14.3 Dust Storms: Revised Algorithm

The newest dust emission algorithm was developed for use over North America based upon climatological satellite observations of airborne dust. This second procedure is also invoked by setting the namelist parameter ICHEM=3 in the file SETUP.CFG and the potential dust emission points are also defined in the input CONTROL file by the two additional fields after the release height: the threshold friction velocity and the dust emission factor, the product of the soil dust density and emission area.

1. In the revised dust emission algorithm, the threshold friction velocity and emission factors vary for each location and dust locations are predefined by month based upon a climatology of dust sources developed from satellite data. In this approach, the dust emissions are defined by:
   • Q = K A (U_*-U_*t)
   
   where the positive value in the threshold friction velocity field (cm/s) triggers the use of the new algorithm. The second field in the CONTROL file is the product of the soil-dust density (K) and emission area (A).


2. To configure the model, sample monthly control files are provided for the continental U.S. or all global locations. The threshold velocities tabulated in the continental and global files were derived from different meteorological data sources.


3. For the current example, we have already extracted 21 locations (based upon the 0.25 degree grid used to analyze the satellite data) in the Sevier Lake region of Utah that are frequent dust sources to the Salt Lake City region from the dust_conus.zip file. Open the Setup Run and retrieve the pre-configured control file CONTROL_dust2.txt from the \Tutorial\dust directory. Note that the two additional fields now show a varying threshold friction velocity and the product of the dust density and emission area (KA). This approach provides more user control of the emission configuration. Use the same namelist file for all dust simulations by retrieving the SETUP_dust.txt file. You should also restore the original one-degree ASCDATA.CFG file from the bdyfiles directory. Save the changes and run the model through the standard run menu. The special runs menu is not required because the CONTROL is already configured for dust emissions.


4. Once the run completes go right to the Convert to DATEM step. Although the model results show a higher correlation (0.63) than the previous simulation, and the model under-prediction has increased to about a factor of 1500!

There are a lot of uncertainties in these calculations, especially when more detailed soil and land-use information is required. In this case, the climatological values for U_* and KA may not be representative for 2010, perhaps because of flooding in previous years built up erodible sediment that was not present during the period (2003-2006) of the climatological analysis of the dust emission sources. Emission adjustments will be examined in the next section.