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Expert: A microburst sank Concordia

NOAA satellite analysis confirms skipper's account of the knockdown off Brazil that doomed the tall ship

A NOAA scientist who researches microbursts has fingered one of the violent downdrafts as the likely culprit in the Feb. 17 sinking of the tall ship Concordia. He reached that conclusion after analyzing satellite imagery of the weather that day.

Concordia went down within a half-hour of her encounter with the microburst, and all 64 on board were rescued.

Ken Pryor, a researcher with NOAA's Center for Satellite Applications and Research in Camp Springs, Md., has confirmed what William Curry, captain of the 188-foot three-masted Canadian school ship, says he and his crew observed when Concordia went down 290 nautical miles south-southeast of Rio de Janeiro, Brazil. A microburst - a violent downdraft that strikes suddenly and without warning - knocked the ship on its side and it was unable to recover.

Concordia sank within a half-hour. Two merchant ships plucked the vessel's 64 students and crew from life rafts 40 hours later.

Pryor's analysis of infrared imagery from a Geostationary Operational Environmental Satellite identified a convective storm - one with powerful updrafts, "overshooting  tops" (dome-like crowns topping off thunderheads and indicating severe thunderstorm activity), and the clincher, a "dry-air notch" where the storm sucked dry air up into the clouds, providing fuel for a powerful downburst. Pryor says he found these conditions, ideal for generating microbursts, in the vicinity of Concordia just 13 minutes before the knockdown.

A microburst forms like this, according to Pryor. The storm's updraft sucks dry air into the clouds. The dry air causes precipitation to evaporate, which cools the air, making it heavier and causing it to sink and create downdrafts, which become microbursts when very localized and powerful. These microbursts hit the water and spread out horizontally, creating a burst of high wind along the surface.

"I would say [the overshooting tops] were directly overhead, if not in very close proximity" to Concordia around the time of her sinking, Pryor says. The dry-air notch - the point at which the storm system was drawing warm, dry air in - was 10 to 20 miles from the ship on the storm's southwest flank, "which isn't out of order, considering the size of the storm complex," he says. "I would say [the downdraft] was in very close proximity to Concordia. Based on the observations of the crew, it occurred almost on top of them - overhead."

Capt. Curry, in an e-mail to Soundings, describes erratic wind conditions, also suggesting a downdraft over, or very nearly over, the ship. "When the concentrated rain patch passed over the ship, the wind increased to 25 knots and the vessel sailed normally for a few minutes," he says. "Then the mate reported that the wind backed suddenly (hauled ahead) and he put the helm up to bear away. Subsequently, the wind veered to the starboard quarter, and the anemometer showed an increasing wind speed. The mate lost track of the anemometer as it climbed through 30 knots, as the vessel was rapidly increasing her heel to port and, seconds later, capsized to an angle of 90 to 100 degrees."

The brief but violent encounter tore the storm-reefed mizzen in at least two places, Curry says, and the upper square topsail was torn from head to foot. "The wind gust that knocked the Concordia down lasted only minutes, at most," he says.

Winds prior to the microburst were 16 to 20 knots, seas 4 to 6 feet. The mate on watch had seen the squall approaching, noted it and reported no sign of strong winds (froth or whitecaps) below the squall line and a concentrated circle of rain about a mile in diameter. Concordia already was running with just 40 percent of her sails up, Curry says. "Only the 'lowers' were set, which included storm-reefed mizzen, mainsail, main staysail, fore staysail, inner jib, and upper and lower square topsails," he says.

If the downdraft was, in fact, overhead or close to it, it could explain why the ship didn't recover, Pryor says. Curry, while reserving final judgment until the investigation is complete, concurs. "As you can appreciate, after over 500,000 successful miles at sea and a long 18-year history of weathering many storms, the Concordia had proven herself a very able sea-boat able to stand up to hurricane-force winds," he says. "But 40-plus knots of wind directed downward after the vessel had heeled to deck-edge immersion angle is another story."

Pryor says his meteorological findings remain a hypothesis, but he says in other reports of microbursts - the sinking of the Lady D, for one - radar has confirmed his satellite imagery analysis, revealing conditions favorable to microbursts.

The Lady D was a pontoon water taxi that capsized and sank in a severe thunderstorm in March 2004 in Baltimore Harbor, resulting in five deaths. Pryor says his analysis of both the satellite and radar imagery of conditions over Baltimore Harbor that day also show a convective storm system, a dry-air notch - confirmed by both the radar and the imagery - and a downdraft that he says capsized the Lady D. (The National Transportation Safety Board puts part of the blame for the capsize on a Coast Guard stability standard for the Lady D that was flawed, allowing it to carry more passengers than it should have been carrying.)

Pryor notes that in the cases of both the Lady D and the Concordia, winds generated by the downdrafts were 35 to 45 knots, not the 100 knots or more often associated with a microburst. Pryor says the 100-knot downburst is very unusual. "I've documented several hundred downbursts," he says. "By far, the majority of downbursts at the surface were 35 to 50 knots."

The imagery Pryor analyzes is a graphic representation of the difference between water vapor temperatures and air temperatures on the surface and in the clouds, as measured by satellite-based infrared sensors.

Pryor says he can say with some confidence that a downburst sank Concordia, based on crew observations as well as his own observations with satellite imagery. "It's all consistent with a downburst," he says.

When Concordia sank, some questioned the vessel's stability. Canada's Globe and Mail newspaper reported March 2 that Terry Davies, founder and chairman of Classes Afloat, which operates the school ship, says it underwent a stability test during construction that showed it could recover from a 110-degree knockdown.

The Transportation Safety Board of Canada and the Barbados Maritime Ship Registry, headquartered in London, continue their investigations into the loss of the Canada-based, Barbados-registered tall ship.

This article originally appeared in the June 2010 issue.