  The finite-difference dynamical core based on the equal-interval
latitude-longitude mesh has been widely used for numerical simulations of the
Atmospheric General Circulation Model (AGCM). Previous work utilizes different
filtering schemes to alleviate the instability problem incurred by the unequal
physical spacing at different latitudes, but they all incur high communication
and computation overhead and become a scaling bottleneck. This paper proposes a
highly scalable 3d dynamical core based on a new leap-format finite-difference
computing scheme. It generalizes the usual finite-difference format with
adaptive wider intervals and is able to maintain the computational stability in
the grid updating. Therefore, the costly filtering scheme is eliminated. The
new scheme is parallelized with a shifting communication method and implemented
with fine communication optimizations based on a 3D decomposition. With the
proposed leap-format computation scheme, the communication overhead of the AGCM
is significantly reduced and good load balance is exhibited. The simulation
results verify the correctness of the new leap-format scheme. The new
leap-format dynamical core scales up to 196,608 CPU cores and achieves the
speed of 7.4 simulation-year-per-day (SYPD) and 2.0x speedup on average over
the latest implementation at a high resolution of 25KM.
