著者

TAKAMURA Nao WADA Akiyoshi

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2020-035, (Released:2020-05-26)

被引用文献数

3

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In August 2016, a total of eight typhoons formed in the western North Pacific, and four of which landed on northern and eastern Japan. These typhoons were associated with heavy rainfall and strong winds and caused massive damages in the Japanese archipelago. Moreover, five of the eight typhoons underwent extratropical transition (ET), which was more frequent than an average of 2.1 typhoons per year during August. To clarify the characteristics of the typhoon tracks that caused such unusual landfall and frequent ET in August 2016, we conducted k-means cluster and cyclone phase space (CPS) analyses for typhoons that occurred in August and September. Composite analysis and case study were also conducted to clarify the synoptic environments around the typhoons. To examine the unusual characteristics in August 2016, we compared the results of the analyses for this period with those in August from 2001 to 2015 and those in September 2016. The k-means cluster analysis showed that the direction of the typhoon tracks in August 2016 were more northward than that of the typhoons in August from 2001 to 2015 and those in September 2016. Moreover, the CPS analysis revealed that ET in August 2016 was characterized by a more indistinct structural change from a warm-core structure to a cold-core structure with a shorter duration than ET in August from 2001 to 2015. The synoptic environments around the typhoons in August 2016 were characterized by enhanced undulations of the upper-tropospheric jet stream, increased amplitudes of the mid-tropospheric trough, and relatively warm air around the typhoons in the lower troposphere. These synoptic environments explained the unusual landfall of typhoons with a more northward track and the more frequent ET and more indistinct structural evolution of ET in August 2016.

著者

YANASE Wataru ARAKI Kentaro WADA Akiyoshi SHIMADA Udai HAYASHI Masahiro HORINOUCHI Takeshi

出版者

公益社団法人 日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2022-041, (Released:2022-06-30)

被引用文献数

6

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Torrential rain in Typhoon Hagibis caused a devastating disaster in Japan in October 2019. The precipitation was concentrated in the northern half of Hagibis during extratropical transition (ET). To elucidate the mechanisms of this asymmetric precipitation, synoptic- and meso-scale processes were analyzed mainly using the Japan Meteorological Agency Non-Hydrostatic Model. The present study demonstrates that the asymmetric processes were different depending on the ET stages. When Hagibis was close to the baroclinic zone at middle latitudes around 12 October (the frontal stage), heavy precipitation in the northeastern part of Hagibis was attributed to warm frontogenesis and a quasi-geostrophic ascent, as reported in many previous studies. In contrast, when Hagibis was moderately distant from the baroclinic zone around 11 October (the prefrontal stage), heavy precipitation in the northern part occurred in slantwise northward ascending motion in the outer region. This slantwise motion developed in a region with strong westerly vertical shear, which was enhanced between Hagibis and a westerly jet stream. Based on the analyses of potential vorticity and absolute angular momentum, this region was characterized by reduced moist symmetric stability in the lower and middle troposphere accompanied by inertial instability in the upper troposphere and conditional instability in the lower troposphere. These results provide additional insights into the time evolution of asymmetric processes during ET in the absence of a distinct upper-tropospheric trough, particularly the slantwise motion in the prefrontal stage.

著者

FUDEYASU Hironori SHIMADA Udai OIKAWA Yoshinori EITO Hisaki WADA Akiyoshi YOSHIDA Ryuji HORINOUCHI Takeshi

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2022-031, (Released:2022-04-21)

被引用文献数

4

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This study investigated the atmospheric and oceanic contributions to the genesis of Typhoon Faxai in 2019. Our statistical analysis using the tropical cyclone genesis score (TGS) attributed the tropical disturbance that developed into Faxai (Pre-Faxai) to easterly waves (EWs). The EW score evaluated by a grid version of the TGS (Grid-EW) averaged around the occurrence of Pre-Faxai was approximately twice as large as the climatological mean; it was the second largest value in the past 38 years. The Pre-Faxai area with high Grid-EW scores could be traced back to the eastern North Pacific (ENP) around August 25, 2019. The lower-troposphere environment characterized by high Grid-EW scores was favorable for vortex formation because it provided a containment area for moisture entrained by the developing circulation or lofted by the deep convection therein. The Pre-Faxai area with high Grid-EW scores moved westward because of the background easterly flow over the ENP, then entered the western North Pacific (WNP). The Typhoon Intensity Forecast Scheme (TIFS) showed that the important environments for its genesis were ocean conditions and the vertical wind shear. The oceanic conditions contributed to the development of Pre-Faxai as it traveled over the WNP. The enhancement of vertical wind shear and subsequent suppression of the development of Pre-Faxai were caused by the lower-troposphere easterly winds associated with high EW scores; they were also caused by upper-troposphere westerly winds associated with an upper cold low northwest of Pre-Faxai. When the vertical shear decreased with weakening of the upper cold low, Pre-Faxai reached tropical storm intensity on September 4. Therefore, TGS and TIFS detected Pre-Faxai 10 days before the typhoon arose, an indication that monitoring environmental factors such as EW and vertical wind shear are important for disaster prevention.