Log wind profile


Log wind profile

The Log wind profile is a semi-empirical relationship used to describe the vertical distribution of horizontal wind speeds above the ground within the atmospheric surface layer. The relationship is well described in the planetary boundary layer literature [cite book|author=Oke, T.R.|title=Boundary Layer Climates|edition=|publisher=Methuen|year=1987|id=] .

The logarithmic profile of wind speeds is generally limited to the lowest 100 meters (325') of the atmosphere (i.e., the surface layer of the atmospheric boundary layer). In the free atmosphere, geostrophic wind relationships should be used. The equation to estimate the wind speed (u) at height z (meters) above the ground is:

u_z = frac{u_*}{kappa} left [ln left(frac{z-d}{z_0} ight) + varphi(z,z_0,L) ight]
where u_* is the friction (or shear) velocity (m s-1), kappa is von Karman's constant (~0.41), d is the zero plane displacement, z_0 is the surface roughness (in meters), and varphi is a stability term where L is the Monin-Obukhov stability parameter. Under neutral stability conditions, z/L = 0 and varphi drops out.

Zero-plane displacement (d) is the height in meters above the ground at which zero wind speed is achieved as a result of flow obstacles such as trees or buildings. It is generally approximated as 2/3 of the average height of the obstacles. For example, if estimating winds over a forest canopy of height h = 30 m, the zero-plane displacement would be d = 20 m.

Roughness length (z_0) is a corrective measure to account for the effect of the roughness of a surface on wind flow, and is between 1/10 and 1/30 of the average height of the roughness elements on the ground. Over smooth, open water, expect a value around 0.0002 m, while over flat, open grassland z_0 ≈ 0.03 m, cropland ≈ 0.1-0.25 m, and brush or forest ≈ 0.5-1.0 m (values above 1 m are rare and indicate excessively rough terrain).

The log wind profile is generally considered to be a more reliable estimator than the Wind profile power law, which is commonly used when neutral conditions are assumed and roughness information is not available.

Applications

Log wind profiles are generated and used in a good many atmospheric pollution dispersion models. [cite book|author=Beychok, Milton R.|title=Fundamentals Of Stack Gas Dispersion|edition=4th Edition|publisher=author-published|year=2005|id=ISBN 0-9644588-0-2]

ee also

* Compilation of atmospheric dispersion models

References


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