Drag Coefficient and Turbulence Mixing Length of Local Climate Zone-Based Urban Morphologies Derived Using Obstacle-Resolving Modelling
Nagel, Tim ; Schoetter, Robert ; Bourgin, Victor ; Masson, Valéry ; Onofri, Emma
Large-eddy microscale simulations of eleven local climate zone-based (LCZ) urban morphologies with various building plane and frontal area density are used to investigate the flow characteristics and provide vertical profiles of velocity, sectional drag coefficient, and turbulence mixing length. The urban morphologies are procedurally generated to mimick real urban districts. The simulations are performed with the MesoNH-IBM meteorological research model, which allows to represent explicitly the obstacles and to account for the impact of the large scale turbulence structures on the urban canopy layer (UCL). The results show that, in heterogeneous building height UCLs, the streamwise velocity profile is not exponential, the mixing length is not constant and the equivalent sectional drag coefficient formula based on bulk morphology parameters is not valid. Comparatively to an non-urban mixing length increasing linearly with the distance from the ground, the UCL mixing length is higher for $$z/h_\textrm{mean} \in [0 - \approx 0.75]$$, because of the turbulent structures generated by the buildings and lower above, because of the shear generated at the building roofs. These differences extend up to several times the mean building height. The vertical profile of the dispersive momentum flux (DMF) in the UCL is in agreement with the literature; positive DMF is found upstream of the buildings whereas negative DMF is localized downstream. Although the DMF is lower than the turbulent momentum flux for most of the LCZs, it is not negligible for midrise and highrise LCZs. The large-scale atmospheric boundary-layer turbulence has a negligible influence on most of the investigated horizontally-averaged quantities. This suggests that considering a neutral stratification and a wind flow aligned with the buildings, most of the turbulence within the UCL is generated by the buildings themselves.</p>
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