Atmospheric Chemistry and Physics

Abstract. To study the meteorology and chemistry that is associated with the early stages of the North American Mon-soon, the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) is applied for the first time at high resolution (4 km grid spacing, allowing for explicit rep-resentation of convection) over a large region (continental US and northern Mexico) for a multi-week (15 July to 7 Au-gust 2006) integration. Evaluation of model results shows that WRF-Chem reasonably represents the large-scale me-teorology and strong convective storms, but tends to overes-timate weak convection. In the upper troposphere, the WRF-Chem model predicts ozone (O3) and carbon monoxide (CO) to within 10–20 % of aircraft and sonde measurements. Com-parison of UT O3 and CO frequency distributions between WRF-Chem and satellite data indicates that WRF-Chem i