UW WRF-GFS Aerosol Settings

Aerosols can be added to the UW WRF-GFS runs in the RRTMG radiation scheme. As noted in WRF 3.6 updates:

aer_opt = 2: A new way to input aerosol for RRTMG and Goddard
radiation options. Either AOD at 550 nm or AOD plus Angstrom exponent,
single scattering albedo and cloud asymmetry parameters can be input
via constant values from namelist or 2D input via auxiliary input
stream 15. Aerosol type may be set. (Thanks to Jose Ruiz-Arias of
Universidad of Jaen, Spain)

We first used these with the 2018081600 run. When smoke in the domain is deemed significant we will use the aer_opt = 2 settings,
adding the predicted 2-d aerosol optical depth (AOD) at 550 nm (AOD5502D in WRF)
and the predicted 2-d aerosol Angstrom exponent (ANGEXP2D in WRF) from
NASA's GEOS-5 system.

Please check the log file to see when these fields are activated and deactivated. The aerosols will only be activated in summer months: July through September. For the year 2020, they were most recently activated with the 2020082012 runs.

WRF's aerosol single-scattering albedo (AERSSA2D) and aerosol asymmetry factor (AERASY2D)
are set to the recommended fixed values of 0.85 and 0.9, respectively. See the latest
36/12/4 km namelist.input and 4/3 km namelist.input for a complete list of the aerosol settings.

The GEOS-5 system's output variables are related to WRF's values as described in this table:

Due to time-lag in NASA GEOS-5 system, forecast values from a 12-hour old GEOS-5 run are used in
the WRF runs. The AOD5502D and ANGEXP2D fields are updated every 3 hours of integration time.
Since NASA runs the system out to 10 days, the fields are available and are used for the extended runs also.