LES validation of urban flow, part II: eddy statistics and flow structures

Time-dependent three-dimensional numerical simulations such as large-eddy simulation (LES) play an important role in fundamental research and practical applications in meteorology and wind engineering. Whether these simulations provide a sufficiently accurate picture of the time-dependent structure of the flow, however, is often not determined in enough detail. We propose an application-specific validation procedure for LES that focuses on the time dependent nature of mechanically induced shear-layer turbulence to derive information about strengths and limitations of the model. The validation procedure is tested for LES of turbulent flow in a complex city, for which reference data from wind-tunnel experiments are available. An initial comparison of mean flow statistics and frequency distributions was presented in part I. Part II focuses on comparing eddy statistics and flow structures. Analyses of integral time scales and auto-spectral energy densities show that the tested LES reproduces the temporal characteristics of energy-dominant and flux-carrying eddies accurately. Quadrant analysis of the vertical turbulent momentum flux reveals strong similarities between instantaneous ejection-sweep patterns in the LES and the laboratory flow, also showing comparable occurrence statistics of rare but strong flux events. A further comparison of wavelet-coefficient frequency distributions and associated high-order statistics reveals a strong agreement of location-dependent intermittency patterns induced by resolved eddies in the energy-production range. The validation concept enables wide-ranging conclusions to be drawn about the skill of turbulence-resolving simulations than the traditional approach of comparing only mean flow and turbulence statistics. Based on the accuracy levels determined, it can be stated that the tested LES is sufficiently accurate for its purpose of generating realistic urban wind fields that can be used to drive simpler dispersion models

LES validation of urban flow, part I: flow statistics and frequency distributions

Essential prerequisites for a thorough model evaluation are the availability of problem-specific, quality-controlled reference data and the use of model-specific comparison methods. The work presented here is motivated by the striking lack of proportion between the increasing use of large-eddy simulation (LES) as a standard technique in micro-meteorology and wind engineering and the level of scrutiny that is commonly applied to assess the quality of results obtained. We propose and apply an in-depth, multi-level validation concept that is specifically targeted at the time-dependency of mechanically induced shear-layer turbulence. Near-surface isothermal turbulent flow in a densely built-up city serves as the test scenario for the approach. High-resolution LES data are evaluated based on a comprehensive database of boundary-layer wind-tunnel measurements. From an exploratory data analysis of mean flow and turbulence statistics, a high level of agreement between simulation and experiment is apparent. Inspecting frequency distributions of the underlying instantaneous data proves to be necessary for a more rigorous assessment of the overall prediction quality. From velocity histograms local accuracy limitations due to a comparatively coarse building representation as well as particular strengths of the model to capture complex urban flow features with sufficient accuracy are readily determined. However, the analysis shows that further crucial information about the physical validity of the LES needs to be obtained through the comparison of eddy statistics, which is focused on in part II. Compared with methods that rely on single figures of merit, the multi-level validation strategy presented here supports conclusions about the simulation quality and the model's fitness for its intended range of application through a deeper understanding of the unsteady structure of the flow

Modelling short-term maximum individual exposure from airborne hazardous releases in urban environments. Part ΙI: Validation of a deterministic model with wind tunnel experimental data

The capability to predict short-term maximum individual exposure is very important for several applications including, for example, deliberate/accidental release of hazardous substances, odour fluctuations or material flammability level exceedance. Recently, authors have proposed a simple approach relating maximum individual exposure to parameters such as the fluctuation intensity and the concentration integral time scale. In the first part of this study (Part I), the methodology was validated against field measurements, which are governed by the natural variability of atmospheric boundary conditions. In Part II of this study, an in-depth validation of the approach is performed using reference data recorded under truly stationary and well documented flow conditions. For this reason, a boundary-layer wind-tunnel experiment was used. The experimental dataset includes 196 time-resolved concentration measurements which detect the dispersion from a continuous point source within an urban model of semi-idealized complexity. The data analysis allowed the improvement of an important model parameter. The model performed very well in predicting the maximum individual exposure, presenting a factor of two of observations equal to 95%. For large time intervals, an exponential correction term has been introduced in the model based on the experimental observations. The new model is capable of predicting all time intervals giving an overall factor of two of observations equal to 100%

Improved representation of particle size and solubility in model simulations of atmospheric dispersion and wet-deposition from Fukushima

Radionuclides released into the atmosphere following the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident were detected by ground-based monitoring stations worldwide. The inter-continental dispersion of radionuclides provides a unique opportunity to evaluate the ability of atmospheric dispersion models to represent the processes controlling their transport and deposition in the atmosphere. Co-located measurements of radioxenon (133Xe) and caesium (137Cs) concentrations enable individual physical processes (dispersion, dry and wet deposition) to be isolated. In this paper we focus on errors in the prediction of 137Cs attributed to the representation of particle size and solubility, in the process of modelling wet deposition. Simulations of 133Xe and 137Cs concentrations using the UK Met Office NAME (Numerical Atmospheric-dispersion Modelling Environment) model are compared with CTBTO (Comprehensive Nuclear-Test-Ban Treaty Organisation) surface station measurements. NAME predictions of 137Cs using a bulk wet deposition parameterisation (which does not account for particle size dependent scavenging or solubility) significantly underestimate observed 137Cs. When a binned wet deposition parameterisation is implemented (which accounts for particle size dependent scavenging) the correlations between modelled and observed air concentrations improve at all 9 of the Northern Hemisphere sites studied and the respective RMSLE (root-mean-square-log-error) decreases by a factor of 7 due to a decrease in the wet-deposition of Aitken and Accumulation mode particles. Finally, NAME simulations were performed in which insoluble submicron particles are represented. Representing insoluble particles in the NAME simulations improves the RMSLE at all sites further by a factor of 7. Thus NAME is able to predict 137Cs with good accuracy (within a factor of 10 of observed 137Cs values) at distances greater than 10,000 km from FDNPP only if insoluble submicron particles are considered in the description of the source. This result provides further evidence of the presence of insoluble Cs-rich microparticles in the release following the accident at FDNPP and suggests that these small particles travelled across the Pacific Ocean to the US and further across the North Atlantic Ocean towards Europe

High-resolution global climate simulations: representation of cities

Ensemble runs of high-resolution (~10 km; N1280) global climate simulations (2005–2010) with the Met Office HadGEM3 model are analysed over large urban areas in the south-east UK (London) and south-east China (Shanghai, Hangzhou, Nanjing region). With a focus on urban areas, we compare meteorological observations to study the response of modelled surface heat fluxes and screen-level temperatures to urbanisation. HadGEM3 has a simple urban slab scheme with prescribed, globally fixed bulk parameters. Misrepresenting the magnitude or the extent of urban land cover can result in land-surface model bias. As urban land-cover fractions are severely under-estimated in China, this impacts surface heat-flux partitioning and quintessential features such as the urban heat island. Combined with the neglect of anthropogenic heat emissions, this can result in misrepresentation of heat-wave intensities (or cold spells) in cities. The model performance in urban areas could be improved if bulk parameters are modelled instead of prescribed, but this necessitates the availability of local morphology data on a global level. Improving land-cover information and providing more flexible ways to account for differences between cities (e.g. anthropogenic emission; morphology) is essential for realistic future projections of city climates, especially if model output is intended for urban climate services

Characterizing physical and social composition of cities to inform climate adaptation: case studies in Germany

Cities are key to climate change mitigation and adaptation in an increasingly urbanized world. As climate, socio-economic and physical compositions of cities are constantly changing, these need to be considered in their urban climate adaptation. To identify these changes, urban systems can be characterized by physical, functional and social indicators. Multi-dimensional approaches are needed to capture changes of city form and function, including patterns of mobility, land use, land cover, economic activities, and human behaviour. In this paper we examine how urban structure types provide one way to differentiate cities in general and to what extent socio-economic criteria have been considered regarding the characterisation of urban typologies. In addition, we analyse how urban structure types are used in local adaptation strategies and plans to derive recommendations and concrete targets for climate adaptation. To do this, we examine indicators, background data used, and cartographic information developed for and within such urban adaptation plans, focusing in particular on the German cities of Karlsruhe and Berlin. The comparative analysis provides new insights into how present adaptation plans consider physical and social structures including issues of human vulnerability within cities. Based on the analysis we make recommendations on how to improve the consideration of both physical and socio-economic aspects of a city to support pathways for adaptatio

Developing a research strategy to better understand, observe, and simulate urban atmospheric processes at kilometer to subkilometer scales

A Met Office/Natural Environment Research Council Joint Weather and Climate Research Programme workshop brought together 50 key international scientists from the UK and international community to formulate the key requirements for an Urban Meteorological Research strategy. The workshop was jointly organised by University of Reading and the Met Office

Urban signals in high-resolution weather and climate simulations: role of urban land-surface characterisation

Two urban schemes within the Joint UK Land Environment Simulator (JULES) are evaluated offline against multi-year flux observations in the densely built-up city centre of London and in suburban Swindon (UK): (i) the 1-tile slab model, used in climate simulations, (ii) the 2-tile canopy model MORUSES (Met Office–Reading Urban Surface Exchange Scheme), used for numerical weather pre- diction over the UK. Offline, both models perform better at the suburban site, where differences between the urban schemes are less pronounced due to larger vegetation fractions. At both sites, the outgoing short- and longwave radiation is more accurately represented than the turbulent heat fluxes. The seasonal varia- tions of model skill are large in London, where the sensible heat flux in autumn and winter is strongly under-predicted if the large city-centre magnitudes of anthro- pogenic heat emissions are not represented. The delayed timing of the sensible heat flux in the 1-tile model in London results in large negative bias in the morning. The partitioning of the urban surface into canyon and roof in MORUSES improves this as the roof-tile is modelled with a very low thermal inertia, but phase and amplitude of the gridbox-averaged flux critically depend on accurate knowledge of the plan-area fractions of streets and buildings. Not representing non-urban land- cover (e.g. vegetation, inland water) in London results in severely under-predicted latent heat fluxes. Control runs demonstrate that the skill of both models can be greatly improved by providing accurate land-cover and morphology information and using representative anthropogenic heat emissions, which is essential if the model output is intended to inform integrated urban services

Evaluation of fast atmospheric dispersion models in a regular street network

The need to balance computational speed and simulation accuracy is a key challenge in designing atmospheric dispersion models that can be used in scenarios where near real-time hazard predictions are needed. This challenge is aggravated in cities, where models need to have some degree of building-awareness, alongside the ability to capture effects of dominant urban flow processes. We use a combination of high-resolution large-eddy simulation (LES) and wind-tunnel data of flow and dispersion in an idealised, equal-height urban canopy to highlight important dispersion processes and evaluate how these are reproduced by representatives of the most prevalent modelling approaches: (i) a Gaussian plume model, (ii) a Lagrangian stochastic model and (iii) street-network dispersion models. Concentration data from the LES, validated against the wind-tunnel data, were averaged over the volumes of streets in order to provide a high-fidelity reference suitable for evaluating the different models on the same footing. For the particular combination of forcing wind direction and source location studied here, the strongest deviations from the LES reference were associated with mean over-predictions of concentrations by approximately a factor of 2 and with a relative scatter larger than a factor of 4 of the mean, corresponding to cases where the mean plume centreline also deviated significantly from the LES. This was linked to low accuracy of the underlying flow models/parameters that resulted in a misrepresentation of pollutant channelling along streets and of the uneven plume branching observed in intersections. The agreement of model predictions with the LES (which explicitly resolves the turbulent flow and dispersion processes) greatly improved by increasing the accuracy of building-induced modifications of the driving flow field. When provided with a limited set of representative velocity parameters, the comparatively simple street-network models performed equally well or better compared to the Lagrangian model run on full 3D wind fields. The study showed that street-network models capture the dominant building-induced dispersion processes in the canopy layer through parametrisations of horizontal advection and vertical exchange processes at scales of practical interest. At the same time, computational costs and computing times associated with the network approach are ideally suited for emergency-response applications

On aspects of large-eddy simulation validation for near-surface atmospheric flows

The availability of suitable and reliable reference data together with the application of modelspecific comparison methods are the essential ingredients to establish confidence in the capabilities of a numerical model and to truly assess its strengths and limitations. This thesis is motivated by the striking lack of proportion between the increasing use of large-eddy simulation (LES) as a standard modeling technique in micro-meteorological research as opposed to the level of scrutiny that is commonly applied to the quality of the generated numerical predictions. With this study, I suggest and apply a novel validation strategy for LES consisting of a multilevel hierarchy of comparative analysis methods. Unlike standard LES validation procedures that are based on the comparison of low-order statistical moments, the new approach advocated here specifically aims at the time-dependent nature of the problem. The sequence in which statistical quantities are compared mirrors the increase of information provided by the analysis methods. The target area is turbulent flow in the near-surface atmospheric boundary layer. The test scenario for the validation approach is urban flow in the city of Hamburg, Germany. Qualified reference data are generated in the boundary-layer wind tunnel facility at the University of Hamburg through high-resolution flow measurements in a scale-reduced model. Fine-meshed numerical simulations are conducted at the U.S. Naval Research Laboratory in Washington, D.C., with implicit LES. On the basis of an initial exploratory data analysis of mean flow and turbulence statistics, a high level of agreement between simulation and experiment is apparent. Inspecting frequency distributions of the underlying instantaneous data, however, proves to be necessary for a more rigorous assessment of the overall prediction quality. From histograms, local accuracy limitations caused by under-resolution as well as particular strengths of the model to capture complex urban flow features are readily determined. Further crucial information about the physical validity of the LES need to be obtained from eddy statistics. Comparisons of temporal autocorrelations, integral time scales, and auto-spectral energy densities show that the simulation reliably reproduces statistical characteristics of the energy and flux-carrying roughness sublayer structures. At higher elevations, however, inflow generation artifacts are reflected in dubiously short fluctuation time scales and energy peaks that are dislocated toward high frequencies. With the comparison of scale-dependent flow statistics, to which the preceding diagnostics have been blind, the emphasis eventually shifts to structure identification. The quadrant analysis of the vertical turbulent momentum flux discloses strong similarities between ejection-sweep patterns and the occurrence of rare, but extreme, flux events in roof-level vicinity and above the canopy layer. Further scale-wise comparisons of wavelet-coecient frequency distributions and associated high-order statistics reveal consistent location-dependent intermittency patterns induced by eddies in the energy-production range. Compared with usual methods that rely on single figures of merit, the detailed, multi-level validation strategy presented in this thesis allows to draw more wide-ranging and tenable conclusions about the quality of the simulation and to specify the model’s fitness for purpose in greater detail. The proposed validation concept has the potential to be used as a starting point for communitywide activities aiming at the formulation and harmonization of best-practice standards for the quality assurance of micro-meteorological eddy-resolving simulations