Verifying urban canopy parameterization effects in a mesoscale model with wind tunnel data

<p>Airflow within and above urban canopy layers are modelled by different approaches in a wind tunnel and in a numerical mesoscale model. For the experimental approaches in the wind tunnel, the combination of spires, roughness elements and a physical model generates a scaled boundary layer flow with velocity and turbulence characteristics that are consistent with microscale urban canopy flows in reality. A wind tunnel is comparable in resolution with an obstacle resolving microscale model, therefore data comparisons are frequently done for this scale. However, for many applications numerical models of 1 km resolution are used, resolving mesoscale atmospheric phenomena but not microscale ones. Parameterizations are then used to represent physical processes and obstacle influences on the atmospheres. Due to the coarse resolution, a direct comparison of mesoscale model results and wind tunnel is difficult.</p> <p>In this study, we use wind tunnel data as validation datasets to evaluate the urban canopy parameterization effects on airflow in a mesoscale model. We have developed a multi-layer urban canopy parameterization using nudging, implemented in the atmospheric model METRAS. The extended model is tested in an idealized case, in which the model domain is designed using realistic topographical data for the Hamburg city center but not resolving buildings. To simplify the city structure, two important canopy morphological parameters are used: building surface fraction and building height. Experiments with a similar model configuration were carried out in parallel in the Blasius wind tunnel facility of the Environmental Wind-Tunnel Laboratory of the University of Hamburg at a model scale of 1:500. Based on the realistic building surface fraction and building height, a pyramid-like model for the urban canopy is placed in the wind tunnel. The set-ups of the numerical model runs and the wind tunnel experiments are designed following two principles: first, keeping the set-up in both approaches as equivalent as possible, in terms of meteorological conditions, roughness lengths, simulation durations, etc.; secondly, taking into account the limitations of the microscale wind tunnel datasets and keeping as many characteristics of atmospheric processes as possible.</p> <p>The METRAS results show a good agreement with the wind tunnel datasets, in terms of representing building effects such as the reduction of mean wind speeds in the building wake, enhanced turbulence intensities and turbulent fluctuation characteristics for a sufficiently fine scale. However, for coarser resolution, the result comparability reduces and the agreement is less. Thus, we conclude that sub-grid scale canopy effects can be parameterized sufficiently well for their impacts on the average flow, but any detailed changes can only be simulated with a sufficiently high resolution.</p>

A Novel Multiphysics Multiscale Multiporosity Shale Gas Transport Model for Geomechanics/Flow Coupling in Steady and Transient States

Summary A novel multiphysics multiscale multiporosity shale gas transport (M3ST) model was developed to investigate shale gas transport in both transient and steady states. The microscale model component contains a kerogen domain and an inorganic matrix domain, and each domain has its own geomechanical and gas transport properties. Permeabilities of various shale cores were measured in the laboratory using a pulse decay permeameter (PDP) with different pore pressure and confining stress combinations. The PDP-measured apparent permeability as a function of pore pressure under two effective stresses was fitted using the microscale M3ST model component based on nonlinear least squares fitting (NLSF), and the fitted model parameters were able to provide accurate model predictions for another effective stress. The parameters and petrophysical properties determined in the steady state were then used in the transient-state,continuum-scale M3ST model component, which performed history matching of the evolutions of the upstream and downstream gas pressures. In addition, a double-exponential empirical model was developed as a powerful alternative to the M3ST model to fit laboratory-measured apparent permeability under various effective stresses and pore pressures. The developed M3ST model and the research findings in this study provided critical insights into the role of the multiphysics mechanisms, including geomechanics, fluid dynamics and transport, and the Klinkenberg effect on shale gas transport across different spatial scales in both steady and transient states.

Geometrical Assessment of Sunlit and Shaded Area of Urban Trees Based on Aligned Orthographic Views

To quantify the ecosystem services of trees in urban environments, it is necessary to assess received direct solar radiation of each tree. While the Sky View Factor (SVF) is suitable for assessing the total incoming short- and longwave radiation fluxes, its information is limited to specific points in space. For a spatial analysis, it is necessary to sample the area for SVF. A new geometrical method, Area View Factor (AVF), for the calculation of sunlit areas is proposed. AVF is the ratio of the unhidden, projected surface of an object to the whole projected surface of an object in a complex environment. Hereby, a virtual, orthographic camera is oriented in accordance to the sun’s position in the 3D model domain. The method is implemented in the microscale model SkyHelios, utilizing efficient rendering techniques to assess AVF of all urban trees in parallel. The method was applied to Rieselfeld in Freiburg, Germany. The assessed sunlit area is compared to the SVF at the top of each tree and solar altitude angle, revealing a strong relationship between sunlit areas to solar altitude angles. This study shows that AVF is an efficient methodology to assess received direct radiation of urban trees. Based on AVF, it is possible to identify urban areas with shaded and sunlit trees, but it can also be applied to other objects in complex environments. Therefore, AVF is applicable for urban architecture or energetic research questions.

Evaluating the Impact of a Wall-Type Green Infrastructure on PM10 and NOx Concentrations in an Urban Street Environment

Nature-based solutions can represent beneficial tools in the field of urban transformation for their contribution to important environmental services such as air quality improvement. To evaluate the impact on urban air pollution of a CityTree (CT), an innovative wall-type green infrastructure in passive (deposition) and active (filtration) modes of operation, a study was conducted in a real urban setting in Modena (Italy) during 2017 and 2018, combining experimental measurements with modelling system evaluations. In this work, relying on the computational resources of CRESCO (Computational Centre for Research on Complex Systems)/ENEAGRID High Performance Computing infrastructure, we used the air pollution microscale model PMSS (Parallel Micro-SWIFT-Micro SPRAY) to simulate air quality during the experimental campaigns. The spatial characteristics of the impact of the CT on local air pollutants concentrations, specifically nitrogen oxides (NOx) and particulate matter (PM10), were assessed. In particular, we used prescribed bulk deposition velocities provided by the experimental campaigns, which tested the CT both in passive (deposition) and in active (filtration) mode of operation. Our results showed that the PM10 and NOx concentration reductions reach from more than 0.1% up to about 0.8% within an area of 10 × 20 m2 around the infrastructure, when the green infrastructure operates in passive mode. In filtration mode the CT exhibited higher performances in the abatement of PM10 concentrations (between 1.5% and 15%), within approximately the same area. We conclude that CTs may find an application in air quality hotspots within specific urban settings (i.e., urban street canyons) where a very localized reduction of pollutants concentration during rush hours might be of interest to limit population exposure. The optimization of the spatial arrangement of CT modules to increment the “clean air zone” is a factor to be investigated in the ongoing development of the CT technology.

How to develop and apply a model data standard on microscale model data.

<p>Numerical modeling makes it possible to represent complex processes in small-scale and complex areas like cities. For resolving obstacles, grid sizes in the order of meters are needed. Due to small grid sizes and numerical restrictions, such high-resolution investigations require a great deal of resources. Therefore, a re-use of the results by others would enhance the value of any of these model results. However, the subsequent use of model results is still poorly developed. Comparisons of model data, dissemination of results, or reproduction of simulations are hampered by inconsistent data structures, non-standardized variable names, and lack of information on model setup. In general, to ensure the reusability and accessibility of model data, data standards should be used. The most common data standard for atmospheric model output data are the CF conventions, a data standard for netCDF files, but this standard is currently not extended to cover the model output of obstacle resolving models (ORM).</p><p>The AtMoDat (Atmospheric Model Data) project developed a model data standard (ATMODAT standard) which ensure FAIR (Findability, Accessibility, Interoperability, and Reuse) and well documented data. We involved the micro-scale modelling community in this process with a web based survey (http://uhh.de/orm-survey) to find out which micro-scale ORMs are currently in use, their model specifics (e.g. used grid, coordinate system), and the handling of the model result data. Furthermore, the survey provides the opportunity to include suggestions and ideas, what we should consider in the development of the standard. We already identified typical variables used by ORMs (i.e. building structures, wall temperatures) and will propose them to be included in the CF convention.  The application of this standard is tested on the model output of the ORM MITRAS. The standard and experiences with its application will be presented.</p>

Numerical Study on the Dependency of Microstructure Morphologies of Pulsed Laser Deposited TiN Thin Films and the Strain Heterogeneities during Mechanical Testing

Numerical study of the influence of pulsed laser deposited TiN thin films’ microstructure morphologies on strain heterogeneities during loading was the goal of this research. The investigation was based on the digital material representation (DMR) concept applied to replicate an investigated thin film’s microstructure morphology. The physically based pulsed laser deposited model was implemented to recreate characteristic features of a thin film microstructure. The kinetic Monte Carlo (kMC) approach was the basis of the model in the first part of the work. The developed kMC algorithm was used to generate thin film’s three-dimensional representation with its columnar morphology. Such a digital model was then validated with the experimental data from metallographic analysis of laboratory deposited TiN(100)/Si. In the second part of the research, the kMC generated DMR model of thin film was incorporated into the finite element (FE) simulation. The 3D film’s morphology was discretized with conforming finite element mesh, and then incorporated as a microscale model into the macroscale finite element simulation of nanoindentation test. Such a multiscale model was finally used to evaluate the development of local deformation heterogeneities associated with the underlying microstructure morphology. In this part, the capabilities of the proposed approach were clearly highlighted.

Applying urban climate model in prediction mode—evaluation of MUKLIMO_3 model performance for Austrian cities based on the summer period of 2019

Extreme heat events are natural hazards affecting many regions of the world. This study uses an example of the six largest cities in Austria to demonstrate the potential of urban climate model simulations applied in prediction mode providing detailed information on thermal conditions. For this purpose, the urban climate model MUKLIMO_3 of the German Meteorological Service (DWD) coupled with the hydrostatic numerical weather prediction model, ALARO, is used to simulate the development of the urban heat island (UHI) in Austrian cities for the summer period of 2019 with a horizontal resolution of 100 m. In addition to the evaluation of UHI predicting skills, other relevant variables, such as humidity and wind characteristics on hourly basis, are also analysed in this paper. Model evaluation confirmed that the MUKLIMO_3 microscale model had the capacity to simulate the main thermal spatiotemporal patterns in urban areas; however, a strong dependence on the input data from the mesoscale model was found. Our results showed large benefit in prediction of maximum air temperatures in urban areas, while the relative humidity predictions of MUKLIMO_3 appear to be much less plausible and show large variety of model prediction skills. Urban climate model simulations using real atmospheric conditions can facilitate better quantification and understanding of day-to-day intra-urban variations in microclimate as well as provide a basis for evaluation of the microclimate prediction skills of mesoscale numerical models with urban extensions.

Non-eddy-resolving modelling and parametrization of turbulent convection over sea ice leads and evaluation with airborne observations

<p><span><span>The polar ocean regions are characterised by a large variety of interactions between sea ice surfaces</span><span>, open water</span><span>, and the atmosphere. Especially between late autumn and spring, leads (open-water channels in sea ice) may play a crucial role within this system: Due to large temperature differences between the surface of leads and the near-surface atmosphere, strong turbulent convective plumes are generated with an enhanced turbulent transport of heat, moisture, and momentum. In consequence, lead-generated convection has a strong impact on the characteristics of the polar atmospheric boundary layer (ABL). </span></span></p><p><span><span>We apply a plume- but non-eddy-resolving, microscale model to study the convection over three different leads, which had been observed during the aircraft campaign STABLE over the Arctic Marginal Sea Ice Zone in March 2013. Model simulations are performed using a local and a non-local turbulence closure. The latter represents a lead-width-dependent approach for </span><span>the </span><span>turbulent fluxes </span><span>based on large eddy simulation </span><span>and it is</span><span> designed for an idealised, </span><span>lead-perpendicular</span><span>, and near-neutral inflow in an ABL of 300m </span><span>thickness</span><span>. </span><span>The observed cases from STABLE are also characterised by lead-perpendicular inflow conditions</span><span>, but the ABL is much shallower than in the ideali</span><span>s</span><span>ed cases and the inflow stratification is </span><span>partly</span><span> (slightly) stable. </span><span>Our main goal is to study the quality of both parametrizations and to evaluate, if the non-local parametrization shows advantages as compared to the local closure.</span></span></p><p><span><span>We show that the basic</span><span> observed features of the lead-generated convection are represented with both closures </span><span>despite some minor differences that will be explained</span><span>. However, the advantages of the non-local closure become clearly obvious by the physically more realistic representation of regions with observed vertical entrainment or where the observations hint at counter-gradient transport. Moreover, we also show that some weaknesses of the simulations can be </span><span>almost </span><span>overcome by introducing two further modifications </span><span>of</span><span> the non-local closure. We consider our results as another important step in the development of atmospheric turbulence parametrizations </span><span>for </span><span>non-eddy-resolving, microscale simulations of</span><span> strongly inhomogeneous convective </span><span>boundary layers</span><span>.</span></span></p>

Effect of Wet Deposition on Secondary Inorganic Aerosols Using an Urban-Scale Air Quality Model

We investigated the effects of wet deposition on secondary inorganic aerosols (SIAs) in urban areas by coupling the wet deposition module with the three-dimensional computational fluid dynamics atmospheric chemistry model (CFD-Chem). We developed a wet deposition model for the microscale model by improving on the global modeling initiative scheme. We evaluated the model by comparing it to the observed washout ratio from the total wet deposition. The simulated washout ratio calculated using the wet scavenging coefficient (WSC) based on the theoretical calculation is six times lower than that observed, suggesting that the wet deposition amount of SIAs from below-cloud scavenging might be underestimated. When we applied the WSC based on field measurements, the washout ratio was much improved; however, it was slightly overestimated compared to the observed rate. Therefore, we estimated the optimal WSC for SIAs in the urban area using a linear regression approach. We conducted a model using the wet deposition of SIAs in a megacity to understand the effects of wet deposition on the SIA concentration using estimated optimal WSCs. The simulated results indicate that washout processes decrease the surface aerosol concentration, showing that reductions in the average surface concentrations from washout processes were from 7.1% to 11.2%. The simulation results suggest that washout processes can reduce the particulate matter concentration in urban areas, indicating that washout processes should be considered in the microscale model, although the modeling domain can only simulate washout processes.