Driving physical mechanisms of flow and dispersion in urban canopies

The paper summarizes results from recent full‐scale and wind‐tunnel studies and discusses the complexity of urban canopy layer (UCL) flow and related challenges for urban modeling. Wind‐tunnel data for idealized street‐canyon intersections demonstrate that street‐level flow and dispersion patterns are significantly altered for configurations with non‐uniform building heights. For most wind directions, concentration maxima were upto a factor of 2 higher for cases with taller buildings near the intersection. Enhanced vertical downward mixing in the wakes of the taller buildings apparently causes higher street‐level winds close by, but simultaneously, other regions become sheltered from active mixing and poor ventilation. The analysis of Joint Urban 2003 (JU2003) full‐scale data also pointed out that high‐momentum fluid can be effectively mixed down in the wakes of high‐rise buildings, and that UCL flow is predominantly dynamically driven by roof‐level winds. Building‐height variability is thus, a key factor for UCL dynamics and mixing. Additionally, a linear model, which assumes that the along‐ and across‐canyon velocity components are directly proportional to their above roof‐level counterparts, has then been tested. The complexity of UCL flow patterns cannot be captured by such a simple model, but it may be useful for estimating street‐level winds. For the along‐canyon component, more than 70% of the calculated values were within a factor of 2 of the measured values, while for the across‐canyon component less than 50% of the predictions agreed within a factor of 2. The practical applicability of the model was further assessed using JU2003 wind‐tunnel data and a comparison between JU2003 wind‐tunnel and full‐scale flow profiles is also presented. It becomes obvious that UCL flow patterns are significantly altered by changes in upwind wind direction of less than 10° . While the field and laboratory results are qualitatively in good agreement, noticeable differences exist at certain sites. Copyright © 2007 Royal Meteorological Society