Contributions of the Large-Scale Environment to the Typhoon Genesis of Faxai (2019)

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  • FUDEYASU Hironori
    Graduate School of Education, Yokohama National University, Yokohama, Japan Typhoon Science and Technology Research Center, Yokohama National University, Yokohama, Japan
  • SHIMADA Udai
    Typhoon Science and Technology Research Center, Yokohama National University, Yokohama, Japan Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, Japan
  • OIKAWA Yoshinori
    Japan Meteorological Agency, Tokyo, Japan
  • EITO Hisaki
    Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, Japan
  • WADA Akiyoshi
    Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, Japan
  • YOSHIDA Ryuji
    CIRES, University of Colorado Boulder, Colorado, USA NOAA Earth System Research Laboratory, Colorado, USA RIKEN Center for Computational Science, Kobe, Japan Research Center for Urban Safety and Security, Kobe University, Kobe, Japan
  • HORINOUCHI Takeshi
    Typhoon Science and Technology Research Center, Yokohama National University, Yokohama, Japan Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japan

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  • 台風Faxai（2019）の発生に対する大規模環境場の寄与

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Abstract

<p>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, and 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 due to the background easterly flow over the ENP and 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, which indicates that monitoring environmental factors such as EW and vertical wind shear are important for disaster prevention.</p>

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