## 4 Performance Evaluation of ROMS
### Real Experiment: ROMS-NWP  
Experiments were conducted considering the real topography and external forces of the NWP. A 2048 \(\times\) 2048 grid was constructed with an interval of approximately 1 minute in the region of 117\({}^{\circ}\) to 160\({}^{\circ}\) longitude and 20\({}^{\circ}\) to 25\({}^{\circ}\) latitude surrounding the Korean Peninsula (Figure 12). The depth of the model was interpolated to the ultra-high resolution grid using ETOPO2 [14] data, and the vertical grid was divided into equal intervals of 30 layers to 5000 m depth from surface.  
GFS data and HYbrid Coordinate Ocean Model (HYCOM, [[https://nccss.hycom.org/thredds/nccss](https://nccss.hycom.org/thredds/nccss)]([https://nccss.hycom.org/thredds/nccss](https://nccss.hycom.org/thredds/nccss))) data were used for the initial field and external forces. The experiment was simulated for 24 h from 0:00 to 24:00 on 23 August 2018. This is the period when the twin typhoons of Soulik and Cimaron severely damaged the Korean Peninsula and represents an appropriate integration time required for performance analysis. GFS data for surface wind, heat flux, freshwater flux, surface air pressure, cloud, albedo and so on were used for the surface external forces required by ROMS. HYCOM data was also used for an initial field and lateral forces of the U, V, SSH, T, and S were interpolated to fit the ultra-high resolution model grid.  
The time interval of the model integration was 20 s. Like the WRF-NWP experiment, the computational performance was evaluated without analyzing the accuracy of the simulation results due to the short simulation time. Figure 13 shows the sea-surface temperature field from the simulation results in the KNL system. The distribution of the sea-surface temperature is well simulated for the period when typhoon Soulik passed near the Jeju island of South Korea.  
NWP experiments were performed with a 2048 \(\times\) 2048 \(\times\) 30 grid configuration (refer to Table 3). This configuration is suitable for evaluating the overall performance of the system because it considers the impact of the I/O performance to evaluate real topography and external forces. We used 128, 256, and 512 cores for comparing the performances of parallel systems according to the experimental results. In addition, a 1024 ultra-high resolution experiment was performed on the KNL system to test its performance limits.  
The NWP ultra-high resolution experiment without the parallelization of I/O could not be simulated with over 60 cores per node in the KNL system due to the lack of system memory. Thus, only 32 cores per node were used in this case, but it showed a higher performance than that of 64 cores per node in the benchmark experiment of ROMS (see Figure 11).  
Figure 12: Model area and depth of ROMS for the real experiment.  
The results of each experiment are summarized in Figure 14. The KNL system performs at an overall rate of approximately 44% compared to the performances of SKL and CLS. This is slightly slower than the benchmark experiment of ROMS (47%) due to the loads of I/O accessing. However, the effect of the I/O loads on parallel performance is small in the ROMS experiments compared with the WRF ideal experiment (54%) and the real experiment (37%) in Chapter 2. Even the 1024 cores experiment shows a continuous increase in the parallel performance of the KNL system; thus, it is the more suitable system for an ultra-high parallel numerical model.  
Figure 14: Performances of ROMS-NWP with respect to the number of cores for the real experiment: (SKL+CLS)/(2*KNL) = 44%.  
Figure 13: Real experimental results using ROMS during the period of typhoons Soulik and Cimaron: colors = SST, arrows = sea surface currents.