Aerodynamic roughness length: Difference between revisions
From Glossary of Meteorology
imported>Perlwikibot (Created page with " {{TermHeader}} {{TermSearch}} <div class="termentry"> <div class="term"> == aerodynamic roughness length == </div> <div class="definition"><div class="short_definition...") |
imported>Perlwikibot No edit summary |
||
Line 9: | Line 9: | ||
</div> | </div> | ||
<div class="definition"><div class="short_definition">( | <div class="definition"><div class="short_definition">(''Also called'' the roughness length, ''z''<sub>0</sub>.) The height above the displacement plane at which the mean wind becomes zero when extrapolating the logarithmic wind- speed [[profile]] downward through the [[surface layer]].</div><br/> <div class="paragraph">It is a theoretical height that must be determined from the wind-speed profile, although there has been some success at relating this height to the arrangement, spacing, and physical height of individual roughness elements such as trees or houses. The average [[wind speed]] <div class="inline-formula">[[File:ams2001glos-Aex02.gif|link=|ams2001glos-Aex02]]</div> in the surface layer can be written using [[Monin–Obukhov similarity theory]] as <div class="display-formula"><blockquote>[[File:ams2001glos-Ae12.gif|link=|center|ams2001glos-Ae12]]</blockquote></div> where ''z'' is height above ground, ''d'' is height of the displacement plane above ground, ''L'' is the Monin–Obukhov length [[scale]], ''k'' is [[von Kármán's constant]], ψ is a [[stability]] correction factor (= 0 for statically neutral conditions), and ''u''<sub>*</sub> is the [[friction velocity]]. To determine aerodynamic roughness, most experimentalists prefer to make [[wind profile]] measurements during statically neutral conditions (windy, [[overcast]], negligible [[temperature]] [[advection]]) so that ψ = 0 in the equation above. <br/>''Compare'' [[Charnock's relation]].</div><br/> </div> | ||
</div> | </div> | ||
Revision as of 14:34, 20 February 2012
aerodynamic roughness length
(Also called the roughness length, z0.) The height above the displacement plane at which the mean wind becomes zero when extrapolating the logarithmic wind- speed profile downward through the surface layer.
It is a theoretical height that must be determined from the wind-speed profile, although there has been some success at relating this height to the arrangement, spacing, and physical height of individual roughness elements such as trees or houses. The average wind speed in the surface layer can be written using Monin–Obukhov similarity theory as where z is height above ground, d is height of the displacement plane above ground, L is the Monin–Obukhov length scale, k is von Kármán's constant, ψ is a stability correction factor (= 0 for statically neutral conditions), and u* is the friction velocity. To determine aerodynamic roughness, most experimentalists prefer to make wind profile measurements during statically neutral conditions (windy, overcast, negligible temperature advection) so that ψ = 0 in the equation above.
Compare Charnock's relation.
Compare Charnock's relation.