Wind shear

From Wikipedia, the free encyclopedia

You have new messages (last change).
It has been suggested that this article or section be merged with wind gradient. (Discuss)
for the Marvel Comics character, click here

Wind shear is a difference in wind speed and/or direction between two points in the atmosphere. It can be either vertical or horizontal. Airplane pilots generally regard windshear to be a change in airspeed of 15 knots per thousand feet, and/or a change in azimuth of 30 degrees or greater per thousand feet.

Wind shear can affect aircraft airspeed during take off and landing in disastrous ways. An explanation in regard to vertical wind shear is discussed in wind gradient. It is also a key factor governing the severity of thunderstorms. An additional hazard is turbulence often associated with wind shear. Wind shear also generally inhibits tropical cyclone development.

Contents

[edit] Where and when it is observed

Microburst schematic from NASA. Note the downward motion of the air until it hits ground level, then spreads outward in all directions. The wind regime in a microburst is completely opposite to a tornado.
Microburst schematic from NASA. Note the downward motion of the air until it hits ground level, then spreads outward in all directions. The wind regime in a microburst is completely opposite to a tornado.

Weather situations where shear is observed include:

  • Weather fronts. Significant shear is observed, when the temperature difference across the front is 5 °C or more, and the front moves at 15 kt or faster. Because fronts are three-dimensional phenomena, frontal shear can be observed at any altitude between surface and tropopause, and therefore be seen both horizontally and vertically.
  • Low Level Jets. When a nocturnal low-level jet forms above the boundary layer ahead of a cold front, significant low level wind shear can develop. This is also known as nonconvective wind shear.
  • Mountains. When winds blow over a mountain, vertical shear is observed on the lee side. If the flow is strong enough, strong eddies known as rotors (vertically oriented circulations to the lee of mountains) can form, which are dangerous to ascending and descending aircraft.[1]
  • Inversions. When on a clear and calm night, a radiation inversion is formed near the ground, the friction does not affect wind above the inversion top. Change in wind can be 90 degrees in direction and 40 kt in speed. Even a nocturnal low level jet can sometimes be observed. Density difference causes additional problems to aviation.
  • Downbursts. When an outflow boundary moves away from a thunderstorm due to a shallow layer of rain-cooled air spreading out at ground level, both speed and directional wind shear can result. The stronger the outflow boundary, the stronger the resultant vertical wind shear.

[edit] Effects on aircraft

Effect of wind shear of plane trajectory. Note how merely correcting for the initial gust front can have dire consequences.
Effect of wind shear of plane trajectory. Note how merely correcting for the initial gust front can have dire consequences.

In the United States, a string of fatal accidents near thunderstorms downed passenger airliners during final descent and initial ascent in New York (1975), New Orleans (1982), and Dallas-Fort Worth (1985). Air Force One landed five minutes before one of the strongest downbursts ever recorded in the Washington, D.C. area at Andrews Air Force Base, with President Ronald Reagan aboard.[2] Ultimately, the culprit in these disasters was deemed to be wind shear. Strong low-level outflow from thunderstorms causes rapid changes in the wind character just above ground level. Initially, this outflow causes a headwind which causes the plane's acceleration, and causes a pilot to reduce engine power if they are unaware of the wind shear. As the plane passes into the region of the downdraft, the localized headwind disappears, suddenly dropping the plane's airspeed, perhaps too much if the pilot has reduced engine power. Then, when the plane passes through the other side of the downdraft, the previous headwind becomes a tailwind. These sudden switches from headwind to downdraft to tailwind can cause a plane to crash if these switches occur too close to the ground. A pilot can correct for this wind change by increasing engine power, if there is enough time to react.[3]

As the result of the accidents in the 1970s and 1980s, the Federal Aviation Agency mandated in 1988 that all commercial aircraft were to have onboard windshear detection systems by 1993. Three airlines, United Airlines, Continental Airlines, and Northwest Airlines received extensions until the end of 1995 so to install predictive wind shear sensors rather than reactive systems in their aircraft. The results of these efforts was immediate. Between 1964 and 1985, wind shear directly caused or contributed to 26 major civil transport aircraft accidents in the U.S. that led to 620 deaths and 200 injuries. Of these accidents, 15 occurred during take-off, 3 during flight, and 8 during landing. Since 1995, the number of major civil aircraft accidents caused by wind shear has dropped to approximately 1 each 10 years due to the mandated onboard detection, as well as the addition of Doppler radar units on the ground.

[edit] Effects on sound propagation

Vertical wind shear can have a pronounced effect upon sound propagation in the lower atmosphere, sound waves being "bent" by a refraction phenomenon. This effect is important in understanding roadway noise and aircraft noise propagation and can be significant in the design of noise barriers.[4]

[edit] Effects on tropical cyclones

Tropical cyclones require low values of vertical wind shear so that their warm core can remain stacked above their surface circulation center, and further development as a warm-core cyclone can continue. Strongly sheared tropical cyclones tend to either level in intensity or dissipate due to the breakdown of their internal heat engine. [5]

[edit] Effects on thunderstorms and severe weather

Severe thunderstorms, which can spawn tornadoes and hailstorms, require wind shear to organize the storm in such a way as to maintain the thunderstorm for a longer period of time by separating the storm's inflow from its rain-cooled outflow. Thunderstorms in an atmosphere with virtually no vertical wind shear weaken as soon as they send out an outflow boundary in all directions, which quickly cuts off its inflow of relatively warm, moist air and subsequently kills the thunderstorm.[6]

[edit] See also

[edit] References

  1. ^ National Center for Atmospheric Research. T-REX: Catching the Sierra’s waves and rotors Retrieved on 2006-10-21.
  2. ^ National Weather Service Forecast Office, Riverton, Wyoming. Downburst. Retrieved on 2006-10-22.
  3. ^ NASA Langley Air Force Base. Making the Skies Safer From Windshear. Retrieved on 2006-10-22.
  4. ^ Washington State Department of Transportation. GROUND PLANE WIND SHEAR INTERACTION ON ACOUSTIC TRANSMISSION. Retrieved on 2006-10-21.
  5. ^ University of Illinois. Hurricanes. Retrieved 2006-10-21.
  6. ^ University of Illinois. Vertical Wind Shear Retrieved on 2006-10-21.

[edit] External links

Retrieved from "http://en.wikipedia.org../../../w/i/n/Wind_shear.html"