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Ryosuke Shibuya, Yukari Takayabu, and Hirotaka Kamahori

Abstract

This study examines disastrous historical precipitation cases that generate extreme precipitation simultaneously over a wide area in Japan (as in July 2018), defined as widespread extreme precipitation events. A statistically significant large-scale environment conducive for widespread extreme precipitation events over western Japan is investigated based on composite analysis. During a widespread precipitation event, a zonally elongated positive anomaly of the column-integrated water vapor extends from East China to western Japan. In the lower troposphere, a dipole of a geopotential height anomaly exists with positive and negative values at the east and west of the precipitation area, respectively. It is found that the negative geopotential anomaly is enhanced over East China at 2 days before the event and moves toward the precipitating area mainly due to the potential vorticity (PV) production term by diabatic heating, analogous to a diabatic Rossby wave. The temporal evolution of the dynamical forced vertical velocity is well in phase with that the PV production term, suggesting the importance of the coupling between the dynamical forced motion and diabatic heating. This result provides a physical understanding of the reason why both the background moisture and the baroclinicity are essential in the composited atmospheric fields and another view to the importance of the feedback parameter between the dynamical motion and diabatic heating.

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Hiroaki Ueda, Hirotaka Kamahori, and Nobuo Yamazaki

Abstract

Using the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) four-dimensional data assimilation (4DDA) upper-air data, the large-scale heat source (Q 1) and moisture sink (Q 2) over the western and eastern Tibetan Plateau are examined for a 4-month period from 1 May to 31 August 1998. The computations were performed on the sigma–pressure hybrid coordinate, named η-coordinate, since the analysis area includes high-elevation mountains.

Over the western Tibetan Plateau in May, there is a deep layer of heating occupying the whole troposphere with the maximum value exceeding 3 K day−1 around 400–600 hPa. The smaller magnitude of the apparent moisture sink is confined in the lower troposphere 1 km above the ground surface. Vertically integrated heat sources of 103 W m−2 over the western Tibetan Plateau are accompanied by a moisture sink of about half that (60 W m−2). These results indicate that the latent heat release associated with condensation plays an important role in the total heating besides the sensible heat supply from the land surface. Later in July, the moisture sink over the eastern Tibetan Plateau nearly equals the heat source indicating the dominance of moist processes associated with summertime monsoon rains.

The contrasting features of the heat source and moisture sink are closely related to the circulation fields. Throughout May and June, we observe strong upward motion along the western and southwestern slopes of the western Tibetan Plateau, while there is salient subsidence motion over the eastern Tibetan Plateau. The analyses of static stability and lifting condensation level indicate that the release of latent heat relevant to moist convection is a dominant factor for tropospheric heating after the monsoon onset, while the premonsoon period (May) is composed of both convective rainfall and dry thermal convection.

Thus, the heating mechanism prior to the onset of the monsoon, especially over the western Tibetan Plateau, can be characterized by the hybrid nature of “wet” processes due to condensation heating and “dry” processes associated with the sensible heat flux from the elevated mountain surface.

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Masashi Nagata, Lance Leslie, Yoshio Kurihara, Russell L. Elsberry, Masanori Yamasaki, Hirotaka Kamahori, Robert Abbey Jr., Kotaro Bessho, Javier Calvo, Johnny C. L. Chan, Peter Clark, Michel Desgagne, Song-You Hong, Detlev Majewski, Piero Malguzzi, John McGregor, Hiroshi Mino, Akihiko Murata, Jason Nachamkin, Michel Roch, and Clive Wilson

The Third Comparison of Mesoscale Prediction and Research Experiment (COMPARE) workshop was held in Tokyo, Japan, on 13–15 December 1999, cosponsored by the Japan Meteorological Agency (JMA), Japan Science and Technology Agency, and the World Meteorological Organization. The third case of COMPARE focuses on an event of explosive tropical cyclone [Typhoon Flo (9019)] development that occurred during the cooperative three field experiments, the Tropical Cyclone Motion experiment 1990, Special Experiment Concerning Recurvature and Unusual Motion, and TYPHOON-90, conducted in the western North Pacific in August and September 1990. Fourteen models from nine countries have participated in at least a part of a set of experiments using a combination of four initial conditions provided and three horizontal resolutions. The resultant forecasts were collected, processed, and verified with analyses and observational data at JMA. Archived datasets have been prepared to be distributed to participating members for use in further evaluation studies.

In the workshop, preliminary conclusions from the evaluation study were presented and discussed in the light of initiatives of the experiment and from the viewpoints of tropical cyclone experts. Initial conditions, depending on both large-scale analyses and vortex bogusing, have a large impact on tropical cyclone intensity predictions. Some models succeeded in predicting the explosive deepening of the target typhoon at least qualitatively in terms of the time evolution of central pressure. Horizontal grid spacing has a very large impact on tropical cyclone intensity prediction, while the impact of vertical resolution is less clear, with some models being very sensitive and others less so. The structure of and processes in the eyewall clouds with subsidence inside as well as boundary layer and moist physical processes are considered important in the explosive development of tropical cyclones. Follow-up research activities in this case were proposed to examine possible working hypotheses related to the explosive development.

New strategies for selection of future COMPARE cases were worked out, including seven suitability requirements to be met by candidate cases. The VORTEX95 case was withdrawn as a candidate, and two other possible cases were presented and discussed.

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