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

Abstract

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 1° × 1°), the coarse global model (resolution 4° × 5°), and the intermediate global model (resolution 2° × 2.5°) 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.