Papers - Sugimoto, Norihiko

Super-rotation independent of horizontal diffusion reproduced in a Venus GCM

Sugimoto N., Fujisawa Y., Komori N., Kashimura H., Takagi M., Matsuda Y.

Earth, Planets and Space (Earth, Planets and Space)  75 ( 1 )  2023.12

Research paper (scientific journal), Joint Work, Lead author, Corresponding author, Accepted,  ISSN  13438832

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Horizontal diffusion and resolution are important factors to generate and maintain the super-rotation in the general circulation model (GCM) because waves which transport angular momentum are sensitive to them. Here, we investigated how the super-rotation depends on the magnitude of horizontal hyper diffusion using a Venus atmospheric GCM with medium and high resolutions. In both the runs, we found a parameter range where the structure of fully developed super-rotation is almost independent of the magnitude of horizontal diffusion. Spectral analysis shows that unrealistically strong super-rotation is developed when medium-scale disturbances are dissipated by stronger horizontal diffusion. On the other hand, artificially weak super-rotation is also realized because spurious small-scale disturbances are accumulated when the horizontal diffusion is too weak. Graphical Abstract: [Figure not available: see fulltext.]

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Vertical-Wind-Induced Cloud Opacity Variation in Low Latitudes Simulated by a Venus GCM

Karyu H., Kuroda T., Itoh K., Nitta A., Ikeda K., Yamamoto M., Sugimoto N., Terada N., Kasaba Y., Takahashi M., Hartogh P.

Journal of Geophysical Research: Planets (Journal of Geophysical Research: Planets)  128 ( 2 )  2023.02

Research paper (scientific journal), Joint Work, Accepted,  ISSN  21699097

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Venusian cloud structure and variation are strongly linked to atmospheric dynamics. Past near-infrared measurements have found cloud variation such as zonal-wavenumber-1 cloud marking and cloud discontinuity. However, their formation mechanism is still not well understood. To investigate the Venusian cloud structure and its variation, we have developed a Venus GCM incorporating cloud condensation, evaporation, sedimentation, and simple atmospheric chemistry to represent the H2SO4 cycle. The GCM takes into account cloud particles with radii of 0.3, 1.0, 1.26, and 3.13 μm (Modes 1, 2, 2', and 3, respectively) based on past in situ observations. The simulated latitudinal trends of the cloud top and bottom structures are qualitatively consistent with past observations. Zonally averaged cloud mass loading was the largest and smallest in low and middle latitudes, respectively, and maintained by a mechanism similar to that of past 2-D numerical studies. At the equator, the column integrated optical depth at 1 μm varied between 33 and 50, which is in good agreement with past observations. This variation consists of two types of cloud mass loading changes between 46 and 52 km. One is a rapid small-scale variation induced by gravity waves. The other is a quasi-periodic zonal-wavenumber-1 variation coupled with an equatorial Kelvin wave, which is similar to the observed cloud marking. Our results showed that the vertical wind associated with the Kelvin wave is essential for maintaining the quasi-periodic variation, along with the condensation/evaporation by the temperature variation. The vertical-wind-induced cloud generation also suggests a relationship to the cloud discontinuity.

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The first assimilation of Akatsuki single-layer winds and its validation with Venusian atmospheric waves excited by solar heating

Fujisawa Y., Murakami S.y., Sugimoto N., Takagi M., Imamura T., Horinouchi T., Hashimoto G.L., Ishiwatari M., Enomoto T., Miyoshi T., Kashimura H., Hayashi Y.Y.

Scientific Reports (Scientific Reports)  12 ( 1 )  2022.12

Research paper (scientific journal), Joint Work, Accepted

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The planetary missions including the Venus Climate Orbiter ‘Akatsuki’ provide new information on various atmospheric phenomena. Nevertheless, it is difficult to elucidate their three-dimensional structures globally and continuously only from observations because satellite observations are considerably limited in time and space. We constructed the first ‘objective analysis’ of Venus’ atmosphere by assimilating cloud-top horizontal winds on the dayside from the equator to mid-latitudes, which is frequently obtained from Akatsuki's Ultraviolet Imager (UVI). The three-dimensional structures of thermal tides, found recently to play a crucial role in maintaining the super rotation, are greatly improved by the data assimilation. This result is confirmed by comparison with Akatsuki's temperature observations. The momentum transport caused by the thermal tides and other disturbances are also modified by the wind assimilation and agrees well with those estimated from the UVI observations. The assimilated dataset is reliable and will be open to the public along with the Akatsuki observations for further investigation of Venus’ atmospheric phenomena.

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Akatsuki LIR observing system simulation experiments evaluated by thermal tides in the Venus atmosphere

Sugimoto N., Fujisawa Y., Komori N., Ando H., Kouyama T., Takagi M.

Geoscience Letters (Geoscience Letters)  9 ( 1 )  2022.12

Research paper (scientific journal), Joint Work, Lead author, Corresponding author, Accepted

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Impacts of temperature assimilation on Venusian thermal tides are investigated by the observing system simulation experiments assuming Akatsuki Longwave Infrared Camera (LIR) observations. Synthetic temperature data are prepared by a Venusian general circulation model (VGCM) to test if LIR temperature data resolves a discrepancy in the structure of thermal tides between observations and the VGCM. They are assimilated at 70 km altitude with several combinations of frequency and horizontal region. The result shows that the three-dimensional structure of thermal tides is significantly improved not only in temperature but also in horizontal wind, even if observations are available only at a limited frequency of 6-hourly or on the dayside. The zonal–mean zonal wind and temperature fields are also modified at 60–80 km altitudes globally through the vertical momentum transport of thermal tides. It would be promising to assimilate Akatsuki LIR observations to produce realistic objective analysis of the Venus atmosphere.

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A Sensitivity Study of the Thermal Tides in the Venusian Atmosphere: Structures and Dynamical Effects on the Superrotation

Suzuki A., Takagi M., Ando H., Imai M., Sugimoto N., Matsuda Y.

Journal of Geophysical Research: Planets (Journal of Geophysical Research: Planets)  127 ( 6 )  2022.06

Research paper (scientific journal), Joint Work, Accepted,  ISSN  21699097

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In order to resolve discrepancy between recent observational and numerical studies on the thermal tides in the Venusian atmosphere, we investigated by means of a general circulation model how the thermal tides are affected by the static stability in and above the upper cloud layer by using three different distributions of the static stability. The results show that the vertical structure of the semidiurnal tide, which propagates vertically, is strongly affected by the static stability. The diurnal tide, which has an equivalent barotropic structure in 62–73 km altitudes, becomes weaker with the higher static stability although its vertical structure is almost unchanged. The horizontal distribution of the thermal tides with the realistic static stability distribution, which is consistent with radio occultation measurements, agrees with the observations at the cloud top. The meridional angular momentum flux associated with the thermal tides is equatorward in low latitudes near the altitude where the equatorial zonal-mean wind takes its maximum. This result is consistent with the recent Akatsuki UVI observations, suggesting that the thermal tides could contribute to the maintenance of the superrotation in the equatorial region near the cloud top. In the most realistic case, the zonal-mean zonal wind is effectively accelerated at rates of 0.2–0.5 m s−1 day−1 in low latitudes at altitudes of 52–76 km by both the meridional and vertical angular momentum transports. The thermal tides also induce significant meridional heat flux, which cannot be ignored in the dynamical effect on the zonal-mean zonal wind.

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A GCM Study on the 4-Day and 5-Day Waves in the Venus Atmosphere

Takagi M., Ando H., Sugimoto N., Matsuda Y.

Journal of Geophysical Research: Planets (Journal of Geophysical Research: Planets)  127 ( 4 )  2022.04

Research paper (scientific journal), Joint Work, Accepted,  ISSN  21699097

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The 4-day and 5-day waves observed at the cloud top in the Venus atmosphere are expected to play important roles in the maintenance of the general circulation. However, they have not been reproduced in Venus general circulation models (GCMs) so far, and their structures and excitation mechanisms remain poorly understood. Using an improved Venus GCM, we succeeded in reproducing the planetary-scale 3.3-day and 5.8-day waves, which are considered to correspond to the 4-day and 5-day waves, respectively. We also obtained the 4.8-day wave, whose structure is similar to that of the 5.8-day wave. The three-dimensional structures of these waves consist of a Kelvin wave in low latitudes and one or two Rossby waves in mid- and high latitudes that are connected at the critical latitudes. This fact indicates that they are excited by the Rossby-Kelvin instability. The Kelvin mode of the 3.3-day wave could be observed at the cloud top, while those of the 5.8-day and 4.8-day waves cannot be observed due to the critical levels. The mid-latitude Rossby mode of the 5.8-day wave is dominated by mid-latitude vortices symmetric about the equator at the cloud top; this result is quite consistent with the observations. The equatorward angular momentum flux induced by these waves could contribute to the Venus superrotation. The Rossby modes of the 5.8-day and 4.8-day waves have baroclinic structures in the cloud layer inducing significant poleward heat flux. The planetary-scale structures symmetric about the equator observed in the Venus atmosphere could be explained by these waves.

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Road to the first star: Venus orbiter from Japan (49)‒ first challenges to data assimilation for the Venus atmosphere:Combination of Akatsuki observations and numerical simulations ‒

Sugimoto N., Fijisawa Y., Ando H., Takagi M., AFES-Venus team, ALEDAS-V team

Planetary People - The Japanese Society for Planetary Sciences (The Japanese Society for Planetary Sciences)  31 ( 1 ) 88 - 93 2022.03

Research paper (scientific journal), Joint Work, Lead author, Corresponding author,  ISSN  0918273X

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<p>金星探査機「あかつき」の観測データを最大限に活用するために，我々の研究グループでは金星大気の全球的な流れを地球シミュレータ上で計算する数値モデル「AFES-Venus」と，局所アンサンブルカルマンフィルタを用いて観測データをAFES-Venusへと同化するデータ同化システム「ALEDAS-V」を開発してきました．前回からの進展として，AFES-Venusでは，超長時間積分による静止状態からのスーパーローテーションの再現，超高解像度計算による熱潮汐波からの大気重力波の自発的な放射の発見，雲物理過程の導入，低安定度層などの温度構造の再現，加熱や安定度の改良によるケルビン波や熱潮汐波の現実的な再現，などに成功しました．また，ALEDAS-Vでは，「あかつき」やVenus Expressの紫外画像で得られた水平風を同化し，金星大気初の現実的かつ時空間的に偏りのないデータセットの公開への準備が進めているほか，仮想的な観測データを同化することにより観測の「有効性」を検証するための観測システムシミュレーション実験なども実施しています．これらの成果を簡単に紹介し，今後の展望についての報告をしたいと思います．</p>

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Dynamical Effect on Static Stability of the Venus Atmosphere Simulated Using a General Circulation Model: A Comparison With Radio Occultation Measurements

Ando H., Takaya K., Takagi M., Sugimoto N., Imamura T., Sagawa H., Tellmann S., Pätzold M., Matsuda Y., Häusler B., Limaye S., Choudhary R.K., Antonita M.

Journal of Geophysical Research: Planets (Journal of Geophysical Research: Planets)  127 ( 3 )  2022.03

Research paper (scientific journal), Joint Work, Accepted,  ISSN  21699097

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Distributions of temperature and static stability in the Venus atmosphere consistent with recent radio occultation measurements are reproduced using a general circulation model. A low-stability layer is maintained at low- and mid-latitudes at 50–60 km altitude and is sandwiched by high- and moderate-stability layers extending above 60 and below 50 km, respectively. In the polar region, the low-stability layer is located at 46–63 km altitude and the relatively low-stability layer is also found at 40–46 km altitude. To investigate how these thermal structures form, we examine the dynamical effects of the atmospheric motions on the static stability below 65 km altitude. The results show that the heat transports due to the mean meridional circulation and disturbances are important in low-latitudes. The mid- and high-latitudes above ∼47 km are destabilized by radiative processes and stabilized by the disturbances, which are mainly associated with baroclinic Rossby-type waves. Below ∼47 km altitude, the polar region is destabilized by the dynamical processes which induce the appreciable equatorward heat transport, which might be related to Rossby waves in the sub-cloud region, although the suppression of convective adjustment at cloud heights leads to stabilization at 43–47 km.

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Kelvin Wave and Its Impact on the Venus Atmosphere Tested by Observing System Simulation Experiment

Sugimoto N., Fujisawa Y., Shirasaka M., Abe M., Murakami S., Kouyama T., Ando H., Takagi M., Yamamoto M.

Atmosphere (Atmosphere)  13 ( 2 )  2022.02

Research paper (scientific journal), Joint Work, Lead author, Corresponding author, Accepted

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At the cloud top of the Venus atmosphere, equatorial Kelvin waves have been observed and are considered to play an important role in the super-rotation. We were able to reproduce the wave in a general circulation model (GCM) by conducting an observing system simulation experiment (OSSE) with the help of a data assimilation system. The synthetic horizontal winds of the Kelvin wave produced by the linear wave propagating model are assimilated at the cloud top (~70 km) in realistic conditions, assuming they are obtained from cloud tracking of ultra-violet images (UVI) taken by the Venus orbiters. It is demonstrated using Eliassen–Palm (EP) fluxes that the reproduced Kelvin wave transports angular momentum and plays an important role in the magnitude and structure of the super-rotation, causing the acceleration and deceleration of zonal wind of ~0.1 m/s day−1. The conditions required in order to reproduce the Kelvin wave have also been investigated. It is desirable to have 24 hourly dayside satellite observations in an equatorial orbit, such as the Akatsuki Venus climate orbiter. The results of this type of data assimilation study will be useful in the planning of future observation missions to the atmospheres of planets.

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Generation of gravity waves from thermal tides in the Venus atmosphere

Sugimoto N., Fujisawa Y., Kashimura H., Noguchi K., Kuroda T., Takagi M., Hayashi Y.Y.

Nature Communications (Nature Communications)  12 ( 1 )  2021.12

Research paper (scientific journal), Joint Work, Lead author, Corresponding author, Accepted

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Gravity waves play essential roles in the terrestrial atmosphere because they propagate far from source regions and transport momentum and energy globally. Gravity waves are also observed in the Venus atmosphere, but their characteristics have been poorly understood. Here we demonstrate activities of small-scale gravity waves using a high-resolution Venus general circulation model with less than 20 and 0.25 km in the horizontal and vertical grid intervals, respectively. We find spontaneous gravity wave radiation from nearly balanced flows. In the upper cloud layer (~70 km), the thermal tides in the super-rotation are primary sources of small-scale gravity waves in the low-latitudes. Baroclinic/barotropic waves are also essential sources in the mid- and high-latitudes. The small-scale gravity waves affect the three-dimensional structure of the super-rotation and contribute to material mixing through their breaking processes. They propagate vertically and transport momentum globally, which decelerates the super-rotation in the upper cloud layer (~70 km) and accelerates it above ~80 km.

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Quasi-Periodic Variation of the Lower Equatorial Cloud Induced by Atmospheric Waves on Venus

Ando H., Takagi M., Sagawa H., Sugimoto N., Sekiguchi M., Matsuda Y.

Journal of Geophysical Research: Planets (Journal of Geophysical Research: Planets)  126 ( 6 )  2021.06

Research paper (scientific journal), Joint Work, Accepted,  ISSN  21699097

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A quasi-periodic variation of the lower cloud amount induced by atmospheric waves was reproduced in our Venus general circulation model for the first time. This result agrees well with previous 2.3-μm infrared nightside measurements. The cloud amount variation is mainly caused by a temperature fluctuation with a period of ∼5.5 Earth days near the cloud base, which is associated with a Kelvin-like gravity wave with a zonal wavenumber-1 structure. Our result might be able to explain the mechanism of the periodical variation of the brightness in the Venusian low-latitudes shown by the previous 2.3-μm infrared nightside measurements. It is also suggested that the temporal variation of the mass loading due to the temperature fluctuation is related to the abrupt appearance and disappearance of the large cloud particle (Mode 3) and explains the sharp discontinuity of the lower cloud found by the recent Akatsuki and ground-based infrared nightside measurements.

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Observing system simulation experiment to reproduce kelvin wave in the venus atmosphere

Sugimoto N., Fujisawa Y., Shirasaka M., Hosono A., Abe M., Ando H., Takagi M., Yamamoto M.

Atmosphere (Atmosphere)  12 ( 1 ) 1 - 16 2021.01

Research paper (scientific journal), Joint Work, Lead author, Corresponding author, Accepted

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Planetary-scale 4-day Kelvin-type waves at the cloud top of the Venus atmosphere have been reported from the 1980s, and their significance for atmospheric dynamics has been pointed out. However, these waves have not been reproduced in Venus atmospheric general circulation models (VGCMs). Recently, horizontal winds associated with the planetary-scale waves at the cloud top have been obtained from cloud images taken by cameras onboard Venus orbiters, which could enable us to clarify the structure and roles of Kelvin-type waves. In order to examine this possibility, our team carried out an idealized observing system simulation experiment (OSSE) with a data assimilation system which we developed. The wind velocity data provided by a CCSR/NIES (Center for Climate System Research/National Institute for Environmental Studies) VGCM where equatorial Kelvin-type waves were assumed below the cloud bottom was used as idealized observations. Results show that 4-day planetary-scale Kelvin-type waves are successfully reproduced if the wind velocity between 15◦ S and 15◦ N latitudes is assimilated every 6 h at 70 km altitude. It is strongly suggested that the Kelvin-type waves could be reproduced and investigated by the data assimilation with the horizontal wind data derived from Akatsuki ultraviolet images. The present results also contribute to planning future missions for understanding planetary atmospheres.

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Orbit Design Optimization for Planetary Crosslink Radio Occultation

Yamamoto Tomotaka, Ikari Satoshi, Ando Hiroki, Imamura Takeshi, Hosono Asako, Abe Mirai, Fujisawa Yukiko, Sugimoto Norihiko, Kawabata Yosuke, Funase Ryu, Nakasuka Shinichi

JOURNAL OF THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES (THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES)  69 ( 5 ) 179 - 186 2021

Research paper (scientific journal), Joint Work, Accepted,  ISSN  1344-6460

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<p>Crosslink radio occultation (RO) using multiple spacecraft is a promising method to observe the planetary atmosphere with higher spatial and temporal resolution than conventional RO measurements between a spacecraft and a ground station. In this paper, an optimal orbit configuration design method for crosslink RO is discussed. Orbit elements of two spacecraft are explored under two evaluation functions: values of observation and fuel costs for orbit injection. Then, generated Pareto optimal solutions are analyzed based on three categories classified by orbit characteristics, and we proposed an orbit design guideline to enhance observational value against fuel cost. The qualitative understandings for crosslink RO orbit design by this paper can contribute to future mission design. </p>

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Thermal structure of the Venusian atmosphere from the sub-cloud region to the mesosphere as observed by radio occultation

Ando H., Imamura T., Tellmann S., Pätzold M., Häusler B., Sugimoto N., Takagi M., Sagawa H., Limaye S., Matsuda Y., Choudhary R.K., Antonita M.

Scientific Reports (Scientific Reports)  10 ( 1 )  2020.12

Research paper (scientific journal), Joint Work, Accepted

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© 2020, The Author(s). We present distributions of the zonal-mean temperature and static stability in the Venusian atmosphere obtained from Venus Express and Akatsuki radio occultation profiles penetrating down to an altitude of 40 km. At latitudes equatorward of 75°, static stability derived from the observed temperature profiles is consistent with previous in-situ measurements in that there is a low-stability layer at altitudes of 50–58 km and highly and moderately stratified layers above 58 km and below 50 km, respectively. Meanwhile, at latitudes poleward of 75°, a low-stability layer extends down to 42 km, which has been unreported in analyses of previous measurements. The deep low-stability layer in the polar region cannot be explained by vertical convection in the middle/lower cloud layer, and the present result thus introduces new constraints on the dynamics of the sub-cloud atmosphere. The Venusian atmosphere is in striking contrast to the Earth’s troposphere, which generally has a deeper low-stability layer at low latitudes than at mid- and high latitudes.

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Venusian Cloud Distribution Simulated by a General Circulation Model

Ando H., Takagi M., Sugimoto N., Sagawa H., Matsuda Y.

Journal of Geophysical Research: Planets (Journal of Geophysical Research: Planets)  125 ( 7 )  2020.07

Research paper (scientific journal), Joint Work, Accepted,  ISSN  21699097

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We construct a simple cloud model for a Venus general circulation model (GCM), which includes condensable gases of H2O and H2SO4 vapors, and condensation, evaporation, and sedimentation of sulfuric acid cloud particles. The zonally averaged mass loading of the cloud reproduced in the model takes its maximum and minimum in high and middle latitudes, respectively. This latitudinal distribution is consistent with the infrared measurements. The thick cloud is formed in high latitudes at 43–55 km altitudes by vertical winds associated with disturbances enhanced in the low static stability layer. The moderately thick cloud in low latitudes is attributed mainly to the transport of H2SO4 vapor by the mean meridional circulation. The horizontal cloud distribution in low latitudes has zonal wave numbers 1 and 2 structures, which change in time significantly. These characteristics of the low-latitude cloud would be associated with atmospheric waves in the cloud layer. The mixing ratio of H2O vapor increases with latitude in the cloud layer due to the vertical wind disturbances in the low static stability layer in high latitudes. This latitudinal trend is qualitatively consistent with the infrared measurements. The mixing ratio of H2SO4 vapor increases with latitude in the subcloud layer because a large amount of the cloud is evaporated there due to the sedimentation of cloud particles in the thick lower cloud in the polar region. The present results suggest that the Venus cloud distribution in the lower cloud layer is strongly affected by waves and/or disturbances as well as the mean meridional circulation.

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Planetary-scale streak structure reproduced in high-resolution simulations of the Venus atmosphere with a low-stability layer

Kashimura H., Sugimoto N., Takagi M., Matsuda Y., Ohfuchi W., Enomoto T., Nakajima K., Ishiwatari M., Sato T., Hashimoto G., Satoh T., Takahashi Y., Hayashi Y.

Nature Communications (Nature Communications)  10 ( 1 )  2019.12

Research paper (scientific journal), Joint Work, Accepted

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© 2019, The Author(s). Cloud patterns are important clues for revealing the atmospheric circulation of Venus. Recently, a planetary-scale streak structure has been discovered in middle- and lower-cloud images of Venus’ night-side taken by IR2, the 2-μm camera, on board the Akatsuki orbiter. However, its formation mechanism has not been investigated. Here we succeed, for the first time, in reproducing the patterns of the observed streak structure, as regions of strong downward flows that develop in high-resolution global simulations of the Venus atmosphere. The streaks are formed in both hemispheres with equatorial symmetry, which is caused by equatorial Rossby-like and Kelvin-like waves with zonal wavenumber one. The low-stability layer that has been suggested by past observations is essential for reproducing the streak structure. The streaks of downward flow result from the interaction of the meridionally tilted phase lines of the Rossby-like waves and the characteristics of baroclinic instability produced around the low-stability layer.

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Impact of Data Assimilation on Thermal Tides in the Case of Venus Express Wind Observation

Sugimoto N., Kouyama T., Takagi M.

Geophysical Research Letters (Geophysical Research Letters)  46 ( 9 ) 4573 - 4580 2019.05

Research paper (scientific journal), Joint Work, Lead author, Corresponding author, Accepted,  ISSN  00948276

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©2019. American Geophysical Union. All Rights Reserved. Impacts of horizontal winds assimilation on thermal tides are investigated by using the Venus atmospheric general circulation model for the Earth Simulator local ensemble transform Kalman filter data assimilation system. The assimilated data are horizontal winds derived from Venus ultraviolet images taken by the Venus Monitoring Camera onboard the Venus Express orbiter. The results show that three-dimensional structures of the thermal tides are significantly improved not only in the horizontal winds but also in the temperature field, even though the observations are available only at the cloud top level on the southern dayside hemisphere approximately once an Earth day. The reproduced temperature fields agree well with recent radio occultation measurements of the Venus Climate Orbiter Akatsuki. The zonal mean fields of the zonal wind and temperature are also improved globally. This study would enable reanalysis of past Venus observations.

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Fully Developed Superrotation Driven by the Mean Meridional Circulation in a Venus GCM

Sugimoto N., Takagi M., Matsuda Y.

Geophysical Research Letters (Geophysical Research Letters)  46 ( 3 ) 1776 - 1784 2019.02

Research paper (scientific journal), Joint Work, Lead author, Corresponding author, Accepted,  ISSN  00948276

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©2019. American Geophysical Union. All Rights Reserved. Fully developed superrotation is reproduced for the first time in very long term simulations with a dynamical Venus general circulation model (GCM) driven by a zonally averaged component of the realistic solar heating only. Starting from a motionless state with temperature distribution including a low static stability layer in the lower cloud layer, the fast superrotating zonal flow of ~100 m/s is established after 500 Earth years. In this experiment, the mean meridional circulation is responsible for generating the superrotation, because effects of thermal tides, topography, radiation process, and cloud physics are excluded in the present simulations. Sensitivity experiments indicate that the vertical eddy viscosity smaller than 0.02 m 2 /s is necessary for the fast superrotation to appear. The low static stability layer also strongly affects the superrotation with high-latitude jets through angular momentum transport by baroclinic/barotropic instability in the cloud layer.

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Observing system simulation experiment for radio occultation measurements of the Venus atmosphere among small satellites

SUGIMOTO Norihiko, ABE Mirai, KIKUCHI Yukako, HOSONO Asako, ANDO Hiroki, TAKAGI Masahiro, GARATE LOPEZ Itziar, LEBONNOIS Sebastien, AO Chi

Journal of Japan Society of Civil Engineers, Ser. A2 (Applied Mechanics (AM)) (Japan Society of Civil Engineers)  75 ( 2 ) I_477 - I_486 2019

Research paper (scientific journal), Joint Work, Accepted

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<p>We have developed the Venus AFES (atmospheric GCM (general circulation model) for the Earth Simulator) LETKF (local ensemble transform Kalman filter) data assimilation system (VALEDAS) to make full use of observations. In this study, radio occultation measurements among small satellites are evaluated by the observing system simulation experiment (OSSE) of VALEDAS. Idealized observations are prepared by a French Venus Atmospheric GCM in which the cold collar is realistically reproduced. Reproducibility of the cold collar in VALEDAS is tested by several types of observations. The results show that the cold collar is successfully reproduced by assimilating at least 2 or 3 vertical temperature profiles in the polar region every 4 or 6 hours. Therefore, the radio occultation measurements among three satellites in polar orbits would be promising to improve the polar atmospheric structures at about 40–90 km altitudes.</p>

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Data assimilation system for the Venus atmosphere and its preliminary results

Sugimoto Norihiko

NCTAM papers, National Congress of Theoretical and Applied Mechanics, Japan (National Committee for IUTAM)  65 ( 0 ) 95 2019

Research paper (conference, symposium, etc.), Joint Work

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<p>We have developed the Venus atmosphere local ensemble transform Kalman filter data assimilation system (VALEDAS) to make full use of the observation. This system has been tested for GCM simulations excluding the thermal tides. Here, we further proceed to the data assimilation with horizontal winds derived from Venus ultraviolet images taken by the Venus Monitoring Camera onboard the Venus Express orbiter using GCM simulations including the thermal tides. The preliminary results suggest that the system would provide useful information for updating the future missions and improving the Venusian GCMs and produce first analysis datasets from the Venus Climate Orbiter ‘Akatsuki’.</p>

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Local Time Dependence of the Thermal Structure in the Venusian Equatorial Upper Atmosphere: Comparison of Akatsuki Radio Occultation Measurements and GCM Results

ANDO Hiroki, TAKAGI Masahiro, FUKUHARA Tetsuya, IMAMURA Takeshi, SUGIMOTO Norihiko, SAGAWA Hideo, NOGUCHI Katsuyuki, TELLMANN Silvia, PATZOLD Martin, HAUSLER Bernd, MURATA Yasuhiro, TAKEUCHI Hiroshi, YAMAZAKI Atsushi, TODA Tomoaki, TOMIKI Atsushi, CHOUDHARY Rajkumar, KUMAR Kishore, RAMKUMAR Geetha, ANTONITA Maria

Journal of Geophysical Research: Planets (American Geophysical Union)  123 ( JE005640 ) 11pp - 2280 2018.07

Research paper (scientific journal), Joint Work, Accepted,  ISSN  21699097

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©2018. American Geophysical Union. All Rights Reserved. Temperature profiles of the Venus atmosphere obtained by the Akatsuki radio occultation measurements showed a prominent local time dependence above 65-km altitude at low latitudes equatorward of 35°. A zonal wavenumber 2 component is predominant in the temperature field, and its phase (i.e., isothermal) surfaces descend with local time, suggesting its downward phase propagation. A general circulation model (GCM) for the Venus atmosphere, AFES-Venus, reproduced the local time-dependent thermal structure qualitatively consistent with the radio occultation measurements. Based on a comparison between the radio occultation measurements and the GCM results, the observed zonal wavenumber 2 structure is attributed to the semidiurnal tide. Applying the dispersion relationship for internal gravity waves to the observed wave structure, the zonally averaged zonal wind speed at 75- to 85-km altitudes was found to be significantly smaller than that at the cloud top. The decrease of the zonal wind speed with altitude is attributed to the momentum deposition by the upwardly propagating semidiurnal tide excited in the cloud layer.

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Three dimensional structures of thermal tides simulated by a Venus GCM

TAKAGI Masahiro, SUGIMOTO Norihiko, ANDO Hiroki, MATSUDA Yoshihisa

Journal of Geophysical Research: Planets (American Geophysical Union)  123 ( JE005449 ) 18pp - 352 2018.01

Research paper (scientific journal), Joint Work, Accepted,  ISSN  21699097

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©2018. American Geophysical Union. All Rights Reserved. Thermal tides in the Venus atmosphere are investigated by using a GCM named as AFES-Venus. The three-dimensional structures of wind and temperature associated with the thermal tides obtained in our model are fully examined and compared with observations. The result shows that the wind and temperature distributions of the thermal tides depend complexly on latitude and altitude in the cloud layer, mainly because they consist of vertically propagating and trapped modes with zonal wave numbers of 1–4, each of which predominates in different latitudes and altitudes under the influence of mid- and high-latitude jets. A strong circulation between the subsolar and antisolar (SS-AS) points, which is equivalent to a diurnal component of the thermal tides, is superposed on the superrotation. The vertical velocity of SS-AS circulation is about 10 times larger than that of the zonal-mean meridional circulation (ZMMC) in 60–70 km altitudes. It is suggested that the SS-AS circulation could contribute to the material transport, and its upward motion might be related to the UV dark region observed in the subsolar and early afternoon regions in low latitudes. The terdiurnal and quaterdiurnal tides, which may be excited by the nonlinear interactions among the diurnal and semidiurnal tides in middle and high latitudes, are detected in the solar-fixed Y-shape structure formed in the vertical wind field in the upper cloud layer. The ZMMC is weak and has a complex structure in the cloud layer; the Hadley circulation is confined to latitudes equatorward of 30°, and the Ferrel-like one appears in middle and high latitudes.

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A dynamical mechanism for Secondary Eyewall Formation in Tropical Cyclones

MIYAMOTO Yoshiaki, NOLAN David, SUGIMOTO Norihiko

Journal of the Atmospheric Sciences (American Meteorological Society)  75 ( 11 ) 3965 - 3986 2018

Research paper (scientific journal), Joint Work, Accepted,  ISSN  00224928

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© 2018 American Meteorological Society. This study proposes that secondary eyewall formation (SEF) of tropical cyclones (TCs) can be attributed to an instability of flow in the free atmosphere coupled with Ekman pumping. Unstable solutions of a 1.5-layer shallow-water system are obtained under fast-wind speed conditions in the free atmosphere. The instability condition derived in the linear model indicates the importance of the ratio of angular velocity to vorticity, and the condition is more likely to be satisfied when the ratio is large and its radial gradient is positive. Thus, fast angular velocity, low absolute vertical vorticity, small negative radial gradient of angular velocity, and large negative gradient of vertical vorticity are favorable. Eigenvalue analyses are performed over a wide range of parameters using a vorticity profile with an infinitesimal secondary maximum. The growth rate increases with vorticity outside the radius of maximum wind (RMW), the radius of the secondary vorticity maximum, its magnitude, and the Rossby number defined by maximum tangential velocity, the RMW, and the Coriolis parameter. Furthermore, the growth rate is positive only between 2 and 7 times the RMW, and it is negative close to or far outside the RMW. These features are consistent with previous studies on SEF. A dimensionless quantity γ obtained from the unstable condition in the linear theory is applied to SEF events simulated by two different full-physics numerical models;γ increases several hours before a secondary peak of tangential velocity forms, suggesting that the initial process of SEF can be attributed to the proposed mechanism.

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Inertia-gravity wave radiation from the elliptical vortex in the f-plane shallow water system

SUGIMOTO Norihiko

Fluid Dynamics Research (The Japan Society of Fluid Mechanics)  49 ( 025508 ) 17pp 2017

Research paper (scientific journal), Single Work, Accepted

Vertical structure of the axi-asymmetric temperature disturbance in the Venusian polar atmosphere: Comparison between radio occultation measurements and GCM results

ANDO Hiroki, IMAMURA Takeshi, SUGIMOTO Norihiko, TAKAGI Masahiro, TELLMANN Silvia, PATZOLD Martin, HAUSLER Bernd, KASHIMURA Hiroki, and MATSUDA Yoshihisa

Journal of Geophysical Research: Planets (American Geophysical Union)  122 ( JE005213 ) 17pp 2017

Research paper (scientific journal), Joint Work, Accepted

Development of an ensemble Kalman filter data assimilation system for the Venusian atmosphere

SUGIMOTO Norihiko, YAMAZAKI Akira, KOUYAMA Toru, KASHIMURA Hiroki, ENOMOTO Takeshi, TAKAGI Masahiro,

Scientific Reports (Nature Publishing Group)  7 ( 9321 ) 9pp 2017

Research paper (scientific journal), Joint Work, Lead author, Corresponding author, Accepted

Wave analysis in the atmosphere of Venus below 100-km altitude, simulated by LMD Venus GCM

LEBONNOIS Sébastien, SUGIMOTO Norihiko, GILLI Gabriella

Icarus 278   38 - 51 2016.06

Research paper (scientific journal), Joint Work, Accepted

Generation and backreaction of spontaneously emitted inertia-gravity waves

SUGIMOTO Norihiko, Riwal Plougonven

Geophysical Research Letters (American Geophysical Union)  43   3519 - 3525 2016.04

Research paper (scientific journal), Joint Work, Lead author, Corresponding author, Accepted

The puzzling Venusian polar atmospheric structure reproduced by a general circulation model

ANDO Hiroki, SUGIMOTO Norihiko, TAKAGI Masahiro, KASHIMURA Hiroki, IMAMURA Takeshi, MATSUDA Yoshihisa

Nature Communications (Nature Publishing Group)  7 ( 10398 ) 8pp 2016.02

Research paper (scientific journal), Joint Work, Accepted

A theoretical study on the spontaneous radiation of inertia-gravity waves using the renormalization group method. Part I: Derivation of the renormalization group equations

YASUDA Yuki, SATO Kaoru, SUGIMOTO Norihiko

Journal of the Atmospheric Sciences (American Meteorological Society)  72 ( 3 ) 957 - 983 2015

Research paper (scientific journal), Joint Work, Accepted

A theoretical study on the spontaneous radiation of inertia-gravity waves using the renormalization group method. Part II: Verification of the theoretical equations by numerical simulation

YASUDA Yuki, SATO Kaoru, SUGIMOTO Norihiko

Journal of the Atmospheric Sciences (American Meteorological Society)  72 ( 3 ) 984 - 1009 2015

Research paper (scientific journal), Joint Work, Accepted

Cyclone-anticyclone asymmetry in gravity wave radiation from a co-rotating vortex pair in rotating shallow water

SUGIMOTO Norihiko, ISHIOKA Keiichi, KOBAYASHI Hiromichi, SHIMOMURA Yutaka

Journal of Fluid Mechanics 772   80 - 106 2015

Research paper (scientific journal), Joint Work, Lead author, Corresponding author, Accepted

Inertia-gravity wave radiation from the merging of two co-rotating vortices in the f-plane shallow water system

SUGIMOTO Norihiko

Physics of Fluids 27   121701 2015

Research paper (scientific journal), Single Work, Lead author, Last author, Corresponding author, Accepted

Inverse insolation dependence of Venus' cloud-level convection

IMAMURA Takeshi, HIGUCHI Takehito, MAEJIMA Yasumitsu, TAKAGI Masahiro, SUGIMOTO Norihiko, IKEDA Kohei, ANDO Hiroki

Icarus 228   181 - 188 2014.01

Research paper (scientific journal), Joint Work, Accepted

Waves in a Venus general circulation model

SUGIMOTO Norihiko, TAKAGI Masahiro, MATSUDA Yoshihisa,

Geophysical Research Letters (American Geophysical Union)  41   7461 - 7467 2014

Research paper (scientific journal), Joint Work, Accepted

Baroclinic modes in the Venus atmosphere simulated by GCM

SUGIMOTO Norihiko, TAKAGI Masahiro, MATSUDA Yoshihisa,

Journal of Geophysical Research: Planets (American Geophysical Union)  119   1950 - 1968 2014

Research paper (scientific journal), Joint Work, Accepted

A Reynolds-averaged turbulence modeling approach to the maintenance of the Venus superrotation

YOSHIZAWA Akira, KOBAYASHI Hiromichi, SUGIMOTO Norihiko, YOKOI Nobumitsu, SHIMOMURA Yutaka

Geophysical and Astrophysical Fluid Dynamics 107 ( 6 ) 614-639 2013

Research paper (scientific journal), Joint Work, Accepted

Baroclinic modes in the atmosphere on Venus simulated by AFES

SUGIMOTO Norihiko, TAKAGI Masahiro, MATSUDA Yoshihisa, TAKAHASHI O. Yoshiyuki, ISHIWATARI Masaki, HAYASHI Yoshi-Yuki,

Theoretical and Applied Mechanics Japan (National Committee for IUTAM)  61   11-21 2013

Research paper (scientific journal), Joint Work, Accepted

Toward high resolution simulation for the atmosphere on Venus by AFES (Atmospheric GCM For the Earth Simulator)

SUGIMOTO Norihiko, TAKAGI Masahiro, MATSUDA Yoshihisa, TAKAHASHI O. Yoshiyuki, ISHIWATARI Masaki, HAYASHI Yoshi-Yuki,

Proceedings of International Conference on Simulation Technology (JSST 2012)    315-322 2012

Research paper (international conference proceedings), Joint Work, Accepted

Spontaneous Gravity Wave Radiation in a Shallow Water System on a Rotating Sphere

SUGIMOTO Norihiko, ISHII Katsuya

Journal of the Meteorological Society of Japan (Meteorological Society of Japan)  90 ( 1 ) 101-125 2012

Research paper (scientific journal), Joint Work, Accepted

Numerical modeling for Venus atmosphere based on AFES (Atmospheric GCM For the Earth Simulator)

SUGIMOTO Norihiko, TAKAGI Masahiro, MATSUDA Yoshihisa, TAKAHASHI O. Yoshiyuki, ISHIWATARI Masaki, HAYASHI Yoshi-Yuki,

System Simulation and Scientific Computing” Communications in Computer and Information Science (CCIS) (Springer-Verlag)     70-78 2012

Research paper (international conference proceedings), Joint Work, Accepted

Visualization of Huge Climate Data with High Speed Spherical Self-Organizing Map

TACHIBANA Kanta, SUGIMOTO Norihiko, SHIOGAMA Hideo, NOZAWA Toru

Journal of Advanced Computational Intelligence & Intelligent Informatics (JACIII) (Japan Society for Fuzzy Theory and Intelligent Informatics (SOFT))  13 ( 3 ) 210-216 2009

Research paper (scientific journal), Joint Work, Accepted

High speed method of detecting vortex without empirical conditions - Application to risk-assessment of tropical cyclone -

SUGIMOTO Norihiko, PHAM Minh Tuan, TACHIBANA Kanta, YOSHIKAWA Tomohiro, FURUHASHI Takeshi

Theoretical and Applied Mechanics Japan (National Committee for IUTAM)  57   297-307 2009

Research paper (scientific journal), Joint Work, Accepted

High speed non-empirical tropical cyclone identification method in huge climatology data

SUGIMOTO Norihiko, PHAM Minh Tuan, TACHIBANA Kanta, YOSHIKAWA Tomohiro, FURUHASHI Takeshi

"Hurricanes and Climate Change", Proceedings of 1st International Summit on Hurricanes and Climate Change, (Springer-Verlag)     251-263 2008

Research paper (international conference proceedings), Joint Work, Accepted

Parameter Sweep Experiments on Spontaneous Gravity Wave Radiation From Unsteady Rotational Flow in an F-plane Shallow Water System

SUGIMOTO Norihiko, ISHIOKA Keiichi, ISHII Katsuya

Journal of the Atmospheric Sciences (American Meteorological Society)  65 ( 1 ) 234 - 249 2008

Research paper (scientific journal), Joint Work, Accepted

2D Numerical simulation of slow flow past circular cylinders

SUGIMOTO Norihiko, HASHIYA Hiroaki, ISHII Katsuya

Theoretical and Applied Mechanics Japan (National Committee for IUTAM)  56   273-283 2008

Research paper (scientific journal), Joint Work, Accepted,  ISSN  ISSN 1348-0693

A First Attempt to Apply High Speed Spherical Self-organizing Map to Huge Climate Datasets

SUGIMOTO Norihiko, TACHIBANA Kanta

SOLA (Science Online Letters on the Atmosphere) (Meteorological Society of Japan)  4   41-44 2008

Research paper (scientific journal), Joint Work, Accepted

Spontaneous Gravity Wave Radiation from Unsteady Flows in Shallow Water System on a Rotating Sphere

SUGIMOTO Norihiko, ISHII Katsuya

"Frontiers of Computational Science II", Proceedings of International Symposium on Frontiers of Computational Science 2008 (Nagoya University)     181-188 2008

Research paper (international conference proceedings), Joint Work, Accepted

Application of high speed spherical self-organizing map to climate research

SUGIMOTO Norihiko, TACHIBANA Kanta, SHIOGAMA Hideo, NOZAWA Toru

Proceedings of Joint 4th International Conference on Soft Computing and Intelligent Systems and 9th International Symposium on Advanced Intelligent Systems (IEEE Computational Intelligence Society)     1574-1579 2008

Research paper (international conference proceedings), Joint Work, Accepted

結合コンパクト差分を用いた, 球面浅水系におけるジェット流からの重力波放射の数値実験

SUGIMOTO Norihiko, ISHII Katsuya

京都大学数理解析研究所講究録「複雑流体の数理と数値シミュレーション」 (京都大学数理解析研究所)   ( 1539 ) 15-26 2007

Research paper (bulletin of university, research institution), Joint Work

Balance regimes for the stability of a jet in an f-plane shallow water system

SUGIMOTO Norihiko, ISHIOKA Keiichi, YODEN Shigeo

Fluid Dynamics Research (The Japan Society of Fluid Mechanics)  39 ( 5 ) 353-377 2007

Research paper (scientific journal), Joint Work, Accepted

Gravity wave radiation from unsteady rotational flow in an f-plane shallow water system

SUGIMOTO Norihiko, ISHIOKA Keiichi, YODEN Shigeo

Fluid Dynamics Research (The Japan Society of Fluid Mechanics)  39 ( 11-12 ) 731-754 2007

Research paper (scientific journal), Joint Work, Accepted

Source models of gravity waves in an f-plane shallow water system

SUGIMOTO Norihiko, ISHIOKA Keiichi, ISHII Katsuya

"Frontiers of Computational Science", Proceedings of International Symposium on Frontiers of Computational Science 2005 (Springer-Verlag)     221-225 2007

Research paper (international conference proceedings), Joint Work, Accepted

浅水系における非定常な渦流からの重力波放射 -パラメータ走査実験-

SUGIMOTO Norihiko, ISHIOKA Keiichi

九州大学応用力学研究所研究集会報告「地球流体における波動と対流現象の力学」 (九州大学応用力学研究所)     11-18 2006

Research paper (bulletin of university, research institution), Joint Work

浅水系における非定常なジェット流からの重力波放射 -パラメータスイープ実験-

SUGIMOTO Norihiko, ISHIOKA Keiichi

京都大学数理解析研究所講究録「非線形波動の数理と応用」 (京都大学数理解析研究所)   ( 1483 ) 72-83 2006

Research paper (bulletin of university, research institution), Joint Work

ｆ 平面浅水系におけるジェットの安定性に見られるバランス領域について

SUGIMOTO Norihiko, ISHIOKA Keiichi, YODEN Shigeo

第20回大気圏シンポジウムプロシーディング (宇宙航空研究開発機構　宇宙科学研究本部)  20   13-16 2006

Research paper (bulletin of university, research institution), Joint Work

浅水系におけるジェットの非定常運動に伴う重力波放射について

SUGIMOTO Norihiko, ISHIOKA Keiichi, YODEN Shigeo

京都大学数理解析研究所講究録「波の非線形現象の数理と応用」 (京都大学数理解析研究所)   ( 1430 ) 216-234 2005

Research paper (bulletin of university, research institution), Joint Work

Froude Number Dependence of Gravity Wave Radiation From Unsteady Rotational Flow in f-plane Shallow Water System

SUGIMOTO Norihiko, ISHIOKA Keiichi, YODEN Shigeo

Theoretical and Applied Mechanics Japan (National Committee for IUTAM)  54   299-305 2005

Research paper (scientific journal), Joint Work, Accepted,  ISSN  ISSN 1348-0693

f 平面浅水系における非定常なジェット流からの重力波放射のパラメータスイープ実験

SUGIMOTO Norihiko, ISHIOKA Keiichi

第19回大気圏シンポジウムプロシーディング (宇宙航空研究開発機構　宇宙科学研究本部)  19   124-127 2005

Research paper (bulletin of university, research institution), Joint Work

浅水系における非定常な渦流からの重力波放射

SUGIMOTO Norihiko, ISHIOKA Keiichi, YODEN Shigeo

第18回大気圏シンポジウムプロシーディング (宇宙航空研究開発機構　宇宙科学研究本部)  18   26-29 2004

Research paper (bulletin of university, research institution), Joint Work

浅水系におけるバランスした渦流からの重力波放射

SUGIMOTO Norihiko, ISHIOKA Keiichi, YODEN Shigeo

九州大学応用力学研究所研究集会報告「地球流体における波動と対流現象の力学」 (九州大学応用力学研究所)     68-74 2004

Research paper (bulletin of university, research institution), Joint Work

f 平面浅水系におけるジェット流の不安定擾乱について

SUGIMOTO Norihiko, YODEN Shigeo

京都大学数理解析研究所講究録「非線形波動現象の構造と力学」 (京都大学数理解析研究所)   ( 1271 ) 51-60 2002

Research paper (bulletin of university, research institution), Joint Work