Toward the achievement of reliable global kilometer-scale (k-scale) climate simulations, we improve the Nonhydrostatic ICosaherdral Atmospheric Model (NICAM) by focusing on moist physical processes. A goal of the model improvement is to establish a configuration that can simulate realistic fields seamlessly from the daily-scale variability to the climatological statistics. Referring to the two representative configurations of the present NICAM, of which each has been used for climate-scale and sub-seasonal-scale experiments, we try to find the appropriate partitioning of fast/local and slow/global-scale circulations. In a series of sensitivity experiments at 14-km horizontal mesh, (1) the tuning of terminal velocities of rain, snow, and cloud ice, (2) the implementation of turbulent diffusion by the Leonard term, and (3) enhanced vertical resolution are tested. These tests yield reasonable convection triggering and convection-induced tropospheric moistening, and result in better performance than in previous NICAM climate simulations. In the mean state, double Intertropical Convergence Zone bias disappears, and the zonal contrast of equatorial precipitation, top-of-atmosphere radiation balance, vertical temperature profile, and position/strength of subtropical jet are dramatically better reproduced. Variability such as equatorial waves and the Madden–Julian oscillation (MJO) is spontaneously realized with appropriate spectral power balance, and the Asian summer monsoon, boreal-summer MJO, and tropical cyclone (TC) activities are more realistically simulated especially around the western Pacific. Meanwhile, biases still exist in the representation of low-cloud fraction, TC intensity, and precipitation diurnal cycle, suggesting that both finer spatial resolutions and the further model development are warranted.

Typhoon Faxai hit Japan in 2019 and severely damaged the Tokyo metropolitan area. To mitigate such damages, a good track forecast is necessary even before the typhoon formation. To investigate the predictability of the genesis and movement of a precursor vortex and its relationship with the synoptic-scale flow, 1600-member ensemble simulations of Typhoon Faxai were performed using a 14-km mesh global nonhydrostatic atmospheric model, which started from 16 different initial days (i.e., 1600 members in total). The results show that the model could predict an enhanced risk of a Faxai-like vortex heading toward Japan two weeks before landfall, which was up to 70%. The reason for the enhancement was a rapid increase in the members reproducing a precursor vortex from 15 to 12 days before landfall in Japan. In addition, the upper-tropospheric trough played an essential role in the track simulation of Faxai.

The eastward movement speed of Madden–Julian Oscillation (MJO) events simulated in a 30-year simulation on a global cloud resolving model, nonhydrostatic icosahedral atmospheric model (NICAM), following the atmospheric model intercomparison project (AMIP) protocol, but with a slab ocean, was analyzed and compared with the observation. The simulation reproduced the observed tendency of the MJO to decelerate when they are embedded within stronger Walker circulation, intensified by background sea surface temperature states with larger zonal gradients between the warmer western Pacific and the cooler Indian ocean and eastern Pacific. However, the simulated MJO events displayed a slow bias and occurred disproportionately during El Niño events. These biases were associated with an overestimation of the western Walker circulation cell strength, which was partially counteracted during El Niño events. Our results highlight the importance of accurately reproducing the mean atmospheric circulation for the realistic reproduction of MJO in long term simulations.

NICAM (Nonhydrostatic Icosahedral Atmospheric Modeling) has been used to conduct global storm resolving simulations with a mesh size of O(km) over the globe (Satoh, M. et al. 2017). Using the supercomputer “Fugaku”, we explore studies in the following directions: 1. Large-ensemble simulations (1000 members), 2. longer-duration simulations (100 years: HighResMIP; Kodama, C. et al. 2021), 3. higher-resolution simulations (less than a kilometer dx; Miyamoto et al. 2013), 4. high-resolution atmosphere-ocean coupled model simulations (atmosphere 3.5 km × ocean 0.1 deg: NICOCO; Miyakawa, T. et al. 2017), and 5. large ensemble data assimilations with NICAM-LETKF (Yashiro, H. et al. 2020). In this talk, we first review the current activities of NICAM on Fugaku. As the most uncertain component of atmospheric models in general, we intercompared the cloud properties of the DYAMOND simulation data (Stevens, B. et al. 2019; Roh, W. et al. 2021). We found that the domain averaged outgoing long-wave radiation is relatively similar across the models, but the net shortwave radiation at the top of the atmosphere shows significant differences among the models (Figure). The vertical profiles of cloud concentration are widely divergent among models, and cloud water content exhibits larger intermodel differences than cloud ice. This result implies more focused evaluations of clouds are required for improving the global storm resolving models. The forthcoming satellite “EarthCARE” (Illingworth, A. et al. 2015) provides a comprehensive dataset for cloud evaluations of atmospheric models, particularly by the first cloud Doppler radar from space. We present possible strategies for the new era of satellite collaboration studies with global storm resolving models.

Future changes in tropical cyclone properties are an important component of climate change impacts and risk for many tropical and mid-latitude countries. In this study we assess the performance of a multi-model ensemble of climate models, at resolutions ranging from 250km to 25km. We use a common experimental design including both atmosphere-only and coupled simulations run over the period 1950-2050, with two tracking algorithms applied uniformly across the models. There are overall improvements in tropical cyclone frequency, spatial distribution and intensity in models at 25 km resolution, with several of them able to represent very intense storms. Projected tropical cyclone activity by 2050 generally declines in the South Indian Ocean, while changes in other ocean basins are more uncertain and sensitive to both tracking algorithm and imposed forcings. Coupled models with smaller biases suggest a slight increase in average TC 10m wind speeds by 2050.