a Three-Dimensional Global Tracer Transport Model Formulated on Episodic Consideration

A three-dimensional Global Episodic Tracer Transport Model (GETTM) with simple chemistry has been developed. This model calculates the concentrations of chemical species in the global atmosphere on 18 levels and 128 by 64 horizontal grids. The flux-form Bott's advection scheme (Bott 1989a) has been further developed for spherical coordinates and a nonuniform vertical grid mesh to describe the advection in GETTM. An efficient iterative flow-adjusting method has been developed to handle the mass inconsistency problem in the air continuity equation incurred in real applications of Eulerian advection algorithms. GETTM parameterizes the effects of subgrid diffusive transport, cloud vertical transport, aqueous transformation, wet deposition, and dry deposition on atmospheric chemical species in an event -based manner by using temporally and spatially varying meteorology. A Delta-Eddington radiative transfer model is used to compute the radiative fluxes and photolysis rates. GETTM simulations of atmospheric distributions of ^{222}Rn and short -lived sulfur compounds provide preliminary integrated tests of the model descriptions of all relevant processes over the global domain. Branch runs of the standard NCAR CCM2 have been executed and their archived meteorology with high time resolution is used to obtain the 33-day GETTM results of ^{222}Rn and sulfur. GETTM-simulated surface concentrations and mean vertical profile of ^{222}Rn are in reasonable agreement with available observations and other modeling studies, indicating that the model has described horizontal and vertical transport processes reasonably well. GETTM-derived sulfur budgets and turn-over times are in good agreement with those of another global sulfur model (Langner and Rodhe, 1991) and RADM. GETTM-simulated average concentrations of H_2O_2, DMS, H_2S, SO_2, SO_sp{4}{2-} and deposition rates of SO_2 and SO_sp{4}{2-} are generally consistent with observations and other modeling studies. These sulfur simulation results suggest that GETTM describes the deposition, transport, and sulfur chemistry processes reasonably well. Sensitivities of model results to some approximations of the simple sulfur chemistry are tested.