A New Parametrization of Mixing Length in an Urban Canopy Derived from a Large-Eddy Simulation Database for Tokyo
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Average horizontal wind velocity in an urban canopy is mainly determined by a balance between flow deceleration caused by the drag force of buildings and flow acceleration from the momentum flux gradient in the canopy. To express the transport of momentum in an urban canopy, mixing length is often used to calculate diffusivity there. A new parametrization for mixing length is introduced for a one-dimensional multilayer urban canopy model (UCM). A database from large-eddy simulations using actual urban morphology for Tokyo is used for this parametrization. The derived mixing length is described as a function of the non-dimensional height raised to the power of \(q\), where \(q < 1\). The \(q\) value and constants of the function also depend on the selection of canopy height. The mixing length profile is closely related to that of the average plane area index of the buildings in the study area. Recalculation of mean horizontal wind velocity using the new parametrization of mixing length for Tokyo slightly improved the multilayer UCM results.
KeywordsLarge-eddy simulation database Multilayer urban canopy model Plane area index Urban canopy height Vertical diffusivity
This study was supported by the Research Program on Climate Change Adaptation, promoted by the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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