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Cooper and Caulcott Ltd
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How Uncertain are Numerical Models?
Neill Cooper
email: webnsc@cplusc.co.uk
Overheads of a talk given to the NSCA Dispersion Model
User Group on 11th May, 2000
Background
Issues
- Models are approximate
- Data is approximate
- Validation data is sparse
- The atmosphere is turbulent
- Plume behaviour is unpredictable
- Modelling of single episodes is important
Sources of
uncertainty include
- Input Data Errors
- Insufficient data
- Meteorology
- Emission data (Talk 2)
- Atmospheric Turbulence
- Model Simplifications
- Future weather
- Spatial and Temporal Averaging
Categories
of uncertainty
- Parameter Uncertainty
- Conceptual Model Uncertainty
- Scenario (Future) Uncertainty
- Variability (Turbulence)
Parameter
Uncertainty
- Tests with a simple ‘R91’ type model
- Undertake a Monte Carlo study
Uncertainty
Study: input values
- Source Q : 1.0 ±
20%
- 10m Wind speed u : 5 ± 2 ms-1
- Boundary layer height A : 750±
250m
- Roughness length z0 : 0.1 to 0.4m
- Stability category : B C or D
- Effective source height z : 25
to 50 m
- Receptor height H : 10m ±
5m
- Number of runs : 100
Uncertainty
Study: results
- Concentration 100m downwind varies by a factor
of 17.
- Variation 1 and 10 km downwind about a factor
of 5
- Important parameters:
- 100 m: source height, receptor
height
- 1 km: wind speed
- 10 km: wind speed; boundary layer
height
Conceptual
Model Uncertainty
- Most models use a Gaussian cross-section for
plumes.
- Model intercomparisons show large differences.
Scenario
(Future) Uncertainty
Variability
- Turbulence
Quantifying
uncertainty
- Parameter Uncertainty - gives a factor of 2 on
the concentration from a single plume
- Conceptual Model Uncertainty - ?
- Scenario (Future) Uncertainty - ?
- Variability (Turbulence) - factor of 2
How accurate
are models?
- QQ plots show excellent agreement
- Scatter plots show poorer agreement
- Validation of ADMS and AERMOD give
average differences of about a factor of 2 for single, short term
releases.
Conclusions
For a single release, the
uncertainty in numerical model results is about a factor of 2 or 3.
© 2000 Neill S Cooper
Last changed on June 12, 2000
Classification of the uncertainty in numerical models can be found
here.
Estimates of the uncertainty in atmospheric dispersion models can
be found here.
Atmospheric Dispersion Modelling page
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