The effect of mass inconsistency of the meteorological field generated by a common meteorological model on air quality modeling

The widely used mesoscale meteorological model, MM5 and a recently released US EPA air quality model, Models-3/CMAQ were employed to investigate the effect of mass inconsistency in air quality modeling. During a 24-h simulation using a hydrostatic MM5 output, the mass inconsistent error, as measured by trace species concentration, grew by 242%. The nonhydrostatic simulation produced approximately 6 times larger mass inconsistent error than the hydrostatic model. This is because the nonhydrostatic model is not strictly mass consistent by neglecting terms representing pressure increase due to heating in a pressure perturbation equation, another form of the continuity equation. In both the hydrostatic and nonhydrostatic models, mass inconsistency was produced due to inconsistent numerical schemes and integral time steps between the meteorological model and the air quality model. Temporal interpolation of meteorological data in air quality modeling partly caused the mass inconsistency as well. The mass inconsistent error was removed successfully by conserving the mixing ratio of the trace species in both the hydrostatic and the nonhydrostatic cases.