The role of horizontal resolution in assessing the transport of carbon monoxide using WRF-Chem

Anthropogenic emissions are transported both locally and globally. Depending on the magnitude of the transport, the emissions can have varying impacts on air quality and atmospheric chemistry. Transport occurs most efficiently if emissions are lofted to the middle or upper troposphere. Major mechanisms producing vertical transport are the warm conveyor belt (WCB) of middle latitude cyclones and deep convection that may be embedded within. Global chemical transport models appear to adequately simulate large-scale WCB transport, but the convection is parameterized which does not capture the fine-scale transport due to deep convection. This study uses the WRF-Chem chemical transport model to simulate a middle latitude cyclone in East Asia at three different horizontal resolutions (45, 15, and 5 km grid spacing). The cyclone contains a typical WCB with an embedded squall line and passes through an area of large surface CO concentrations. We use model output from WRF-Chem to compare differences between the large-scale transport of the WCB (the 45 km simulation) and the smaller-scale transport due to convection (the 15 km simulation). Forward trajectories are calculated from WRF-Chem output using HYSPLIT. At 45 km grid spacing, the WCB exhibits gradual ascent, lofting surface CO to 6 -- 7 km. Upon reaching the warm front, the WCB and associated CO turn eastward and diffuse over the Pacific Ocean. Convective transport at 5 km resolution with explicitly resolved convection occurs very rapidly, with surface CO lofted to altitudes greater than 10 km in 1 h or less. As the surface CO reaches the tropopause it spreads horizontally both eastward and westward with little diffusion. The coarse 45 km spaced domain also contained a short wave trough and an extensive area of convection, not related to the cyclone, that were responsible for slowly lifting considerable CO to the upper troposphere. We also compute CO mass fluxes to compare the differences in vertical transport due to the different grid spacings. In the domain with explicit convection, upward CO fluxes exceed 100,000 t when the squall line is at peak intensity, while fluxes from the two coarser resolutions are an order of magnitude smaller. Specific areas of interest were defined to examine the role of convective transport within the entire 5 km domain. Convection encompasses only a small portion of the domain, but is responsible for ∼ 40% of the upward CO transport. These results indicate that fine-scale resolution is critically important when examining the transport of surface emissions in areas of deep convection.