Computation of global photochemistry with SMVGEAR II

A computer model was developed to simulate global gas-phase photochemistry. The model solves chemical equations with SMVGEAR II, a sparse-matrix, vectorized Gear-type code. To obtain SMVGEAR II, the original SMVGEAR code was modified to allow computation of different sets of chemical reactions for urban, free-tropospheric, and stratospheric regions during the same model run. SMVGEAR was also modified to allow grid cells in each region of the atmosphere to be reordered according to stiffness of the chemical equations, each time interval. Reordering speeds solutions by a factor of more than two compared to not reordering. Two 30 day simulations of chemistry coupled (with feedback) to radiation field calculations over a global grid, were performed. In one simulation, results were obtained for 49,680 grid cells (15-layers general circulation model grid), and in the other, results were obtained for 96,048 cells (29-layer grid). In both simulations, 200 “urban” (assumed below 900 mb) chemical equations were solved., 169 “free tropospheric” (225–900 mb) equations were solved, and 115 “stratospheric” (above 225 mb) equations were solved. The times required for the two 30 day simulations on a single processor of a Cray 90 were 6.2 and 11.6 h, respectively, at an average speed of approximately 362 megaflops.