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Izuru Takayabu and Shin-ichi Takehiro

Abstract

The concept of wave over-reflection is applied to the unstable normal mode of the Eady problem. In order to consider propagation properties of the Rossby waves trapped at the boundaries, two thin boundary layers are introduced at the top and bottom of the fluid layer of the original model. It is shown that the Rossby waves in the boundary layers are always over-reflected as long as their critical levels exist in the constant-shear evanescent region and the waves are transmitted to the opposite boundary layer. The dispersion relation obtained by using laser formula and quantization qualitatively coincides with that of the normal mode. Although the growth rate is systematically overestimated, the short-wave cutoff is well described by the over-reflection solution.

The mechanism of over-reflection obtained in this study is understood by conservation of momentum. Since the pseudomomentum of the transmitted and the incident waves have opposite signs to each other, the amplitude of the reflected wave should become larger than that of the incident wave to satisfy constant momentum flux constraints. This mechanism corresponds to that of over-reflection in a linear shear flow of the shallow water model shown in a paper by Takehiro and Hayashi.

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Hiroaki Kawase, Yukiko Imada, Hiroshige Tsuguti, Toshiyuki Nakaegawa, Naoko Seino, Akihiko Murata, and Izuru Takayabu
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Ryuhei Yoshida, Yumi Onodera, Takamasa Tojo, Takeshi Yamazaki, Hiromitsu Kanno, Izuru Takayabu, and Asuka Suzuki-Parker

Abstract

A physical vegetation model [the Two-Layer Model (2LM)] was applied to estimate the climate change impacts on rice leaf wetness (LW) as a potential indicator of rice blast occurrence. Japan was used as an example. Dynamically downscaled data at 20-km-mesh resolution from three global climate models (CCSM4, MIROC5, and MRI-CGCM3) were utilized for present (1981–2000) and future (2081–2100) climates under the representative concentration pathway 4.5 scenario. To evaluate the performance of the 2LM, the LW and other meteorological variables were observed for 108 days during the summer of 2013 at three sites on the Pacific Ocean side of Japan. The derived correct estimation rate was 77.4%, which is similar to that observed in previous studies. Using the downscaled dataset, the changes in several precipitation indices were calculated. The regionally averaged ensemble mean precipitation increased by 6%, although large intermodel differences were found. By defining a wet day as any day in which the daily precipitation was ≥ 1 mm day−1, it was found that the precipitation frequency decreased by 6% and the precipitation intensity increased by 11% for the entire area. The leaf surface environment was estimated to be dry; leaf wetness, wet frequency, and wet times all decreased. It was found that a decrease in water trap opportunities due to reduced precipitation frequency was the primary contributor to the LW decrease. For blast fungus, an increased precipitation intensity was expected to enhance the washout effect on the leaf surface. In the present case, the infection risk was estimated to decrease for Japan.

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Hiroshi Niino, Osamu Suzuki, Hiroshi Nirasawa, Tokunosuke Fujitani, Hisao Ohno, Izuru Takayabu, Nobuyuki Kinoshita, and Yoshimitsu Ogura

Abstract

On the evening of 11 December 1990, two supercell storms bit the Chiba Prefecture, southeast of Tokyo, and spawned two tornadoes in Mobara and Kamogawa. The Mobara tornado caused the most severe tornado damage since 1960 in Japan over a damage swath of 6.5 km in length and 500 m in average width. A detailed damage survey revealed that the tornado moved north-northeastward at a speed of about 16 m s−1. The maximum wind speed near the ground, estimated from damage to structures, was more than 78 m s−1.

The storms were initiated in the warm sector of a developing extratropical cyclone, about 6–7 h prior to the tornadogenesis. They moved straightforwardly northeastward at a speed of about 16 m s−1 throughout their life cycles including their supercell phases.

The mesocyclone in the Mobara storm had been detected by a single-Doppler radar for 44 min. Vertical vorticity of the mesocyclone amplified to 2 × 10−2 s−1 almost simultaneously between 1 and 5 km AGL, about 20 min prior to the tornadogenesis. About 4 min before the tornadogenesis a small mesocyclone formed in the south edge of the major mesocyclone, and this new mesocyclone produced the tornado.

The Kamogawa storm was in supercell phase for more than 2 h. A mesocyclone was detected both by surface wind records and by the Doppler radar. The vorticity of the mesocyclone amplified and weakened at least two times. Traces of surface pressure, temperature, and precipitation rate at Tateyama Observatory in the storm path, about 20 km southwest of Kamogawa, showed that the center of the mesocyclone was located in cooler air behind the gust front and the major precipitation preceded the mesocyclone. A barometer located near the center of the tornado damage path in Kamogawa recorded two pressure dips, indicating that the centers of the mesolow and the tornado were about 5 km apart.

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Sachie Kanada, Tetsuya Takemi, Masaya Kato, Shota Yamasaki, Hironori Fudeyasu, Kazuhisa Tsuboki, Osamu Arakawa, and Izuru Takayabu

Abstract

Intense tropical cyclones (TCs) sometimes cause huge disasters, so it is imperative to explore the impacts of climate change on such TCs. Therefore, the authors conducted numerical simulations of the most destructive historical TC in Japanese history, Typhoon Vera (1959), in the current climate and a global warming climate. The authors used four nonhydrostatic models with a horizontal resolution of 5 km: the cloud-resolving storm simulator, the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model, the Japan Meteorological Agency (JMA) operational nonhydrostatic mesoscale model, and the Weather Research and Forecasting Model. Initial and boundary conditions for the control simulation were provided by the Japanese 55-year Reanalysis dataset. Changes between the periods of 1979–2003 and 2075–99 were estimated from climate runs of a 20-km-mesh atmospheric general circulation model, and these changes were added to the initial and boundary conditions of the control simulation to produce the future climate conditions.

Although the representation of inner-core structures varies largely between the models, all models project an increase in the maximum intensity of future typhoons. It is found that structural changes only appeared around the storm center with sudden changes in precipitation and near-surface wind speeds as the radius of maximum wind speed (RMW) contracted. In the future climate, the water vapor mixing ratio in the lower troposphere increased by 3–4 g kg−1. The increased water vapor allowed the eyewall updrafts to form continuously inside the RMW and contributed to rapid condensation in the taller and more intense updrafts.

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Izuru Takayabu, Roy Rasmussen, Eiichi Nakakita, Andreas Prein, Hiroaki Kawase, ShunIchi Watanabe, Sachiho A. Adachi, Tetsuya Takemi, Kosei Yamaguchi, Yukari Osakada, and Ying-Hsin Wu
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Ryo Mizuta, Akihiko Murata, Masayoshi Ishii, Hideo Shiogama, Kenshi Hibino, Nobuhito Mori, Osamu Arakawa, Yukiko Imada, Kohei Yoshida, Toshinori Aoyagi, Hiroaki Kawase, Masato Mori, Yasuko Okada, Tomoya Shimura, Toshiharu Nagatomo, Mikiko Ikeda, Hirokazu Endo, Masaya Nosaka, Miki Arai, Chiharu Takahashi, Kenji Tanaka, Tetsuya Takemi, Yasuto Tachikawa, Khujanazarov Temur, Youichi Kamae, Masahiro Watanabe, Hidetaka Sasaki, Akio Kitoh, Izuru Takayabu, Eiichi Nakakita, and Masahide Kimoto

Abstract

An unprecedentedly large ensemble of climate simulations with a 60-km atmospheric general circulation model and dynamical downscaling with a 20-km regional climate model has been performed to obtain probabilistic future projections of low-frequency local-scale events. The climate of the latter half of the twentieth century, the climate 4 K warmer than the preindustrial climate, and the climate of the latter half of the twentieth century without historical trends associated with the anthropogenic effect are each simulated for more than 5,000 years. From large ensemble simulations, probabilistic future changes in extreme events are available directly without using any statistical models. The atmospheric models are highly skillful in representing localized extreme events, such as heavy precipitation and tropical cyclones. Moreover, mean climate changes in the models are consistent with those in phase 5 of the Coupled Model Intercomparison Project (CMIP5) ensembles. Therefore, the results enable the assessment of probabilistic change in localized severe events that have large uncertainty from internal variability. The simulation outputs are open to the public as a database called “Database for Policy Decision Making for Future Climate Change” (d4PDF), which is intended to be utilized for impact assessment studies and adaptation planning for global warming.

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