National Center for Atmospheric Research P.O. Box 3000 Boulder, CO 80307

1986-11 For Release: July 9, 1986

Researchers Stalk Windshear During Rainstorms in Humid Southeast

HUNTSVILLE— Four years ago today, Pan American World Airways Flight 759 crashed in a heavy rainstorm within a few seconds after liftoff from International Airport, demolishing 15 houses in the suburb of Kenner, killing all

145 people on the plane8 on and the ground. Cause: wind shear. Two years ago on May 31, 1984, United Airlines Flight 663, with 105 people on board, narrowly escaped disaster on takeoff from Denver's Stapleton International Airport when its underside scraped the tops of several 15-foot high antennas, one of which was later found jammed in the plane's tail section. Two 47-inch holes were ripped in the jetliner's belly. Cause: wind shear. Nearly one year ago, on August 2, 1985, Delta Airlines Flight 191 crashed on approach during a driving rainstorm about one-quarter mile from the Dallas-Fort Worth International Airport, killing 133 people. Cause: wind shear. To learn more about the meteorological conditions that give rise to such disasters, federal and university scientists conducting a large field project termed Microburst and Severe Thunderstorm (MIST), at Huntsville, Alabama. The study, which is funded by the National Science Foundation, will continue through July 31. The scientific team is led by Theodore Fujita, University of Chicago, who was the first person to predict that microburst wind shears exist and can cause airplanes to crash. "The purpose of MIST is to investigate the meteorological structure of microbursts from thunderstorms and their environment in a warm, moist region of the ," says James Wilson, field project manager for the National Center for Atmospheric Research (NCAR). Microbursts are brief, violent downdrafts that occur near the ground and can interfere with an aircraft's flight path. NCAR, a private nonprofit research laboratory sponsored by the National Science Foundation, has supplied three Doppler radars for scanning convective thunderstorm systems and a network of 50 solar-powered portable weather stations. Designed and developed by NCAR, the weather stations are spaced

The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research under the sponsorship of the National Science Foundation. Researchers Stalk Windshear 2 from one to three miles apart in a region covering northern Alabama and the adjoining portion of central Tennessee to gather ground-level meteorological information about microbursts. This information will be correlated with wind shear and precipitation data from the radar scans. In addition to Fujita, University of Chicago, other University participants in MIST include the Gregory Forbes, Pennsylvania State University; Roger Wakimoto, the University of California at Los Angeles, and V. N. Bringi, Colorado State University. Other support facilities include a four-engine Lockheed P-3 research aircraft, operated by the National Oceanic and Atmospheric Administration (NOAA), which is gathering airborne measurements of meteorological and cloud physics conditions of the microburst environment. The MIST research project is part of a larger comprehensive program termed the Cooperative Huntsville Meteorological Experiment, which is sponsored by the National Aeronautics and Space Administration (NASA), the Federal Aviation Administration (FAA) and the National Science Foundation. The NASA component of the field project, called the Satellite Precipitation and Cloud Experiment (SPACE), focuses on observations of the physical processes that leads to the formation of small convective systems during the pre-storm period. Two high-altitude aircraft, a NASA U-2 and an ER-2, have been making numerous flights at 60,000 feet altitude. A South Dakota School of Mines and Technology T-28 storm penetrating aircraft, outfitted with four-inch armor plating to prevent hail damage, is flying into storm clouds to gather information on liquid water content and hail size during storms. A companion experiment, FLOWS (for FAA-Lincoln Laboratories Operational Weather Studies) is focusing on the development and testing of automatic computer detection of wind shear using Doppler weather radars. The National Center for Atmospheric Research is managed by the University Corporation for Atmospheric Research, a consortium of 55 universities with graduate programs in the atmospheric sciences or related fields.

For further information, please contact: Joan Vandiver Frisch, Manager, Media Relations National Center for Atmospheric Research (NCAR) P.O. Box 3000, Boulder, Colorado 80307 (303) 497-8721

NOTE: See attached background information and wind shear accident statistics NCAR Background Sheet on Microbursts, Wind Shear— July 1986

Definitions:

Wind Shear- A situation in which the wind changes its speed and/or direction over a small distance. When an airplane flies through a wind change of this kind, especially when at low altitude, the pilot must adjust for it quickly to maintain course and altitude. Usually a wind shear poses little danger. But should it occur over a distance as short as about 1 to 3 kilometers, a pilot may have only a matter of 30 seconds or less to recover safely from its effects. It is in connection with convective storms that the particular type of wind shear known as "microburst" occurs.

Microburst- A relatively simple form of disturbance in atmospheric flow, characterized by a strong downdraft associated with a thunderstorm and sometimes with a summertime cumulus cloud small enough to produce no tell-tale lightning or heavy rain. The downdraft usually has a visible rain shaft associated with it. When and where the downdraft reaches the earth's surface, it spreads out horizontally, much like a stream of water from a kitchen tap striking the bottom of the sink. Sometimes the rain shaft evaporates before reaching the ground, leaving the downdraft invisible.

But regions of the sky where microbursts may lie hidden are something else to reckon with. They typically look benign. The potentially lethal low-altitude wind shear of a microburst may be packaged in a harmless-looking rain shaft, or perhaps in the rain-free air below cloud base with no visible clues to warn of its presence. Sometimes there are tell-tale signs of the presence of a microburst that pilots can be trained to recognize. One of these is the sight of localized blowing dust near the ground (termed a gust front).

But, too often, microbursts can also occur in the heavy rain of a thunderstorm where they are obscured from view. This, it turned out, was the case of the microburst that brought down Flight 759 in Kenner, Louisiana, on July 9, 1982.

Accidents

In 1982, the National Academy of Sciences re-examined plane crash reports over an 18-year period and estimated that microbursts may have accounted for 28 airplane accidents involving 491 deaths and 206 injuries between 1964 and 1982. Since then, the only major airline crash in the United States that has involved wind shear was the August 2, 1985, Delta Airlines crash at Dallas.

10 Worst Aircraft Accidents InvolvingShear,1975-1985 Wind

Rank Date Plane Location Dead/Injured 1 . 7/9/82 Pan Am B-727 New Orleans 154 kil led,9 injured 2 . 8/2/85 Delta L1011 Dallas/Fort Worth133 killed 3. 6/24/75 Eastern B-727 New York (Kennedy)112 killed,12 injured 4. 1/30/74 Pan Am B-707 Pago Pago, Samoakilled 96 5 injured 5. 6/23/76 A1legheny DC-9 Philadelphi a 87 injured 6 . 3/1/64 Paradise L-1049 Lake Tahoe, N.Y.k i11 85ed 7. 7/23/73 Ozark FM-227B St. Louis 38 ki11ed, 6 injured 8 . 11/27/73 Delta DC-9 Chatanooga, Tenn. 42 injured 9. 12/17/73 Iberia DC-10 Boston 16 injured 1 0 . 8/7/75 Continental B-727Denver 15 injured

SOURCE: National Center for Atmospheric Research NCAR Background Information on Microbursts, Wind Shear - Page 2

The National Center for Atmospheric Research (NCAR) in Boulder, Colorado, has been involved in two major field projects to study the microburst phenomena within the past four years.

In the summer of 1982, the Joint Airport Weather Studies (JAWS) Project, directed by James Wilson and John McCarthy of NCAR in collaboration with Theodore Fujita, University of Chicago, was conducted at Denver's Stapleton International Airport.

Two years later, in the summer of 1984, the FAA wanted NCAR to provide operational protection for Stapleton Airport in a clearly and recently demonstrated high microburst risk environment. In addition, the objectives of NCAR were to test a number of very short term weather prediction, detection and warning concepts utilizing results from the JAWS analysis, and to determine whether products developed from these concepts were operationally effective. This was known as the CLAWS (Classify, Locate, Avoid Wind Shear) project at Denver, Colorado.

Field Research Results- The results of both of these programs have provided further insight into the airflow characteristics of microbursts, as well as new understanding of aircraft performance in microbursts, and have pointed to a need for adequate detection and warning of the event in the context of aviation safety.

JAWS Project Results- Microbursts are common in Denver. Approximately 160 microbursts occurred over the 1,600 square kilometer research network in 90 days of operation. This averages 1.75 microbursts per day. Furthermore, NCAR scientists Wilson and McCarthy estimated that 20 aircraft are likely to encounter microbursts at Stapleton Airport below 500 feet at ground level in a typical summer. Such encounters should be considered serious and life-threatening to the passengers and crew of these aircraft.

Based on statistics gleaned from approximately 75 microbursts detected by Doppler radar during the JAWS Project, the following description of microburst characteristics and behavior was determined:

Typical horizontal dimension: 1 (.6 to to 3 kilometers 1.9 miles).

Microburst lifetime: 5 to 15 minutes, with the period of severe wind shear from 2 to 4 minutes.

Average Wind Differential (the headwind-to-tailwind shear for an aircraft in flight through a microburst): 25m/s (56 miles/hour).

Maximum wind differential (observed by Doppler radar): 48 m/s (107 miles/hour).

Frequency of occurrence: about one per day in the JAWS-monitored area covering 1600 square kilometers (615 square miles).

Detection Systems

Currently, two kinds of wind-shear detection systems are available. One of these is the Low-level Wind Shear Alert System (LLWSAS) of the Federal Aviation Administration (FAA). The system is an array of wind measuring devices (anemometers and vanes) situated around each airport. Readings from these are centrally processed to determine if any sudden wind shift (in speed or direction) is occurring, and, if so, the system alerts the air traffic controller in the NCAR Background Information on Microbursts, Wind Shear - Page 3 tower, who in turn notified the pilot. The JAWS project found that the current LLWSAS is somewhat deficient in detecting microbursts, due primarily to an inadequate resolution of the system. The other consists of on-board, commercially available, direct sensors.

The best technological solution for the detection and warning of hazardous wind shear appears to be the use of Doppler radars either at the airport or on-board the aircraft. Only by examining the region where an aircraft might fly, before the encounter occurs, can full protection result. One or more Doppler radars at an airport could provide extraordinary protection. The FAA is currently seeking to develop a NEXRAD-type radar just for that purpose, to be installed and made operational at 100 airports by 1992.

NCAR scientsts believe that the best way to protect those airports without a Doppler radar system is to develop an airborne microwave Doppler radar or Doppler laser system that looks several miles ahead of the airplane, and hence could see the wind shear long before it would have to be entered. Thus the pilot could program a missed approach for landing in plenty of time, or in a takeoff situation could delay the departure until conditions improve. The National Aeronautics and Space Administration (NASA) is currently planning to develop such a system. Research at NCAR and at the National Oceanic and Atmospheric Administration (NOAA), will contribute significantly to the design of the NASA system.

-The End-

For further information on these projects, please contact: Joan Vandiver Frisch, Manager, Media Relations and Information Services National Center for Atmospheric Research, Boulder, Colorado 80307 (303) 497-8721