Forecasting Microbursts & Downbursts

Fernando Caracena
NOAA/Forecast Systems Laboratory

"Windshear Revisited by Capt. William W. Melvin (from Air Line Pilot Magazine, Nov. 1994, excerpted from SAE Paper 901995)

Too many windshear accidents have been analyzed wih emphasis on pilot error without attempting to understand why the errors were made. In most cases, the analyses were flawed, and no substantial pilot error existed. This has caused considerable misunderstanding of serious aspects of windshear hazards that still exist in pilot training literature. These misunderstandings pose human factor problems for pilots when they have to deal with windshear. Many pilots have been trained to aviod large supercell-type thunderstorms in the belief that this will prevent encounters with microbursts. Yet no evidence exists that any of the known microburst encounters have occurred in supercell storms. Dr. Ted Fujita and Dr. Fernando Caracena recognized authorities in this field~ have repeatedly emphasized that microbursts are frequently generated from benign-appearing cells. Many "experts" who disagree with Drs. Fujita and Caracena have emphasized the supercell storms with warnings of dangers of gust fronts. These so-called experts are leading pilots down the primrose path for microburst encounters."
wet/dry microbursts
The type of windshear that is most dangerous to aviation is the
type spawned by a microburst in an isolated rainshower or
thunderstorm. The crital time for an aircraft is during landing or
take off. In an extremely dry environment, little or no rain
may reach the surface, but the winds may exceed hurricane force,
and may approach the speeds of a weak to moderate tornado.
In a wet environment, the microburst may be imbedded in heavy
rain, but its onset may be so sudden as to catch pilots unaware.

Microburst animation.


Microburst definitions

The definition of a microburst depends on its operational use. If wind damage is a concern, then the magnitudes of the wind gusts are important. If aviation is the area of concern, then critical values of the horozontal windshear and magnitude of the downdraft are the important considerations. In field experiments the operational definitions screen out the important events, allowing researcher to focus their attention..

A microburst is a three-dimensional circulation pattern of damaging winds driven outward near the surface by the ground impact of an unusually strong convective downdraft. Its horizontal extent is 5 km or less; and its lifetime is only a few rninutes. It may contain lmbedded and leading edge vortices that rotate along a horizontal axis, reaching tornadic strength, presenting an extreme hazard to aircraft taking off and landing. The entire structure of downdraft, severe winds, and imbedded and leading edge vortices constitutes the microburst¹s circulation pattern.

Fujita (1985):
A downburst is a strong downdraft which induces an outburst of damaging winds on or near the ground. Damaging winds, either straight or curved, are highly divergent
MACROBURST: A large downburst with its outburst winds extending
in excess of 4 km (2.5 miles) in horizontal dimension. An intense
macroburst often causes widespread, tornado-like damage. Damaging
winds, lasting 5 to 30 minutes, could be as high as 60 m/sec (134 mph).

MICROBURST: A small downburst with its outburst, damaging winds
extending only 4 km (2.5 miles) or less. In spite of its small
horizontal scale, an intense microburst could induce damaging
winds as high as 75 m/sec (168 mph).

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forecasting methodologies

There is at present an insufficient observational database from which to develop a comprehensive forecasting scheme for microbursts; however, there are enough data from the Joint Airport Weather Studies (JAWS) project on which to base and design an objective forecating algorithm for dry microbursts. Data from the MIcroburst and Severe Thunderstorm (MIST) project plus case studies of the Gulf Coast and Florida area provide sounding based thresholds for wet microbursts. The development of a comprehensive forecast scheme for microbursts is for the moment on hold, until the required field project data are taken.

Microbursts in classic severe storm environments
The aircraft accident rate due to microbursts in association with classic severe local storms is practically nonexistent for commercial aviation, since the system is apparently well designed to protect passenger jets against well-organized, long-lived, and highly reflecting storms and therefore automatically protects aircraft from any microburst components of these storms. By simply avoiding classic severe storm types, such as squall lines and supercells, pilots are already avoiding any microburst components of these storms. However, aircraft are not well protected from wet and dry microbursts that are an unexpected component of isolated airmass-type thunderstorms, and rainshowers.

The predictability of microbursts in a dry environment
Krum (1954) first described the typical sounding associated with dry thunderstorms that produce strong downdrafts. Prediction of dry microbursts from local soundings was explored qualitatively first by Brown et al. (1982) then by Wakimoto (1985) based on JAWS (Joint Airport Windshear Studies) project data. A preliminary, quantitative prediction scheme also based on JAWS project data was proposed by Caracena et al. (1983a) and Caracena and Flueck (1986 and 1987a and b), demonstrating that the virga-type microbursts in that form in dry sub-cloud environments can be forecast in terms of upper air data. What remained to be found were a means of forecasting microbursts in a wet enviroment in association with heavy rain. These were the type of conditions involved at New Orleans International Airport at 21 10 UTC 9 July 1982, when Pan American Airways Flight 759 crashed after attempting to take off through a wet microburst (Caracena et al., 1983b).

Predicting wet microbursts
Atkins and Wakimoto (1991) analyzing data from the 1986 MIST (MIcroburst and Severe Thunderstorm) project conducted in northern Alabama found that in all five days when wet microburst occurred, the equivalent potential temperature differences between the surface value and the minimum aloft were 20 deg K or greater. On the other three days with thunderstorms, but without microbursts, there were equivalent potential temperature differences of 13 deg K or less. Atkins and Wakimoto (1991) also examined data from other well documented wet microburst cases, such as happened near Chicago (Fujita 1985), near Edmund, Oklahoma (Eilts and Doviak 1987), and southern Florida (Caracena and Maier 1987). They found that in all these microburst cases, the equivalent potential differences were greater than 20 deg K.

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Forecasting the Potential For Central Florida Microbursts

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