Documentation of the Evaluation of CALPUFF and Other Long ...

The main conclusion of the SRL75 CALPUFF evaluation is that the fitted Gaussian plume
evaluation approach can be a poor and misleading indicator of LRT dispersion model
performance. In fact, the whole concept of a well‐defined Gaussian plume at far downwind
distances (e.g., > 50 km) is questionable since wind variations and shear can destroy the
Gaussian distribution. Thus, we recommend that future studies no longer use the fitted
Gaussian plume evaluation methodology for evaluating LRT dispersion models and adopt
alternate evaluation approaches that are free from a priori assumption regarding the
distribution of the observed tracer concentrations.
Cross Appalachian Tracer Experiment (CAPTEX)
The Cross Appalachian Tracer Experiment (CAPTEX) performed five tracer releases from either
Dayton, Ohio or Sudbury, Ontario with tracer concentrations measured at hundreds of
monitoring sites deployed in the northeastern U.S. and southeastern Canada out to distances of
1000 km downwind of the release sites. Numerous CALPUFF sensitivity tests were performed
for the third (CTEX3) and fifth (CTEX5) CAPTEX tracer releases from, respectively, Dayton and
Sudbury. The performance of the six LRT models was also intercompared using the CTEX3 and
CTEX5 field experiments.
CAPTEX Meteorological Modeling
MM5 meteorological modeling was conducted for the CTEX3 and CTEX5 periods using modeling
approaches prevalent in the 1980’s (e.g., one 80 km grid with 16 vertical layers) that was
sequentially updated to use a more current MM5 modeling approach (e.g., 108/36/12/4 km
nested grids with 43 vertical layers). The MM5 experiments also employed various levels of
four dimensional data assimilation (FDDA) from none (i.e., forecast mode) to increasing
aggressive use of FDDA.
CALMET sensitivity tests were conducted using 80, 36 and 12 km MM5 data as input and using
CALMET grid resolutions of 18, 12 and 4 km. For each MM5 and CALMET grid resolution
combination, additional CALMET sensitivity tests were performed to investigate the effects of
different options for blending the meteorological observations into the CALMET STEP1 wind
fields using the STEP2 objective analysis (OA) procedures to produce the wind field that is
provided as input to CALPUFF:
• A – RMAX1/RMAX2 = 500/1000
• B – RMAX1/RMAX2 = 100/200
• C – RMAX1/RMAX2 = 10/100
• D – no meteorological observations (NOOBS = 2)
Wind fields estimated by the MM5 and CALMET CTEX3 and CTEX5 sensitivity tests were paired
with surface wind observations in space and time, then aggregated by day and then aggregated
over the modeling period. The surface wind comparison is not an independent evaluation since
many of the surface wind observations in the evaluation database are also provided as input to
CALMET. Since the CALMET STEP2 OA procedure is designed to make the CALMET winds at the
monitoring sites better match the observed values, one would expect CALMET simulations
using observations to perform better than those that do not. However, as EPA points out in
their 2009 IWAQM reassessment report, CALMET’s OA procedure can also produce
discontinuities and artifacts in the wind fields resulting in a degradation of the wind fields even
though they may match the observed winds better at the locations of the observations (EPA,
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