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| Enlarge ImageThis buoy is equipped with an array of seven sensors called ASIMET (for Air-Sea Interaction Meteorology). The sensors measure sunlight, heat, precipitation, humidity, barometric pressure, wind speed and direction, air temperature, sea surface temperature, and salinity. (Photo by Al Plueddemann, Woods Hole Oceanographic Institution) |
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| Enlarge ImageThe buoys are atop moorings that are anchored to the seafloor. Instruments on the mooring line measure temperature, salinity, currents, chemicals and other conditions in the ocean. (Illustration by Jayne Doucette, Woods Hole Oceanographic Institution) |
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| Enlarge ImageBob Weller, now chair of the WHOI Physical Oceanography Department, has led a nearly two-decade effort to develop sensor packages that can precisely measure meteorological data in the open ocean. (Photo by Sean Whelan, Woods Hole Oceanographic Institution) |
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| Enlarge ImageThat's not a pot o' gold, but a buoy collecting valuable data on conditions at sea. (Photo by Sebastien Bigorre, Woods Hole Oceanographic Institution) |
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| Enlarge ImageMembers of the WHOI Upper Ocean Processes Group conduct at-sea repairs on an ASIMET buoy in the Gulf Stream that was probably damaged by a ship. (Photo by Patrick Rowe, Woods Hole Oceanographic Institution) |
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Related Multimedia |
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ASIMET buoy designs over the years
In search of the perfect buoy design: rugged, roomy and aerodynamic, with low power requirements. |
» View Slideshow
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Which sensor is which?
A clickable buoy shows the variety of sensors on ASIMET buoys
Painting by E. Paul Oberlander, Flash by Jayne Doucette, Woods Hole Oceanographic Institution | » View Flash
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Related Links |
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» Outposts in the Ocean
An article by Bob Weller about ocean-observing buoys in the January 2000 Oceanus magazine
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» OceanSITES
The Upper Ocean Processes Group is part of an international group of scientists using advanced buoys to study many aspects of the ocean
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Earth is often called the blue marble. But it’s more like a marble
cake: a swirling batter of air, sea, and dirt stirred by our spinning
planet and baking under the sun.
Every day, sunlight streams into the atmosphere, reflecting off clouds and dust particles, warming the
air, and making the wind blow. Heat from the sun evaporates water, leaving the sea
surface cooler, saltier, and denser and setting currents in motion. Winds also whip whitecaps into currents. Falling
rain makes the sea surface fresher and (depending where it lands) warmer or
cooler.
All of these physical transactions combine to create our climate, and
all of them change over every minute and every meter. Designing equipment to
track climate dynamics—in minute detail and at levels of precision unmatched in
the world—has obsessed Bob Weller and the Upper Ocean Processes (UOP) group at
Woods Hole Oceanographic Institution.
Two decades ago, few researchers thought it was even possible to
obtain such measurements in the roiling ocean, Weller said. The doubters, however, were "land-based
people, who have the option of cleaning radiometers every day, and
[their instruments] don’t rock and roll like buoys do. So they had
always been
pretty skeptical of ocean data.” Weller set out to change those
perceptions.
Dwelling in the details
The result of this obsession is an array of seven sensors called ASIMET (for
Air-Sea Interaction Meteorology). The sensors measure heat, sunlight, wind speed and direction, precipitation, humidity,
barometric pressure, air temperature, sea surface temperature, and salinity.
The $60,000 systems are currently bolted on a half-dozen ocean
buoys and fastened to
the masts on the bows of a handful of research and volunteer commercial ships around the
globe. Left alone for up to a year, the instruments send back enough data for
Weller’s team to calculate, minute by minute, exactly how much heat, fresh
water, and momentum moves between atmosphere and ocean.
“The kind of attention that the UOP group
puts on every parameter has been tremendous for the community,” said
Don Conlee, chief scientist for the National Data Buoy Center at the
Stennis Space Center in Mississippi. “Woods Hole has helped to define
what the achievable
accuracy and quality can be, and it gives the rest of us a benchmark to
aim
for.”
Cul-de-sacs and culminations
ASIMET began life in 1988 when scientists realized that
large inaccuracies in sea-surface measurements were holding back advances in
understanding ocean-atmosphere interactions. The ASIMET program, then called
just Improved Meteorology (IMET), sought to create an integrated system of
sensors that would simultaneously take measurements of all the various factors that computer-driven models needed
to calculate climate.
By
1993, Weller’s group was testing prototypes at sea. By 1995, the first
generation of IMET was up and running. Seven sensors housed in plastic tubes
shaped like tennis-ball canisters made the necessary measurements. The system drew
about 7.5 watts of power and ran partially on solar panels.
Ever
since, ASIMET designers have made continuous improvements to the system
despite
investigating plenty of dead-ends along the way. They fabricated a
grating to create more space for instruments atop buoys, but they
abandoned it after finding it
disrupted airflow. Solar panels sounded like a good idea but were
foiled by
clouds and beaten by rough waves. Hourly broadcasts to satellites
blasted
instruments with static, drowning out measurements.
Eleven years later, ASIMET sensors are shielded inside
titanium canisters and use 92 percent less power, freeing the system from
dependence on solar panels. They measure more precisely than ever, too, yielding
estimates of heat flux that are 90 percent more accurate than in the days
before IMET. An evolving system
Leaving ASIMET systems alone on buoys in the open ocean introduced
the team to brutal design challenges. One of the most painful lessons came in
the early 1990s off Iceland. A buoy snapped loose after it began to pitch at
the same frequency as storm waves, loading the buoy’s mooring line with 7,000
pounds of pressure every 15 seconds. Now, a design program ensures buoy
moorings are “detuned” before they go out. In his office, Weller still keeps
the broken pear link that had connected that buoy to its mooring line.
A fully equipped ASIMET buoy is a far cry from a lobster
trap or channel marker. The base is a 10-foot-wide puck of yellow Surlyn foam.
From it rises a 10-foot scaffold that bristles with instruments. A thickened wind
vane runs down one aluminum support, brandishing two satellite antennas and
pointing the buoy into the wind.
On the tower top, 12 or more instruments
cluster at the upwind edge where the airflow is least disturbed. Below, a
watertight trapdoor leads to a hold stuffed with knee-high stacks of batteries.
Sharing space are two microprocessors that control the instruments, archive the
data, and send out hourly updates via satellite.
'Burn-ins' to avoid 'burn-outs'
At
weekly meetings, the UOP group hashes out equipment problems.
With Weller consulting a whiteboard list of cruise deadlines, about a
dozen men
and women supply mooring, hardware, software, and logistical details.
Brainstorming is fed by fresh donuts or, recently, an authentic German
layer cake courtesy of Frank Bahr, a WHOI research specialist who
oversees
ASIMET deployments on volunteer commercial ships.
Even after 18 years of sweating the details, calibration is
an incessant worry. It doesn’t matter how accurate a freshly polished sensor is
on the WHOI dock. It has to measure unflaggingly for a year at a time while it
gets battered by waves, chilled in gales, or encrusted with salt spray and
seabird poop.
The team has three main approaches to the problem of
consistent measurements: lab calibration (or “burn-in”), redundancy, and
calibration at sea. Burn-in involves incessantly monitoring and retuning an
instrument for six weeks on land before a cruise. All ASIMET deployments
include at least two full sets of instruments, each set keeping an eye on the
other. Technicians carry a third complete set on all cruises in case a sensor malfunctions
on the way out. Before and after each buoy deployment, the researchers dedicate
an entire day at sea to checking the buoy’s instruments against the ship’s
onboard readings.
Opportunisitic and strategic deployments
Now that the system is precise, reliable, and frugal with
power, Weller dreams of deploying it on more buoys at more places around the
globe. Although blanketing the ocean with thousands of ASIMET buoys isn’t
practical, Weller notes that buoys in perhaps 10 carefully chosen regions per
ocean could lead to huge improvements in understanding how the ocean affects
global climate.
Weller’s list includes some regions he already has covered,
such as in the heart of the Gulf Stream and in a perpetually overcast region
off Chile. Sites on his wish list include the region downwind of the Sahara,
where windborne dust might be either blocking sunlight or heating the
atmosphere; and the brutally rough Southern Ocean where present buoy designs
simply can’t survive the winter.
For the future, Weller’s group is still looking for the
perfect radiometer, perhaps the system’s most finicky sensor and one crucial to
measuring both heat and light. Wind gauges are still unreliable in very light
and very heavy winds. David Hosom, one of the original IMET engineers, now
emeritus at WHOI, covets a $4,500 rain gauge that measures individual drops of
rain. And the team is adding another crucial climate measurement: carbon
dioxide. In October 2006, Weller and his group departed Chile to install a buoy
equipped with a CO2 sensor made by Chris Sabine at the NOAA Pacific
Marine Environmental Laboratory in Seattle.
At their first stop, in a collaboration with the Chilean
Navy Hydrographic and Oceanographic Service, the ship installed a
tsunami-warning buoy. But a sharp-eyed observer sailing past at 75ºW might
notice some extra cylindrical instruments lining the buoy tower. With rising
fuel costs cutting into research budgets, collaborating just makes sense,
Weller said. “If you’ve already got a ship driving around doing tsunami buoys,
why not put on some ASIMET instruments?”
—Hugh
Powell
Posted: March 30, 2007 [top] |
