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During their flights aboard the Twin Otter to measure the interactions between the boreal forest and the lower atmosphere, the BOREAS team encountered eight vortices during the summer of 1994—an average of one for every 1800 km (1100 miles) flown (MacPherson and Betts 1997). They ranged in diameter from 150 to 300 meters (500—1000 feet), and reached some 1500 meters (5000 feet) in height.

Functioning a little like "mini-tornadoes," these vortices provide a mechanism in low wind conditions for the surface to transfer enormous amounts of heat and carbon dioxide up into the atmosphere. The BOREAS team measured fluxes of heat and carbon monoxide anywhere from 30 to 60 times greater than average during these vortex events.

"The vortices are a kind of dry-air convection," Betts says. [Here, "convection" refers to the upward motion of warm air, which rises because it is less dense than adjacent cooler air, just as warm air rises above a wood stove.]
 

  dust devil

Small atmospheric vortices in arid regions, like this one in Australia, are called "dust devils." In the boreal forests there are few fine dust and sand particles to be lofted into the air by vortices, so they remain invisible, threatening low-flying aircraft. (Photograph by John Roenfeldt, courtesy Australian Severe Weather)

vortex schematic

"There are a number of different ways in which the atmosphere organizes to transfer heat away from the surface," he adds. "The fact that the boreal ecosystem uses these vortices is an interesting curiosity—but very important when you’re flying 20 meters above the forest canopy."

References

  1. MacPherson, J. Ian and Alan K. Betts, 1997: Aircraft encounters with strong coherent vortices over the boreal forest, Journal of Geophysical Research, 102, pp. 29,231-34.
  2. Betts, Alan K., Pedro Viterbo, Anton Beljaars, Hua-Lu Pan, Song-You Hong, Mike Goulden, and Steve Wofsy, 1998: Evaluation of land-surface interaction in ECMWF and NCEP/NCAR reanalysis models over grassland (FIFE) and boreal forest (BOREAS), Journal of Geophysical Research, 103, pp. 23,079-85.
  3. Betts, A. K., M. L. Goulden, and S.C. Wofsy, 1999: Controls on evaporation in a boreal spruce forest. J. Climate, 12, 1601-1618.
  4. Viterbo, P. and A.K. Betts, 1999: The impact on ECMWF forecasts of changes to the albedo of the boreal forests in the presence of snow. J. Geophys. Res. (In press, BOREAS special issue).
  5. Betts, Alan K., 1999: Personal interview.
  6. Hall, Forrest G, 1999: Personal interview.

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A column of rising warm air forms the center of a vortex, surrounded by spiraling winds. (Diagram courtesy Mars Pathfinder Project, Jet Propulsion Lab)

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