A nested grid formulation for chemical transport over Asia: Applications to CO

[1] A global three-dimensional chemical transport model (GEOS-CHEM) was modified to permit treatment of a limited spatial regime with resolution higher than that adopted for the global background. Identified as a one-way nested grid formulation, the model was applied to a simulation of CO over Asia during spring 2001. Differences between results obtained using the nested grid (resolution 1degrees x 1degrees), the coarse global model (resolution 4degrees x 5degrees), and the intermediate global model (resolution 2degrees x 2.5degrees) are discussed. The higher-resolution model allows for more efficient, advection-related, ventilation of the lower atmosphere, reflecting the significance of localized regions of intense upward motion not resolved in a coarser-resolution simulation. Budget analysis suggests that upward transfer to higher altitudes through large-scale advection provides the major sink for CO below 4 km. Horizontal advection, mainly through the north boundary, contributes a net source of CO to the window domain despite the polluted nature of the study region. The nested-grid model is shown to provide good agreement with measurements made during the Transport and Chemical Evolution over the Pacific (TRACE-P) campaign in spring 2001, notably better than the low-resolution model in simulating frontal lifting process and differences across the boundary separating the regions of cyclonic and anticyclonic flow. The high-resolution window approach also allows us to differentiate transport mechanisms for individual subregions of China on a much finer scale than was possible previously. Suggestions are made as to how to allow for subgrid vertical advective motions in the low-resolution model through a carefully designed and broadly tested eddy diffusion treatment.