View source for Aerodynamic roughness length

== aerodynamic roughness length ==

(''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]].<br/>

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 [[File:ams2001glos-Aex02.gif|link=|ams2001glos-Aex02]] in the surface  layer can be written using [[Monin-obukhov similarity theory|Monin&ndash;Obukhov similarity theory]] as <blockquote>[[File:ams2001glos-Ae12.gif|link=|center|ams2001glos-Ae12]]</blockquote> where ''z'' is height above ground, ''d'' is height of the displacement plane above ground, ''L'' is the  Monin&ndash;Obukhov length [[scale]], ''k'' is [[von K&#x000e1;rm&#x000e1;n's constant]], &#x003c8; is a [[stability]] correction factor (=0 for statically neutral conditions), and ''u''<sub>&#x0002a;</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 &#x003c8; = 0 in the equation above. <br/>''Compare'' [[Charnock's relation]].<br/> 

<p>''Term edited 25 September 2019.''</p>