Below follows a description of the OML model. Alternative sources of information:
Contents:
Model name
OML Acronym for the Danish term "Operationelle Meteorologiske Luftkvalitetsmodeller"
Contact address
Aarhus University
Department of Environmental Science
P.O. Box 358
DK-4000 Roskilde
Denmark
Att. Per Løfstrøm or Anne Sofie Lansø
E-mail: oml@envs.au.dk
URL: http://www.au.dk/oml-international
Model versions and status
The following version is currently distributed:
OML-Multi 7.0, Windows (in Danish and English). Released August 2020.
Intended field of application
The OML model is a modern Gaussian plume model intended to be used for distances up to about 20 km from the source. The source is typically one or more stacks, and possibly also area sources. Typically, the OML model is applied for regulatory purposes. In particular, it is the recommended model to be used for environmental impact assessments when new industrial sources are planned in Denmark. The model can be used for both high and low sources; it is not suitable for complex terrain conditions. The model requires information on emission and meteorology on an hourly basis. It computes a time series of concentrations at user-specified receptor points, from which statistics are extracted and presented to the user.
Model type and dimension
OML is a modern Gaussian plume model, based on boundary layer scaling instead of relying on Pasquill stability classification. It belongs to the same class of models as UK-ADMS, AERMOD and HPDM.
Model description summary
The model describes dispersion of a passive, or possibly buoyant, gas from a number of sources. It is characteristic for the OML model that it does not use traditional discrete stability categories, but instead describes dispersion processes in terms of basic boundary-layer scaling parameters, such as friction velocity, Monin-Obukhov length, and the convective velocity scale. Thus, before being used by the model, meteorological measurements must be processed by a pre-processor. In the OML model, the Gaussian dispersion parameters sigma-y and sigma-z are not - as in conventional operational models - functions only of stability category and distance from the source. Instead, they are continuous functions of several boundary layer parameters. The dispersion parameters are regarded as the result of contributions from several mechanisms: convective turbulence, mechanical turbulence, plume buoyancy and building downwash. Their dependence on source height is taken explicitly into account. The plume rise is modelled by methods proposed by Briggs (1984) supplemented by a number of extensions. In contrast to most conventional models, penetration of the plume into the atmosphere above the mixing layer is not simulated as an on/off process. Instead, the extent of plume penetration is considered.
Limitations
The model is Gaussian. It thus essentially assumes that the plume has a straight centre line, pointing along the assigned wind direction. The concentration distribution is Gaussian, both horizontally and vertically. Under low wind conditions, a Gaussian model does not perform well, because the basic assumptions underlying the model are violated. The model assumes stationary conditions.
In its current version, the model does not take account of deposition.
The model calculates hourly averaged concentration values. Conversion to shorter averaging times is not simple.
The model does not account for changes in turbulence regimes acting on a plume due to changes in surface characteristics, e. g. from land to water or vice versa.
The model is not a complex terrain model, although it does include some simple algorithms to describe dispersion over slightly hilly terrain.
The handling of building effects is based on simple methods, whereas in reality, aerodynamics in the wake of a building is an extremely complex matter. The primary intent of the building effect algorithm used in OML-Point is to improve concentration estimates applicable for distances beyond ca. five building heights downwind. Concentration estimates close to buildings should not be considered reliable.
Resolution
Temporal resolution
The model is designed to work with input and output in the form of one-hour averages
Horizontal resolution
Concept not applicable (the model is a Gaussian plume model)
Vertical resolution
Concept not applicable (the model is a Gaussian plume model)
Schemes
Advection
Gaussian plume
Turbulence
Boundary layer scaling
Deposition
The version 6.0 does not account for deposition. Version 6.2 performs very simple and conservative estimates based on average concentrations (without deposition) and an average deposition velocity supplied by the user. Version 7.0 estimates ammonia deposition from animal housing located in Denmark with outlet heights less than 15 m, and is a simplified version of OML-DEP. This part of deposition interface in version 7.0 is in Danish only.
Chemistry
None
Solution technique
Not applicable (Gaussian plume model)
Input requirements
Emissions
Specify emission strengths etc. for point sources and area sources. Further indicate data concerning buildings close to the source (in order to estimate building downwash).
Meteorology
Before being used by the model, meteorological measurements must be processed by a pre-processor, the OML meteorological preprocessor.For use in Denmark, processed meteorological data are available off-the-shelf for many locations. The OML meteorological preprocessor has typically as input hourly meteorological measurements from a synoptic or analogous surface station, and twice-daily vertical profiles of temperature from a nearby radiosonde station. Output is in this case hourly values of turbulence parameters: most essentially sensible heat flux, Monin-Obukhov length, friction velocity and mixing height. More specialised versions of the preprocessor have been designed for non-standard input such as mast measurements instead of synoptic surface data. The main elements of the meteorological preprocessor have been documented in a number of publications (e.g. Berkowicz, R. and Prahm, L.P. (1982): Sensible heat flux estimated from routine meteorological data by the resistance method. J.App.Met. 21, 1845-1864; and:Olesen, H.R. and Brown, N. (1992, 2. edition): The OML meteorological preprocessor - a software package for the preparation of meteorological data for dispersion models. MST LUFT-A 122. National Environmental Research Institute, DK-4000 Roskilde, Denmark.)
Topography
Specified in the form of terrain heights at receptor locations.
Initial conditions
Not applicable
Boundary conditions
Not applicable
Other input requirements
Receptor data: position and height of receptors
Output quantities
The usual output from a model run is a number of summary tables. OML-Multi allows far more flexibility in output than OML-Point. Output tables include averages, 99-percentiles and many other statistics related to EU limit values. Tables can be produced on a monthly, yearly or overall basis, calculated at each receptor. OML-Multi can display the results in the form of simple maps. Further, the tables may be exported and subjected to further data processing such as graphical presentation etc. by third party software.
User interface availability
The OML-Multi model is designed for the Windows operating system (any version of Windows). OML-Multi allows the user to choose either English or Danish language for the menus and help text.
User community
The OML model is being widely used in Denmark by non-expert users in local environmental agencies, by consulting engineers and by large industries. If the meteorological input is available, basic use of the model can be mastered in an hour of work. Familiarity with the model requires somewhat more. Users outside Denmark will normally need processed meteorological data from local stations. The preprocessing of meteorological data is a rather specialised task, which involves running the preprocessor, and possibly adapting it to local conditions.
Previous applications
The model is being widely applied in Denmark, and has also found use in some other countries. In Sweden, the Swedish Meteorological and Hydrological Institute (SMHI) maintains a model based on the same core code as OML.
Documentation status
The various parts of the model and the associated meteorological preprocessor have been described in many publications, the most detailed of which are reports by the Danish National Environmental Research Institute. A brief user's guide accompanies the model, and the model includes a comprehensive help text system. Detailed descriptions of the model are given by Olesen et al. (2007a), Berkowicz et al. (1986) and Olesen et al. (1992). General descriptions of the model and the contexts in which it is used is given by Olesen (1995).
Validation and evaluation
Model validation against reference dataset
The model has been tested against experimental data sets from
Results of the evaluations performed with three of these data sets have been reported in a European model evaluation exercise in 1994 which allowed several models to be compared on a similar basis, using the so-called "Model Validation Kit":
Olesen, H.R., 1995, The model validation exercise at Mol. Overview of results. Workshop on Operational Short-range Atmospheric Dispersion Models for Environmental Impact Assessment in Europe, Mol, Belgium, Nov. 1994, Int. J. Environment and Pollution, Vol. 5, Nos. 4-6, pp. 761-784.
Computer requirements
CPU time
The model can run on any Windows PC. As a guide to the computational resources required, the computer time used by OML-Multi for calculations with one source in a net of 400 receptor points and with one year of meteorology is on the order of one second with a modern PC.
Availability
The following versions exist:
The price of OML-Multi is Dkr 18900 excl. VAT (approximately 2550 Euro). A demo version of OML-Multi (with limited lifetime) is available upon request.
References
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