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