Comparing turbulent mixing of atmospheric oxidants across model scales

Abstract

Vertical mixing processes as simulated by atmospheric models affect the prediction of vertical profiles of reactive trace gases and oxidants and our understanding of atmospheric chemistry throughout the boundary layer. Specifically, the vertical transport of these oxidants is important for understanding where in situ chemistry occurs. Simulations of the vertical transport are influenced by many factors, including the horizontal and vertical grid resolution, planetary boundary layer (PBL) schemes, and the use of cumulus parameterization to represent sub-grid scale cloud processes. Here, we compare simulations using the Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) with the NCAR Large Eddy Simulation (LES) model and the NASA P-3B measurements over the Baltimore-Washington region to evaluate the performance of WRF-Chem in simulating vertical mixing of chemical oxidants in the PBL. For two different PBL schemes, WRF-Chem simulates a weaker convective environment and lower PBL height than LES. Comparing WRF simulations, the Yonsei University (YSU) PBL scheme simulates higher and drier PBL than the Mellor-Yamada-Janjic (MYJ) PBL scheme. The stronger vertical mixing in the LES model simulates higher concentrations of chemical oxidants aloft than WRF-Chem, leading to a constant O3 vertical profile up to the top of the LES domain (4.8 km). Weaker vertical mixing in WRF-Chem prohibits upward transport of the key chemical precursors, therefore resulting in low in situ chemical production of O3 and OH aloft in the WRF-Chem simulations with both PBL schemes and reducing the oxidative capacity in the buffer or cloud layer.