Coherent structures in urban roughness sublayer turbulence

Data measured within and above an urban street canyon is used to analyse the importance of coherent structures in turbulent transfer. The atmosphere was probed simultaneously at six different heights from the base of the street canyon up to 2.2 times the mean building height. Integral length scales, joint probability densities, quadrant measures and wavelet-based conditional sampling techniques are used to picture momentum and sensible heat exchange in the urban roughness sublayer. Layers directly at roof height show smallest length scales. With increasing distance from roofs, length scales increase, both up to higher levels above the roofs and also down to the base of the street canyon. Quadrant analysis indicates that turbulent momentum and sensible heat fluxes are strongly sweep-dominated in large parts of the street canyon and close to the roofs. Only at the highest measurement levels do ejections slightly dominate exchange. Exchange is intermittent where length scales are largest, namely in the lower street canyon and at higher levels in the urban roughness sublayer. At rooftop, exchange is more efficient and least intermittent. A wavelet-based detection is used to reconstruct time-height cross-sections of paired events measured at multiple levels simultaneously. Two types of sharp scalar fronts were identified—sweeps followed by ejections and ejections followed by sweeps. Both are important in transporting momentum and sensible heat. Copyright © 2007 Royal Meteorological Society

[1]  J. Finnigan.  Turbulence in Waving Wheat , 1979 .

[2]  J. Finnigan,et al.  A Wind-Tunnel Study of Airflow in Waving Wheat: An EOF Analysis of the Structure of the Large-Eddy Motion , 2000, Boundary-Layer Meteorology.

[3]  R. Shaw,et al.  Two-Point Correlation Analysis Of Neutrally Stratified Flow Within And Above A Forest From Large-Eddy Simulation , 2000 .

[4]  C. Feigenwinter.  The vertical structure of turbulence above an urban canopy , 1999 .

[5]  A. Thom,et al.  Turbulence in and above Plant Canopies , 1981 .

[6]  A. Christen,et al.  Energy and radiation balance of a central European city , 2004 .

[7]  M. Irvine,et al.  The Control Of Coherent Eddies In Vegetation Canopies: Streamwise Structure Spacing, Canopy Shear Scale And Atmospheric Stability , 2000 .

[8]  X. J. Zhang,et al.  Evidence of pressure-forced turbulent flow in a forest , 1992 .

[9]  M. Rotach.  Turbulence within and above an urban canopy , 1991 .

[10]  R. Shaw,et al.  Structure of the Reynolds Stress in a Canopy Layer , 1983 .

[11]  Andreas Christen,et al.  Atmospheric turbulence and surface energy exchange in urban environments : results from the Basel Urban Boundary Layer Experiment (BUBBLE) , 2005 .

[12]  J. Finnigan,et al.  A wind tunnel study of air flow in waving wheat: Two-point velocity statistics , 1994 .

[13]  Matthias Roth,et al.  Turbulent transfer relationships over an urban surface. I. Spectral characteristics , 1993 .

[14]  Christopher Baker,et al.  Large‐eddy simulation of turbulent flow in a street canyon , 2004 .

[15]  W. Willmarth,et al.  Measurements of the structure of the Reynolds stress in a turbulent boundary layer , 1973, Journal of Fluid Mechanics.

[16]  M. Raupach,et al.  Experiments on scalar dispersion within a model plant canopy part I: The turbulence structure , 1986 .

[17]  H. Bergström,et al.  Turbulent exchange above a pine forest II. Organized structures , 1989 .

[18]  T. Oke,et al.  Relative Efficiencies of Turbulent Transfer of Heat, Mass, and Momentum over a Patchy Urban Surface , 1995 .

[19]  S. Yates,et al.  Atmospheric volatilization of methyl bromide, 1,3-dichloropropene, and propargyl bromide through two plastic films: transfer coefficient and temperature effect , 1999 .

[20]  R. Shaw,et al.  Observation of organized structure in turbulent flow within and above a forest canopy , 1989 .

[21]  Timothy R. Oke,et al.  Aerodynamic Properties of Urban Areas Derived from Analysis of Surface Form , 1999 .

[22]  T. Oke,et al.  Venting of Heat and Carbon Dioxide from Urban Canyons at Night , 2005 .

[23]  J. Monteith,et al.  Boundary Layer Climates. , 1979 .

[24]  Matthias Roth,et al.  Review of atmospheric turbulence over cities , 2007 .

[25]  P. Holmes,et al.  Turbulence, Coherent Structures, Dynamical Systems and Symmetry , 1996 .

[26]  R. Shaw,et al.  Conditional analysis of temperature and humidity microfronts and ejection/sweep motions within and above a deciduous forest , 1992 .

[27]  Experimental determination of the turbulent kinetic energy budget within and above an urban canopy , 2004 .

[28]  Susumu Oikawa,et al.  Turbulence characteristics and organized motion in a suburban roughness sublayer , 1995 .

[29]  Michael Schatzmann,et al.  BUBBLE – an Urban Boundary Layer Meteorology Project , 2005 .

[30]  R. A. Antonia,et al.  Conditional Sampling in Turbulence Measurement , 1981 .

[31]  J. Finnigan.  Turbulence in plant canopies , 2000 .

[32]  Mathias W. Rotach,et al.  On the influence of the urban roughness sublayer on turbulence and dispersion , 1999 .

[33]  C. Torrence,et al.  A Practical Guide to Wavelet Analysis. , 1998 .

[34]  B. Feddersen.  Wind tunnel modelling of turbulence and dispersion above tall and highly dense urban roughness , 2005 .

[35]  C. Feigenwinter,et al.  Detection and analysis of coherent structures in urban turbulence , 2005 .

[36]  R. A. Antonia,et al.  Rough-Wall Turbulent Boundary Layers , 1991 .

[37]  Manabu Kanda,et al.  Large-Eddy Simulation of Turbulent Organized Structures within and above Explicitly Resolved Cube Arrays , 2004 .