Experimental determination of the turbulent kinetic energy budget within and above an urban canopy

Many aspects of the turbulent exchange processes above rough surfaces are summarized in the budget of turbulent kinetic energy (TKE). With the increasing need of accurately predicting dispersion within and close to urban canopies for air quality, health and emergency response, detailed knowledge on the magnitude of turbulent fluctuations and the underlying physical processes that create, relocate and destroy kinetic energy become important. This may help to improve model parameterizations and to develop more appropriate scaling methods. However, little is known on the TKE budget within the urban roughness sublayer, and there is especially a lack of experimental data on its behaviour within the urban canopy layer.

In this contribution we focus on results from a measurement site which was operated in the framework of the Basel Urban Boundary Layer Experiment BUBBLE. A tower with six ultrasonic anemometers, arrayed in a vertical profile, was operated within and above a typical European street canyon.

The mean wind profile at the tower shows a distinct inflection point at 1.1h. The inflected velocity profile produces an instability, which is associated with high vorticity and high turbulent kinetic energy in the region just at and above roof top. As a consequence the magnitude of shear production is highest above roof top. Above roof-level, shear production is by far the most important source for TKE. It decreases rapidly inside the street canyon, both in absolute and relative numbers. At the zeroplane displacement height (0.7h) it becomes nearly zero. Higher wind speeds in the canyon under along canyon flow result in a small second maximum at canyon floor.

Over a compact urban surface, mainly roof areas contribute to buoyant production. In most situations, buoyant production of TKE is negligible within the canyon and small (relative to shear production ) in the roughness sublayer above roof top.

Over the whole vertical profile the TKE budget is not in local equilibrium, thus the produced turbulent kinetic energy has to be vertically relocated by a number of transport processes: (i) turbulent flux of TKE, (ii) dispersive flux of TKE, (iii) viscous transport of TKE and (iv) pressure transport of TKE. The only transport term that can be measured directly is (i), the turbulent flux. The measurements show, that it relocates TKE from the region above rooftop (z/h > 1.2) down into the upper part of the street canyon. The transport is associated with intermittent large structures. Pressure transport is the most important non-measured term. The pressure transport was estimated as residual term and seems to relocate TKE from the roof top and the upper canopy in higher layers of the roughness sublayer and also down into the very bottom of the street canyon.

Dissipation was determined via the inertial subrange of velocity spectra. This inertial subrange method is not of universal applicability, however it is shown, that it can provide useful estimates of the dissipation of TKE. Dissipation rate is highest at roof top and decreases in both directions.