  The Atmospheric General Circulation Model (AGCM) has been an important
research tool in the study of climate change for decades. As the demand for
high-resolution simulation is becoming urgent, the scalability and simulation
efficiency is faced with great challenges, especially for the
latitude-longitude mesh-based models. In this paper, we propose a highly
scalable 3D atmospheric general circulation model based on leap-format, namely
AGCM-3DLF. Firstly, it utilizes a 3D decomposition method allowing for
parallelism release in all three physical dimensions. Then the leap-format
difference computation scheme is adopted to maintain computational stability in
grid updating and avoid additional filtering at the high latitudes. A novel
shifting window communication algorithm is designed for parallelization of the
unified model. Furthermore, a series of optimizations are conducted to improve
the effectiveness of large-scale simulations. Experiment results in different
platforms demonstrate good efficiency and scalability of the model. AGCM-3DLF
scales up to the entire CAS-Xiandao1 supercomputer (196,608 CPU cores),
attaining the speed of 11.1 simulation-year-per-day (SYPD) at a high resolution
of 25KM. In addition, simulations conducted on the Sunway TaihuLight
supercomputer exhibit a 1.06 million cores scalability with 36.1% parallel
efficiency.
