Development and evaluation of the 2-way coupled WRF-CMAQ model

Description

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 at the U.S. EPA. The new model system runs as a single executable with 2-way data communication between the WRF and CMAQ components via buffer files. This design requires minimal changes to either model, which allows for easy updating and maintenance of compatibility with the “offline” 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, and (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. Model simulations of the 2-way WRF-CMAQ have been evaluated for a summer month in the eastern U.S. and an outbreak of wildfires 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 WRF-CMAQ also includes an experimental implementation of indirect effects where aerosols from CMAQ are activated as cloud condensation nuclei that 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. We are just starting a DOE-funded project to evaluate the capability of the 2-way WRF-CMAQ model to accurately represent the effects of aerosol loading on radiative forcing over the past 25 years during which there were substantial reductions in aerosol precursor emissions (e.g. SO2, NOx) in North America and Europe. Model evaluation will utilize long-term aerosol, cloud, and radiation measurements collected at the SGP site to evaluate high-resolution model simulations for North America and measurements from the NSA site to evaluate coarser resolution Northern Hemisphere model simulations. In addition, we will use field campaign data from VOCALS and CARES to investigate modeled interactions of aerosols, clouds, and radiation on an event basis.