Mass Conservative Coupling of Non-Hydrostatic Meteorological Models with Air Quality Models

Abstract

High-resolution data produced by recent non-hydrostatic meteorological models (MMs) are expected to significantly improve the characterization of transport in air quality models (AQMs). However, the use of data from non-hydrostatic MMs presents new problems for the air quality modeler. First, there is the consistency issue. The wind components together with air density satisfy the continuity equation in MMs. On the other hand, AQMs rely on the species continuity equation to enforce the principle of mass conservation. Though the continuity equation does not appear explicitly in their formulation, AQMs are expected to maintain a uniform mass mixing ratio field for an inert tracer after transport with the winds produced by the MM. This expectation could only occur if the two models used the same discretization, i.e., grid, time step, and finite difference forms. However, the models may not share the same grid structure and the forms used for advection in AQMs are usually very different from those in MMs. Also, since the outputs of the MM are stored less frequently than the AQM time step, the input variables cannot be reconstructed exactly at the desired instants. Consequently, the winds and the air density used in AQMs may not be consistent (i.e., they do not satisfy the continuity equation) and the uniform tracer field cannot be maintained. The perturbation of uniform fields is usually more pronounced with data from non-hydrostatic MMs than with data from hydrostatic MMs for the same domains. These perturbations grow in time and may generate instabilities in AQM solutions. A second issue, species or tracer mass conservation, presents itself because of the attempts to establish consistency and produce stable results. In some existing AQMs, the conservation of species mass was sacrificed in order to obtain stable results. However, large mass conservation errors are not tolerable in AQMs used to establish source-receptor relationships for the design of emission control strategies. Therefore, it is desirable to establish consistency in a mass-conservative manner.