12.3 Physics Ensemble

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 sections. The model normally runs with its default options, unless a SETUP.CFG namelist file is found. Although various physical parameterization options were reviewed in previous sections, the actual namelist parameters or their values were not discussed in detail. This information is available through the HELP menu button or from the HYSPLIT online user guide . In the model physics ensemble, a script is run that automatically sequences through the most common parameter variations. This automated process will be reviewed in this section.

1. Before running a simulation, again delete all the ensemble files left over from the previous section. The start by retrieving the saved ensemble_control.txt and ensemble_setup.txt settings into the GUI menu. For this simulation, from the Setup Run menus, change the release height back to 10 m and change the output file from ensemble to a unique name such as ensphys.


2. The Special Runs / Ensemble / Physics menu will sequence through 15 different calculations: variations in how the mixed layer depth is calculated, various turbulence and stability equations, varying the ratio of vertical to horizontal turbulence, and varying the Lagrangian time scale. Note that these variations are hardcoded into the script and not all combinations may be valid for different meteorological data.


3. Open the Advanced / Configuration Setup / Concentration / Menu #4 and change the maximum number of particles to 10000.


4. Press Execute Script to start the process. As each simulation completes, its sequence number (1-15) is shown in the upper text box, and the name and value of the modified namelist variable is shown in the lower box. No data entries are required, the simulation sequence is entirely controlled by the script. The physics ensemble menu will stay open as long as the model is running. The following variations, shown by sequence number, are programmed in the script:



{file}.001 : idsp = 2	Mass correcting dispersion calculation
{file}.002 : kmixd = 1	Mixed layer depth from temperature profile
{file}.003 : kmixd = 3	Mixed layer depth from modified Richardson number
{file}.004 : kmix0 = 50	Minimum mixed layer depth set to 50 m
{file}.005 : kxmix = 1	Averaged vertical mixing in the PBL
{file}.006 : kdef = 1	Use horizontal velocity deformation for mixing
{file}.007 : kbls = 2	Determine stability from temperature profile
{file}.008 : kblt = 1	Beljaars equations for turbulence
{file}.009 : kblt = 3	Use TKE for turbulence
{file}.010 : kblt = 5	Hanna equations for turbulence
{file}.011 : vscales = 200	Fixed vertical Lagrangian time scale
{file}.012 : vscales = -1	Variable Lagrangian time scale
{file}.013 : 008 + 012	Beljaars with variable time scale
{file}.014 : 009 + 012	TKE with variable time scale
{file}.015 : 010 + 012	Hanna with variable time scale



5. When the last simulation has finished, the calculations complete message will be shown. Following the same procedure as previously with the meteorological ensemble, select the Display / Ensemble / Create Files menu tab to create the probability files. Then go to the Display / Ensemble / View Map menu and select the 90th percentile map and page forward to the 26th from 1500-1800 which shows a peak value, greater than 10^-9 contour, near sampling station 510, Little Valley, NY.


6. 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 26 18 (26th 15-18) shows the 90th percentile concentration (upper horizontal line) to be about 1.5x10^-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 (2) have concentrations greater than that value at that location.


7. If we again manually superimpose the measurements, the physics ensemble shows a smaller concentration variation between members and some of the members maintain higher concentrations for several sampling time periods after the peak, unlike the meteorological grid ensemble.


8. To determine which members produced the highest concentrations, look closely at the second graphic, the ensemble member plot, which shows that member 9 produced some of the highest concentrations during those time periods; the simulation using the TKE profile. 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 (\hysplit\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. Do not delete the HYSPLIT physics ensemble output files, they will be used in another example.