All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 138 18 0
PDF Downloads 9 8 0

On the Importance of Precision for Short-Range Forecasting and Climate Simulation

View More View Less
  • 1 National Center for Atmospheric Research, Boulder, Colo. 80302
Full access

Abstract

This paper presents results of experiments designed to test the importance of arithmetic precision in short-term forecasts and long-term climate simulations with the NCAR global circulation model. It is expected that the next-generation computers will have a sizable speed gain when using lower-precision arithmetic. To determine how precision affects the model results, we compare several short- and long-term experiments using 48-bit mantissa arithmetic (normal for the CDC 6600 and 7600 computers) with corresponding experiments using 24- and 21-bit mantissa arithmetic. The errors due to the lower precision are much smaller than typical observational errors. In addition, it appears that in the short-term experiments the rapid error growth of the model dominates the round-off error accumulation resulting from the lower-precision arithmetic. Therefore, the lower precision used by the next-generation computers should not have a detrimental effect on short-term forecast accuracy. The long-term climate simulation experiments indicated a very similar conclusion. Even though there were some differences in the results of the experiments, climate indicators such as zonal wind, zonal temperature or eddy transport are quite similar.

Abstract

This paper presents results of experiments designed to test the importance of arithmetic precision in short-term forecasts and long-term climate simulations with the NCAR global circulation model. It is expected that the next-generation computers will have a sizable speed gain when using lower-precision arithmetic. To determine how precision affects the model results, we compare several short- and long-term experiments using 48-bit mantissa arithmetic (normal for the CDC 6600 and 7600 computers) with corresponding experiments using 24- and 21-bit mantissa arithmetic. The errors due to the lower precision are much smaller than typical observational errors. In addition, it appears that in the short-term experiments the rapid error growth of the model dominates the round-off error accumulation resulting from the lower-precision arithmetic. Therefore, the lower precision used by the next-generation computers should not have a detrimental effect on short-term forecast accuracy. The long-term climate simulation experiments indicated a very similar conclusion. Even though there were some differences in the results of the experiments, climate indicators such as zonal wind, zonal temperature or eddy transport are quite similar.

Save