[A-AS08] Weather, Climate, and Environmental Science Studies using High-Performance Computing

[AAS08-08] Challenge to Kilometer-scale Atmospheric Climate Simulations with NICAM

*Daisuke Takasuka^1,2, Chihiro Kodama², Tamaki Suematsu³, Tomoki Ohno⁴, Yohei Yamada², Tatsuya Seiki², Hisashi Yashiro⁵, Masuo Nakano², Hiroaki Miura⁶, Akira T Noda², Tomoe Nasuno², Tomoki Miyakawa¹, Ryusuke Masunaga² (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology, 3.RIKEN Center for Computational Science, 4.Meteorological Research Institute, 5.National Institute for Environmental Studies, 6.Department of Earth and Planetary Science, The University of Tokyo, Tokyo)

Keywords:Global convection-resolving model, Climate simulation

The recent increase in computing power enables us to perform a global kilometer-scale (K-scale) “convection-resolving” model (GCRM) simulation on the climate time scale. While this approach is expected to reduce some uncertainties related to unresolved cloud processes in the climate system, we also found systematic model biases in both climatological statistics and variabilities, irrespective of grid spacing at least for O(1-10)-km scale. Considering that this situation partly comes from the poor constraint of a moisture-convection relation depending on model physics at a K-scale, we have revised the treatment of cloud microphysics and unresolved turbulent diffusion in the Nonhydrostatic ICosahedral Atmospheric Model (NICAM) for improved climate simulations on the K-scale. After this revision, we are now performing a 10-year AGCM-type simulation at 3.5-km horizontal mesh from January 2011 on the supercomputer “Fugaku”, and we have accomplished about 5-year integration. A preliminary analysis reveals that both the model revisions and 3.5-km horizontal resolution can seamlessly improve the reproducibility of atmospheric fields over a wide range of spatio-temporal scales: the climatological mean precipitation and OLR distributions, mid-latitude circulations, the Asian summer monsoon, spontaneous MJO realization, tropical cyclone intensity, and precipitation diurnal cycle. However, we also face challenges associated with the poor representation of low clouds in higher resolution simulations, which should be resolved in the ocean-coupled climate run in terms of the energy balance. This work can be a clue for understanding the merits and issues of K-scale GCRM climate simulations and providing a possible strategy for the future model development.