Turbulent organized structure over a reduced urban scale model

It is well known that the turbulent organized structure (TOS) accounts for the most of the momentum or scalar fluxes in a surface layer (SL). We investigated the TOS in the SL over an urban scale model. The present study focused on (1) the atmospheric stability effect, and (2) the horizontal scale of the TOS. The scale model is constructed in outdoor, which has 50 x 100m horizontal extent and with 1.5-meter (=H) cubic roughness blocks arranged uniformly. Eight sonic anemometers are used. At first, 5 anemometers were aligned vertically (H, 1.5H, 2H, 3H and 4H) to obtain the vertical profiles of turbulent statistics. We performed the quadrant analysis at each height to examine the relative contribution of sweep (high speed downdraft) and ejection (low speed updraft) to the momentum flux in a specific period (30 minutes). The result reflected the effect of stability. The fraction of the sweep mode was larger than that of the ejection mode at the range of the measurement heights in the stable atmosphere. In the unstable condition, the ejection mode was crossover the sweep mode above the height 3H. This characteristic has been observed also in a field observation of a densely built up residential area in Tokyo. Second, in order to obtain the horizontal distribution of TOS, the instruments were aligned at a same height (2H) above the adjacent eight blocks in a span wise direction of the horizontal mean flow. It was observed that high speed and low speed streaks were aligned in the span wise direction. The horizontal scales of those streaks are about 3 or 4 times larger than that of the surface roughness blocks. The horizontal distributions of temperature and vertical velocity fluctuations also had the same coherent structures.