Characteristics of Microbursts in the Continental United States

Microbursts-powerlul downdrafts generally associated with thunderstonns that occur in hot, humid weather-have caused a number of aircraft crashes. To prevent future accidents, air traffic controllers must be able to detect, and predict, microburst events. All microbursts are not alike, however; several distinct weather patterns can produce microbursts. Thus a categorization of the different types of microbufsts is an essential part of understanding these hazardous phenomena. Using this categoriza-tion, the relative hazard to aviation ofthe various types ofmicrobursts can be assessed. Wind shear is a major cause of air carrier accidents in the United States-seven crashes due" to wind shear since 1970 have killed 575 people. Most of these accidents have been caused by a particular form of wind shear, called a microburst. Microbursts are small-scale, low-altitude, intense downdrafts that impactthe surface ofthe earth and cause strong divergent outflows of wind. They arise from thunderstorms and are usually, but not always, associated with heavy rainfall. Anumber of meteorologically distinct phenomena , all ofwhich give rise to strong surface outflows, are being called microbursts. The current interest within the scientific and aviation communities in understanding micro-bursts makes it important to categorize these phenomena according to their meteorological nature and true aviation hazard potential. This categorization is essential to discovering exactly what atmospheric conditions lead to the development of microbursts, and to building a coherent base ofknowledge on which automated algorithms for the detection and prediction of microbursts can draw [1]. HISTORY One of the first illustrations of the hazard posed by thunderstorm downdrafts impacting the surface to planes flying at low altitudes appeared in 1961 [21. This study was based primarily on the wind pattern encountered by a BOAC Argonaut plane taking offfrom the Kano, Nigeria, airport in June 1956 [3]. In this type ofwind pattern (Fig. 1), the plane experiences a headwind as it approaches the thunderstorm outflow. This headwind causes an increase oflift and an increase in altitude. A pilotwho is unaware ofwhat is to follow maytry to compensate for the increased lift, especially if it occurs during a landing. (To avoid overshooting the runway, the pilot must maintain a precise glides10pe.) But then a downdraft causes the plane to lose altitude, and a tailwind , experienced after the plane flies through the center of the diverging outflow, decreases the lift and causes a further loss of altitude. Although it was not recognized back in 1961, a pilot's reaction to …