S. Trini Castelli

Application of a model system for the study of transport and diffusion in complex terrain to the TRACT experiment

Abstract The transport and diffusion processes of a tracer gas released near the ground in the Rhine valley region, in Central Europe, during the 1992 TRACT field experiment, are simulated by a computational model system for complex terrain. This system (RMS) is composed of the prognostic mesoscale model RAMS, the Lagrangian stochastic dispersion model SPRAY and the interface code MIRS, which links RAMS to SPRAY. Three flow simulations were performed, with different initialisations and the one showing the best agreement with the measured flow was selected for the simulation of the TRACT tracer experiment. Tracer concentrations measured by an array of samplers at ground level and by an airplane aloft, are used to evaluate the 3-D concentration field simulated by the model system. The analysis of the simulation results generated by RMS shows that our model system very well reproduces the general behaviour of the contaminant plume, the temporal and spatial distribution of the concentration and the location of the concentration maxima.

Turbulence fields for atmospheric dispersion models in horizontally non-homogeneous conditions

Abstract When atmospheric pollutant dispersion is simulated over complex terrain, the turbulence input parameters are often prescribed according to standard parameterisation based on surface layer quantities. These last are available from literature. In the past, several different parameterisations for the first few moments of the turbulence velocity statistic and for the Lagrangian time scale have been developed and tested in different stability conditions. The main shortcomings for using these parameterisations are their inadequateness in predicting the turbulence field in horizontally non-homogeneous boundary layer (like for example in complex terrain or in urban heat island) and the essentially local nature of the prescribed turbulence. The purpose of this work is to suggest proper methods for predicting turbulence field for dispersion model over complex terrain and, more generally, in horizontally non-homogeneous conditions. The modelling system RMS (RAMS–MIRS–SPRAY) is applied to the present study. We introduced new non-local turbulence closures in the RAMS circulation model, which provides both mean wind and turbulence fields. Then, through the interface code (MIRS), the input quantities needed to the Lagrangian stochastic dispersion model (SPRAY) are computed. The model system is validated against a tracer experiment performed in a wind tunnel over a schematic two-dimensional valley.

Evaluation of the environmental impact of two different heating scenarios in urban area

In this work, the impact of two different house-heating systems on pollution scenarios in the Turin town centre is evaluated by means of the three-dimensional modelling system RMS. The first scenario (present situation) refers to the NOx ground level concentration due to an existing Combined Heat and Power Plant (CHP-plant) and to domestic heating, whereas the second one (future situation) considers the possible advantages of substituting the domestic heating of a central town district with a second Combined Heat and Power Plant to be installed in the district itself. A five consecutive days simulation has been carried out in a particularly severe dispersion situation, characterised by thermodynamic stability and low wind conditions. This period was selected on the basis of in situ observations and its choice was then confirmed as appropriate by means of a statistical analysis of the meteorological model outputs. The main result of the study is that hourly NOx maxima g.l.c. in the centrally heated district is lower in the future scenario than in the present one; while maxima keep approximately the same in the rest of the town. Then, top-ten ground level concentrations of daily maxima over all the domain decrease in the new scenario.

Comparison of atmospheric modelling systems simulating the flow, turbulence and dispersion at the microscale within obstacles

Three different modelling techniques to simulate the pollutant dispersion in the atmosphere at the microscale and in presence of obstacles are evaluated and compared. The Eulerian and Lagrangian approaches are discussed, using RAMS6.0 and MicroSpray models respectively. Both prognostic and diagnostic modelling systems are considered for the meteorology as input to the Lagrangian model, their differences and performances are investigated. An experiment from the Mock Urban Setting Test field campaign observed dataset, measured within an idealized urban roughness, is used as reference for the comparison. A case in neutral conditions was chosen among the available ones. The predicted mean flow, turbulence and concentration fields are analysed on the basis of the observed data. The performances of the different modelling approaches are compared and their specific characteristics are addressed. Given the same flow and turbulence input fields, the quality of the Lagrangian particle model is found to be overall comparable to the full-Eulerian approach. The diagnostic approach for the meteorology shows a worse agreement with observations than the prognostic approach but still providing, in a much shorter simulation time, fields that are suitable and reliable for driving the dispersion model.

Application of a modified version of RAMS model to simulate the flow and turbulence in the presence of buildings: the MUST COST732 exercise

A modified version of the atmospheric model RAMS6.0 is used to simulate the flow in the MUST experiment, within the framework of the COST732 Action. A standard version of the k-e turbulence closure model and its renormalisation group version, RNG k-e, were implemented in the model. Simulations were performed to test the suitability of using the model to reproduce the flow and turbulence in the presence of buildings in a complex configuration. Wind speed and turbulent kinetic energy profiles from measurements and simulations with different turbulence schemes were compared. Preliminary results emphasise the difference between the different turbulence schemes.

Proposal of a new Lagrangian particle model for the simulation of dense gas dispersion

A Lagrangian particle dispersion model for dense gas dispersion is proposed. It is a new version of MicroSpray oriented to simulate dense gas dispersion in urban or industrial environment, and includes the treatment of obstacles, complex terrain and low wind. The dense gas descent is computed by a 3D dynamical plume rise/descent model, the gravity spreading and bottom boundary condition are accounted for by empirical formulations. The differences between simulations obtained with the new model and with the standard one are presented. The model is validated against tracer field data gathered in the Thorney Island experiment 8.

Turbulence closure in atmospheric circulation model and its influence on the dispersion

In this work the comparisons between measured and numerically simulated tracer experiments are presented for two different datasets. Different versions of the model system RMS, including the Regional Atmospheric Modelling System (RAMS) and the dispersion model SPRAY, accounting for alternative turbulence closures are used. These last are the Mellor and Yamada model level 2.5 and two versions of the E-l model, the standard isotropic and anisotropic versions. Moreover, simulations performed with the CALPUFF dispersion model coupled with RAMS are also presented. The wind fields and the ground level concentrations produced by the two model systems are shown and compared.

A dispersion study of the aerosol emitted by fertiliser plants in the region of Serra do Mar Sierra, Cubatao, Brazil

The dispersions of inhalable particulate matter emitted by the fertiliser plants at Cubatao (Brazil) were simulated using the Lagrangian dispersion model SPRAY. Wind fields and other meteorological variables were obtained with the model RAMS. The results of these simulations were compared with the source apportionment performed by chemical mass balance on samples of the local aerosol.

Development of a Lagrangian Particle Model for Dense Gas Dispersion in Urban Environment

A new version of the Lagrangian particle model MicroSpray is proposed. It simulates the dense gas dispersion in situations characterized by the presence of buildings, other obstacles, complex terrain, and possible occurrence of low wind speed conditions. Its performances are compared to an atmospheric CFD model output and to a field experiment (Thorney Island).

Development and Application of MicroRMS Modelling System to Simulate the Flow, Turbulence and Dispersion in the Presence of Buildings

A modelling system for the simulation of the flow and dispersion from the mesoscale down to the urban microscale is under development. This modelling system is a microscale version of the regional off-line system RMS (RAMS-MIRS-SPRAY) — MicroRMS. A modified version of RAMS6.0 is used, in which a Cartesian grid and the ADaptive Aperture method are implemented for defining the presence of buildings in arbitrarily steep topography and where alternative versions of the k-e turbulence closure model were incorporated. After RAMS, the Lagrangian stochastic dispersion model MicroSPRAY is applied, specially devoted to simulate accidental gaseous releases at microscales, including the presence of obstacles and buildings. At present, the efforts are focused on the development of a micro-version of the interface code MIRS, calculating the surface and boundary layers’ parameters and the Lagrangian variables. The first step in the project was to harmonize the treatment of buildings between RAM6.0 and MicroSPRAY approaches. Here we present the first tests of MicroRMS prototype, applied to the MUST exercise of Cost732 Action, a flow and dispersion field test carried out in the Great Basin Desert (USA) in 2001, where 120 standard containers were set up in a regular array of obstacles.