Improvement of SMVGEAR II on vector and scalar machines through absolute error tolerance control

The computer speed of SMVGEAR II was improved markedly on scalar and vector machines with relatively little loss in accuracy. The improvement was due to a method of frequently recalculating the absolute error tolerance instead of keeping it constant for a given set of chemistry. To test the effects of the modification on speed and accuracy, three Eulerian model simulations were performed on two vector machines, the Gray C-90 and Gray J-916, and a scalar machine, the Silicon Graphics Origin 2000. The first simulation was an urban air pollution case in which gas chemistry, dynamical meteorology, radiation, and species transport were solved together. The second and third simulations were global cases in which chemistry, dynamical meteorology, radiation, and species transport were solved together. For the urban case, the new algorithm reduced SMVGEAR II's computer time by 50-67%, depending on the machine used. For the global cases, chemistry time was reduced by 14-44%. In all simulations, normalized gross errors were less than 1%, and the time required to solve chemistry dropped to near or below 50% of the total model simulation time. Thus, gas chemistry may no longer be the computationally-limiting process in many atmospheric models. The simulations show the usefulness of SMVGEAR II on a scalar workstation with sufficient memory bandwidth. (C) 1998 Elsevier Science Ltd. All rights reserved.