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Role of Typhoon Prapiroon (Typhoon No SOLA, 2019, Vol. 15A, 37−42, doi:10.2151/sola.15A-007 37 Role of Typhoon Prapiroon (Typhoon No. 7) on the Formation Process of the Baiu Front Inducing Heavy Rain in July 2018 in Western Japan Qoosaku Moteki Dynamic Coupling of Ocean-Atmosphere-Land Research Program (DCOP), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Kanagawa, Japan cessor rain event” (PRE), which is a heavy rainfall event affected Abstract the low-level moisture flow to the north of TCs (Bosart et al. 2012; Galarneau et al. 2010; Schumacher and Galarneau 2012), Heavy rain in western Japan was broadly induced by the stag- and the “moisture road,” which is a large northward moisture flux nation of the Baiu front during 5−7 July 2018. This study hypoth- associated with high potential vorticity beside TCs (Yoshida and esizes that cold air advection over the Sea of Japan intensified by Itoh 2012), are proposed. Studies and disaster controls on heavy Typhoon Prapiroon (Typhoon No. 7) was one of the triggering rainfall directly induced by TCs have continuously been addressed factors for the formation process of the Baiu front over western as a part of JMA forecast operations. Japan. Typhoon Prapiroon passed over the Sea of Japan on 4 July According to a press release from the JMA (JMA 2018), heavy and became extratropical at approximately 40°N on 5 July. During rainfall during the latter period was induced by the Baiu front its passage, the strong southward pressure gradient force to the formed over western Japan as a result of cold air flowing into the north of Typhoon Prapiroon broke down the convergence line of Sea of Japan from the Sea of Okhotsk after 5 July. However, the the Baiu front that remained at approximately 45°N before 4 July Baiu front before 3 July remained at approximately 45°N and the and thick cold air from the Okhotsk High flowed over the Sea of cold air of the Okhotsk High remained to the north of Hokkaido. Japan. The Okhotsk High expanded toward the Sea of Japan and Thus, the Okhotsk High rapidly expanded southward on 4 July enhanced cold air advection to the north of western Japan. As a and the Baiu front jumped from 45°N (Hokkaido) to 35°N (western result, the Baiu front was stationary at approximately 35°N after Japan). Why did the Okhotsk High drastically expanded over the 5 July. In addition, the westerly jet in the east of an upper-level Sea of Japan? Such drastic process should require some strong trough deepened along the typhoon track was associated with the forcing and Typhoon Prapiroon passing over the Sea of Japan on 4 adiabatic component of the ascending motion over the isentropic July could be a candidate of the forcing. The southward expanding upslope and was suggested to contribute to the maintenance of process of the Okhotsk High should be investigated to understand Baiu frontal convection. the formation mechanism behind the stagnation of the Baiu front (Citation: Moteki, Q., 2019: Role of Typhoon Prapiroon in western Japan. (Typhoon No. 7) on the formation process of the Baiu front induc- The objective of this study is to explain the formation process ing heavy rain in July 2018 in western Japan. SOLA, 15A, 37−42, of the Baiu front remains over almost the same location in western doi:10.2151/sola.15A-007.) Japan during the latter period for 3 days (5−7 July). We focus on a role of Typhoon Prapiroon passing over the Sea of Japan from 4−5 July in triggering the drastic southward expansion of the Okhotsk 1. Introduction High and propose a scenario explaining the formation process of the Baiu front during the latter period. Heavy rainfall events, which are referred as the Heavy Rain Event of July 2018 (Japan Meteorological Agency [JMA], 2018; Tokyo Climate Center, JMA, 2018), occurred over wide areas of 2. Data western Japan from 28 June to 8 July and unprecedented amounts of 3-day accumulated precipitation were recorded at more than Precipitation estimates are obtained from the Global Satellite 100 operational weather stations. A total of 221 facilities and 6296 Mapping of Precipitation (GSMaP) standard product of (Aonashi buildings in 15 prefectures were completely destroyed by floods, et al. 2009; Kubota et al. 2007; Shige et al. 2009; Ushio et al. 2009; landslides, and mudflows associated with heavy rainfall, and more Yamamoto and Shige 2015). The spatial resolution is 0.1° × 0.1° than 200 people died. and the temporal resolution is 1 hour. The Japanese 55-year reanal- The Heavy Rain Event of July 2018 in western Japan can be ysis (JRA-55) from 1958−2012 (Ebita et al. 2011; Kobayashi divided into 3 periods (preceding (28 June−1 July), middle (2−4 et al. 2015) is used to investigate the large-scale environment. July) and latter (5−8 July) periods) on the basis of causes of heavy The dataset has a 1.25° horizontal resolution, 38 levels (including rainfall. The local heavy rainfall events during the preceding the surface and levels from 1−1000 hPa), and 6-h intervals. The period occurred in many areas to the south of the Baiu front Typhoon Prapiroon position is derived from the JMA best-track when it remained over the Sea of Japan. Heavy rainfall during data. The positions of the synoptic-scale fronts are derived from the middle period (Figs. 1a and 1b) was induced in association the JMA weather chart at 00 UTC on each day. with the passage of Typhoon Prapiroon over the Okinawa region. Heavy rainfall during the latter period (Figs. 1e and 1f) was directly induced by the Baiu front remaining over western Japan. 3. Why did the Baiu front suddenly jump from Many previous studies have revealed the mechanisms of heavy Hokkaido to western Japan? rainfall events to south of the Baiu front similar to those in the preceding period (Kunoki et al. 2015; Manda et al. 2014; Miyama The Baiu front, which induced heavy rainfall during the latter et al. 2012; Moteki et al. 2006; Moteki 2004; Moteki et al. 2004; period (5−7 July), appeared over western Japan just after the pas- Sato et al. 2016). From the perspective of heavy rainfall events sage of Typhoon Prapiroon over the Sea of Japan (Fig. 1). Before that are indirectly affected of tropical cyclones (TCs), the “prede- 4 July (Figs. 1a and 1b), although the Baiu front analyzed in the JMA weather chart is located to north of 40°N over Hokkaido, it suddenly jumps to south of 35°N over western Japan on 5 July Corresponding author: Qoosaku MOTEKI, Dynamic Coupling of Ocean- Atmosphere-Land Research Program (DCOP), Japan Agency for Marine- (Fig. 1c). On 5 July, Typhoon Prapiroon became extratropical and Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho Yoko- moved eastward over the Sea of Japan, and a surface pressure suka City, Kanagawa, 237-0061, Japan. E-mail: [email protected]. ©The Author(s) 2019. This is an open access article published by the Meteorological Society of Japan under a Creative Commons Attribution 4.0 International (CC BY 4.0) license (http://creativecommons.org/license/by/4.0). 38 Moteki, Role of Typhoon Prapiroon on the Formation Process of the Baiu Front Fig. 1. The 24-hour accumulated rainfall (mm; colored) and daily averaged SLP (hPa; contours) with JRA-55 on 3−8 July 2018. The red circles and green rectangles represent the JMA best-track positions of Typhoon Prapiroon and those of Typhoon Prapiroon becoming extratropical, respectively. The blue bold lines represent the synoptic-scale front with the JMA weather chart. trough dominated the Japanese islands. On 5−6 July, a heavy rain- the Okhotsk High moved southward behind Typhoon Prapiroon as fall area of more than 100 mm/h associated with the Baiu front it became extratropical (Supplement 2). Strong cold air advection was distributed over western Japan, and the Baiu front was almost was maintained until 8 July (Figs. 2d, 2e, and 2f). meridionally stationary from 130°E−135°E. On 7−8 July, the Cold air over the Sea of Japan, which was formed by cold air heavy rainfall area was reduced although the Baiu front continued advection after the passage of Typhoon Prapiroon, was associated to be stagnant (Figs. 1e and 1f). with a significant surface pressure ridge (Fig. 1 and Supplement 2) After the passage of Typhoon Prapiroon, cold air advection and continuous northerly winds were maintained by the southward from the Sea of Okhotsk intensified over the Sea of Japan (Fig. PGF (Fig. 4). Although warm air advection was dominant over the 2). On 3 July (Fig. 2a), warm air advection was dominant over Sea of Japan from 27 June to 4 July, the northerly wind area to the the Sea of Japan, and the Baiu front was located at approximately north of 45°N was found to expand toward 35°N on 5 July (Fig. 45°N. That is, the southern edge of the cold northerly flow from 4a). This drastic expansion of the northerly wind area corresponds the Okhotsk High was positioned at approximately 45°N (Supple- well to the strong southward PGF more than 1 × 10−3 m/s2 during ment 1). On 4 July (Fig. 2b), cold air advection toward the center the passage of Typhoon Prapiroon. That is, the strong southward of Typhoon Prapiroon appeared north of the Sea of Japan. PGF north of Typhoon Prapiroon could be a trigger for the drastic The 12-hours forward trajectories of cold air originated from southward expansion of the Okhotsk High to the Sea of Japan.
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