|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 1994an average of one for every 1800 km (1100 miles) flown
(MacPherson and Betts 1997). They ranged in diameter from 150 to 300
meters (5001000 feet), and reached some 1500 meters (5000 feet) in
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.]
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
"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 curiositybut very important when youre flying 20
meters above the forest canopy."
- 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.
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.
- Betts, A. K., M. L. Goulden, and
S.C. Wofsy, 1999: Controls on evaporation in a boreal spruce forest.
J. Climate, 12, 1601-1618.
- 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).
- Betts, Alan K., 1999:
- Hall, Forrest G, 1999: Personal
Improving the Forecast Models
A column of rising warm air forms the center of a vortex,
surrounded by spiraling winds. (Diagram courtesy Mars Pathfinder Project, Jet Propulsion Lab)