The Wayback Machine - https://web.archive.org/web/20110718120142/http://www-frd.fsl.noaa.gov/~caracena/micro/MBVoring.htm

The Microburst as a Vortex Ring

The a vortex-ring stretched at the base of an intense downdraft explains the observed behavior of a microburst: intense, narrowly focused surface winds expand in a ring, crest and abruptly dissipate (see Fujita, 1985). The stretching of the vortex transfers energy downscale from the larger downdraft to the smaller vortex circulation, spinning it progressively faster until instabilities rapidly break the vortex down into turbulence.


Caracena (1982) hypothesized that a microburst could be a vortex ring traveling downward along the edge of a downdraft.


Didden and Ho, (1985) observed vortex ring instabilities in the laboratory when they directed a neutrally buoyant jet against a flat plate. They observed that vortex ring instabilities increase the outflow speed at the outer edge of the surface boundary layer by 1.6 times over the nozzle exit velocity of the jet.
�


Brain Waranauskas�s photographs of a dry microburst during the JAWS project taken in rapid succession show the rapid decay of its vortex circulation. Compare these pictures with their laboratory analog.

Fujita (1986) analyzing flight recorder data of Delta Flight 191, which crashed at Dallas-Fort Worth International Airport (DFW) the afternoon of� 2 August 1985, found that the aircraft encountered not only a microburst, but that this microburst contained imbedded horizontal vorteces.


A vertical cross section of the microburst encountered by Delta Flight 191 by Caracena et al. (1986), analyzed based on the flight data recorder.
�

Fujita (1985) observed that somtimes an in-cloud mesocyclone forms above a developing microburst. Roberts and Wilson (1989) report that vorticity about a vertical axis and in-cloud convergence are Doppler radar precursors of a microburst (see also, Wilson et al., 1984).
�

Multiple microbursts

Some investigators have reported that not only one, but several microburst have happend in succession over the same area minutes apart. A microburst periodicity was observed in the crash of Pan American Flight 769 (see Caracena et al., 1983; Fujita, 1983), and has also been observed in field experiments. The periodicity of the vortex ring instability explains the wide variety of observed microburst life times and characteristics. Most microbursts last less than 5 min, but others have been observed to last four or even six times as long (Wilson et al, 1984). Long-lived microbursts may be simply a series of discrete micobursts forming over the same site.
�