A new type of ultra-high resolution atmospheric global circulation model is developed. The new model is designed to perform ''cloud resolving simulations'' by directly calculating deep convection and meso-scale circulations, which play key roles not only in the tropical circulations but in the global circulations of the atmosphere. Since cores of deep convection have a few km in horizontal size, they have not directly been resolved by existing atmospheric general circulation models (AGCMs). In order to drastically enhance horizontal resolution, a new framework of a global atmospheric model is required; we adopted nonhydrostatic governing equations and icosahedral grids to the new model, and call it Nonhydrostatic ICosahedral Atmospheric Model (NICAM). In this article, we review governing equations and numerical techniques employed, and present the results from the unique 3.5-km mesh global experiments-with O(10 9 ) computational nodesusing realistic topography and land/ocean surface thermal forcing. The results show realistic behaviors of multi-scale convective systems in the tropics, which have not been captured by AGCMs. We also argue future perspective of the roles of the new model in the next generation atmospheric sciences.

This article reviews the development of a global non-hydrostatic model, focusing on the pioneering research of the Non-hydrostatic Icosahedral Atmospheric Model (NICAM). Very high resolution global atmospheric circulation simulations with horizontal mesh spacing of approximately O (km) were conducted using recently developed supercomputers. These types of simulations were conducted with a specifically designed atmospheric global model based on a quasi-uniform grid mesh structure and a non-hydrostatic equation system. This review describes the development of each dynamical and physical component of NICAM, the assimilation strategy and its related models, and provides a scientific overview of NICAM studies conducted to date.

Results from global simulations using a nonhydrostatic icosahedral‐grid AGCM with cloud‐resolving resolutions on an aqua planet are discussed. Results depend on the resolution. Simulations with grid intervals of 7 km and 3.5 km include many realistic features in the tropics: hierarchical cloud structures, a Madden‐Julian Oscillation (MJO)‐like intraseasonal oscillation, and diurnal precipitation cycles. Global cloud‐resolving simulations show promise for future climate research. Such models avoid the liabilities associated with cumulus parameterization.

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The shear instability problem in the spherical shallow water system is investigated for three types of wind profiles that are observed at the upper cloud level in Venus. Destabilized Kelvin modes are obtained for all profiles, even when the wind profile is barotropically and inertially stable. The eigenfunctions of these unstable modes are a hybrid of Kelvin modes and continuous modes, which have singularity at the critical latitude. Destabilized Rossby-Kelvin modes are also obtained for the barotropically unstable profile with strong jets. When Lamb parameter ⑀ ϭ (2a⍀) 2 /gH is large, together with other destabilized gravity modes, these modes have the property of inertial instability modes, which are described by preceding studies on the tropical inertial instability. The destabilizing mechanism of unstable modes is described using resonance theory.It is found that the angular-momentum flux is equatorward for almost all growing modes obtained in this study; this result is consistent with what the resonance theory predicts. This momentum transport may contribute to the mechanism of producing the superrotation of the Venus atmosphere based on the meridional circulation. The destabilized Kelvin modes may be considered as a source of the 4-day waves observed in the equatorial region at the cloud top of Venus.