Detection and Analysis of Organized Motions in Urban Turbulence

Turbulent transport within and above a canopy is dominated to a large extent by large-scale intermittent coherent structures. Numerous authors have studied these organized motions over a wide range of canopies and in the laboratory by means of quadrant analysis and other methods of conditional sampling. It is a general conclusion of all these studies, that gusts (sweeps) dominate over bursts (ejections) within and close above the canopy, where as in the high roughness sublayer and surface layer, bursts contribute most to the turbulent transport of momentum and sensible heat. The large-scale motions or ejection-sweep cycles are known to manifest themselves as temperature ramps in turbulent time series.

In this study, the fluctuations of the horizontal wind components u’ and v’, the vertical component w’ and temperature T’, sampled with 20.83 Hz and measured simultaneously at three levels (z/h=1.5, 2.1 and 3.2, with z as the sensor height and h the height of the roughness elements) over an urban canopy in the inner city of Basel, Switzerland, are analyzed. By combining the results from spectral analysis, quadrant analysis and wavelet analysis, patterns of the organized structures (first- and second order moments) under unstable conditions have been calculated and their dominating time scale has been determined. The conditionally sampled fluxes have been compared to the conventionally (Reynolds-)averaged fluxes.

The results show the remarkably stable patterns of the coherent structures to be very similar to the profiles found over vegetation canopies, however the dominating time scale of around 90 seconds is significantly larger due to the rougher surface and the higher roughness elements. In contrary to studies made over vegetation canopies, no pronounced tilt of the microfronts could be observed in the present data set. We found that structures are efficient in transporting sensible heat, e.g. up to 80 % of the Reynolds-averaged flux at z/h=1.5. The contribution of the detected events to the transport of momentum is low, decreases even with increasing height and the association with the temperature ramp is lost at the uppermost measurement level at z/h=3.5 for the conditionally sampled horizontal wind component and the friction velocity.