Modeling aerosol direct and indirect effects in the 2-way coupled WRF-CMAQ

A new 2-way coupled meteorology and air quality model composed of the Weather Research and Forecasting (WRF) model and the Community Multiscale Air Quality (CMAQ) model is being developed and tested by the Atmospheric Modeling and Analysis Division at the USEPA. The new model system runs as a single executable with 2-way data communication between the WRF and CMAQ components via IOAPI_3 buffer files. This design requires minimal changes to either model which allows for easy updating and maintenance of compatibility with the “off-line” system. The main purposes of the coupled model are: 1) to allow efficient frequent data exchange for high resolution (down to 1 km grid cell size) simulations, 2) to allow feedback of gases and aerosols from CMAQ to WRF where they can affect radiation and microphysics processes, 3) to allow for more integrated treatment of chemical and physical processes.

The direct effects of aerosols on shortwave radiation and the direct effects of tropospheric ozone on longwave (LW) radiation have been implemented in the CAM and RRTMG radiation schemes. A new Mie scattering algorithm has been developed for a wider range of wavelengths including LW. New mixing state treatments for aerosols containing black carbon and other constituents such as sulfate and organic carbon have been developed and tested. New model simulations of the 2-way WRF-CMAQ using the latest versions of both models have been evaluated for a summer month in the eastern US and an outbreak of wild fires in California in 2008. Comparisons between runs with and without direct feedbacks show significant impacts on solar radiation, 2-m temperature, PBL height, and ozone and PM2.5 concentrations, especially in areas affected by smoke plumes.

The 2-way WRF-CMAQ also includes an experimental implementation of indirect effects where aerosols from CMAQ are activated as cloud condensation nuclei which determine the droplet number concentration for the cloud microphysics model. The resulting effective droplet radius is used in the radiation model to compute cloud optical properties. The indirect effects are being tested by evaluation of cloud radiative forcing compared to satellite measurements.