Wind gradient
[9] This turbulence causes vertical mixing between the air moving horizontally at various levels, which has an effect on the dispersion of pollutants,[1] dust and airborne sand and soil particles.
[4][11] Because of the relatively smooth water surface, wind speeds do not decrease as much close to the sea as they do on land.
[13][14] For engineering purposes, the wind gradient is modeled as a simple shear exhibiting a vertical velocity profile varying according to a power law with a constant exponential coefficient based on surface type.
The cross-isobar angle of the diverted ageostrophic flow near the surface ranges from 10° over open water, to 30° over rough hilly terrain, and can increase to 40°-50° over land at night when the wind speed is very low.
[23] Atmospheric stability occurring at night with radiative cooling tends to contain turbulent eddies vertically, increasing the wind gradient.
[24] In the convective boundary layer, strong mixing diminishes vertical wind gradient.
[26] For engineering purposes, a power law wind speed profile may be defined as follows:[11][15]
[27] The wind gradient can create a large bending moment in the shaft of a two-bladed turbine when the blades are vertical.
where: The Hellmann exponent depends upon the coastal location and the shape of the terrain on the ground, and the stability of the air.
Examples of values of the Hellmann exponent are given in the table below:[30] In gliding, wind gradient affects the takeoff and landing phases of flight of a glider.
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.
[32] 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.
[33] Wind gradient 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.
"[36] The mast head instruments indication of apparent wind speed and direction is different from what the sailor sees and feels near the surface.
[38] Sailors may also adjust the trim of the sail to account for wind gradient, for example using a boom vang.
[40] This is consistent with another source, which shows that the change in wind speed is very small for heights over 2 meters[41] and with a statement by the Australian Government Bureau of Meteorology[42] according to which differences can be as little as 5% in unstable air.
Wind gradient can have a pronounced effect upon sound propagation in the lower atmosphere.
This effect is important in understanding sound propagation from distant sources, such as foghorns, thunder, sonic booms, gunshots or other phenomena like mistpouffers.
[46] These effects were first quantified in the field of highway engineering to address variations of noise barrier efficacy in the 1960s.
[47] When the sun warms the Earth's surface, there is a negative temperature gradient in atmosphere.
[49] The radius of curvature of the sound path is inversely proportional to the velocity gradient.
[50] A wind speed gradient of 4 (m/s)/km can produce refraction equal to a typical temperature lapse rate of 7.5 °C/km.
[55] In the case of transverse sound propagation, wind gradients do not sensibly modify sound propagation relative to the windless condition; the gradient effect appears to be important only in upwind and downwind configurations.
