Sep 12, 2018

The new U.S. EPA modeling guideline (Appendix W) officially took effect on May 21, 2017. One impactful change in the guideline is the approval to utilize prognostic meteorological data through the U.S. EPA's Mesoscale Model Interface Program (MMIF) pre-processor to generate inputs for regulatory modeling applications using AERMET and AERMOD. Section 8.4.2(a) of Appendix W states: “When processing prognostic meteorological data for AERMOD, the Mesoscale Model Interface Program (MMIF) should be used to process data for input to AERMET.” This new option provides stakeholders with an alternative to using the standard NWS airport data in situations where nearby observational data is not available or where meteorological conditions change rapidly with distance.  

While there are several meteorological grid models, including the MM5 model that can be used to develop inputs for air quality models, the most commonly used meteorological grid model by the U.S. EPA and the modeling community is the Weather Research and Forecasting (WRF) model. The WRF model pulls in observations and archived meteorological model data from the region around the modeling site, and uses exactly the same physical equations that are used in weather forecasting to model the historical weather conditions at the specific project location. 

To assist modelers with using WRF, MMIF and AERMET to process AERMOD-ready meteorological data, the U.S. EPA has published a MMIF guidance document on the use of prognostic meteorological data and the MMIF program. The U.S. EPA has also prepared AERMOD-ready meteorological data using AERMET and WRF-MMIF outputs by running WRF simulations for a CONUS domain and processing WRF outputs using MMIF. These datasets can be obtained through points of contact at multi­jurisdictional organizations and state agencies. However, caution should be taken when using this data as the U.S. EPA's WRF-MMIF-AERMET processed AERMOD-ready data have many limitations including:

  • The data is only available for a CONUS domain
  • The data is only available for years 2013-2015
  • A 12 km resolution was used in WRF processing
  • A 0.5 m/s calm wind threshold was used in the MMIF processing
  • AERMET version 15181 was used

These datasets would therefore not be the best representation of your modeling domain if the project's domain is outside of the defined CONUS domain, or if the project requires data beyond the years of 2013 to 2015. Also, to obtain more representative meteorological data, a higher WRF grid resolution is needed to better capture the conditions at the modeling site. MMIF does not interpolate WRF output results, and it only looks for the closest grid point to the source site.

Another thing to be aware of is the discrepancy between Appendix W and the latest MMIF guidance. Appendix W section states “For NWS ASOS data, especially data using the 1-minute ASOS winds, a wind speed threshold option is allowed with a recommended speed of 0.5 m/s. When using prognostic data processed by MMIF, a 0.5 m/s threshold is also invoked by MMIF for input to AERMET.” This statement contradicts the latest MMIF Guidance, section 3.4 which recommends “For both MMIF to AERMET and MMIF to AERMOD, the user should set the minimum wind speed, AER_MIN_SPEED, to zero, since the input is prognostic data and does not have a functional minimum threshold as found in an anemometer.” BREEZE developers communicated with the U.S. EPA regarding this discrepancy and received confirmation that the MMIF guidance (i.e., a minimum wind speed of 0.0 m/s) should be followed when using MMIF to process prognostic data. As such, even if the U.S. EPA processed CONUS WRF data for 2013-2015 with a 12 km is required, the data should be processed again with MMIF and the latest AERMET version (18081) because the U.S. EPA-processed data uses a 0.5 m/s calm wind threshold in MMIF and was processed using AERMET version 15181.

The BREEZE Data Team can help to perform refined WRF modeling for locations around the world. We account for the site-specific conditions as well as options for the project domains, so the data provided is customized to fit the required domain, year, and grid resolution. Please contact the BREEZE Data Team at or +1 (972) 661-8881 if you have any questions about processing WRF data, are interested in purchasing AERMOD-ready meteorological data processed using WRF, or if you want to ensure that the data provided by the U.S. EPA is appropriate for your modeling needs.