Ozone Modeling: Background
From EPA Ozone modeling guidance:
Mean Normalized Bias (MNB)
This performance statistic averages the model/observation residual, paired in time, normalized by observation, over all monitor times/locations. A value of zero would indicate that the model over predictions and model under predictions exactly cancel each other out.Oil and gasMean Normalized Gross Error (MNGE)
This performance statistic averages the absolute value of the model/observation residual, paired in time, normalized by observation, over all monitor times/locations. A value of zero would indicate that the model exactly matches the observed values at all points in space/time. Biogenics
In EPA guidance for the 1-hour ozone attainment demonstrations (U.S. EPA, 1991a), several statistical goals were identified for operational model performance. These goals were identified by assessing past modeling applications of ozone models and determining common ranges of bias, error, and accuracy (Tesche et al., 1990). The 1-hour guidance noted that because of differences in the quality of the applications considered, it was inappropriate to establish "rigid criterion for model acceptance or rejection" (i.e., no pass/fail test). It was recommended that these ranges should be used in conjunction with the additional qualitative procedures to assess overall model performance.
With the additional experience of another decade of photochemical modeling, it is clear that there is no single definitive test for evaluating model performance. All of the tests identified in Sections 15.2 and 15.3 have strengths and weaknesses. Further, even within a single performance test, it is not appropriate to assign “bright line” criteria that distinguish between adequate and inadequate model performance. In this regard, EPA recommends that a “weight of evidence” approach (like that described in Section 4) be used to determine whether a particular modeling application is valid for assessing the future attainment status of an area.
Traditionally, the statistical range for adequate performance of a photochemical model was a MNB value of <= 15% and MNGE of <= 25%. These ranges were based on the experience actual model performance over years by different agencies modeling a variety of ozone conditions.
- Section 15: Guidance on the Use of Models and Other Analyses in Attainment Demonstrations for the 8-hour Ozone NAAQS Model Sub-domain
- Summary Statistics for June1- July 31, 2002
- Observed Ozone vs. Modeled Ozone June 1-July 31, 2002
