Information about Wind Shear

Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Wind shear can be broken down into vertical and horizontal components, with horizontal wind shear seen across weather fronts and near the coast, and vertical shear typically near the surface, and sometimes at higher levels in the atmosphere.

Wind shear itself is a microscale meteorological phenomenon, but it may be associated with mesoscale or synoptic scale weather features. It is commonly observed near microbursts and downbursts, weather fronts, low level wind maxima known as low level jets, near mountains, radiation inversions, buildings, wind turbines, and sailboats. Wind shear has a significant effect during take-off and landing of aircraft, and was a significant cause of aircraft accidents involving large loss of life within the United States.

Sound propagation is affected by wind shear, which can bend the wave front, causing sounds to be heard where they normally would not, or vice versa. Strong vertical wind shear within the troposphere also inhibits tropical cyclone development, but helps to organize individual thunderstorms into living long life cycles and producing severe weather. The meteorological concept of thermal wind deals with how differences in wind with height are dependent on horizontal temperature differences.

Definition

Wind shear refers to the variation of wind over either horizontal or vertical distances. Airplane pilots generally regard significant windshear to be a horizontal change in airspeed of 30 knots (15 m/s) and/or vertical speed changes greater than 152 meters/500 feet per minute. Low level wind shear can affect aircraft airspeed during take off and landing in disastrous ways.^[1] It is also a key factor in the creation of severe thunderstorms. The additional hazard of turbulence is often associated with wind shear.

Where and when it is strongly observed

Weather situations where shear is observed include:

• Weather fronts. Significant shear is observed, when the temperature difference across the front is 5 Â°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 vertical wind shear can develop near the lower portion of the low level jet. 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, turbulent eddies known as rotors associated with lee waves may form, which are dangerous to ascending and descending aircraft.^[2]
• 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 at the leading edge of the three dimensional boundary. The stronger the outflow boundary, the stronger the resultant vertical wind shear.

In the horizontal

Weather fronts

Main article: Weather fronts

Weather fronts are boundaries between two masses of air of different densities which normally are convergence zones in the wind field and are the principal cause of significant weather. Within surface weather analyses, they are depicted using various colored lines and symbols. The air masses usually differ in temperature and may also differ in humidity. Wind shear in the horizontal occurs near these boundaries. Cold fronts feature narrow bands of thunderstorms and severe weather, and may be preceded by squall lines and dry lines. Cold fronts are sharper surface boundaries with more significant horizontal wind shear than warm fronts. When a front becomes stationary, it can degenerate into a line which separates regions of differing wind speed, known as a shear line. In the tropics, tropical waves move from east to west across the Atlantic and eastern 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 local land breeze and sea breeze boundaries.^[3]

Near coastlines

The magnitude of winds offshore are nearly double the wind speed observed onshore. This is attributed to the differences in friction between land masses and offshore waters. Sometimes, there are even directional differences, particularly if local sea breezes contaminate the wind on shore during daylight hours.^[4]

In the vertical

Thermal wind

Main article: Thermal wind

Thermal wind is a meteorological term not referring to an actual wind, but a difference in the geostrophic wind between two pressure levels and , with ; in essence, wind shear. It is only present in an atmosphere with horizontal gradients of temperature (or in an ocean with horizontal gradients of density), i.e. baroclinicity. In a barotropic atmosphere, where temperature is uniform, the geostrophic wind is independent of height. The name stems from the fact that this wind flows around areas of low (and high) temperature in the same manner as the geostrophic wind flows around areas of low (and high) pressure.

The thermal wind equation is

,

where the are geopotential height fields with , is the Coriolis parameter, and is the upward-pointing unit vector in the vertical direction. The thermal wind equation does not determine the wind in the tropics. Since f is small or zero there, the equation reduces to stating that is small.^[5]

Effects on tropical cyclones

Main article: Tropical cyclogenesis

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.^[6]

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 of time by separating the storm's inflow from its rain-cooled outflow. An increasing nocturnal 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 quickly cuts off its inflow of relatively warm, moist air and subsequently kills the thunderstorm.^[8]

Planetary boundary layer

The atmospheric effect of surface friction with winds aloft force surface winds to slow and back counterclockwise near the surface of the Earth blowing inward across isobars, when compared to the winds in frictionless flow well above the Earth's surface.^[9] This layer where friction slows and changes the wind is known as the planetary boundary layer, and is thickest during the day and thinnest at night. Daytime heating thickens the boundary layer as winds at the surface become increasingly mixed with winds aloft due to insolation, or solar heating. Radiative cooling overnight further enhances wind decoupling between the winds at the surface and the winds above the boundary layer and thereby increases wind shear. These wind changes force wind shear between the boundary layer and the wind aloft, and is most emphasized at night.

Effects on flight

Main articles: Aeronautics and Gliding

Gliding

In gliding, wind gradient affects the takeoff and landing phases of flight of a glider. Wind gradient can have a noticeable effect on ground 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.^[10]

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.^[11]

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 wingspan, which exposes them to a greater wind speed difference for a given 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.^[11][12]

Soaring

Soaring related to wind shear, also called dynamic soaring, is a technique used by soaring birds including albatrosses. If the wind shear is of sufficient magnitude, a bird can climb into the wind gradient, trading ground speed for height, while maintaining airspeed.^[13] By then turning downwind, and diving through the wind gradient, they can also gain energy.^[14]

Impact on passenger aircraft

Strong outflow from thunderstorms causes rapid changes in the three-dimensional wind velocity just above ground level. Initially, this outflow causes a headwind that increases airspeed, which normally causes a pilot to reduce engine power if they are unaware of the wind shear. As the aircraft passes into the region of the downdraft, the localized headwind diminishes, reducing the aircraft's airspeed, and increasing its sink rate. Then, when the aircraft passes through the other side of the downdraft, the headwind becomes a tailwind, reducing airspeed further, leaving the aircraft in a low-power, low-speed descent. This can lead to an accident if the aircraft is too low to affect a recovery before ground contact.^[15] As the result of the accidents in the 1970s and 1980s, in 1988 the U.S. Federal Aviation Administration mandated that all commercial aircraft have on-board windshear detection systems by 1993. 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. Since 1995, the number of major civil aircraft accidents caused by wind shear has dropped to approximately one every ten years due to the mandated on-board detection, as well as the addition of Doppler radar units on the ground. (NEXRAD)

Sailing

Main article: Sailing

Wind shear affects sailboats in motion by presenting a different wind speed and direction at different heights along the mast. Sailmakers may introduce sail twist in the design of the sail, where the head of the sail is set at a different angle of attack from the foot of the sail in order to change the lift distribution with height. The effect of wind shear can be factored into the selection of twist in the sail design, but this can be difficult to predict since wind shear may vary widely in different weather conditions. Sailors may also adjust the trim of the sail to account for wind gradient, for example using a boom vang.^[16]

Sound propagation

Wind shear can have a pronounced effect upon sound propagation in the lower atmosphere. The audibility of sounds from distant sources, such as thunder or gunshots, is very dependent on the amount of shear. Shear can have a pronounced effect upon sound propagation in the lower atmosphere, where waves can be "bent" by refraction phenomenon. The result of these differing sound levels is key in (noise pollution) considerations, for example from roadway noise and aircraft noise, and must be considered in the design of noise barriers.^[17] 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.^[18]



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, creating an acoustic shadow at some distance from the source.^[19] In the 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,^[20] because they could not hear the sounds of battle only six miles downwind.^[21]

Effects on architecture

Main article: Wind Engineering

Wind engineering is a field of engineering devoted to the analysis of wind effects on the natural and built environment. It includes strong winds which may cause discomfort as well as extreme winds such as tornadoes, hurricanes and storms which may cause widespread destruction. Wind Engineering draws upon meteorology, aerodynamics and a number of 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.

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.^[22] The wind gradient can create a large bending moment in the shaft of a two bladed turbine when the blades are vertical.^[23] The reduced wind gradient over water means shorter and less expensive wind turbine towers can be used in shallow seas.^[24]

See also

• Air safety
• Convergence zone
• Hodograph
• NEXRAD
• Low level windshear alert system (LLWAS)
• Microburst
• Refraction
• Tropical cyclone
• Thunderstorm
• Turbulence

References

External links

• National Science Digital Library - Wind shear