Large Eddy Simulations have been performed for fully developed turbulent flow within and above explicitly resolved simple cube arrays. The model (LES-CITY) was validated in comparison with laboratory experiments. The systematic influence of cubic density on turbulent flow characteristics was investigated through numerical experiments in a wide range of cubic area density (0 % to 44%). The following results were obtained. (1) The influence of the heterogeneity of surface geometry: the dispersive momentum flux is quite large within the canopy layer due to a mean stream such as re-circulation, while it is small but not negligible above the canopy due to Turbulent Organized Structures (TOS). The spatial variation of temporally averaged momentum in the roughness sub-layer is as large as 10-20 % of the total kinematic surface drag. (2) The intermittency of flow: the temporally and spatial-ensemble averaged flow structure confirm the existence of conventionally described canyon flow regimes; isolated, interfacial and wake. However, the intermittency of the canyon flow for any cubic densities is quite large and the stream patterns are never persistent. (3) TOS: TOS similar to those observed in turbulent surface layer flows are simulated, which are characterized by longitudinally-elongated low speed streaks and the corresponding shorter streamwise vortices. The streaks in sparse and dense canopy flows are likely to be aligned to the street-line and to the roof-lines, respectively. Such heterogeneity of TOS partially accounts for the large spatial variation of momentum flux. (4) The difference from vegetation flow: the TOS and the resulting turbulent statistics of urban flow above the canopy resemble those in surface layers, contrast to the mixing layer analogy of vegetation flows. The re-circulation within the canopy makes significant influence on the turbulent statistics such as negative Reynolds stress, large dispersive flux and standard deviations.

Key words: LES-CITY, urban obstacles, turbulent organized structure, surface heterogeneity, intermittency