17.1 Dynamic or Lagrangian Sampling




Although the output is a simple text file, in terms of alternate display options, the dynamic or Lagrangian sampling is very different than any of the other display options. The model can be configured to move a sampler through the computational domain, either passively, moving with the wind as a balloon, or as an aircraft with a defined direction and speed. Start with the same configuration used for many of the aircraft sampling examples. We will make a few modifications. Start by retrieving conc_case_control.txt and the namelist file conc_case_setup.txt.

  1. We are going to configure the model to match the time and locations of the same 914 m AGL aircraft sampling flight that was used in several previous examples. Note the end of the last sample was on the 26th 0348 to 0354, which means we only need to run the model from the 25th 17Z to the 26th 04Z, a run duration of 11 hours. Open the Concentration / Setup Run menu to make that change.

  2. Next open the Concentration Grid menu. Several changes are required. If the sampling start time was set to 26 03 change it to an earlier time, such as 83 09 26 00 00 so that the entire last aircraft pass, which starts at 0248, will be captured by the simulation. The last change required is that the sampling needs to be changed from averaging (00) to snapshot (01), resulting in the last line showing 01 01 00. The middle 01 doesn't matter, because we will not be examining the conventional binary output file.

  3. In a snapshot grid definition, the particle masses are summed to the concentration grid each time step. As the dynamic sampler passes through the snapshot grid, the concentration of the grid cell occupied by the sampler at that time step is summed to the dynamic sampler. Then the concentration grid is set to zero before the next time step calculation. To configure the dynamic sampling option, select the Advanced / File Edit / Dynamic Sampling menu tab.

  4. In the first menu, define the number of samplers that will be followed at the same time. In our situation, we only need to define one aircraft pass. A pass is a segment that can be defined by a single velocity vector. Pressing the Configure Samplers button opens another menu, where by selecting the radio-button opens the configuration menu for the selected sampler.

  5. Review the data from the sampling aircraft again. We will start the dynamic sampling from the beginning of the pass defined by the data record:
    • 1983 09 26 0248 0006 40.86 -81.81 468.0

    On the first line set the aircraft start position to 40.86 -81.81 914.0. The aircraft flight level was at 914 m MSL. From the latitude-longitude positions it is possible to determine the aircraft direction, 290 degrees, and speed 50 m/s. Enter those on the second line. The third line is the start time of the flight leg and the second line defines the start time of the sampling, in this case they are both at the same time: 83 09 26 02 48. The next two lines define the sampler averaging time and disk output interval in minutes, in this case both values are set to 06 minutes. The output will be written to a file called LAGOUT.TXT. Save each of the menus to close the menus. The dynamic sampling information is written to a file called LAGSET.CFG, when found in the working directory turns on the dynamic sampling option.

  6. Now run the model and when the simulation completes, open the LAGOUT.TXT file to see how the model results compare with the measurements. Two issues to note. First there is no units conversion factor option, hence values are g/m3. Second, the output times are at the end of the sample, while the measurement file times are at the beginning of each sample. The maximum calculated concentration occurs at:
    • 83 9 26 3 12 41.079 -82.617 667.0 0.1444E-07

    which corresponds to a concentration of 14440 pg/m3. The maximum measurement value for that flight segment also occurred for a sample collected for the same time period and approximately the same position but it was about twice as large:
    • 1983 09 26 0306 0006 41.09 -82.52 29936.4

    As with many other previous examples, your results may differ slightly from the results shown here.

The results shown here permit model simulations to be configured in a way to match aircraft or balloon borne sampling platforms. There are limitations to this approach in terms of multiple pollutants and concentration grids. Currently only simple single pollutant simulations are permitted.