Surface Roughness and Wind Speed

Surface roughness describes how features such as trees, buildings and uneven ground obstruct airflow. Within the atmospheric boundary layer, the lowest part of the atmosphere directly influenced by the surface, these obstacles increase drag and turbulence, modifying both average wind speed and its variation with height.

Surface drag and momentum transfer

The ground slows the air through friction, while obstacles exert additional drag. Their wakes generate mechanical turbulence, which mixes slower air near the surface with faster air above.

• Turbulence transfers horizontal momentum downward, where surface drag removes it from the airflow.
• Under otherwise comparable conditions, a rougher surface generally produces lower mean wind speeds at a given near-surface height than a smoother surface.

How wind speed changes with height

Mean wind speed generally increases with height as the direct influence of surface drag weakens. Roughness therefore changes the vertical wind profile, rather than reducing wind speed uniformly throughout the boundary layer.

• Aerodynamic roughness length is a parameter representing the surface's effect on airflow; it is not simply the physical height of obstacles.
• Under approximately neutral atmospheric stability, mean wind speed follows an approximately logarithmic profile within the surface layer above the immediate influence of individual obstacles.
• Atmospheric stability also matters: unstable conditions promote vertical mixing, while stable conditions suppress it.

Implications for urban ventilation

Buildings and vegetation create a spatially uneven wind field. Sheltered zones can have weak airflow, while gaps and passages can locally accelerate winds.

• Greater turbulence does not necessarily mean greater mean wind speed at pedestrian level.
• Urban ventilation depends on building height, spacing and orientation as well as surface roughness; reduced airflow can limit the removal of heat and pollutants.