If you are not continuing from the previous section, first Reset, then retrieve
conc_case_control.txt and conc_case_setup.txt.
We found that both the computation of the stability and the estimate of the resulting turbulence was a function of the mixed layer or boundary layer depth (used interchangeably in these discussions). The default approach is to use the mixed layer depth that is output from the meteorological model. This field is not always available and if missing, the depth is computed from the temperature profile as the height at which the potential temperature exceeds the surface temperature by two degrees. An alternative approach uses a modified Richardson number to estimate the top of the mixed layer.
- To compute the plume using a mixed layer depth computed from the temperature profile, open menu #7, press Reset, and check the radio-button marked From temperature profile, save the change, run the model, display the results, and then compare the resulting plume with the default calculation using the WRF estimated depths. The temperature profile result is very similar to the default.
- The modified Richardson number approach is based upon a method first outlined by Vogelezand and Holtslag that uses excess temperature for convective cases that is computed from virtual potential temperature and horizontal and vertical friction velocity to estimate the mixed layer depth. To use this approach, open menu #7 and check the radio-button marked modified Richardson #, save the change, run the model, display the results, and then compare the resulting plume with the default calculation using the WRF estimated depths. The Richardson number approach is almost identical to the default.
- To determine the sensitivity of the calculation to the mixed layer depth, we can force the model to use a constant user set value. A review of the last profile in the MESSAGE file for this last calculation shows the tracer mixed up to about 2500 m. To double the depth, open menu #7 and check the Set as Constant radio-button and enter the value 5000. The button highlight will disappear when the value is changed from its default, but the calculation will use this value and the resulting plume looks similar to the calculation using the temperature profile except that it is not as wide.
- To contrast the previous calculation, reduce the mixed layer depth to half its original value. Change the Set as Constant field to 1250, rerun the model, and now the resulting plume is again similar in structure, but now concentrations are higher by about a factor of two.
In summary, the results show that the magnitude of the air concentration prediction is directly related to the mixed layer depth only in situations of strong vertical mixing, when the particles interact with this mixing boundary.
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