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Hoon Park, Song-You Hong, Hyeong-Bin Cheong, and Myung-Seo Koo

Abstract

This study describes an application of the double Fourier series (DFS) spectral method developed by Cheong as an alternative dynamical option in a model system that was ported into the Global/Regional Integrated Model System (GRIMs). A message passing interface (MPI) for a massive parallel-processor cluster computer devised for the DFS dynamical core is also presented. The new dynamical core with full physics was evaluated against a conventional spherical harmonics (SPH) dynamical core in terms of short-range forecast capability for a heavy rainfall event and seasonal simulation framework. Comparison of the two dynamical cores demonstrates that the new DFS dynamical core exhibits performance comparable to the SPH in terms of simulated climatology accuracy and the forecast of a heavy rainfall event. Most importantly, the DFS algorithm guarantees improved computational efficiency in the cluster computer as the model resolution increases, which is consistent with theoretical values computed from a dry primitive equation model framework. The current study shows that, at higher resolutions, the DFS approach can be a competitive dynamical core because the DFS algorithm provides the advantages of both the spectral method for high numerical accuracy and the gridpoint method for high performance computing in the aspect of computational cost.

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Song-You Hong, Masao Kanamitsu, Jung-Eun Kim, and Myung-Seo Koo

Abstract

This study investigates the effects of the diurnal cycle on monsoonal circulations over Asia in summer with a focus on precipitation. To this end, two sets of experiments are designed in a regional climate modeling framework forced by reanalysis data. The control experiment is a normal integration in which radiation is computed hourly, whereas the no-diurnal experiment is an experimental integration in which the daily averaged solar flux is computed once a day. Analysis of the results from the two experiments reveals that the diurnal cycle enhances the daily averaged sensible heat flux over land and the latent flux over oceans, which means that daytime net solar heating exceeds nighttime cooling in terms of the effects in surface climate and monsoonal circulations. Seasonal precipitation increased by about 3% over land and 11% over oceans. The surface hydroclimate over land is strongly influenced by the interaction between land and the atmosphere, and results in cooler surface temperatures except over the Tibetan Plateau. Over oceans, a robust increase in precipitation results from enhanced planetary boundary layer mixing. The diurnal cycle over the Tibetan Plateau region is found to decrease surface albedo by melting snow during the daytime, which contributes to the formation of the thermal low near the surface and the Tibetan high in the upper troposphere. The resultant monsoonal precipitation is modulated by an increase (decrease) in precipitation over northern (southern) India. This modulation results in the summer monsoon over East Asia being shifted northward.

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Song-You Hong, Myung-Seo Koo, Jihyeon Jang, Jung-Eun Esther Kim, Hoon Park, Min-Su Joh, Ji-Hoon Kang, and Tae-Jin Oh

Abstract

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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