NASA - National Aeronautics and Space Administration

We describe and evaluate a system for regional modeling of atmospheric composition with the Met Office Unified Model (UM), suitable for climate, weather forecasting and air quality applications. In this system, named NUMAC (“Nested UM with Aerosols and Chemistry”), a global model provides boundary conditions for regional models nested within it, using the Met Office's Regional Nesting Suite for multi-scale simulations. The regional models, which can run at convection-permitting or cloud-resolving scales, use the same code as the global model. The system includes double-moment prognostic aerosol microphysics with interactive chemistry of sulfur species, ozone, NOx, and CO as in the UK Earth System Model. Double-moment prognostic cloud microphysics is optional. To test NUMAC, we compare simulations to surface and aircraft measurements from NASA's Korea-United States Air Quality campaign over South Korea. The performance of the regional model, which we run at 5 km resolution, is similar to the well-evaluated global model when the regional and global models use the same emissions. Most species such as ozone, NOx, OH, or PM2.5 are simulated within a factor of 2 of observations most of the time, though they are biased low compared to monitors in polluted areas (observed surface dry PM2.5 averages 28 μgm −3 but we simulate 17 μgm −3). Meteorology and clouds are represented satisfactorily. With higher-resolution emissions, many of the low model biases are reduced, but a tuning was required to keep NO concentrations realistic, indicating shortcomings in the chemistry scheme. We demonstrate the potential of NUMAC for studies of aerosol-cloud interactions. Plain Language Summary Unified atmospheric simulation systems, defined as those that are routinely used to represent air quality, weather and climate with the same code, are rare. The UK Met Office Unified Model (UM) is one such system, but until recently, aerosol-cloud interactions were represented only in global climate simulations. Effects of aerosol-cloud interactions and atmospheric chemistry on Earth's radiation balance are large and must be included in global and regional climate predictions. However, partly because clouds and emissions are spatially inhomogeneous, their effects on climate are currently poorly understood. Simulations with high grid resolution can help us understand the relevant processes better. We document here a system for representing air quality, weather and climate with the UM, including chemistry-aerosol-cloud interactions, at high spatial grid resolution on regional scales. We test our system against surface and aircraft measurements of atmospheric chemical species such as ozone and particulate matter made in May 2016 in Korea. The model represents most species well, but has some shortcomings we must address in future work. Overall, however, we judge that it is ready for studies of how atmospheric chemical species and particulate matter affect air quality, weather and climate, and can be further improved as it is used.