A numerical study of the effects of large eddies on photochemistry in the convective boundary layer

The SMVGEAR II (Sparse-Matrix Vectorized Gear) chemistry solver has been integrated into the RAMS (Regional Atmospheric Modeling System) mesoscale model for the purpose of conducting large-eddy simulations (LES) with reactive photochemistry in the convective boundary layer (CBL). This new coupled model (dubbed LESchem) computes the LES dynamics and chemical transformations each time step in a directly coupled fashion. The simplified isoprene chemistry mechanism currently used in LESchem solves 77 kinetic reactions and 15 photolytic reactions for 45 trace gas species. The numerical studies reported here are idealized 2-hour coupled simulations of the midday CBL with no mean wind on a 9.8 km by 9.8 km by 4 km domain with a horizontal grid resolution of 200 m and a vertical resolution of 100 m. Two different photochemical scenarios are discussed: isoprene and nitric oxide (NO) coemitting continuously and uniformly from the bottom surface representing homogeneously mixed vegetation emissions; and isoprene alone continuously emitting homogeneously from the surface into an initially high background NO mixing ratio, representing a fresh urban plume that has advected over a forested area. The results from these coupled LES-photochemical simulations are then compared to coupled mesoscale-photochemical simulations produced by the same LESchem model, but which had been configured for mesoscale, first-order closure simulations. All of the coupled model development work and the numerical simulations reported here were performed on a personal computer (PC), but the model code is portable and has been installed on an Alpha-based workstation, as well.