The Budget of Turbulent Kinetic Energy in the Urban Roughness Sublayer |
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Authors: | Andreas Christen Mathias W Rotach Roland Vogt |
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Institution: | 1.Department of Geography, Atmospheric Science Program,University of British Columbia,Vancouver,Canada;2.Federal Office for Meteorology and Climatology,Zurich,Switzerland;3.Institute of Meteorology, Climatology and Remote Sensing, Department of Environmental Sciences,University of Basel,Basel,Switzerland |
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Abstract: | Full-scale observations from two urban sites in Basel, Switzerland were analysed to identify the magnitude of different processes
that create, relocate, and dissipate turbulent kinetic energy (TKE) in the urban atmosphere. Two towers equipped with a profile
of six ultrasonic anemometers each sampled the flow in the urban roughness sublayer, i.e. from street canyon base up to roughly
2.5 times the mean building height. This observational study suggests a conceptual division of the urban roughness sublayer
into three layers: (1) the layer above the highest roofs, where local buoyancy production and local shear production of TKE
are counterbalanced by local viscous dissipation rate and scaled turbulence statistics are close to to surface-layer values;
(2) the layer around mean building height with a distinct inflexional mean wind profile, a strong shear and wake production
of TKE, a more efficient turbulent exchange of momentum, and a notable export of TKE by transport processes; (3) the lower
street canyon with imported TKE by transport processes and negligible local production. Averaged integral velocity variances
vary significantly with height in the urban roughness sublayer and reflect the driving processes that create or relocate TKE
at a particular height. The observed profiles of the terms of the TKE budget and the velocity variances show many similarities
to observations within and above vegetation canopies. |
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Keywords: | Atmospheric turbulence Dispersion Dissipation Plane mixing layer analogy Turbulence Turbulent kinetic energy Urban canopy Urban roughness sublayer Velocity variances |
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