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. The two-way coupled Weather Research and Forecasting and Community Multiscale Air Quality (WRF-CMAQ) model has been developed to morerealistically represent the atmosphere by accounting for complexchemistry–meteorology feedbacks. In this study, we present a comparativeanalysis of two-way (with consideration of both aerosol direct and indirecteffects) and offline coupled WRF v3.4 and CMAQ v5.0.2 over the contiguousUS. Long-term (5 years from 2008 to 2012) simulations using WRF-CMAQ with bothoffline and two-way coupling modes are carried out with anthropogenicemissions based on multiple years of the U.S. National Emission Inventoryand chemical initial and boundary conditions derived from an advanced Earthsystem model (i.e., a modified version of the Community Earth SystemModel/Community Atmospheric Model). The comprehensive model evaluations show that both two-way WRF-CMAQ and WRF-only simulations perform well for majormeteorological variables such as temperature at 2 m, relative humidity at 2 m, wind speed at 10 m, precipitation (except for against the National Climatic Data Center data), and shortwave and longwave radiation. Both two-way and offline CMAQ also show good performance for ozone (O3) and fine particulate matter (PM2.5). Due to the consideration of aerosoldirect and indirect effects, two-way WRF-CMAQ shows improved performanceover offline coupled WRF and CMAQ in terms of spatiotemporal distributionsand statistics, especially for radiation, cloud forcing, O3, sulfate, nitrate, ammonium, elemental carbon, tropospheric O3 residual, and column nitrogen dioxide (NO2). For example, the mean biases have been reduced by more than 10 W m−2 for shortwave radiation and cloud radiative forcing and by more than 2 ppb for max 8 hO3. However, relatively large biases still exist for cloud predictions,some PM2.5 species, and PM10 that warrant follow-up studies tobetter understand those issues. The impacts of chemistry–meteorologicalfeedbacks are found to play important roles in affecting regional airquality in the US by reducing domain-average concentrations of carbonmonoxide (CO), O3, nitrogen oxide (NOx), volatile organiccompounds (VOCs), and PM2.5 by 3.1 % (up to 27.8 %), 4.2 % (up to16.2 %), 6.6 % (up to 50.9 %), 5.8 % (up to 46.6 %), and 8.6 %(up to 49.1 %), respectively, mainly due to reduced radiation,temperature, and wind speed. The overall performance of the two-way coupledWRF-CMAQ model achieved in this work is generally good or satisfactory andthe improved performance for two-way coupled WRF-CMAQ should be consideredalong with other factors in developing future model applications to informpolicy making.

geoscientific model development

A comparative study of two-way and offline coupled WRF v3.4 and CMAQ v5.0.2 over the contiguous US: performance evaluation and impacts of chemistry–meteorology feedbacks on air quality