Parametrization of Drag and Turbulence for Urban Neighbourhoods with Trees

Abstract

Urban canopy parametrizations designed to be coupled with mesoscale models must predict the integrated effect of urban obstacles on the flow at each height in the canopy. To assess these neighbourhood-scale effects, results of microscale simulations may be horizontally-averaged. Obstacle-resolving computational fluid dynamics (CFD) simulations of neutrally-stratified flow through canopies of blocks (buildings) with varying distributions and densities of porous media (tree foliage) are conducted, and the spatially-averaged impacts on the flow of these building-tree combinations are assessed. The accuracy with which a one-dimensional (column) model with a one-equation (k-l) turbulence scheme represents spatially-averaged CFD results is evaluated. Individual physical mechanisms by which trees and buildings affect flow in the column model are evaluated in terms of relative importance. For the treed urban configurations considered, effects of buildings and trees may be considered independently. Building drag coefficients and length scale effects need not be altered due to the presence of tree foliage; therefore, parametrization of spatially-averaged flow through urban neighbourhoods with trees is greatly simplified. The new parametrization includes only source and sink terms significant for the prediction of spatially-averaged flow profiles: momentum drag due to buildings and trees (and the associated wake production of turbulent kinetic energy), modification of length scales by buildings, and enhanced dissipation of turbulent kinetic energy due to the small scale of tree foliage elements. Coefficients for the Santiago and Martilli (Boundary-Layer Meteorol 137: 417-439, 2010) parametrization of building drag coefficients and length scales are revised. Inclusion of foliage terms from the new parametrization in addition to the Santiago and Martilli building terms reduces rootmean-square difference (RMSD) of the column model streamwise velocity component and turbulent kinetic energy relative to the CFD model by 89 % in the canopy and 71 % above Electronic supplementary material The online version of this article (123 158 E. S. Krayenhoff et al.the canopy on average for the highest leaf area density scenarios tested: 0.50 m 2 m −3 . RMSD values with the new parametrization are less than 20 % of mean layer magnitude for the streamwise velocity component within and above the canopy, and for above-canopy turbulent kinetic energy; RMSD values for within-canopy turbulent kinetic energy are negligible for most scenarios. The foliage-related portion of the new parametrization is required for scenarios with tree foliage of equal or greater height than the buildings, and for scenarios with foliage below roof height for building plan area densities less than approximately 0.25.

Application of MORUSES single‐layer urban canopy model in a tropical city: Results from Singapore

“…Although the presence of vegetation inside canyons produces an impact on flow and radiation within (e.g. Lemonsu et al ., ; Krayenhoff et al ., ; ; Wang et al ., ; Redon et al ., ), these effects are not calculated by MORUSES. Instead, vegetation is simulated separately from the canyon tile.…”

Impacts of in‐canyon vegetation and canyon aspect ratio on the thermal environment of street canyons: numerical investigation using a coupled WRF‐VUCM model

“…However, urban tree effects are not parametrized in their study. Krayenhoff et al () represents urban trees within street canyons and above rooftops using a leaf area density profile and parametrizes the associated physical processes for the multi‐layer UCM (Salamanca and Martilli, ). Ryu et al () includes urban tree parametrization in the single‐layer UCM (Kusaka et al , ; Wang et al , ) using a similar representation method commonly used in microscale models.…”

The Impact of Planting Trees on NOx Concentrations: The Case of the Plaza de la Cruz Neighborhood in Pamplona (Spain)

“…In addition, the fraction of pollutant removed from air by means of the deposition to the leaves is represented as a mass sink in the transport equation (S d , Equation 4). This approach to model vegetation has been evaluated by Santiago et al [24], Krayenhoff et al [37], and Santiago et al [28], and it is also similar to those employed in other CFD studies [23,25,31]. The mathematical expressions are the following,…”

“…These experiments have been extensively used to validate simulations with RANS by Foudhil et al [47] and with Large Eddy Simulations by Dupont and Brunet [48]. Our validation results have been presented in Krayenhoff et al [37] and Santiago et al [28]. In addition, the current modelling of urban vegetation was evaluated by using CODASC wind-tunnel dataset (COncentration DAta of Street Canyons -www.windforschung.de/CODASC.htm) [49,50] by simulating a street canyon with and without vegetation.…”