(BRAVO) Study: Final Report. - Desert Research Institute

Final Report — September 2004
• Whether any findings of biases in trajectory models and wind fields are valid for
Big Bend during summer and fall 1999 only (the BRAVO study period) or
whether similar biases exist for other regions of the country and for other seasons
and other years in south Texas.
The models examined were the Atmospheric Transport and Dispersion (ATAD),
HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) ver. 4.5, and CAPITA
Monte Carlo models. Input meteorological data included rawinsonde data, rawinsonde data
plus the four BRAVO wind profilers, and three modeled gridded wind fields (EDAS, FNL,
and MM5). Details of the input meteorology are discussed in Section 8.2 and details of the
three back-trajectory models are given in Section 8.3.
Big Bend is a particularly challenging receptor site at which to generate back
trajectories due to its proximity to both complex terrain and to data-sparse areas. Analyses of
back trajectories by several methods, including plotting trajectories by all methods for each
day and examination of the residence time probability density functions, showed that there
are some systematic differences between the results of different back trajectory model/input
data combinations at Big Bend National Park during BRAVO.
9.2.1 Effect of Wind Field Choice
Most of the differences appear to be due to the choice of input wind field rather than
the choice of back trajectory model, although there are also some differences due to model
alone. Using only rawinsonde data (ATAD model only) gives trajectories arriving at Big
Bend from the most southerly direction, EDAS data results in the most easterly trajectories,
and those generated using MM5 and FNL data fall between these extremes. These general
directional biases are consistent across models and are more pronounced during summer than
fall. The BRAVO tracer data clearly indicate that ATAD with rawinsonde data alone results
in trajectories that are too southerly, especially during the summer. This is most likely due to
wind sounding data scarcity in the region. Supplementing the rawinsonde data with
information from the four BRAVO wind profilers causes the ATAD/rawinsonde trajectories
to be more similar to those generated using the gridded wind fields. The MM5 data have a
higher mean wind speed than EDAS/FNL and will therefore be somewhat more likely to
attribute concentrations to source areas farther away from Big Bend than will the EDAS/FNL
wind fields.
Figure 9-5 illustrates the effect of choice of wind field on the back trajectories
generated by one model, the CAPITA Monte-Carlo model. Each line represents the path of
one of the particles that is followed by the model. The left panel shows that on September
15 th the trajectories generated using the EDAS and MM5 wind fields are similar, although the
previously-mentioned greater reach of the MM5 trajectories is apparent. The lower panel
below the map shows the height histories of the particle trajectories, starting with Time = 0 at
Big Bend, and reveals that the MM5 wind field places the particles at higher altitudes, where
the wind speeds tend to be greater.
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