2.6 Wet Deposition Options (10 min)




In this section we will run a series of hypothetical calculations based upon the Fukushima accident simulation described in one of the earlier sections but this time varying the wet deposition to determine the sensitivity of the airborne mass to the wet removal constants. The wet deposition options have been changing over the last few model updates:

Version   Within-   Below-   Notes
1-221   3.2E+5   5.0E-5   A
222-644   4.0E+4   5.0E-5   B
645-656   1.0E+6   1.0E-6   C
657-xxx   8.0E-5   8.0E-5   D

A. Original Method
B. Original Method using parameters optimized for Fukushima (J. Geophys. Res., Vol. 117, D05107, doi:10.1029/2011JD017205)
C. Revision #1 the within-cloud scavenging ratio equation uses the cloud-layer rather than the pollutant layer to compute the removal
D. Revision #2 the scavenging coefficient method as an option for in-cloud removal (coefficients less than one). Using coefficients greater than one for in-cloud, uses the Revision #1 scavenging ratio method. The below-cloud removal method is unchanged.

  1. To insure the most sensitivity to wet deposition, we need to determine the time period that shows precipitation at the release point. Use the program xtrct_stn to extract the time series of one variable from the meteorological file using the following inputs:

    ../files/    Meteorology file directory
    wrf11031400.bin    WRF data file name
    01    Extract data for one variable
    TPP1 01 1000.0    Extract TPP1 at level 01 and convert from m/s to mm/s
    37.4206 141.0329    Location of data extraction
    1    Use linear interpolation (1)
    rain.txt    Output file name
    1    Starting record number in output file
    9999    Maximum number of records to process

    The output file shows that the WRF calculation showed rain to occur between 0700 to 2200 on March 15th at the release point. Therefore the 12 hour period from 1200-2400 on the 15th will be used for the wet deposition test calculations.

  2. Configure the CONTROL for a simple 12 hour duration calculation using continuous emissions such that the total emission mass after 12 hours would equal 100 units if no deposition were to occur:

    1    One pollutatant defined
    CPAR    Cesium particle
    8.33333    Release rate 8.33 /hr x 12 hr = 100 units
    12.0    Emission duration hours

    Otherwise the dry deposition is set to a nominal value of 0.1 cm/s and the wet deposition constants are varied according to the values given in the table shown above.

  3. Now run the model multiple times, each time using a different scavenging method or coefficients. Note that the original method is not available as an option through the current version of the code. Versions 644 or earlier are required. Looking at the MESSAGE at the end of 12 hours for each simulation shows the total mass remaining airborne:

    Method (A), coefficients (A):    43%
    Method (A), coefficients (B):    66%
    Method (B), coefficients (B):    89%
    Method (C), coefficients (C):    75%
    Method (D), coefficients (D):    67%

For more information about scavenging options see the excellent overview by Sportisse (2007). The results here show the revised method, using either the scavenging coefficient or scavenging ratio provides comparable results and is less sensitive to changes in rainfall or the value of the coefficients that the original wet removal method.