Wind shear

Source: Wikipedia, the free encyclopedia.
For other uses, see Wind shear (disambiguation).
Cirrus
uncinus ice crystal plumes showing high-level wind shear, with changes in wind speed and direction

Wind shear (or windshear), sometimes referred to as

atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal wind shear. Vertical wind shear is a change in wind speed or direction with a change in altitude. Horizontal wind shear is a change in wind speed with a change in lateral position for a given altitude.[1]

Wind shear is a

microbursts and downbursts caused by thunderstorms, fronts, areas of locally higher low-level winds referred to as low-level jets, near mountains
, radiation inversions that occur due to clear skies and calm winds, buildings, wind turbines, and sailboats. Wind shear has significant effects on the control of an aircraft, and it has been the only or a contributing cause of many aircraft accidents.

Sound movement through the atmosphere is affected by wind shear, which can bend the wave front, causing sounds to be heard where they normally would not. Strong vertical wind shear within the troposphere also inhibits tropical cyclone development but helps to organize individual thunderstorms into longer life cycles which can then produce severe weather. The thermal wind concept explains how differences in wind speed at different heights are dependent on horizontal temperature differences and explains the existence of the jet stream.[2]

Mojave desert
.

Definition

Wind shear refers to the variation of wind velocity over either horizontal or vertical distances. Airplane pilots generally regard significant wind shear to be a horizontal change in airspeed of 30 knots (15 m/s) for light aircraft, and near 45 knots (23 m/s) for airliners at flight altitude.[3] Vertical speed changes greater than 4.9 knots (2.5 m/s) also qualify as significant wind shear for aircraft. Low-level wind shear can affect aircraft airspeed during takeoff and landing in disastrous ways, and airliner pilots are trained to avoid all microburst wind shear (headwind loss in excess of 30 knots [15 m/s]).[4] The rationale for this additional caution includes:[citation needed]

  • microburst intensity can double in a minute or less,
  • the winds can shift to excessive crosswinds,
  • 40–50 knots (21–26 m/s) is the threshold for survivability at some stages of low-altitude operations, and
  • several of the historical wind shear accidents involved 35–45 knots (18–23 m/s) microbursts.

Wind shear is also a key factor in the formation of severe thunderstorms. The additional hazard of turbulence is often associated with wind shear.[citation needed]

Occurrence

See also: Jet stream
Microburst schematic from NASA. The direction of travel is downward until the air current hits ground level, at which point it spreads outward in all directions. The wind regime in a microburst is completely opposite to a tornado.[citation needed
]

Weather situations where shear is observed include:

Horizontal component

Weather fronts

Main article:
Weather fronts

Weather fronts are boundaries between two masses of air of different

Pacific basins. Directional and speed shear can occur across the axis of stronger tropical waves, as northerly winds precede the wave axis and southeast winds are seen behind the wave axis. Horizontal wind shear can also occur along the local land breeze and sea breeze boundaries.[10]

Near coastlines

Wind shear along the coast with low-level clouds moving towards the east and higher-level clouds moving towards the south-west

The magnitude of winds offshore is nearly double the wind speed observed onshore. This is attributed to the differences in friction between landmasses and offshore waters. Sometimes, there are even directional differences, particularly if local sea breezes change the wind on shore during daylight hours.[11]

Vertical component

Thermal wind

Main article: Thermal wind

Thermal wind is a meteorological term not referring to an actual

high) pressure.[12]

The thermal wind equation is

where the φ are

vertical direction. The thermal wind equation does not determine the wind in the tropics. Since f is small or zero, such as near the equator, the equation reduces to stating that ∇(φ1 − φ0) is small.[12]

This equation basically describes the existence of the jet stream, a westerly current of air with maximum wind speeds close to the tropopause which is (even though other factors are also important) the result of the temperature contrast between equator and pole.[citation needed]

Effects on tropical cyclones

cumulonimbus cloud, or thunderstorm.[13]
See also: Tropical cyclogenesis § Weak vertical wind shear

Tropical cyclones are, in essence, heat engines that are fueled by the temperature gradient between the warm tropical ocean surface and the colder upper atmosphere. Tropical cyclone development requires relatively low values of vertical wind shear so that their warm core can remain above their surface circulation center, thereby promoting intensification. Strongly sheared tropical cyclones weaken as the upper circulation is blown away from the low-level center.[citation needed]

Effects on thunderstorms and severe weather

Main article:
Severe thunderstorm

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. This occurs as the storm's inflow becomes separated from its rain-cooled outflow. An increasing nocturnal, or overnight, low-level jet can increase the severe weather potential by increasing the vertical wind shear through the troposphere. Thunderstorms in an atmosphere with virtually no vertical wind shear weaken as soon as they send out an outflow boundary in all directions, which then quickly cuts off its inflow of relatively warm, moist air and causes the thunderstorm to dissipate.[14]

Planetary boundary layer

Depiction of where the planetary boundary layer lies on a sunny day
See also: Ekman layer, Ekman spiral, Planetary boundary layer, and Surface layer

The atmospheric effect of surface friction with winds aloft forces surface winds to slow and back counterclockwise near the surface of

insolation, or solar heating. Radiative cooling overnight further enhances wind decoupling between the winds at the surface and the winds above the boundary layer by calming the surface wind which increases wind shear. These wind changes force wind shear between the boundary layer and the wind aloft and are most emphasized at night.[citation needed
]

Effects on flight

Main articles: Aeronautics and Gliding
Gliding
Glider ground launch affected by wind shear

In gliding, wind gradients just above the surface affect the takeoff and landing phases of the flight of a glider. Wind gradient can have a noticeable effect on

ground launches, also known as winch launches or wire launches. If the wind gradient is significant or sudden, or both, and the pilot maintains the same pitch attitude, the indicated airspeed will increase, possibly exceeding the maximum ground launch tow speed. The pilot must adjust the airspeed to deal with the effect of the gradient.[16]

When landing, wind shear is also a hazard, particularly when the winds are strong. As the glider descends through the wind gradient on final approach to landing, airspeed decreases while sink rate increases, and there is insufficient time to accelerate prior to ground contact. The pilot must anticipate the wind gradient and use a higher approach speed to compensate for it.[17]

Wind shear is also a hazard for aircraft making steep turns near the ground. It is a particular problem for gliders which have a relatively long

bank angle. The different airspeed experienced by each wing tip can result in an aerodynamic stall on one wing, causing a loss of control accident.[17][18]

Parachuting

Wind shear or wind gradients are a threat to parachutists, particularly to BASE jumping and wingsuit flying. Skydivers have been pushed off of their course by sudden shifts in wind direction and speed, and have collided with bridges, cliffsides, trees, other skydivers, the ground, and other obstacles.[citation needed] Skydivers routinely make adjustments to the position of their open canopies to compensate for changes in direction while making landings to prevent accidents such as canopy collisions and canopy inversion.[citation needed]

Soaring

Soaring related to wind shear, also called

soaring birds like albatrosses, who can maintain flight without wing flapping. If the wind shear is of sufficient magnitude, a bird can climb into the wind gradient, trading ground speed for height, while maintaining airspeed.[19] By then turning downwind, and diving through the wind gradient, they can also gain energy.[20] It has also been used by glider pilots
on rare occasions.

Wind shear can also produce

atmospheric inversion separates two layers with a marked difference in wind direction. If the wind encounters distortions in the inversion layer caused by thermals coming up from below, it will produce significant shear waves that can be used for soaring.[21]

Impact on passenger aircraft
Effect of wind shear on aircraft trajectory. Note how merely correcting for the initial gust front can have dire consequences.
Wreckage of Delta Air Lines Flight 191 tail section after a microburst slammed the aircraft into the ground. Another aircraft can be seen flying in the background past the crash scene.

Windshear can be extremely dangerous for aircraft, especially during takeoff and landing. Sudden changes in wind velocity can cause rapid decreases in airspeed, leading to the aircraft being unable to maintain altitude. Windshear has been responsible for several deadly accidents, including Eastern Air Lines Flight 66, Pan Am Flight 759, Delta Air Lines Flight 191, and USAir Flight 1016.[citation needed]

Windshear can be detected using Doppler radar.[22][23][24] Airports can be fitted with low-level windshear alert systems or Terminal Doppler Weather Radar, and aircraft can be fitted with airborne wind shear detection and alert systems. Following the 1985 crash of Delta Air Lines Flight 191, in 1988 the U.S. Federal Aviation Administration mandated that all commercial aircraft have airborne wind shear detection and alert systems by 1993. The installation of high-resolution Terminal Doppler Weather Radar stations at many U.S. airports that are commonly affected by windshear has further aided the ability of pilots and ground controllers to avoid wind shear conditions.[25]

Sailing

Wind shear affects

Sailors may also adjust the trim of the sail to account for low-level wind shear, for example using a boom vang.[26]

Sound propagation

See also: Speed of sound

Wind shear can have a pronounced effect upon sound propagation in the lower atmosphere, where waves can be "bent" by

aircraft noise, and must be considered in the design of noise barriers.[27] This phenomenon was first applied to the field of noise pollution study in the 1960s, contributing to the design of urban highways as well as noise barriers.[28]

NOAA
)

The speed of sound varies with temperature. Since temperature and sound velocity normally decrease with increasing altitude, sound is refracted upward, away from listeners on the ground, producing an acoustic shadow at some distance from the source.[29] In 1862, during the American Civil War Battle of Iuka, an acoustic shadow, believed to have been enhanced by a northeast wind, kept two divisions of Union soldiers out of the battle,[30] because they could not hear the sounds of battle only six miles downwind.[31]

Effects on architecture

hurricanes, and storms which may cause widespread destruction. Wind engineering draws upon meteorology, aerodynamics, and several specialist engineering disciplines. The tools used include climate models, atmospheric boundary layer wind tunnels, and numerical models. It involves, among other topics, how wind impacting buildings must be accounted for in engineering.[32]

Wind turbines are affected by wind shear. Vertical wind-speed profiles result in different wind speeds at the blades nearest to the ground level compared to those at the top of blade travel, and this, in turn, affects the turbine operation.[33] This low-level wind shear can cause a large bending moment in the shaft of a two-bladed turbine when the blades are vertical.[34] The reduced wind shear over water means shorter and less expensive wind turbine towers can be used in shallow seas.[35]

See also

References

  1. ^ "Vertical wind shear. Retrieved on 2015-10-24".
  2. ^ a b c d "Low-Level Wind Shear". Integrated Publishing. Retrieved 2007-11-25.
  3. FAA FAA Advisory Circular Pilot Wind Shear Guide. Archived 2006-10-14 at the Wayback Machine
    Retrieved on 2007-12-15.
  4. ^ "Wind Shear". NASA. Archived from the original on 2007-10-09. Retrieved 2007-10-09.
  5. ^ "Jet Streams in the UK". BBC. Archived from the original on January 18, 2008. Retrieved 2008-05-08.
  6. ISSN 1520-0493
    .
  7. ISSN 0022-4928
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  8. ^ National Center for Atmospheric Research. T-REX: Catching the Sierra’s waves and rotors Archived 2006-11-21 at the Wayback Machine Retrieved on 2006-10-21.
  9. ^ Fujita, Tetsuya Theodore (1985). ""The Downburst, Microburst, and Macroburst," by Fujita, T. Theodore, SMRP Research Paper Number 210, 1985". swco-ir.tdl.org. Retrieved 2023-10-30.
  10. ^ David M. Roth. Hydrometeorological Prediction Center. Unified Surface Analysis Manual. Retrieved on 2006-10-22.
  11. ^ Franklin B. Schwing and Jackson O. Blanton. The Use of Land and Sea Based Wind Data in a Simple Circulation Model. Retrieved on 2007-10-03.
  12. ^
    ISBN 0-12-354015-1
  13. ISBN 0-412-41160-1
    .
  14. ^ University of Illinois. Vertical Wind Shear Retrieved on 2006-10-21.
  15. American Meteorological Association
    . Retrieved 2015-02-15.
  16. ^ Glider Flying Handbook. U.S. Government Printing Office, Washington D.C.: U.S. Federal Aviation Administration. 2003. pp. 7–16. FAA-8083-13_GFH.
  17. ^
    ISBN 978-0-9605676-4-5
    .
  18. ISBN 0-9605676-3-1
    .
  19. ISBN 0-691-08678-8
    .
  20. ISBN 0-521-44822-0
    .
  21. ISBN 978-3-9808838-2-5
    .
  22. ^ Airborne Wind Shear Detection and Warning Systems. NASA Conference Publication 10050, Part 1. July 1990. p. 214. Retrieved 20 November 2022.
  23. ^ Windshear Training Aid: 4.0 - Windshear Substantiating Data. Federal Aviation Administration. February 1987. p. 4.2–95. Retrieved 20 November 2022.
  24. ^ Ryan, Muhammad (21 October 2020). "Detecting Wind Shear Using Radial Velocity of Doppler Radar". Medium. Retrieved 19 November 2022.
  25. ^ "Terminal Doppler Weather Radar Information". National Weather Service. Retrieved 4 August 2009.
  26. ISBN 1-57409-000-3
    .
  27. ^ Foss, Rene N. (June 1978). "Ground Plane Wind Shear Interaction on Acoustic Transmission". WA-RD 033.1. Washington State Department of Transportation. Retrieved 2007-05-30. {{cite journal}}: Cite journal requires |journal= (help)
  28. S2CID 109914430
    .
  29. ISBN 0-07-136097-2
    .
  30. ISBN 1-56619-913-1
    .
  31. ISBN 0-8078-5783-1
    .
  32. ^ Professor John Twidell. Wind Engineering. Archived 2007-10-25 at the Wayback Machine Retrieved on 2007-11-25.
  33. ISBN 0-470-86899-6
    .
  34. ISBN 0-471-49456-9
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  35. ISBN 3-540-40340-X
    .

External links
Retrieved from "https://en.wikipedia.org/w/index.php?title=Wind_shear&oldid=1220320748"