An object-based porosity approach for modelling turbulent exchanges in urban canopies at neighbourhood-scale

<p>Turbulent flows within and above urban and vegetative canopies in the atmospheric boundary layer have profound implications for a variety of important problems in  agricultural  and urban meteorology, such as the spreading of pollens and pollutants. We study such turbulence via Direct Numerical Simulations (DNSs), by using the code developed in (2019 Mortikov),  in which there is a closed channel between two parallel walls and a canopy of constant areal density profile on the lower wall. We impose periodic boundary conditions in the horizontal directions and a no-slip impenetrable boundary condition in the wall-normal direction. For the canopy, we use different formulations of the Forchheimer drag. We assess the role of the canopy on the turbulent flow. In particular, we show the influence of added drag on the mean profiles, balance equations of the second-order moments, and the local anisotropy of the flow.</p><p>We observe that the turbulence transport profile undergoes an abrupt transition at the canopy top and transfer of energy from the roughness sublayer above the canopy to inside the canopy.  <br>The pressure-strain correlation removes energy from the wall-normal fluctuations, which has the least share of the turbulent kinetic energy and distributes it among the other components in the bulk of the canopy. In the inertial range, within and above the canopy, the energy spectra for the streamwise component is steeper than the spanwise and the wall-normal components and is closer to the Kolmogorov -5/3 spectrum as observed in the eddy covariance measurements in the roughness sublayer (2020 Bhattacharjee).</p><p>We thank the DST, CSIR (India), SERC (IISc) for computational resources, the AtMath Collaboration at the University of Helsinki, and ICOS by University of Helsinki for their support. This study was also partially funded by RFBR project number 20-05-00776.</p><p>Reference</p><p>2019, Mortikov, E. V., Glazunov, A. V., & Lykosov, V. N. Numerical study of plane Couette flow: turbulence statistics and the structure of pressure–strain correlations, Russian Journal of Numerical Analysis and Mathematical Modelling, 34(2), 119-132. doi: https://doi.org/10.1515/rnam-2019-0010.</p><p>2020, Bhattacharjee S., Pandit R., Vesala T., Mammarella I., Katul G., and Sahoo G. Anisotropy and multifractal analysis of turbulent velocity and temperature in the roughness sublayer of a forested canopy, arXiv:2010.04194.</p>

The study of wind flow characteristics within urban environments is of prior interest for climate, meteorology and air pollution issues. Previous works have shown that within the urban areas the turbulent transfers of momentum, heat and mass between the Atmospheric Boundary Layer (ABL) and the urban canopy are governed by the interactions between large- and small-scale turbulent structures. Hence, to accurately compute flow characteristics at the neighbourhood scale, it is required to compute canopy-generated turbulence, as well as the turbulent structures prevailing in the ABL. This wide range of scales presents a major issue when performing unsteady simulations of turbulent flows such as Large-Eddy Simulations (LES), since the flow field specified at the inlet conditions the downstream development of the flow. In ABL numerical models, the most common method is to use a multi-scale approach based on successive grid-nesting from mesoscale to the local scale. When performing multi-scale nesting to study urban canopy flows, the urban obstacles are generally explicitly taken into account in the smallest high-resolution domain only, larger domains being not refined enough for a realistic representation of the canopy elements. However, it has been shown that adding roughness elements in the larger domain had a noticeable impact on the flow dynamics and statistics within the small domain (Wiersema et al., Mon. Wea. Rev., 2020). This study addresses the problem of the generation of realistic unsteady atmospheric inflow conditions on an idealised urban-like canopy consisting of a staggered array of cubes of constant height and packing density. Here we investigate the performance of using drag-porosity based simulations as precursor calculations for obstacle-resolving LES. The objective is to show that by using a precursor simulation with a less-costly canopy modelling method as inlet, LES of ABL flows within and above a canopy of explicitly resolved obstacles is possible with minimum flow adjustment in the inlet region. The LES atmospheric model ARPS (Advanced Regional Prediction System) is used with an Immersed Boundary Method (IBM) to model the flow over building-like obstacles. In order to assess the proposed approach, three inflow conditions are explored and compared : (1) a periodic precursor calculation over a large domain covered by cube array modelled with IBM, (2) a synthetic turbulence inflow obtained from purposely deteriorating flow information of the periodic case (without modifying moments and spectra), and (3) a drag-porosity precursor calculation (in which the urban canopy is modelled as a porous media depending on canopy averaged morphological characteristics). To the authors’ knowledge, the use of an urban-designed drag model as precursor calculation for an obstacle-resolving simulation is the first to be performed within a single atmospheric solver.

In this observational study, the role of tree phenology on the atmospheric turbulence parameterization over 10-m-tall and relatively sparse deciduous vegetation is quantified. Observations from the Canopy Horizontal Array Turbulence Study (CHATS) field experiment are analyzed to establish the dependence of the turbulent exchange of momentum, heat, and moisture, as well as kinetic energy on canopy phenological evolution through widely used parameterization models based on 1) dimensionless gradients or 2) turbulent kinetic energy (TKE) in the roughness sublayer. Observed vertical turbulent fluxes and gradients of mean wind, temperature, and humidity, as well as velocity variances, are used in combination with empirical dimensionless functions to calculate the turbulent exchange coefficient. The analysis shows that changes in canopy phenology influence the turbulent exchange of all quantities analyzed in this study. The turbulent exchange coefficients of those quantities are twice as large near the canopy top for a leafless canopy than for a full-leaf canopy under unstable and near-neutral conditions. This turbulent exchange coefficient difference is related to the differing penetration depths of the turbulent eddies organized at the canopy top, which increase for a canopy without leaves. The TKE and dissipation analysis under near-neutral atmospheric conditions additionally shows that TKE exchange increases for a leafless canopy because of reduced TKE dissipation efficiency relative to that when the canopy is in full-leaf stage. The study closes with discussion surrounding the implications of these findings for parameterizations used in large-scale models.

Eddy-covariance observations above the densely built-up Centre of Nanjing were made from December 2011 to August 2012. Separate eddy-covariance systems installed at two levels on a 36-m tower located on a rooftop were operated simultaneously, and observations grouped into two sectors (A, B) according to the prevalent wind directions. For sector A, where the nearby buildings are all below the lower measurement level, the sensible heat and momentum fluxes are generally greater at the upper level. For sector B, where several high-rise buildings are located upwind, the sensible heat and momentum fluxes at the upper level are close to those at the lower level. The analysis shows that the turbulent eddy characteristics differ between the two wind sectors, leading to a different behaviour of turbulent exchange between the two levels. A hypothesis is proposed that addresses the vertical variation of turbulent fluxes in the urban roughness sublayer (RSL). For sector A, the buildings block the flow, change the trajectory of scalars, and distort the footprint of scalar fluxes; this ‘blocking effect’ is believed to lead to a smaller sensible heat flux above the canopy layer. Such an effect should decrease with height in the RSL, explaining the increase of the observed turbulent heat flux with height. In addition, the presence of non-uniform building heights adversely affects turbulence organization around the canopy top, and likely elevates the inflection point of the mean flow to a higher elevation close to the upper measurement level, where larger shear results in a larger momentum flux. For sector B, wake effects from the nearby high-rise buildings strongly reduce turbulence organization at higher elevations, leading to similar sensible heat and momentum fluxes at both measurement levels.

<strong class="journal-contentHeaderColor">Abstract.</strong> In recent years, urban canopy models (UCMs) have been used as fully coupled components of mesoscale atmospheric models as well as offline tools to estimate temperature and surface fluxes using atmospheric forcings. Examples include multi-layer urban canopy models (MLUCMs), where the vertical variability of turbulent fluxes is calculated by solving prognostic momentum and turbulent kinetic energy (TKE, <em>k</em>) equations using length scale (<em>l </em>) and drag parameterizations. These parameterizations are based on the well-established 1.5-order <em>k</em> &minus; <em>l</em>&nbsp; turbulence closure theory and are often informed by microscale fluid dynamics simulations. However, this approach can include simplifications such as the assumption of the same diffusion coefficient for momentum, TKE, and scalars. In addition, the dispersive stresses arising from spatially-averaged flow properties have been parameterized together with the turbulent fluxes while being controlled by different mechanisms. Both of these assumptions impact the quantification of turbulent exchange of flow properties and subsequent air temperature prediction in urban canopies. To assess these assumptions and improve corresponding parameterization, we conducted 49 large-eddy simulations (LES) for idealized urban arrays, encompassing variable building height distributions and a comprehensive range of urban densities (&lambda;_<em>p</em> &isin; [0.0625,0.64]) seen in global cities. We find that the efficiency of turbulent transport (numerically described via diffusion coefficients) is similar for scalars and momentum but 3.5 times higher for TKE. Additionally, the parameterization of the dispersive momentum flux using the <em>k</em> &minus; <em>l</em>&nbsp; closure was a source of error, while scaling with the pressure gradient and urban morphological parameters appears more appropriate. In response to these findings, we propose two changes to MLUCM v2.0: (a) separate characterization for turbulent diffusion coefficient for momentum and TKE; and (b) introduction of an explicit physics-based "mass flux" term to represent the non-Gaussian component of the dispersive momentum transport as an amendment to the existing "eddy diffusivity" framework. The updated one-dimensional model, after being tuned for building height variability, is further compared against the original LES results and demonstrates improved performance in predicting vertical turbulent exchange in urban canopies.

With climate-related hazards such as heat waves, fires, hurricanes, droughts, and floods becoming more frequent in recent years, reducing atmospheric carbon has emerged as a priority. In cities, efforts to reduce atmospheric carbon have primarily focused on lowering the energy consumption of buildings. However, studies suggest that minimizing building energy consumption alone will not be sufficient to meet the targets outlined in the Paris Agreement. To achieve these goals, new building materials and urban designs are being explored with the intent of transforming cities into carbon sinks. Before evaluating the efficacy of carbon sink strategies, it is important to assess the potential amount of carbon that could be dispersed within the urban canopy from various anthropogenic sources, including buildings. For this reason, this study aims to simulate carbon dispersion within the urban canopy using urban building energy modeling and computational fluid dynamics. Urban building energy modeling is employed to estimate carbon emissions from buildings at the neighborhood scale, while computational fluid dynamics is used to simulate carbon dispersion through air movement within the urban canopy. Models for performing urban building energy and computational fluid dynamics simulations are generated from a 3D city model of the Netherlands. In the 3D city model, the geometry of buildings is represented at Level of Detail 2.2. Their materials and internal heat gains are determined using an online database of building stock in Europe. The results identify locations in several neighborhoods of Delft, Netherlands, where significant amounts of carbon emitted by buildings are dispersed within the urban canopy. At these locations, implementing carbon sink strategies is recommended to help address climate change. The analysis of carbon sink strategies is deferred to future research.

A numerical study of the flow field and exchange processes within a canopy of urban-type roughness

Large-eddy simulation of the urban boundary layer using drag-porosity modeling

The representation of complex urban environments and their climates highlights a key advancement in present-day regional climate modeling. City climates differ from their surrounding regional climates in various ways, e.g., through increased temperatures and reduced evapotranspiration. The development of convection-permitting regional climate model (CPRCM) simulations allow us to resolve urban areas and their climatic phenomena, such as the urban heat island effect, in contrast to their rural surroundings more precisely. However, with increasing resolution, the complex terrain and characteristics of cities must be adequately represented.The EU Horizon project Impetus4Change (https://impetus4change.eu) aims to improve the quality, accessibility and usability of near-term climate information and services at local to regional scales. The project utilises dynamically downscaled regional climate models (RCMs) to produce very high-resolution (3 km) simulations for urban areas across Europe. To achieve this, we integrate the Town Energy Balance model (TEB; Masson, 2000), a single-layer urban canopy model, into the RCM REMO to simulate urban climates and urban-rural interactions under transient climate change conditions.Compared to a previously bulk urban form in REMO, TEB introduces distinctive urban features, such as an urban canyon with roofs, walls and roads. This allows for a more realistic representation of vertical radiative trapping and turbulent exchange. The double-nested CPRCM simulations performed include evaluation simulations driven by reanalysis (ERA5.1) for 2000-2009 and historical simulations driven by the general climate model MPI-ESM1-2-HR for 1995-2014, covering two large domains across Northern and Southern Europe. The output is compared to observations and existing 3 km and 12.5 km simulations of REMO without a sophisticated urban canyon scheme to assess the added value of more realistic urban climate representations in Europe.*This project has received funding from the European Union&amp;#8217;s Horizon Europe research and innovation programme under grant agreement No 101081555.ReferenceMasson, V. A Physically-Based Scheme For The Urban Energy Budget In Atmospheric Models. Boundary-Layer Meteorology 94, 357&amp;#8211;397 (2000). https://doi.org/10.1023/A:1002463829265

The mSCOPE model: A simple adaptation to the SCOPE model to describe reflectance, fluorescence and photosynthesis of vertically heterogeneous canopies

Displacement height (d) is an important parameter in the simple modelling of wind speed and vertical fluxes above vegetative canopies, such as forests. Here we show that, aside from implicit definition through a (displaced) logarithmic profile, accepted formulations for d do not consistently predict flow properties above a forest. Turbulent transport can affect the displacement height, and is an integral part of what is called the roughness sublayer. We develop a more general approach for estimation of d, through production of turbulent kinetic energy and turbulent transport, and show how previous stress-based formulations for displacement height can be seen as simplified cases of a more general definition including turbulent transport. Further, we also give a simplified and practical form for d that is in agreement with the general approach, exploiting the concept of vortex thickness scale from mixing-layer theory. We assess the new and previous displacement height formulations by using flow statistics derived from the atmospheric boundary-layer Reynolds-averaged Navier–Stokes model SCADIS as well as from wind-tunnel observations, for different vegetation types and flow regimes in neutral conditions. The new formulations tend to produce smaller d than stress-based forms, falling closer to the classic logarithmically-defined displacement height. The new, more generally defined, displacement height appears to be more compatible with profiles of components of the turbulent kinetic energy budget, accounting for the combined effects of turbulent transport and shear production. The Coriolis force also plays a role, introducing wind-speed dependence into the behaviour of the roughness sublayer; this affects the turbulent transport, shear production, stress, and wind speed, as well as the displacement height, depending on the character of the forest. We further show how our practical (‘mixing-layer’) form for d matches the new turbulence-based relation, as well as correspondence to previous (stress-based) formulations.

&amp;lt;p&amp;gt;The increase in urban air temperature caused by urban heat and climate change can have negative effects on the outdoor thermal comfort (OTC) as well as on the energy demand for air-conditioning. Nature-based solutions, such as the increase in urban biomass, are often proposed to mitigate excessive urban heat. Trees are expected to decrease temperatures due to shade provision on surfaces and evapotranspiration but their canopy blocks wind flow, thus potentially induce warming by reduction of heat removal. Several studies have shown that trees have a varying potential for air temperature reduction throughout the diurnal cycle as well as in different climates. Studies that partition and attribute the temperature reduction to the aforementioned effects are still lacking though, thus making the explanation of the observed differences difficult.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;To address this knowledge gap, we use the mechanistic urban ecohydrological model, Urban Tethys-Chloris (UT&amp;amp;C, Meili et al. 2019), which accounts for radiation, evapotranspiration and roughness effects of trees in the urban canyon. Turning these components on and off by means of virtual experiments allows us to quantify their contribution to the air and surface temperature modification caused by the tree cover. The results are analysed for compact low-rise residential areas (LCZ3) in four different climates (Phoenix, Singapore, Melbourne, Zurich).&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;We find that tree evapotranspiration is able to lower 2 m air temperature at maximum by 3-4&amp;amp;#176;C in all four climates as stomatal closure due to high vapour pressure deficits in dry and hot cities limit the transpirative cooling effect during mid-day. Counterintuitively, tree-radiation interaction increases the 2 m air temperature up to 2&amp;amp;#176;C at noon time even though a decrease in surface temperatures is observed. While the surfaces underneath the tree canopy receive less radiation due to shading, the overall absorbed solar radiation within the canyon increases due to radiation trapping. In the analysed scenarios, the presence of trees leads to a decrease in the city roughness hindering turbulent energy exchange and thus, increasing the 2 m air temperature in all climates during daytime. The tree-radiation and tree-roughness effects on 2 m air temperature during night vary in different climates due to atmospheric stability effects.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Combining the different tree effects as in the real world, leads to a distinct diurnal pattern of air temperature reduction which is consistent with the observations in the literature. The numerical experiment allows reconciling differences in temperature changes induced by trees across the diurnal cycle and in various climates. The results could be used to guide green cover and tree type selection in cities and inform future studies aimed at optimizing the role of urban greening for improving local microclimatic conditions.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Meili, N., Manoli, G., Burlando, P., Bou-Zeid, E., Chow, W. T. L., Coutts, A. M., Daly, E., Nice, K. A., Roth, M., Tapper, N. J., Velasco, E., Vivoni, E. R., and Fatichi, S.: An urban ecohydrological model to quantify the effect of vegetation on urban climate and hydrology (UT&amp;amp;C v1.0), Geosci. Model Dev. Discuss., https://doi.org/10.5194/gmd-2019-225, accepted, 2019&amp;lt;/p&amp;gt;

Influence of foliar density profile on canopy flow: A large-eddy simulation study

The solution of the radiation transfer equation in a vegetation canopy requires the specification of appropriate boundary conditions, especially at the interface between the vegetation layer and the soil. This short paper documents the effect of soil brightness and anisotropy on the field of radiances emerging at the top of the vegetation canopy in the solar domain. The study is based on the analysis of the sensitivity of an advanced radiation transfer model. It is shown that the scattering properties of the soil must be carefully assigned to ensure a correct interpretation of data collected over optically thin vegetation canopies.

This study presents the integration of the Double Canyon Effect Parameterization (DCEP) into the microscale urban climate model PALM, specifically tailored for non-building resolving resolution simulations (~100m), where individual buildings are not explicitly resolved. The DCEP scheme introduces a sophisticated scheme characterizing street canyons by considering building and street width, orientation, and building height distribution. This implementation allows for the efficient representation of buildings while maintaining computational feasibility through parallelized computations, making it suitable for large-scale simulations. The DCEP scheme explicitly accounts for the energetic exchanges between buildings and the atmosphere across multiple vertical layers, capturing the dynamics of urban surfaces as well as the storage of energy within surfaces. To derive the necessary Urban Canopy Parameters (UCPs) for DCEP, traditionally requiring high-resolution data, we employ an approach based on Local Climate Zones (LCZ). The LCZ classification, consisting of 17 classes, provides the characteristics of various urban areas, allowing for a detailed representation of heterogeneity in different zones. This approach streamlines the derivation of UCPs by assigning typical parameters to land-use categories, thereby overcoming the complexity associated with high-resolution data processing. The pre-processor tool "palm_csd" has been expanded to directly read LCZ data in different projections and resolutions, providing users with the flexibility to customize parameters assigned to each LCZ class within defined limits. Simulations were conducted over a large domain to show the impact of the PALM/DCEP coupled model. Comparative analyses were performed between simulations with the base PALM model and the enhanced PALM/DCEP coupled model. The results highlight the significant contributions of the DCEP scheme in enhancing urban and environmental modeling during non-building resolving resolution simulations. This research contributes valuable insights into the adaptability and effectiveness of the DCEP approach in simulating large urban areas where building-resolving resolution is not practical. The findings pave the way for advancements in urban climate modeling, with implications for urban planning and environmental management in diverse urban landscapes.