14.2 Dust Storms: Simplified Algorithm

Background The amount of material resuspended is dependent on both meteorological conditions and properties of the deposit. The basic mechanism behind resuspension is momentum transfer from the atmosphere to the deposit material. The amount of momentum available to be transferred is dependent on meteorological conditions and is often characertized by the friction velocity, U_*. The efficiency of the momentum transfer and the amount of momentum needed to loft particles is dependent on deposit properties. For instance, momentum is transferred more efficiently to larger particles, but the larger particles rarely gain enough momentum to resuspend. Instead the larger particles saltate, or bounce, along the surface. As they saltate, they transfer momentum to smaller particles which do become airborne.

HYSPLIT currently contains two different wind-blown dust emission algorithms and a third is in development. In each algorithm the following information must be determined

• threshold friction velocity U_*t. Emissions occur when friction velocity exceeds this amount.
• locations which contain a deposit from which material can be resuspended. Emissions only occur from these locations.

• threshold friction velocity is hardwired at 28 cm/s.
• Locations are determined from the landuse files. Emissions are considered from areas coded as desert locations in the landuse file.
• Vertical dust flux is calculated from Q = 0.01 U_*⁴ A.

1. If this example is being run through the web, it will be necessary to download the WRF meteorological data file for the 30th and 31st of March. For convenience, an extract of the two daily WRF files have been merged into a single file in the dust directory.
   
2. If continuing on from the last section, start with a GUI reset
3. CONTROL file open the Setup Run menu and retrieve the pre-configured control file CONTROL_dust0.txt from the \Tutorial\dust directory.
   • Dust emissions occur at locations where a "hardwired" threshold friction velocity (28 cm/s) is exceeded. The GUI option Special Runs / Dust-storm invokes a pre-processor to read the CONTROL file with the three starting points (bottom left, upper right, resolution) to create the final CONTROL file for dust emissions by finding all the desert area land-use locations within that domain. The potential dust emission points are defined in the CONTROL file by the two additional fields after the release height: the threshold friction velocity and the emission area (m²) centered about the emission point. To indicate that the simplified emission algorithm is being used, the threshold friction velocity field is set to -1.0.
   
   
   • Note that in the Concentration Grid menu the sampling start time is set for 00 00 00 06 00, which means that sampling will start 6 hours after the start of the simulation, for a 24-hour duration sample. The model results will be compared with the daily averaged (local midnight to midnight) AirNow measurements. A second hourly concentration grid (dust0_byhr.bin) should also be defined. Then save to exit.
   
   
4. Land Use Files The default landuse files have one-degree spatial resolution. For this application it is better to use a higher resolution landuse file since emissions will be determined from areas which are classified as desert. Optional half-degree and tenth-degree files are provided with the HYSPLIT distribution in the /hysplit/bdyfiles/bdyfiles0p5 and /hysplit/bdyfiles/bdyfiles0p1 directories. To use the half-degree resolution files instead of the default ones, you must copy the ASCDATA.CFG file from the bdyfiles0p1 directory to the /hysplit/working directory. The contents of this file are read during the HYSPLIT simulation initialization step and define the contents and structure of the three data files within that directory: LANDUSE.ASC, ROUGHLEN.ASC, and TERRAIN.ASC. If your HYSPLIT installation is different than the default, another option would be to edit your ASCDATA.CFG file to point to the correct directory, with a data resolution of 0.5, and the number of latitude and longitude data points: 360 and 720.


5. SETUP.CFG file The dust storm algorithm is invoked by setting ICHEM=3 in the SETUP.CFG file.
• Next open the Advanced / Configuration Setup / Concentration / menu #10 and check the Enable PM10 dust storm emission algorithm radio-button. Save to exit. This sets ICHEM=3 in the SETUP.CFG file.
• open menu #4 to increase the particle emission rate to 100000 with a maximum of 50000.
• These changes can also be invoked by retrieving the SETUP_dust.txt file. Save the changes to exit.


6. Run model Now go ahead and press the Special Runs / Dust-storm menu which configures the control file and runs the simulation. Using the half-degree resolution data file, 26 dust emitting locations are identified.


7. Plot When the run completes, open the Concentration / Display menu and set the units conversion to 1.0E+06 to convert grams/m³ to ug/m³. Also set the contour interval to fixed values of 500 200 100 50 20 10, then execute the display to show the dust plume.


8. Evaluate The calculation can be compared to the AirNow measurements in the Salt Lake City area for that day. Open the Utilities / Convert to / DATEM menu and set the measured data input file to AirNow_dust.txt and with the conversion factor of 1.0E+06. The statistical results show under-prediction a large under-prediction (about a factor of 42) but with a correlation coefficient of 0.75, which is confirmed by viewing the scatter diagram. Note that the scatter plot can be improved by increasing the conversion factor to 42*1e6=4.2E+07.


• Although there is under-prediction bias, the relatively high correlation coefficient suggests that the concentration differences between stations was well predicted, which can be partly attributed to the model correctly capturing the temporal and spatial variations in concentration.
• Go back to the Concentration / Display menu. In the DATEM plot file field add the Airnow_dust.txt file. Create the plot and notice that the measurements are at the plume edge in a region where there is a high concentration gradient. The under-prediction may be due to relatively small errors in the plume location and timing.
9. For this case we created two output files. We have been using the one which output 24-hour average concentrations. Now let's examine the hourly average output. Go back to the Concentration / Display menu and choose the dust0_byhr.bin file to plot. This will create 24 graphics for each hour of the simulation. Click through them to see the temporal evolution of the emissions and the plume. The first few will show no concentrations.

The dust storm modeling approach shown here required no additional information besides the landuse files supplied with HYSPLIT. In the next section, a customized CONTROL file will be used that is predefined with all potential dust emission locations.

• CONTROL file open the Setup Run menu and retrieve the pre-configured control file CONTROL_dust0.txt


• Change landuse file copy the ASCDATA.CFG file from the bdyfiles0p1 directory to the /hysplit/working directory.


• Run model Now go ahead and press the Special Runs / Dust-storm menu which configures the control file and runs the simulation. Open the new CONTROL file to see that the number of dust emitting locations has increased to 29.


• Plot Follow the previous instructions to obtain a plot of the dust plume.


• Evaluate Repeat the evaluation. The correlation coefficient decreased to 0.62 but the underprediction has also decreased to about 25. Arguably, the scatter plot looks better. Note that the scatter plot can be improved by increasing the conversion factor to 2.5E+07.


• It is difficult to tell with this small amount of data whether using the higher resolution landuse files improved the simulation.

• Modify CONTROL file open the Setup Run menu and retrieve the pre-configured control file CONTROL_dust5.txt from the \Tutorial\dust directory. Note that the values in the fifth line have changed to specify that the emission points have 0.1 degree spatial resolution.
• Run model Now go ahead and press the Special Runs / Dust-storm menu which configures the control file and runs the simulation. Now, 155 dust emitting locations are identified and the emissions area has decreased. Note that this simulation will take more time to run.


• Plot Follow the previous instructions to obtain a plot of the dust plume.


• Evaluate Repeat the evaluation. The correlation coefficient shows a small increase to 0.70 and the under-prediction has decreased to about a factor of 22. and there is improvement in the scatter diagram.

• Q = K ρ g^-1 U_* (U_*²-U_*t²) A

• Q = 0.01 U_*⁴ A.

3 m 10 s     External Meteorology Links:    WRF201003.bin