TRAJECTORIES
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What are the limitations of trajectories?
Trajectories should primarily be used as a diagnostic tool to evaluate the
flow field at various levels of the atmosphere and to compare the fields between
different meteorological models. The error in a trajectory calculation is
primarily due to the fact that meteorological fields, which vary continuously
in space and time, are at times poorly represented by a field defined at
fixed locations (grid resolution) and at fixed temporal intervals. The other
error, more an error in interpretation than calculation, is that a particular
trajectory may have little relationship to the pollutant plume dispersion pattern.
Trajectories only represent the flow path of a single particle at the time of the
initial release. As a pollutant spreads out both horizontally and vertically, due to
dispersion, it may take many different paths in addition to the initial trajectory.
The trajectory shown in the previous example represents only the flow from a
single point started at the initial time, 1500 m above ground. A simple ensemble
of various trajectory pathways (from 500, 1500, and 3000 m AGL) can provide a more
realistic representation of the plume dispersion, which is illustrated below by a concentration
calculation from the initial trajectory starting location. Note that there is no pollutant
shown at the ground for transport pathways corresponding to the levels of upper
trajectories (670 hPa) because the pollutant did not mix up to those levels.
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| Trajectory ensemble. | Concentration calculation. |