Seiya Nishizawa

Impacts of cloud microphysics on trade wind cumulus: which cloud microphysics processes contribute to the diversity in a large eddy simulation?

This study investigated the impact of several cloud microphysical schemes on the trade wind cumulus in the large eddy simulation model. To highlight the differences due to the cloud microphysical component, we developed a fully compressible large eddy simulation model, which excluded the implicit scheme and approximations as much as possible. The three microphysical schemes, the one-moment bulk, two-moment bulk, and spectral bin schemes were used for sensitivity experiments in which the other components were fixed. Our new large eddy simulation model using a spectral bin scheme successfully reproduced trade wind cumuli, and reliable model performance was confirmed. Results of the sensitivity experiments indicated that precipitation simulated by the one-moment bulk scheme started earlier, and its total amount was larger than that of the other models. By contrast, precipitation simulated by the two-moment scheme started late, and its total amount was small. These results support those of a previous study. The analyses revealed that the expression of two processes, (1) the generation of cloud particles and (2) the conversion from small droplets to raindrops, were crucial to the results. The fast conversion from cloud to rain and the large amount of newly generated cloud particles at the cloud base led to evaporative cooling and subsequent stabilization in the sub-cloud layer. The latent heat released at higher layers by the condensation of cloud particles resulted in the development of the boundary layer top height.

Rossby Waves and Jets in Two-Dimensional Decaying Turbulence on a Rotating Sphere

Abstract Jet formation in decaying two-dimensional turbulence on a rotating sphere is reviewed from the viewpoint of Rossby waves. A series of calculations are performed to confirm the behavior of zonal mean flow generation on the parameter space of the rotation rate Ω and Froude number Fr. When the flow is nondivergent and Ω is large, intense easterly circumpolar jets tend to emerge in addition to the appearance of a banded structure of zonal mean flows with alternating flow directions. When the system allows surface elevation, circumpolar jets disappear and an equatorial easterly jet emerges with increasing Fr. The appearance of the intense easterly jets can be understood by the angular-momentum transport associated with the generation, propagation, and absorption of Rossby waves. When the flow is nondivergent, long Rossby waves tend to be absorbed near the poles. In contrast, when Fr is large, Rossby waves can hardly propagate poleward and tend to be absorbed near the equator.

Toward a General I/O Arbitration Framework for netCDF Based Big Data Processing

On the verge of the convergence between high performance computing HPC and Big Data processing, it has become increasingly prevalent to deploy large-scale data analytics workloads on high-end supercomputers. Such applications often come in the form of complex workflows with various different components, assimilating data from scientific simulations as well as from measurements streamed from sensor networks, such as radars and satellites. For example, as part of the next generation flagship post-K supercomputer project of Japan, RIKEN is investigating the feasibility of a highly accurate weather forecasting system that would provide a real-time outlook for severe guerrilla rainstorms. One of the main performance bottlenecks of this application is the lack of efficient communication among workflow components, which currently takes place over the parallel file system. In this paper, we present an initial study of a direct communication framework designed for complex workflows that eliminates unnecessary file I/O among components. Specifically, we propose an I/O arbitrator layer that provides direct parallel data transfer among job components that rely on the netCDF interface for performing I/O operations, with only minimal modifications to application code. We present the design and an early evaluation of the framework on the K Computer using upi?źto 4800 nodes running RIKENs experimental weather forecasting workflow as a case study.

Automatic generation of efficient codes from mathematical descriptions of stencil computation

Programming in HPC is a tedious work. Therefore functional programming languages that generate HPC programs have been proposed. However, they are not widely used by application scientists, because of learning barrier, and lack of demonstrated application performance. We have designed Formura which adopts application-friendly features such as typed rational array indices. Formura users can describe mathematical concepts such as operation over derivative operators using functional programming. Formura allows intuitive expression over array elements while ensuring the program is a stencil computation, so that state-of-the-art stencil optimization techniques such as temporal blocking is always applied to Formura-generated program. We demonstrate the usefulness of Formura by implementing a preliminary below-ground biology simulation. Optimized C-code are generated from 672 bytes of Formura program. The simulation was executed on the full nodes of the K computer, with 1.184 Pflops, 11.62% floating-point-instruction efficiency, and 31.26% memory throughput efficiency.

Classification of Polar-Night Jet Oscillations and Their Relationship to Fast and Slow Variations in a Global Mechanistic Circulation Model of the Stratosphere and Troposphere

Abstract Polar-night jet oscillation (PJO), which is a low-frequency intraseasonal oscillatory variation in the winter stratosphere, is analyzed statistically with a 14 000-yr-long dataset obtained with an idealized global mechanistic circulation model of the stratosphere and troposphere. After performing an empirical orthogonal function (EOF) analysis on the low-pass-filtered time series of the northern polar temperature, 10 647 PJO events are identified and classified into four groups. About 80% of them are two groups of warm events while the rest are two groups of cold events, which are newly identified variations with opposite sign from the warm events by the same EOF analysis. All of them show slow downward propagations of a positive or negative temperature anomaly, with a relatively short or long lifetime. Composite analysis with such a large number of samples shows that each group has its own typical relationship to unfiltered relatively fast variations in the polar stratosphere known as stratosphe...

A flexible I/O arbitration framework for netCDF‐based big data processing workflows on high‐end supercomputers

On the verge of the convergence between high‐performance computing and Big Data processing, it has become increasingly prevalent to deploy large‐scale data analytics workloads on high‐end supercomputers. Such applications often come in the form of complex workflows with various different components, assimilating data from scientific simulations as well as from measurements streamed from sensor networks, such as radars and satellites. For example, as part of the Flagship 2020 (post‐K) supercomputer project of Japan, RIKEN is investigating the feasibility of a highly accurate weather forecasting system that would provide a real‐time outlook for severe guerrilla rainstorms. One of the main performance bottlenecks of this application is the lack of efficient communication among workflow components, which currently takes place over the parallel file system.In this paper, we present an initial study of a direct communication framework designed for complex workflows that eliminates unnecessary file I/O among components. Specifically, we propose an I/O arbitration layer that provides direct parallel data transfer (both synchronous and asynchronous) among job components that rely on the netCDF interface for performing I/O operations. Our solution requires only minimal modifications to application code. Moreover, we propose a configuration file–based approach that allows users to specify the desired data transfer pattern among workflow components, offering a general solution for different application contexts. We present a preliminary evaluation of the proposed framework on the K Computer (running on up to 4800 compute nodes) using RIKENs experimental weather forecasting workflow as a case study.

CONeP: A cost-effective online nesting procedure for regional atmospheric models

Abstract We propose a cost-effective online nesting procedure (CONeP) for regional atmospheric models to improve computational efficiency. The conventional procedure of online nesting is ineffective because computations are executed sequentially for each domain, and it does not enable users freely to determine the number of computational nodes. However, CONeP can completely avoid this limitation through three actions: 1) splitting the processes into multiple subgroups; 2) making each subgroup manage just one domain; and 3) executing the computations for each domain simultaneously. Since users can assign an optimal number of nodes to each domain, the model with CONeP is computationally efficient. We demonstrate the computational advantage of CONeP over the conventional procedure, comparing the elapsed times with both procedures on a supercomputer. The elapsed time with CONeP is markedly shorter than that observed with the conventional procedure using the same number of computational nodes. This advantage becomes more significant as the number of nesting domains increases.

Simulations of below-ground dynamics of fungi: 1.184 pflops attained by automated generation and autotuning of temporal blocking codes

Stencil computation has many applications in science and engineering, thus many optimization techniques such as temporal blocking have been developed. They are, however, rarely used in real-world applications, since a large amount of careful programming is required for even the simplest of stencils. We introduce Formura, a domain specific language that provides easy access to optimized stencil computations. Higher-order integration schemes can be defined using mathematical notations. Formura generates C code with MPI calls and performs autotuning. Hence its performance is portable to most distributed-memory computers. We show the scientific applicability of Formura by performing magnetohydrodynamics (MHD) and belowground biology simulations. Ability to reach bytes-per-flops ratio only attainable by temporal blocking is demonstrated. We also demonstrate scaling up to the full nodes of the K computer, with 1.184 Pflops, 11.62% floating-pointoperation efficiency, and 31.26% memory throughput efficiency.

A Numerical Study of Convection in a Condensing CO2 Atmosphere under Early Mars-Like Conditions

AbstractCloud convection of a CO2 atmosphere where the major constituent condenses is numerically investigated under a setup idealizing a possible warm atmosphere of early Mars, utilizing a two-dimensional cloud-resolving model forced by a fixed cooling profile as a substitute for a radiative process. The authors compare two cases with different critical saturation ratios as condensation criteria and also examine sensitivity to number mixing ratio of condensed particles given externally.When supersaturation is not necessary for condensation, the entire horizontal domain above the condensation level is continuously covered by clouds irrespective of number mixing ratio of condensed particles. Horizontal-mean cloud mass density decreases exponentially with height. The circulations below and above the condensation level are dominated by dry cellular convection and buoyancy waves, respectively.When 1.35 is adopted as the critical saturation ratio, clouds appear exclusively as intense, short-lived, quasi-period...

Jet formation in decaying two-dimensional turbulence on a rotating sphere

Jet formation in decaying two-dimensional turbulence on a rotating sphere is reviewed from the view point of wave mean-flow interaction for both shallow-water case and non-divergent case as the limit of Fr (Froude number) going to zero. A series of computations have been performed by ourselves to confirm the behavior of zonal mean zonal flow generation on the parameter space of the nondimensional rotation rate Ω and Fr. When the flow is non-divergent and Ω is large, intense retrograde circumpolar jets tend to emerge in addition to a banded structure of mean zonal flows with alternating flow directions in middle and low latitudes. As Fr increases, the circumpolar jets disappear and a retrograde jet emerges in the equatorial region. The appearance of the intense retrograde jets can be understood by the angular momentum transport associated with the propagation and absorption of Rossby waves. When the flow is non-divergent, long Rossby waves tend to be absorbed around the poles. In contrast, when Fr is large, Rossby waves can hardly propagate poleward and tend to be absorbed near the equator. The direction of the equatorial jet, however, is not always retrograde. Our ensemble experiments showed the emergence of a prograde jet, though less likely. This result is contrasted with the previous studies that reported retrograde equatorial jets in most cases for shallow-water turbulence. Furthermore, a mean zonal flow induced by wave-wave interactions was examined using a weakly nonlinear model to clarify the acceleration mechanisms of the equatorial jet. The second-order acceleration is induced by the Rossby waves and mixed Rossby-gravity waves and the acceleration mechanism can be categorized into two types.