5.1 Trajectory Vertical Motion




In most circumstances the input meteorological data will contain a vertical motion field, usually in pressure units. The trajectory and dispersion model calculations can use these data fields directly. If the vertical motion field is missing, or perhaps some special conditions are required for a simulation, the model has an option to replace these fields with an internally calculated vertical velocity based upon an assumption that the pollutant parcel is transported on some other surface. This is accomplished by computing the velocity (Wη) required to maintain a parcel on the selected η surface:

  • Wη = (-∂η/∂t - u ∂η/∂x - v ∂η/∂y) / (∂η/∂z)
given the slope of the surface and its local rate of change and where the surfaces (η) can be either isobaric (p), isosigma (σ), isopycnic (ρ), or isentropic (θ). Note the equation is only an approximation because the velocity fields are not remapped to a new surface and therefore it is possible for a calculation to move off the surface.

  1. The effect of changing the vertical motion method can be evaluated by recalculating the backward trajectory from the trajectory error discussion section. Retrieve the CONTROL file traj_back_control.txt which should show the trajectory setup after changing the output file from tdump_back to tdump_data. Save the change, delete the SETUP.CFG if it exists, run the model, and display the results in pressure units. It should match the previous calculation.

  2. Now repeat the process but configuring the simulation using the isobaric option. Make sure to change the output file name to tdump_isob, save and run the model. Repeat this process for each of the vertical motion options, naming the output file suffix with the name of the vertical motion method.

    • data - uses the vertical velocity field in the meteorological data file
    • isob - isobaric maintains the trajectory on a constant pressure surface
    • isen - isentropic maintains the trajectory on surfaces of constant potential temperature
    • dens - trajectories stay on constant density surfaces
    • sigma - vertical motion is zero and the trajectory stays on the same internal sigma
    • divg - the vertical motion is computed by vertical integration of the velocity divergence

  3. When the calculations are finished, all trajectories can be superimposed on the same plot by using the + symbol between endpoint file names, or by creating the file traj_files.txt of file names which should be entered into the display menu. To show which of the backward trajectories is identical to the reference forward trajectory, add tdump_fwrd to the end of traj_files.txt and replot the results.

At this point it is not possible to determine a correct answer with absolute certainty because there are no tracer measurements at the off-shore starting location for all these back trajectories. However, the only two vertical motion methods that took the trajectory back toward the tracer release location were the calculations using the vertical motion data field and the vertical motion calculated through divergence. Note that the divergence approach is the default when there is no vertical motion in the meteorological data file. The other approaches should only be used for appropriate special applications.

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