Evaluation of the inner-scaling similarity of turbulence over urban-like roughness derived from an outdoor scale model experiment

It is well-known that the turbulent characteristics within the atmospheric surface layer are expressed by universal functions of the inner-scaling parameters (e.g. Monin-Obukhov similarity). Although the universal relations between the turbulent statistics and the inner-scaling parameters have been confirmed in flat and horizontally homogeneous fields, it is not obvious that these relations are applicable for urban surface layers since the inner-scaling parameters depend on surface conditions. The inner-scaling similarity of the turbulent characteristics within the surface layer was evaluated over the array of massive obstacles, mainly focusing on the impact of the roughness. For this purpose, turbulence was measured in the facility of comprehensive outdoor scale model experiment for urban climate (COSMO). This study focuses on two main characteristics of the COSMO experimental site: 1) the building array is composed of cubes of uniform shape and material, which are regularly arranged over a flat plate; 2) the experiment was conducted within the atmospheric boundary layer. Experimental uncertainties that come with urban experiments, such as irregular arrangement of buildings, complexities of land use and the influence of human activities, were eliminated so as to simplify the evaluation of the effects of roughness on the inner-scaled turbulent statistics under atmospheric conditions. The results of the present study illustrated that the inner-scaling similarity was valid for vertical velocity fluctuations and the momentum transfer in the COSMO site, which are consistent with the values observed from a flat and homogeneous field. However, the similarity does not work well for horizontal velocity fluctuations. This failure of the inner-scaling similarity for the horizontal velocity fluctuation is attributed to the outer layer disturbance, which imposes on the horizontal velocity fluctuation, but not to difference of roughness. The contribution of the outer-layer disturbance to modify the inner-scaled velocity fluctuation changed in different roughness.