Doppler radar observations of an Oklahoma downburst
Summary
"Downbursts", first discovered by Dr. T. T. Fujita, are small intense downdrafts at very low altitudes which impact the surface and cause a divergent outflow of wind. They can occur under a variety of meteorological conditions as was demonstrated during the JAWS 1982 field experiment which took place in the vicinity of Stapleton airport in Denver, CO. Many downbursts were detected but most of them were of the type now being called "dry" or "cumulus" or "virga" downbursts. A distinction must be made between these and the "wet" or "thunderstorm" downbursts which are the subject of this study. The two phenomena are very different. They are easy to distinguish: the former come from benign looking cumulus clouds and fall through a very deep and dry subcloud layer and the latter are associated with thunderstorms. Thunderstorm downbursts have been detected throughout the Great Plains and the Midwest, on the east coast, and in Florida, while the virga downbursts have been detected mainly over the high plains east of the Rockies. The word "downburst" was first introduced by Fujita (1976) after the investigation of a plane crash at JFK airport, to describe the situation in which a thunderstorm downdraft becomes hazardous to the operation of jet aircraft on take-off or landing. At first, Fujita (1979) thought that the downburst and the well known thunderstorm downdraft were essentially the same but that, in the same way a funnel cloud aloft is not called a tornado, a mid-level downdraft in a thunderstorm would not be called a downburst. The concept was later refined when it was decided that the downburst must induce "an outburst of damaging winds on or near the ground" (Fujita and Wakimoto, 1981) where "damaging winds" refers to winds that can be estimated on the F-scale (for which there minimum threshold is 18 m/s). These damaging winds can be either straight or curved but they must be highly divergent (Fujita, 1981). Thus, even in its most recent and more meteorological definition, the term downburst is meant to signify a potential human hazard. Whether of not it also signifies a dynamically distinct phenomenon in thunderstorms is a matter of some debate and one which will be investigated in the current work.