- Wind gradient
In common usage, wind gradient, more specifically wind speed gradientcite book | last = Hadlock | first = Charles | title = Mathematical Modeling in the Environment | publisher = Mathematical Association of America | location = Washington | year = 1998 | isbn = 088385709X |pages= |quote=Thus we have a “wind-speed gradient” as we move vertically, and this has a tendencyto encourage mixing between the air at one level and the air at those levels immediately above and below it.] or wind velocity gradient,cite conference
author = Gorder, P.J.
coauthors = Kaufman, K.; Greif, R.
year = 1996
title = Effect of wind gradient on the trajectory synthesis algorithms of the Center-TRACON Automation System (CTAS)
booktitle = AIAA, Guidance, Navigation and Control Conference, San Diego, CA
American Institute of Aeronautics and Astronautics
url = http://pdf.aiaa.org/GetFileGoogle.cfm?gID=10404&gTable=mtgpaper
quote=...the effect of a change in mean wind velocity with altitude, the wind velocity gradient...] or alternatively shear wind, [cite journal
publisher=British Ornithologists’ Union
title=Minimum shear wind strength required for dynamic soaring of albatrosses
quote=...the shear wind gradient is rather weak....the energy gain...is due to a mechanism other than the wind gradient effect.] is the vertical
gradientof the mean horizontal windspeed in the lower atmosphere.cite book | last = Oke | first = T. | title = Boundary Layer Climates | publisher = Methuen | location = London | year = 1987 | isbn = 0415043190 |pages = p. 54 | quote = Therefore the vertical gradient of mean wind speed (dū/dz) is greatest over smooth terrain, and least over rough surfaces.] It is the rate of increase of wind strength with unit increase in height above ground level. [cite book | last = Crocker | first = David | title = Dictionary of Aeronautical English | publisher = Routledge | location = New York | year = 2000 | isbn = 157958201X | pages = p. 104 | quote=wind gradient = rate of increase of wind strength with unit increase in height above ground level;] In metric units, it is often measured in units of meters per second of speed, per kilometer of height (m/s/km), which reduces to the standard unit of shear rate, inverse seconds (s-1).
The atmospheric effect of
surface frictionwith the winds aloftforce the surface windto slow and turn near the surface of the Earth, blowing inward across isobars, when compared to the winds in the nearly frictionless flow well above the Earth's surface. [cite web | title=AMS Glossary of Meteorology, Section E | publisher = American Meterological Association| url=http://amsglossary.allenpress.com/glossary/browse?s=e&p=14 | accessdate =2007-06-03] This layer, where surface friction slows the wind and changes the wind direction, is known as the planetary boundary layer. Daytime solar heating due to insolationthickens the boundary layer as winds at the surface become increasingly mixed with winds aloft. Radiative cooling overnight decouples the winds at the surface from the winds above the boundary layer, increasing vertical wind shear near the surface, also known as wind gradient.
Typically, due to
aerodynamicdrag, there is a wind gradient in the wind flow just a few hundred meters above the earth's surface—the surface layerof the planetary boundary layer. Wind speed increases with increasing height above the ground, starting from zerocite book | last = Wizelius | first = Tore | title = Developing Wind Power Projects | publisher = Earthscan Publications Ltd | location = London | year = 2007 | isbn = 1844072622 | pages = p. 40 | quote = The relation between wind speed and height is called the wind profile or wind gradient.] due to the no-slip condition.cite book | last = Brown | first = G. | title = Sun, Wind & Light | publisher = Wiley | location = New York | year = 2001 | pages = p. 18 | isbn = 0471348775 ] Flow near the surface encounters obstacles that reduce the wind speed, and introduce random vertical and horizontal velocity components at right angles to the main direction of flow. [cite journal
title=CBD-28. Wind on Buildings
author=Dalgliesh, W. A. and D. W. Boyd
work=Canadian Building Digest
quote=Flow near the surface encounters small obstacles that change the wind speed and introduce random vertical and horizontal velocity components at right angles to the main direction of flow.] This
turbulencecauses vertical mixing between the air moving horizontally at one level, and the air at those levels immediately above and below it, which is important in dispersion of pollutantsand in soil erosion.
The reduction in velocity near the surface is a function of surface roughness, so wind velocity profiles are quite different for different terrain types. Rough, irregular ground, and man-made obstructions on the ground, retard movement of the air near the surface, reducing wind velocity.cite book | last = Crawley | first = Stanley | title = Steel Buildings | publisher = Wiley | location = New York | year = 1993 | isbn = 0471842982 | pages = p. 272 ] Because of low surface roughness on the relatively smooth water surface, wind speeds do not increase as much with height above sea level as they do on land.cite book | last = Lubosny | first = Zbigniew | title = Wind Turbine Operation in Electric Power Systems: Advanced Modeling | publisher = Springer | location = Berlin | year = 2003 | isbn = 354040340X | pages = p. 17] Over a city or rough terrain, the wind gradient effect could cause a reduction of 40% to 50% of the
geostrophic windspeed aloft; while over open water or ice, the reduction may be only 20% to 30%. [cite book | last = Harrison | first = Roy | title = Understanding Our Environment | publisher = Royal Society of Chemistry | location = Cambridge | year = 1999 | isbn = 0854045848 | pages = p. 11] cite book | last = Thompson | first = Russell | title = Atmospheric Processes and Systems | publisher = Routledge | location = New York | year = 1998 | isbn = 0415171458 | pages = pp. 102-103 ]
engineeringpurposes, the wind gradient is modeled as a simple shearexhibiting a vertical velocity profile varying according to a power lawwith a constant exponential coefficient based on surface type. The height above ground where surface friction has a negligible effect on wind speed is called the "gradient height" and the wind speed above this height is assumed to be a constant called the "gradient wind speed".cite book | last = Gupta | first = Ajaya | title = Guidelines for Design of Low-Rise Buildings Subjected to Lateral Forces | publisher = CRC Press | location = Boca Raton | year = 1993 | isbn = 0849389690 | pages = p. 49] [cite book | last = Stoltman | first = Joseph | title = International Perspectives on Natural Disasters: Occurrence, Mitigation, and Consequences | publisher = Springer | location = Berlin | year = 2005 | isbn = 1402028504 | pages = 73 ] For example, typical values for the predicted gradient height are 457 m for large cities, 366 m for suburbs, 274 m for open terrain, and 213 m for open sea. [cite book | last = Chen | first = Wai-Fah | title = Handbook of Structural Engineering | publisher = CRC Press | location = Boca Raton | year = 1997 | isbn = 0849326745 | pages = p. 12-50]
Although the power law exponent approximation is convenient, it has no theoretical basis. [cite book | last = Ghosal | first = M. | title = Renewable Energy Resources | chapter = 7.8.5 Vertical Wind Speed Gradient | publisher = Alpha Science International, Ltd | location = City | year = 2005 | isbn = 9781842651254 | pages = pp. 378-379] When the temperature profile is adiabatic, the wind speed should vary
logarithmically with height, [cite book | last = Stull | first = Roland | title = An Introduction to Boundary Layer Meteorology | publisher = Kluwer Academic Publishers | location = Boston | year = 1997 | isbn = 9027727686 | pages = p. 442 | quote = ...both the wind gradient and the mean wind profile itself can usually be described diagnostically by the log wind profile.] Measurements over open terrain in 1961 showed good agreement with the logarithmic fit up to 100 m or so, with near constant average wind speed up through 1000 m.cite journal
author = Thuillier, R.H.
coauthors = Lappe, U.O.
year = 1964
title = Wind and Temperature Profile Characteristics from Observations on a 1400 ft Tower
Journal of Applied Meteorology
American Meteorological Society
volume = 3
issue = 3
pages = 299–306
doi = 10.1175/1520-0450(1964)003<
url = http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0450(1964)003%3C0299%3AWATPCF%3E2.0.CO%3B2
accessdate = 2007-06-10
doi_brokendate = 2008-06-21 ]
The shearing of the wind is usually three-dimensional, [cite book | last = Mcilveen | first = J. | title = Fundamentals of Weather and Climate | publisher = Chapman & Hall | location = London | year = 1992 | isbn = 0412411601 | pages = p. 184] that is, there is also a change in direction between the 'free' pressure-driven geostrophic wind and the wind close to the ground. [cite book | last = Burton | first = Tony | title = Wind Energy Handbook | publisher = J. Wiley | location = London | year = 2001 | pages = p. 20 | isbn = 0471489972 ] This is related to the
Ekman spiraleffect. 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.
After sundown the wind gradient near the surface increases, with the increasing stability.cite journal
author = Köpp, F.
coauthors = Schwiesow, R.L.; Werner, C.
year = 1984
month = 01
title = Remote Measurements of Boundary-Layer Wind Profiles Using a CW Doppler Lidar
Journal of Applied Meteorology and Climatology
American Meteorological Society
volume = 23
issue = 1
pages = p. 153
doi = 10.1175/1520-0450(1984)023<
url = http://ams.allenpress.com/amsonline/?request=get-abstract&doi=10.1175%2F1520-0450(1984)023%3C0148:RMOBLW%3E2.0.CO%3B2
accessdate = 2007-06-09
doi_brokendate = 2008-06-21 ] Atmospheric stability occurring at night with
radiative coolingtends to contain turbulent eddies vertically, increasing the wind gradient.cite book | last = Lal | first = R. | title = Encyclopedia of Soil Science | publisher = Marcel Dekker | location = New York | year = 2005 | isbn = 0849350530 | pages= p. 618] The magnitude of the wind gradient is largely influenced by the height of the convective boundary layer and this effect is even larger over the sea, where there is no diurnal variation of the height of the boundary layer as there is over land.cite conference
author = Johansson, C.
coauthors = Uppsala, S.; Smedman, A.S.
year = 2002
title = Does the height of the boundary layer influence the turbulence structure near the surface over the Baltic Sea?
booktitle = 15th Conference on Boundary Layer and Turbulence
American Meteorological Society
url = http://ams.confex.com/ams/BLT/techprogram/paper_43332.htm
conferenceurl = http://ams.confex.com/ams/BLT/techprogram/program_117.htm ] In the convective boundary layer, strong mixing diminishes vertical wind gradient. [cite book | last = Shao | first = Yaping | title = Physics and Modelling of Wind Erosion | publisher = Kluwer Academic | location = City | year = 2000 | isbn = 9780792366577 |pages = p. 69 | In the bulk of the convective boundary layer, strong mixing diminishes vertical wind gradient...]
The design of buildings must account for wind loads, and these are affected by wind gradient. The respective gradient levels, usually assumed in the Building Codes, are 500 meters for cities, 400 meters for suburbs, and 300 m for flat open terrain. [cite book | last = Augusti | first = Giuliano | title = Probabilistic Methods in Structural Engineering | publisher = Chapman and Hall | location = London | year = 1984 | isbn = 0412222302 |pages = p. 85] For engineering purposes, a power law wind speed profile may be defined as follows:
: = speed of the wind at height : = gradient wind at gradient height : = exponential coefficient
Wind turbines are affected by wind gradient. 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.cite book | last = Heier | first = Siegfried | title = Grid Integration of Wind Energy Conversion Systems | publisher = John Wiley & Sons | location = Chichester | year = 2005 | isbn = 0470868996 | pages = p. 45] The wind gradient can create a large bending moment in the shaft of a two bladed turbine when the blades are vertical. [cite book | last = Harrison | first = Robert | title = Large Wind Turbines | publisher = John Wiley & Sons | location = Chichester | year = 2001 | isbn = 0471494569 | pages = p. 30] The reduced wind gradient over water means shorter and less expensive wind turbine towers can be used in shallow seas. Wind turbines should be tested in a wind tunnelsimulating the wind gradient that they will eventually see, but this is rarely done. [cite book | last = Barlow | first = Jewel | title = Low-Speed Wind Tunnel Testing | publisher = Wiley | location = New York | year = 1999 | isbn = 0471557749 | pages= p. 42 | quote=It would be preferable to evaluate windmills in the wind gradient that they will eventually see, but this is rarely done.]
For wind turbine engineering, an exponential variation in wind speed with height can be defined relative to wind measured at a reference height of 10 meters as:
: = velocity of the wind at height,
[m/s]: = velocity of the wind at height, = 10 meters [m/s]: = Hellman exponent
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 exceedingthe maximum ground launch tow speed. The pilot must adjust the airspeed to deal with the effect of thegradient. [cite book
title = Glider Flying Handbook
year = 2003
publisher = U.S. Federal Aviation Administration
location = U.S. Government Printing Office, Washington D.C.
id = FAA-8083-13_GFH
pages = p. 7-16
When landing, wind gradient is also a hazard, particularly when the winds are strong. [cite book | last = Longland | first = Steven | title = Gliding | publisher = Crowood Press, Limited, The | location = City | year = 2001 | isbn = 1861264143 | pages= p. 125 | quote=The reason for making the increase is because the wind speed increases with height (a `wind gradient')] 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.cite book | last = Piggott | first = Derek | title = Gliding: a Handbook on Soaring Flight | publisher = Knauff & Grove | location = | year = 1997 | isbn = 9780960567645 | pages = pp. 85-86, 130-132 | quote= The wind gradient is said to be steep or pronounced when the change in wind speed with height is very rapid, and it is in these conditions that extra care must be used when taking off or landing in a glider]
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. The different airspeed experienced by each wing tip can result in an aerodynamic stall on one wing, causing a loss of control accident. [cite book | last = Knauff | first = Thomas | title = Glider Basics from First Flight to Solo | publisher = Thomas Knauff | location = | year = 1984 | isbn = 0960567631 ] The rolling moment generated by the different airflow over each wing can exceed the aileroncontrol authority, causing the glider to continue rolling into a steeper bank angle.cite book | last = Conway | first = Carle | title = Joy of Soaring | publisher = Soaring Society of America, Incorporated | location = City | year = 1989 | isbn = 1883813026 If the pilot runs into the wind gradient as he is turning into the wind, it is easy to see that there will be less wind across the lower than the higher wing.]
sailing, wind gradient affects sailboatsby presenting a different wind speed to the sailat different heights along the mast. The direction also varies with height, but sailors refer to this as "wind shear." [cite book | last = Jobson | first = Gary | title = Gary Jobson's Championship Sailing | publisher = International Marine/Ragged Mountain Press | location = City | year = 2004 | isbn = 0071423818 | pages = p. 180 | quote=Wind shear is the difference in direction at varying heights above the water; wind gradient is the difference in wind strength at varying heights above the water.] Wind shear presents a different problem to the sailor, because it causes the apparent wind angle to be different when the sailboat is tacking to one side or the other, requiring the sails to be re-trimmed after each tack.
The mast head instruments indication of apparent wind speed and direction is different from what the sailor sees and feels near the surface.cite book | last = Jobson | first = Gary | title = Championship Tactics: How Anyone Can Sail Faster, Smarter, and Win Races | publisher = St. Martin's Press | location = New York | year = 1990 | isbn = 0312042787 | pages = p. 323 | quote = You'll recognize wind shear if your apparent wind angle is smaller on one tack than on the other...]
Sailmakers may introduce sail twistin 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 distributionwith height. The effect of wind gradient can be factored into the selection of twist in the sail design, but this can be difficult to predict since the wind gradient may vary widely in different weather conditions. Sailorsmay also adjust the trim of the sail to account for wind gradient, for example using a boom vang.cite book | last = Garrett | first = Ross | title = The Symmetry of Sailing | publisher = Sheridan House | location = Dobbs Ferry | year = 1996 | pages = pp. 97-99, 108 | isbn = 1574090003 | quote = Wind speed and direction are normally measured at the top of the mast, and the wind gradient must therefore be known in order to determine the mean windwind speed incident on the sail.]
kitesurfing, the wind gradient is even more important, because the power kiteis flown on 20-30m lines, [cite book | last = Currer | first = Ian | title = Kitesurfing | publisher = Lakes Paragliding | location = City | year = 2002 | isbn = 0954289609 | pages = 27] and the kitesurfer can use the kite to jump off the water, bringing the kite to even greater heights above the sea surface.
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. It is also important in studying noise pollution, for example from roadway noiseand aircraft noise, and must be considered in the design of noise barriers. [cite paper
publisher = Washington State Department of Transportation.
url = http://www.wsdot.wa.gov/Research/Reports/000/033.1.htm
title = Ground Plane Wind Shear Interaction on Acoustic Transmission
accessdate = 2007-05-30
version = WA-RD 033.1
author = Foss, Rene N.
date = June 1978 ] When wind speed increases with altitude, wind blowing towards the listener from the source will refract sound waves downwards, resulting in increased noise levels behind the barrier.cite book | last = Bies | first = David | title = Engineering Noise Control; Theory and Practice | publisher = Spon Press | location = London | year = 2003 | isbn = 0415267137 | pages = p. 235 | quote = As wind speed generally increases with altitude, wind blowing towards the listener from the source will refract sound waves downwards, resulting in increased noise levels.] These effects were first quantified in the field of highway engineering to address variations of noise barrier efficacy in the 1960s. [ [http://www.springerlink.com/content/x1707075n815g604/ C.Michael Hogan, " Analysis of Highway Noise", Journal of Water, Air, & Soil Pollution, Vol. 2, No. 3, Biomedical and Life Sciences and Earth and Environmental Science Issue, Pages 387-392, September 1973, Springer Verlag, Netherlands ISSN 0049-6979] ]
When the sun warms the Earth's surface, there is a negative
temperature gradientin atmosphere. The speed of sounddecreases with decreasing temperature, so this also creates a negative sound speed gradient. [cite book
title=Sound Reinforcement Engineering
publisher=Taylor & Francis
pages=p. 40] The sound wave front travels faster near the ground, so the sound is refracted upward, away from listeners on the ground, creating an
acoustic shadowat some distance from the source. [cite book | last = Everest | first = F. | title = The Master Handbook of Acoustics | publisher = McGraw-Hill | location = New York | year = 2001 | isbn = 0071360972 | pages = pp. 262-263 ] The radius of curvature of the sound path is inversely proportional to the velocity gradient. [cite book
chapter=10. Outdoor sound propagation
series=ME 458: Engineering Noise Control
Penn State University
city=State College, PA
A wind speed gradient of 4 (m/s)/km can produce refraction equal to a typical temperature
lapse rateof 7.5 °C/km. [cite book | last = Uman | first = Martin | title = Lightning | publisher = Dover Publications | location = New York | year = 1984 | isbn = 0486645754 | pages = 196 ] Higher values of wind gradient will refract sound downward toward the surface in the downwind direction, [cite book | last = Volland | first = Hans | title = Handbook of Atmospheric Electrodynamics | publisher = CRC Press | location = Boca Raton | year = 1995 | isbn = 0849386470 | pages = p. 22] eliminating the acoustic shadow on the downwind side. This will increase the audibility of sounds downwind. This downwind refraction effect occurs because there is a wind gradient; the sound is not being carried along by the wind. [cite book | last = Singal | first = S. | title = Noise Pollution and Control Strategy | publisher = Alpha Science International, Ltd | location = | year = 2005 | isbn = 1842652370 | pages = p. 7 | quote = It may be seen that refraction effects occur only because there is a wind gradient and it is not due to the result of sound being convected along by the wind.]
There will usually be both a wind gradient and a temperature gradient. In that case, the effects of both might add together or subtract depending on the situation and the location of the observer. [cite book
publisher=Royal School Of Artillery
series=Basic Science & Technology Section
title=N01-N07 Sound Ranging
quote=...there will usually be both a wind gradient and a temperature gradient.] The wind gradient and the temperature gradient can also have complex interactions. For example, a foghorn can be audible at a place near the source, and a distant place, but not in a sound shadow between them. [cite journal
title=Fog Signals: Areas of Silence and Greatest Range of Sound
journal=Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character
pages= pp. 71–75|doi=10.1098/rspa.1914.0103] In the case of transverse sound propagation, wind gradients do not sensibly modify sound propegation relative to the windless condition; the gradient effect appears to be important only in upwind and downwind configurations.cite journal
author = Malbequi, P.
coauthors = Delrieux, Y.; Canard-caruana, S.
year = 1993
title = Wind tunnel study of 3D sound propagation in presence of a hill and of a wind gradient
journal = ONERA, TP no
volume = 111
pages = p. 5
url = http://adsabs.harvard.edu/abs/1993ONERA....R....M
accessdate = 2007-06-09 ]
For sound propagation, the exponential variation of wind speed with height can be defined as follows:
: = speed of the wind at height , and is a constant : = exponential coefficient based on ground surface roughness, typically between 0.08 and 0.52: = expected wind gradient at height
In the 1862
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, [cite book | last = Cornwall | first = Sir | title = Grant as Military Commander | publisher = Barnes & Noble Inc | location = | year = 1996 | isbn = 1566199131 pages = p. 92] because they could not hear the sounds of battle only six miles downwind. [cite book | last = Cozzens | first = Peter | title = The Darkest Days of the War: the Battles of Iuka and Corinth | publisher = The University of North Carolina Press | location = Chapel Hill | year = 2006 | isbn = 0807857831 ]
Scientists have understood the effect of wind gradient upon
refractionof sound since the mid 1900s; however, with the advent of the U.S. Noise Control Act, the application of this refractive phenomena became applied widely beginning in the early 1970s, chiefly in the application to noise propagation from highways and resultant design of transportation facilities. [Hogan, C. Michael and Gary L. Latshaw, [http://www.worldcatlibraries.org/wcpa/top3mset/2930880 "The Relationship between Highway Planning and Urban Noise"] , Proceedings of the ASCE, Urban Transportation Division specialty conference, May 21/23, 1973, Chicago, Ill., American Society of Civil Engineers]
Wind gradient soaring
Wind gradient soaring, also called
dynamic soaring, is a technique used by soaring birdsincluding albatrosses. If the wind gradient is of sufficient magnitude, a bird can climb into the wind gradient, trading ground speed for height, while maintaining airspeed. [cite book | last = Alexander | first = R. | title = Principles of Animal Locomotion | publisher = Princeton University Press | location = Princeton | year = 2002 | pages = p. 206 | isbn = 0691086788 ] By then turning downwind, and diving through the wind gradient, they can also gain energy. [cite book | last = Alerstam | first = Thomas | title = Bird Migration | publisher = Cambridge University Press | location = Cambridge | year = 1990 | pages = 275 | isbn = 0521448220 ]
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