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This study presents an assessment of three‐dimensional structures of hydrometeors simulated by the NICAM, global nonhydrostatic atmospheric model without cumulus parameterization, using multiple satellite data sets. A satellite simulator package (COSP: the CFMIP Observation Simulator Package) is employed to consistently compare model output with ISCCP, CALIPSO, and CloudSat satellite observations. Special focus is placed on high thin clouds, which are not observable in the conventional ISCCP data set, but can be detected by the CALIPSO observations. For the control run, the NICAM simulation qualitatively captures the geographical distributions of the high, middle, and low clouds, even though the horizontal mesh spacing is as coarse as 14 km. The simulated low cloud is very close to that of the CALIPSO low cloud. Both the CloudSat observations and NICAM simulation show a boomerang‐type pattern in the radar reflectivity‐height histogram, suggesting that NICAM realistically simulates the deep cloud development process. A striking difference was found in the comparisons of high thin cirrus, showing overestimated cloud and higher cloud top in the model simulation. Several model sensitivity experiments are conducted with different cloud microphysical parameters to reduce the model‐observation discrepancies in high thin cirrus. In addition, relationships among clouds, Hadley circulation, outgoing longwave radiation and precipitation are discussed through the sensitivity experiments. Key Points Clouds in global non‐hydrostatic model are assessed using satellite simulators Model qualitatively simulates clouds observed by ISCCP, CALIPSO and CloudSat Sensitivity experiments show improvement of the cirrus bias in the model

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.

In estimates of climate sensitivity obtained from global models, the need to represent clouds introduces a great deal of uncertainty. To address this issue, approaches using a high-resolution global non-hydrostatic model are promising: the model captures cloud structure by explicitly simulating meso-scale convective systems, and the results compare reasonably well with satellite observations. We review the outcomes of a 5-year project aimed at reducing the uncertainty in climate models due to cloud processes using a global non-hydrostatic model. In our project, which was conducted as a subgroup of the Program for Risk Information on Climate Change, or SOUSEI, we use the non-hydrostatic icosahedral atmospheric model (NICAM) to study cloud processes related to climate change. NICAM performs numerical simulations with much higher resolution (about 7 km or 14 km mesh) than conventional global climate models (GCMs) using cloud microphysics schemes without a cumulus parameterization scheme, which causes uncertainties in climate projection.The subgroup had three research targets: analyzing cloud changes in global warming simulations with NICAM with the time-slice approach, sensitivity of the results to the cloud microphysics scheme employed, and evaluating circulation changes due to global warming. The research project also implemented a double-moment bulk cloud microphysics scheme and evaluated its results using satellite observation, as well as comparing it with a bin cloud microphysics scheme. The future projection simulations show in general increase in high cloud coverage, contrary to results with other GCMs. Changes in cloud horizontal-size distribution size and structures of tropical/extratropical cyclones can be discussed with high resolution simulations. At the conclusion of our review, we also describe the future prospects of research for global warming using NICAM in the program that followed SOUSEI, known as TOUGOU.

In relatively coarse-resolution atmospheric models, cumulus parameterization helps account for the effect of subgrid-scale convection, which produces supplemental rainfall to the grid-scale precipitation and impacts the diurnal cycle of precipitation. In this study, the diurnal cycle of precipitation was studied using the new simplified Arakawa-Schubert scheme in a global non-hydrostatic atmospheric model, i.e., the Yin-Yang-grid Unified Model for the Atmosphere. Two new diagnostic closures and a convective trigger function were suggested to emphasize the job of the cloud work function corresponding to the free tropospheric large-scale forcing. Numerical results of the 0.25-degree model in 3-month batched real-case simulations revealed an improvement in the diurnal precipitation variation by using a revised trigger function with an enhanced dynamical constraint on the convective initiation and a suitable threshold of the trigger. By reducing the occurrence of convection during peak solar radiation hours, the revised scheme was shown to be effective in delaying the appearance of early-afternoon rainfall peaks over most land areas and accentuating the nocturnal peaks that were wrongly concealed by the more substantial afternoon peak. In addition, the revised scheme enhanced the simulation capability of the precipitation probability density function, such as increasing the extremely low- and high-intensity precipitation events and decreasing small and moderate rainfall events, which contributed to the reduction of precipitation bias over mid-latitude and tropical land areas.

The definition of a reference state close to the realistic atmosphere in an atmospheric model is essential for deriving prognostic deviations and improving numerical accuracy. In this study, a new dynamical framework allowing easy switching between a one-dimensional (1D) and a three-dimensional (3D) time-independent reference state is developed for the semi-implicit semi-Lagrangian solver in a global non-hydrostatic atmospheric model on Yin–Yang grids. The 3D reference state is introduced with consideration of additional horizontal gradient terms of reference-state terms, which is different from the 1D reference state. It is characterized by reduced magnitude of deviations, more accurate pressure gradient force, as well as alleviated numerical noise. Four idealized benchmark tests and multiple full-physics real-case forecasts are carried out to assess the impact of the 3D and 1D reference states. The 3D reference state shows significant advantages in the simulation of atmospheric transport and wave propagation in the idealized experiments. In the real-case forecasts, batched forecasts from June to August 2021 show a comprehensive improvement in medium-range prediction by using the 3D reference state. The new scheme achieves an enhanced prediction skill for large-scale circulation and extends the effective forecast period by 0.8 days in the Northern Hemisphere.

This article reviews the major outcomes of a 5-year (2011–2016) project using the K computer to perform global numerical atmospheric simulations based on the non-hydrostatic icosahedral atmospheric model (NICAM). The K computer was made available to the public in September 2012 and was used as a primary resource for Japan’s Strategic Programs for Innovative Research (SPIRE), an initiative to investigate five strategic research areas; the NICAM project fell under the research area of climate and weather simulation sciences. Combining NICAM with high-performance computing has created new opportunities in three areas of research: (1) higher resolution global simulations that produce more realistic representations of convective systems, (2) multi-member ensemble simulations that are able to perform extended-range forecasts 10–30 days in advance, and (3) multi-decadal simulations for climatology and variability. Before the K computer era, NICAM was used to demonstrate realistic simulations of intra-seasonal oscillations including the Madden-Julian oscillation (MJO), merely as a case study approach. Thanks to the big leap in computational performance of the K computer, we could greatly increase the number of cases of MJO events for numerical simulations, in addition to integrating time and horizontal resolution. We conclude that the high-resolution global non-hydrostatic model, as used in this five-year project, improves the ability to forecast intra-seasonal oscillations and associated tropical cyclogenesis compared with that of the relatively coarser operational models currently in use. The impacts of the sub-kilometer resolution simulation and the multi-decadal simulations using NICAM are also reviewed.

Previous aqua-planet simulations reported in the literature have shown the existence of both single and double inter-tropical convergence zones (ITCZs). In this study, horizontal grid resolution strongly affects ITCZ morphology as well as the amount of tropical precipitation through its influence on resolved dynamics. The grid adaptation capability of our global model enables simulations that separate the influence of tropical and extra-tropical dynamics on both the ITCZ and tropical precipitation. The presence of single versus double ITCZs in our aqua-planet simulations depends on the resolution of convectively coupled equatorial waves. When the tropical resolution is sufficiently high to resolve prominent equatorial waves a double ITCZ occurs, otherwise a single ITCZ occurs. In contrast, tropical resolution does not affect the magnitude of tropical precipitation in our aqua-planet simulations. Instead the magnitude is sensitive to extra-tropical resolution, through its influence on the strength of baroclinic eddies and their forcing of the Hadley circulation.

Rapid expansion of exotic bamboos has lowered species diversity in Japan's ecosystems by hampering native plant growth. The invasive potential of bamboo, facilitated by global warming, may also affect other countries with developing bamboo industries. We examined past (1975–1980) and recent (2012) distributions of major exotic bamboos (Phyllostachys edulis and P. bambusoides) in areas adjacent to 145 weather stations in central and northern Japan. Bamboo stands have been established at 17 sites along the latitudinal and altitudinal distributional limit during the last three decades. Ecological niche modeling indicated that temperature had a strong influence on bamboo distribution. Using mean annual temperature and sun radiation data, we reproduced bamboo distribution (accuracy = 0.93 and AUC (area under the receiver operating characteristic curve) = 0.92). These results infer that exotic bamboo distribution has shifted northward and upslope, in association with recent climate warming. Then, we simulated future climate data and projected the climate change impact on the potential habitat distribution of invasive bamboos under different temperature increases (i.e., 1.5°C, 2.0°C, 3.0°C, and 4.0°C) relative to the preindustrial period. Potential habitats in central and northern Japan were estimated to increase from 35% under the current climate (1980–2000) to 46%–48%, 51%–54%, 61%–67%, and 77%–83% under 1.5°C, 2.0°C, 3.0°C, and 4.0°C warming levels, respectively. These infer that the risk areas can increase by 1.3 times even under a 1.5°C scenario and expand by 2.3 times under a 4.0°C scenario. For sustainable ecosystem management, both mitigation and adaptation are necessary: bamboo planting must be carefully monitored in predicted potential habitats, which covers most of Japan. We examined past and recent distributions of major exotic bamboos in central and northern Japan. Bamboo stands have been established at 17 sites along the latitudinal and altitudinal distributional limit during the last three decades. Ecological niche modelling projected that potential habitats will increase from 35% under the current climate (1980–2000) to 46–48%, 51–54%, 61–67%, and 77–83% under 1.5°C, 2.0°C, 3.0°C, and 4.0°C warming levels relative to the pre‐industrial period, respectively.

This study evaluates the capability of a non-hydrostatic global climate model with grid stretching (CEU) that uses NCAR Community Atmospheric Model (CAM) physics and EULAG dynamics. We compare CEU rainfall with that produced by CAM using finite volume dynamics (CFV). Both models simulated climate from 1996 to 2000, using the same parameterization schemes. CEU and CFV both simulate well the observed global rainfall pattern. However, with same grid, CEU performs better than CFV in simulating the annual cycles of precipitation over our target region of West Africa. The reason is that it simulates African easterly jet and monsoon circulations better than CFV. CEU simulations with horizontal grid stretching to 0.5° are markedly better than those using CAM’s standard 2.0°×2.5° grid.