On the role of the roughness lenght in urban boundary layers

In an attempt to develop an improved model for the wind-speed profile valid for urban boundary layers (UBLs), the meaning of the roughness length z<inf>0</inf>, i.e. the constant of integration of the vertical gradient of the wind speed, is extended. z<inf>0</inf> is assumed here as a "dynamic" variable rather than as a "geometric" parameter independent of the flow properties. On the basis of experiments conducted in a park located within the city of Rome and in the laboratory, it is found that in neutral conditions the "vertical" profile of the roughness length shows a nearly exponential decrease with height.