service.

Ground zero for EPIC is 95°W, just west of the Galapagos Islands, where the easternmost set of Tropical Atmosphere Ocean (TAO) buoys is stationed. Other instruments will be deployed along this longitude for more than 3,200 kilometers (2,000 miles), centered roughly on the equator.

The geography of South America's west coast leads to strong upwelling off Peru and Ecuador, producing a narrow tongue of shallow, cool surface water that spreads west near the equator (in the absence of El Niño). Because of the cold tongue, "air- sea interactions are actually much stronger than they are in the western Pacific," says Raymond. "The gradients in sea-surface temperature drive the winds to a large extent, and the winds in turn strongly affect the SST distribution."

EPIC will sample the transition zone between the cold tongue and the warm intertropical convergence zone (ITCZ), which should be positioned around 10°N. Among the goals are to study the mechanisms that control oceanic temperature and salinity and the entrainment of cool surface water into the deeper mixed layer. Above sea level, EPIC will analyze the atmospheric boundary layer, the region's vast stratus and stratocumulus decks, and the intense showers and thunderstorms organized in easterly waves. (These waves often spawn tropical cyclones in the northeast Pacific, but the cyclones aren't a direct object of study in EPIC.)

Christopher Bretherton (University of Washington) spent his spring on sabbatical at NCAR working to improve the boundary-layer component of the NCAR Community Climate System Model. As a principal investigator for EPIC, he hopes to better understand why the region's atmosphere is so difficult to represent in the CCSM and other models like it. "Stratocumulus have always been a problem for general circulation models. We don't know much about what maintains the thickness of low-lying stratus clouds," Bretherton says. The cold water and stratus appear to help maintain each other through a positive feedback: when the clouds break, the ocean warms up, which helps the clouds to erode further, or so the theory goes.

Aerosols also play a big role in the stratus equation, although how big is anyone's guess. Aside from their direct role in absorbing or reflecting sunlight, aerosols have at least two indirect effects. For one, their presence leads to more cloud droplets and thus more surface area for sunlight reflection. It's the second indirect effect that Bretherton says "we truly understand very little about." If a cloud's water is subdivided into a greater number of smaller droplets, the cloud is less likely to form drizzle and thus less likely to dissipate. Aerosols from biomass burning in Brazil may be wafting across the Pacific and feeding the ocean's stratus decks in this way. EPIC will try to determine if this process is a primary control on the thickness and reflectivity of the stratocumulus clouds. The EPIC data will also help profile another important control on clouds, the entrainment process by which dry air aloft is mixed down (and entrained into the stratocumulus) by turbulent cloud-level eddies.

To examine feedbacks among clouds, aerosols, and precipitation, scientists from the NOAA Environmental Technology Laboratory, the University of Washington, and UNAM will use radar, lidar, aerosol sampling, and a host of other measurements aboard NOAA's Ron Brown research vessel. The R/V Ron Brown will sail beneath southeast Pacific stratus from the Galapagos Islands to Peru over a two-week period after sampling ocean turbulence and the ITCZ's thunderstorm complexes for three weeks from a fixed site near 10°N. Another research ship, the NSF/Scripps Institution of Oceanography R/V New Horizon, will look at horizontal variations in the subsurface ocean that can affect where the ITCZ storms flare up.

Meanwhile, from a base in Huatulco, Mexico, the NSF/NCAR C-130 aircraft is slated to carry out up to ten long loops along 95°W. Each flight will take a low, vertically serrated path on the outbound leg, dipping into stratocumulus layer, then pass through the top of convection for cloud-physics sampling on the return trip. Eight C-130 flights will explore the ITCZ. A NOAA P-3 is expected to join EPIC if hurricane-hunting duties permit. The operations center in Huatulco will be managed by Gus Emmanuel (UCAR Joint Office for Science Support).

A side benefit of EPIC-2001 could be a better understanding of the El Niño/Southern Oscillation. Some of the most dramatic oceanic and atmospheric shifts induced by ENSO occur as the warm- pool waters extend into the vicinity of 95°W. However, since EPIC is aimed at understanding the cold tongue and its influence under neutral conditions, "we're very relieved that there's not a warm event happening," Bretherton says. "From our point of view, it would have been bad if we'd had a very strong El Niño."

On the Web:
EPIC-2001 home page
EPIC scientific plan
EPIC field catalog

A different stratus study: DYCOMS-II

NCAR's C-130 will already be in the Pacific neighborhood for EPIC, thanks to another field project. DYCOMS-II (Dynamics and Chemistry of Marine Stratocumulus, Phase II: Entrainment Studies) got under way in late July just to the southwest of the California/Mexico border. While this project's focus is north of the EPIC region, DYCOMS-II is interested in the same kind of persistent stratocumulus cloud that also occurs on the south side of the ITCZ.

Several other field studies over the past decade have focused on the mechanics of stratocumulus development in the northeast Pacific. DYCOMS-II was designed to address "a rather narrow set of theoretical questions that arose in part out of attempts to interpret the previous studies," says principal investigator Bjorn Stevens (University of California, Los Angeles). However, he adds, the project has "grown considerably." Along with testing a set of model parameterizations for cloud entrainment, the project is also exploring the interactions among cloud droplets, aerosols, and drizzle. Also on the agenda is a new technique for measuring cloud-level divergence, proposed by Donald Lenschow (NCAR). The NSF/NCAR C-130 is flying in stacks of alternating clockwise and counterclockwise circles to see if the divergence of air from each circle can be accurately detected. "It's a difficult measurement," says Stevens, "largely because mesoscale variations in the wind contribute errors to the measured divergence."

On board the C-130 is a set of very fast response (1000 hertz [Hz] or greater) microphysical probes. The array includes an ultra-fast thermometer developed at the University of Warsaw, a Fast-FSSP (Forward Scattering Spectrometer Probe), and a Gerber Particulate Volume Monitor. In addition, NCAR's Scanning Aerosol Backscatter Lidar (SABL) and the Wyoming Cloud Radar will shed light on cloud morphology and composition.

The "C" in DYCOMS denotes the various fast chemical measurements that will be used to help unravel the dynamics of the layer. Among these instruments are a tunable diode laser that may be able to sample water vapor as often as eight times a second (8 Hz). According to Stevens, the laser "should provide unprecedented measurements of water vapor in and around the marine boundary layer, both in and out of clouds." NCAR chemistry sensors will take readings of ozone, and a newly developed instrument by a group at Drexel University will measure dimethyl sulfide at 25 Hz. The latter, Stevens says, is "thought to be an ideal tracer for estimating mixing between the free atmosphere and the underlying marine layer. It's a real boon to our experimental objectives."

Another novel aspect of DYCOMS-II is the time of day. All but one of the flights will be at night, according to Stevens. "While there were some purely nocturnal flights over [stratocumulus in] the Atlantic, none of these measured clouds in the region where the stratocumulus are topped by relatively dry air. Such conditions are typical off the coast of southern California, but rarely sampled at night."

On the Web:
DYCOMS-II home page

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Edited by Bob Henson, bhenson@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Wed Aug 8 17:05:07 MDT 2001