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This study presents the dependency of the simulation results from a global atmospheric numerical model on machines with different hardware and software systems. The global model program (GMP) of the Global/Regional Integrated Model system (GRIMs) is tested on 10 different computer systems having different central processing unit (CPU) architectures or compilers. There exist differences in the results for different compilers, parallel libraries, and optimization levels, primarily a result of the treatment of rounding errors by the different software systems. The system dependency, which is the standard deviation of the 500-hPa geopotential height averaged over the globe, increases with time. However, its fractional tendency, which is the change of the standard deviation relative to the value itself, remains nearly zero with time. In a seasonal prediction framework, the ensemble spread due to the differences in software system is comparable to the ensemble spread due to the differences in initial conditions that is used for the traditional ensemble forecasting.

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Double-Fourier-Series (DFS) spectral method is applied to a large-size problem of barotropic instability of double-shear flow on the sphere. The computing source is the NEC SX-5 parallel vector processors, with the maximum vector length of 512. It is demonstrated that the DFS spectral model is robust and stable even for such a large-sized intensively nonlinear problem, and can simulate well the multiple scale phenomenon without losing accuracy. In addition to the efficiency on serial computing, represented with OðN 2 log 2 NÞ operations as opposed to OðN 3 Þ for the spherical harmonics spectral method, with N the truncation, the DFS spectral model also preserves the efficiency on parallel computing on vector architecture, due to its nature of two dimensional transform. The parallel performance increased slightly with the resolution, and nearly 33.5 percent (26.8 GFLOPS) of the theoretical peak performance (80 GFLOPS) was achieved in the highest-resolution experiment.Corresponding author: Hyeong-Bin Cheong, Dept. of Environmental Atmospheric Sciences, Pukyong National University, 599-1 Daeyeon 3-cong, Namgu, Pusan 608-737, Korea. E-mail: hbcheong@pknu.ac.kr ( 2004, Meteorological Society of Japan The zonal-mean absolute vorticity, of which initial condition is characterized as two peaks in both hemispheres, evolves with time into a nearly constant value over the hemisphere. On the other hand, the meridional gradient of the absolute vorticity increases around the equator. The kinetic energy per unit mass is calculated for each total wavenumber, where a disturbance field of a single total wavenumber is separated by an 8th-order spherical harmonics filter. Kinetic energy spectrum shows two distinct subranges, each with a constant slope. The subrange, other than the viscous subrange, shows a slightly increasing slope with time and approaches l À3 (l is the total wavenumber) in the matured stage, when a single large vortex is formed. As the resolution increases, the subrange other than the viscous subrange extends to the higher wavenumber domain, due to low viscosity. Numerical convergence of the solution with a fixed viscosity is discussed in terms of time averaged zonal-mean statistics of the zonal-flow.