"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
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.
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.
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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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.
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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Atkins, N.T., and R.M. Wakimoto, 1991: Wet microburst activity over the Southeastern United States: Implications for forecasting. Wea. Forecasting, 6, 470-482.
Bedard, A. J., Jr. and T. J. LeFebvre, 1986: "Surface Measurements of Gust Fronts and Microbursts During the JAWS Project: Statistical Results and Implications for Wind Shear Detection, Prediction, and Modeling," NOAA TM ERL-WPL-135, April 1986.
Braham, R. R., 1952: "The Water Energy Budgets of the Thunderstorm and Their Relation to Thunderstorm Development," Journal of Mete orology, 9, 227-242.
Brown, J., K.R. Knup, and F. Caracena, 1982: Destructive winds from shallow, high-based cumulonimbi, 12th Conf. on Severe Local Storms, Amer. Meteor. Soc., Boston, MA, 272-275.
Caracena, F., 1978: A comparison of two downbursts of different meso scales. Conf. on Wea. Forecasting and Analysis and Aviation Meteorology, 293-300. A.M.S., Silver Spring, MD.
Caracena, F., and J.A. Flueck, 1988: Classifying and predicting microburst activity in the Denver, Colorado, Area. J. of Aircraft, 25, 525-530.
Caracena, F., and J. A. Flueck, 1987a: "A Study in Predicting Microburst Conditions Using Project JAWS Data," JAWS-FAA TR, Boulder, CO.
Caracena, F., J. McCarthy, and I. A. Flueck, 1983a: "Forecasting the Likelihood of Microbursts Along the Front Range of Colorado," 12th Conf. on Severe Local Storms, American Meteorology Society Boston, MA, Oct. 1983, pp. 261-264.
Caracena, F., and J. A. Flueck, 1987b: The classification and prediction of small-scale windshear events in a dry environment. Aerospace Century XXI, Advances in the Astronautical Sciences, 64, 1349-1360.
Caracena, F., R. Ortiz, and J. Augustine, 1987: "The Crash of Delta Flight 191 at Dallas Fort Worth International Airpon on 2 August 1985: Multiscale Analysis of Weather Conditions," National Oceanic and Atmospheric Administration TR ERL 430-ESG-2, Dec. 1987.
Caracena, F., and M. Maier, 1987: Analysis of a microburst in the FACE meteorological mesonetwork in southern Florida. Mon. Wea. Rev., 115, 969-985.
Caracena, F., Maddox, R. A., Purdom, J. F. W., Weaver, J. F., and Green, R. N., 1983b: "Multiscale Analysis of Meteorological Conditions Affecting Pan-American World Airways Flight 759," National Oceanic and Atmospheric Administration TM ERL-ESG-2, Jan.
Dixonj W. J., 1983: ³BMDP Statistical Software, 1983 revised,² University of California Press, Los Angeles .
Eilts, M.D., and R.J. Doviak, 1987: Oklahoma downburst and their asymmetry. J. Climate Appl. Meteor., 26, 69-78.
Flueck, J. A., 1986: "Principles and Prescriptions for Improved Experi mentation in Precipitation Augmentation Research," Precipitation EnhancementA Scientific Challenge, Chap. 16, edited by R. R. Braham, Jr., American Meteorologica! Society Monograph, Vol. 21 No. 43, Boston, MA.
Fujita, T. T., 1985: "The Downburst- Microburst and Macroburst," Sattellite and Mesometeorology Research Project (SMRP) Research Paper 210, Dept. of Geophysical Sciences, Univ. of Chicago, (NTIS PB-148880) Feb. 1985.
Fujita, T.T., 1985: The downburst. SMRP Res. Paper No. 210, NITIS PB 85-148880. 122 pp. [Available from the University of Chicago, Chicago, IL.]
Fujita, T. T., 1983: "Microburst Wind Shear at New Orleans Interna tional Airport, Kenner, Louisiana on July 9, 1982," Univ. of Chicago Satellite and Mesometeorology Research Project Research Paper 199, Jan. 1983.
Fujita, T. T., 1976: "Spearhead Echo and Downburst Near the Approach End of a John F. Kermedy Airport Runway, New York City,² Dept. of Geophysical Sciences, Univ. of Chicago, SMRP Res. Project Research Paper 137, March 1976.
Fujita, T. T., and Byers, H., 1977: "Spearhead Echo and Downburst in the Crash of an Airliner," Monthly Weather Review, 105, 129-146.
Fujita, T. T., and Caracena, F., 1977: "An Analysis of Three Weather Related Aircraft Accidents," Bulletin of the American Meteorological Soctety, 58, 1164 1181.
Fujita, T. T., "DFW Microburst on August 2, 1985," Univ. of Chicago, SMRP Res. Paper 217, (NTIS No. PB-86-131638) Jan 1986.
Joint Airport Weather Studies, "The JAWS Project Operations Summary," JAWS Project Office, NCAR, Boulder, CO, Feb. 1983.
Krumm, W.R., 1954: On the cause of downdrafts from dry thunderstorms over the Plateau Area of the United States. Bull. Amer. Meteor. Soc., 35, 122-125.
McCarthy, J., Wilson, J. W., and Fujita, T. T., 1982: "The Joint Airport Weather Studies Project," Bull. Amer. Meteor. Soc., 63, 15 22.
Tukey, J. W., 1977: "Exploratory Data Analysis,² Addison-Wesley, Reading, MA.
Wakimoto, R.M., 1985: Forecasting microburst activity over the High Plains. Mon. Wea. Rev., 113, 1131-1143.
Wakimoto, R.M., and V.N. Bringi, 1988: Operational detection of microbursts associated with intense convection: The 20 July case during the MIST project. Mon. Wea. Rev., 116, 1521-1539.
Wheeler, M. 1994: Analysis and Review of Downrush Wind Events on 16 August 1994. AMU Memorandum, KSC FL, 22 pp.
Forecasting the Potential For Central Florida Microbursts
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