The plane mixing layer analogy applied to turbulence in the urban roughness sublayer

Parameterizations of fluid mechanical processes explaining the exchange between street canyon air masses and the ambient flow above roof level are of essential interest in urban dispersion models. Unfortunately, as many studies have demonstrated, their implementation by applying classical or modified surface layer theory is inadequate or even fails completely in the lower urban roughness sublayer. Therefore, alternative approaches and simplifications are highly welcome.

In the last 15 years, turbulent exchange in plant canopies have been successfully addressed with the help of the plane mixing layer analogy. However, the permeability of porous plant canopies does not allow a direct analogy with the stiff and bluff urban surface, and many urban specific flow patterns like street canyon vortices are not found in natural canopies.

Extensive turbulence measurements from three experimental urban towers operated during the Basel Urban Boundary Layer Experiment (BUBBLE) allow testing the applicability of the plane mixing layer analogy in the urban roughness sublayer. The analysis of this data-set provides valuable insights into mean flow, turbulent kinetic energy, exchange processes of momentum, energy and mass within the urban roughness sublayer under various flow configurations. We present a generalized ‘urban family portrait', which indeed supports many similarities between the flow in the urban roughness sublayer and a plane mixing layer. Many observed flow characteristics reflect processes that are generally found over rough surfaces and therefore resemble flow properties measured within and above plant canopies, namely an inflected velocity profile, non-negligible turbulent transport terms, a dominance of length scales shed at roof top, high efficiency of the exchange around roof top, and characteristic coherent structures.