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 z0, i.e. the constant of integration of the vertical gradient of the wind speed, is extended. z0 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