Physics Ensemble

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Another component of the uncertainty in the concentration calculation is the variation that can be introduced by using different physical parameterizations, or model options. Most of these were already reviewed in the concentration modeling introductory sections. The model normally runs with its default options, unless a SETUP.CFG namelist file is created in the Advanced / Configuration menu section. Although various physical parameterization options were exercised in the introductory sections, the actual namelist parameters or their values were not addressed. These details are usually available through the context sensitive HELP menu button in each GUI. In the model physics ensemble section, a script is run that automatically sequences through the most common parameter variations. This automated process will be reviewed in this section. Start by retrieving the previously saved captex_control.txt and captex_setup.txt settings into the GUI menu.

  1. The Special Runs / Ensemble / Physics menu will sequence through 20 different calculations stating with the 3D-particle option, Gaussian puffs, Top-Hat puffs with linear growth, Top-Hat puffs with the empirical growth rate, variations in how the mixed layer depth is calculated, setting fixed mixed layer depths: 500, 1000, 1500 m, various turbulence and stability equations, varying the ratio of vertical to horizontal turbulence, and varying the Lagrangian time scale.

  2. The physics ensemble menu will stay open as long as the model is running. WARNING - it may take 2 hours or more to finish all 20 simulations. Press Execute Script to start the process. As each simulation completes, the name and value of the namelist variable will be shown in the text box. The following variations are programmed in the script:

    • {file}.001 : initd = 0 initial distribution is particle (default=0)
    • {file}.002 : initd = 3 distribution gaussian horizontal-puff vertical-particle
    • {file}.003 : initd = 4 distribution top-hat horizontal-puff vertical-particle
    • {file}.004 : kpuff = 1 empirical puff growth rate (default=0 linear)
    • {file}.005 : kmixd = 1 mixed layer from temperature profile (default=0 input data)
    • {file}.006 : kmixd = 2 mixed layer from TKE
    • {file}.007 : kmixd = 500 use a constant 500 m for mixed layer
    • {file}.008 : kmixd = 1000
    • {file}.009 : kmixd = 1500
    • {file}.010 : kmix0 = 125 mimimum mixing depth (default=250)
    • {file}.011 : kzmix = 1 vertical mixing PBL average (default=0 use profile)
    • {file}.012 : kdef = 1 horizontal turbulence from deformation (0 = scale to vertical)
    • {file}.013 : kbls = 2 stability derived wind/temperature profile (default=1 fluxes)
    • {file}.014 : kblt = 1 use Beljaars turbulence parameterizations (default=2 Kanthar)
    • {file}.015 : kblt = 3 use TKE field for turbulence (default TKER=0.18)
    • {file}.016 : tker = 0.36 double vertical turbulence TKER = w'2/(u'2+v'2)
    • {file}.017 : vscale = 100.0 vertical Lagrangian time scale (default = 200 sec)
    • {file}.018 : hscale = 5400.0 horizontal Lagrangian time scale (default = 10800 sec)
    • {file}.019 : vscale = 400.0
    • {file}.020 : hscale = 21600.0

  3. When the last simulation has finished, the calculations complete message will be shown. Then go to the Display / Ensemble / View Map menu and select the 90th percentile map and page forward to the 27th from 0000-0300 which shows a value greater than 10-9 near sampling station 510, Little Valley, NY.

  4. Now open the Box Plots menu and set the location for station 510 to 42.25 -78.80 which will show the box plots time series. The plot for the 27 03 (27th 00-03) shows the 90th percentile concentration (upper horizontal line) to be about 3x10-9, within the range shown on the map for that location. What the 90th percentile value means is that only 10% of the ensemble members (about 3) have concentrations greater than that value at that location.

  5. To determine which members produced the highest concentrations, look closely at the second graphic, the ensemble member plot, which shows that members 5, 6, and 7 produced the highest concentrations at that time period. (HINT - right click on Postscript files to zoom). These were the simulations where the mixing depth computation was manipulated. As the script is running, the namelist variations for each member are written to the file ensemble.txt.

The results shown here represent a wide range of model options. The physics ensemble script (\hysplit4\guicode\conc_phys.tcl) could easily be customized for different model options depending upon the problem under consideration. As should be more obvious by now, there is never going to be just one right answer due to limitations in how well the meteorological data represent the true flow, variability from atmospheric turbulence, and how well the model parameterizations capture these processes.