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 (c) 2007 Royal Meteorological Society.