What distinguishes ECOSIM from existing systems is its capability to allow an integrated quantitative and qualitative analysis of the environment in urban and industrial areas across different environmental domains and sub-domains. In addition to the state of the environment in each domain, the interrelations between the domains and their dynamic behaviour is exploited. Therefore, the individual domains are linked together on large scales and the available multi-media data sources and modelling results are cross-calibrated.

For instance, the system at each local site includes the following numerical modelling tools, which are connected on-line to the users individual monitoring networks:

• meteorological forecasting model;
• air chemistry and dispersion model;
• ground and surface water quality model;
• coastal water pollution model.

Further details of the following models are available:

• MEMO - mesoscale atmospheric model (AUT)
• MODFLOW/MT3D - Groundwater flow and quality modeling (NTUA)
• DYMOS - air pollution dispersion and air chemistry model system (GMD)
• POM - Princeton Ocean Model (UA)

The generic client-server architecture makes it very easy to add any model with a similar structure, i.e., 2 and 3D spatially distributed dynamic simulation models.

MEMO

Introduction

The non-hydrostatic prognostic mesoscale model MEMO (Kunz and Moussiopoulos,1995) is a basic constituent of the European Zooming Model (EZM, previously called EUMAC Zooming Model; Moussiopoulos, 1994 and 1995). The EZM represents one of the most widely used European air quality model systems for urban scale applications (about 15 study cases in the last three years).

MEMO solves the conservation equation for mass, momentum and several scalar quantities in terrain-influenced coordinates. Non-equidistant grid spacing is allowed in all directions. The numerical solution is based on second- order discretization applied on a staggered grid. Special care is taken that conservative properties are preserved within the discrete model equations. The discrete pressure equation is solved with a direct elliptic solver in conjunction with a generalized conjugate gradient method (Flassak and Moussiopoulos, 1988).

Advective terms are treated in MEMO with a monotonicity- preserving discretization scheme with only small implicit diffusion. Turbulent diffusion is described with a two- equation turbulence model. Radiative transfer in the atmosphere is calculated with an efficient scheme based on the emissivity method for longwave radiation and an implicit multilayer method for shortwave radiation (Moussiopoulos, 1987). The surface temperature over land is computed from the surface heat budget equation. The soil temperature is calculated by solving a none- dimensional heat conduction equation for the soil. At lateral boundaries generalized radiation conditions are implemented. The current standard version of MEMO allows performing nested grid simulations.

Among other recent applications, MEMO was used in the study launched by the Greek Ministry of the Environment aiming to assess the environmental impact of constructing the New Athens Airport (Moussiopoulos et al., 1995). Furthermore, it was one of the models used in the "Seven- cities project" in the context of theAuto/Oil study placed by the DGXI of the European Union. Previous applications of MEMO are summarized by Moussiopoulos (1994).

References

1. Flassak Th. and Moussiopoulos N. (1988), Direct solution of the Helmholtz equation using Fourier analysis on the CYBER 205, Environ. Software 3, 12-16.
2. Kunz R. and Moussiopoulos N. (1995), Simulation of the wind field in Athens using refined boundary conditions, Atmospheric Environment 29, 3575-3591.
3. Moussiopoulos N., (1987), An efficient scheme to calculate radiative transfer in mesoscale models, Environ. Software 2, 172-191.
4. Moussiopoulos N., ed. (1994), The EUMAC Zooming Model, EUROTRAC Special Publication, ISS, Garmish- Partenkirchen.
5. Moussiopoulos N. (1995), The EUMAC Zooming Model, a tool for local-to-regional air quality studies, Meteorol. Atmos. Phys. 57, 115-133.
6. Moussiopoulos N., Sahm P., Gikas A., Karagiannidis A., Karatzas, K. and Papalexiou S. (1995), Analysis of air pollutant transport in the Athens basin and in the Spata area with a three-dimensional dispersion model, in Air Pollution III, Vol 3: Urban Pollution (N. Moussiopoulos, H. Power and C.A. Brebbia, eds) Computational Mechanics Publications, Southampton, 141-152.

MODFLOW/MT3D

The models MODFLOW and MT3D are being applied to the Ano Liosia Landfill in the Athens case study.

MODFLOW is a three-dimensional finite-difference ground-water flow model. It has a modular structure that allows it to be easily modified to adapt the code for a particular application.

MODFLOW simulates steady and nonsteady flow in an irregularly shaped flow system in which aquifer layers can be confined, unconfined, or a combination of confined and unconfined. Flow from external stresses, such as flow to wells, areal recharge, evapotranspiration, flow to drains, and flow through river beds, can be simulated. Hydraulic conductivities or transmissivities for any layer may differ spatially and be anisotropic (restricted to having the principal direction aligned with the grid axes and the anisotropy ratio between horizontal coordinate directions is fixed in any one layer), and the storage coefficient may be heterogeneous.

The model requires input of the ratio of vertical hydraulic conductivity to distance between vertically adjacent block centres. Specified head and specified flux boundaries can be simulated as can a head dependent flux across the model's outer boundary that allows water to be supplied to a boundary block in the modelled area at a rate proportional to the current head difference between a "source" of water outside the modelled area and the boundary block. MODFLOW is currently the most used numerical model in the U.S. Geological Survey for ground-water flow problems.

MT3D is a model for simulation of advection, dispersion and chemical reactions of contaminants in groundwater flow systems in either two or three dimensions. The model uses a mixed Eulerian-Lagrangian approach to the solution of the advective-dispersive-reactive equation, based on combination of the method of characteristics and the modified method of characteristics. This approach combines the strength of the method of characteristics for eliminating numerical dispersion and the computational efficiency of the modified method of characteristics.

The MT3D transport model was developed for use with any block-centered finite- difference flow model such as MODFLOW and is based on the assumption that changes in concentration field will not affect the flow field significantly. The MT3D transport model can be used to simulate changes in concentration of single-species miscible contaminants in groundwater considering advection, dispersion and some simple chemical reactions, with various types of boundary conditions and external sources or sinks. The chemical reactions included in the model are equilibrium-controlled linear or non-linear sorption and first-order irreversible decay or biodegradation. Currently, MT3D accommodates the following spatial discretization capabilities and transport boundary conditions:

1. confined, unconfined or variably confined/unconfined aquifer layers;
2. inclined model layers and variable cell thickness within the same layer;
3. specified concentration or mass flux boundaries; and
4. the solute transport effects of external sources and sinks such as wells, drains, rivers, areal recharge and evapotranspiration.

DYMOS - air pollution dispersion and air chemistry model system

At GMD FIRST the DYMOS system (Sydow, 1994) has been developed, a parallelly implemented air pollution simulation system for mesoscale applications.

DYMOS consists of three meteorology/transport models and one air chemistry model for the calculation of photochemical oxidants like ozone. The meteorology/transport models include REWIMET - a hydrostatic mesoscale Eulerian model with a low vertical resolution, GESIMA - a non-hydrostatic mesoscale Eulerian model with a high vertical resolution, and one Lagrangian model. The air chemistry model is CBM-VI dealing with 34 species in 82 reaction equations for simulating the photochemical processes in the lower atmosphere.

Air pollution simulations require extremely large amounts of computing time. In order to make the results of case studies available to users within an acceptable period of time or to enable a smog prediction to be made at all (computing time less than simulation period), the DYMOS system was parallelized (Schmidt and Haenisch, 1994). A message-passing version was developed from the sequential program and implemented on various parallel computers.

By means of the DYMOS system, contracted by the environmental department of the state government of Berlin and the ministry for environment of the state Brandenburg summer smog analyses were carried out concerning the duration of a measuring campaign in July 1994 (Mieth, Unger and Sydow, 1994).

Commissioned by Greenpeace the influence of emissions caused by traffic in Munich on the ozone concentration in the Munich area was analysed for a typical midsummer day in 1994 (Smid, 1996).

In the ECOSIM project the DYMOS models REWIMET, GESIMA and CBM-IV will be used for simulating the air pollution dispersion and air chemistry at the three validation sites. The necessary wind field input will be supplied by the MEMO model.

References

1. P. Mieth, S. Unger and A. Sydow, "Scenario Analysis of a Summer Smog Episode in Berlin", Proc. Second International Conference on Air Pollution, September 27 - 29, 1994, Barcelona, Spain.
2. M. Schmidt, R. Haenisch, "Implementation of an air pollution transport model on parallel hardware", Proc. International Conference on Massively Parallel Processing, June 21 - 23, 1994, Delft, The Netherlands.
3. K. Smid, "Cities cause ozone smog in rural areas", GMD-Spiegel, Special: Simulation Models, Sankt Augustin, 1996.
4. A. Sydow, "Parallel simulation of air pollution", In: K. Brunnstein and E. Raubold (eds.), 13th World Computer Congress 94, Volume 2, Elsevier Science B.V., North-Holland, 1994, pp. 605-612.

POM - Princeton Ocean Model

The Princeton Ocean Model (POM) is an ocean circulation numerical model designed by A.Blumberg and G.Mellor (1987) for both coastal and open ocean studies. It is a public domain model that is being used by a large number of research and academic institutes all over the world.

By June 1996 the POM users group had about 200 members. Due to its ability to simulate both shallow water and deep ocean dynamics, it has been used for a variety of applications ranging from small scale coastal management problems to general circulation studies of the Atlantic Ocean. POM is a three-dimensional, primitive equation numerical model.

The prognostic variables are the three components of the velocity U-V-W the temperature T and the salinity S fields. The equation of state is used for the computation of potential density. Two more prognostic equations are used to calculate turbulent kinetic energy and turbulent macroscale. These equations are part of the Mellor - Yamada 2.5 turbulence closure scheme used for the calculation of vertical diffusivity. Horizontal diffusivities are calculated according to the Smagorinsky formula. A set of vertical integrated equations of continuity and motion are also solved to provide free surface variations.

These equations usually called external mode, are solved with a small time step that obeys the CFL law; for computer time economy, the 3-D equations are solved with a different (larger) time step using the so-called time splitting technique. In the vertical, the model uses the sigma co-ordinate system which is the most appropriate for areas with significant topographic variability. The horizontal grid uses curvilinear orthogonal co-ordinates which are very useful in applications with complex coastline. The horizontal finite difference scheme is staggered, the so-called Arakawa C-grid.

Finally, the model can handle open boundaries through appropriate user defined boundary conditions. One of the most successful applications of POM is the East Coast Ocean Forecast System (ECOFS) that has been producing daily 24-hour forecasts on an operational basis for almost three years at NOAA's National Center for Environmental Prediction.

POM has been extensively used the last few years in a large number of EU funded (DGXII - MAST) research projects in European Seas (eg. MAST-0039-C(A), MAS2-CT93-0055, MAS2-CT94-0107). One of these projects, the MEDMEX (Mediterranean Model Evaluation Experiment) is comparing the most well know ocean circulation models, among them POM, using the Mediterranean sea as a test basin.

For more information, visit the POM home page at http://www.aos.princeton.edu/htdocs.pom/