SOLA, 2019, Vol. 15A, 37−42, doi:10.2151/sola.15A-007 37

Role of Prapiroon (Typhoon No. 7) on the Formation Process of the Baiu Front Inducing Heavy Rain in July 2018 in Western

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 etal. 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 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 . 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. the Sea of Okhotsk on the isentropic surface of 300 K indicate the After 4 July, positive sea level pressure (SLP) anomalies asso- fact that the passage of Typhoon Prapiroon triggers cold air advec- ciated with the Okhotsk High significantly migrated southward tion over the Sea of Japan (Fig. 3). At 12Z on 3 July (Fig. 3a), all (Fig. 4b). After 5 July, the northerly winds over the Sea of Japan trajectories remain to the north of 42.5°N by blocking with the were maintained by a southward PGF of 0.5−1 × 10−3 m/s2 from northward pressure gradient force (PGF) between Typhoon Prap- the Okhotsk High and cold air advection was continued after iroon and the Baiu front located at approximately 45°N. At 18Z on Typhoon Prapiroon became extratropical and moved away from 3 July (Fig. 3b), the several trajectories are found to intrude into the Sea of Japan to the western North Pacific. Meanwhile, a north- the Sea of Japan across the convergence line of the Baiu front in ward PGF of 0.5−1 × 10−3 m/s2 from the Pacific High to the south association with the southward PGF north of Typhoon Prapiroon. of the Japanese islands was also maintained for the period of 5−7 That is, the areas of northerly winds west of Typhoon Prapiroon July. The balanced PGF across the Japanese islands was consistent and north of 45°N merged (Supplement 1) because the strong with the fact that the Baiu front was almost fixed at the same lat- southward PGF north of Typhoon Prapiroon broke down the itudinal position during the latter period of the Heavy Rain Event convergence line of the Baiu front between 12Z and 18Z on 3 July of July 2018 in western Japan. In addition, the fact that there was and the cold air originated from the Sea of Okhotsk advected over no passage of eastward traveling baroclinic wave disturbances as the Sea of Japan. Thus, the passage of Typhoon Prapiroon could an external forcing to break the balanced meridional PGF field play a role in triggering cold air advection over the Sea of Japan during 5−7 July could be one of the factors for the stagnation of from the Sea of Okhotsk. the Baiu front. After the passage of Typhoon Prapiroon on 5 July (Fig. 2c), Cold air over the Sea of Japan became significantly thicker cold air advection became dominant over the Sea of Japan, and due to cold air advection toward the center of Typhoon Prapiroon SOLA, 2019, Vol. 15A, 37−42, doi:10.2151/sola.15A-007 39

Fig. 2. Same as Fig. 1 but for the daily averaged meridional advection of potential temperature (K/s; colored) and potential temperature (K; contours) with JRA-55 from 3−8 July 2018 at the surface. The bold magenta and violet lines represent the synoptic-scale surface pressure trough and synoptic-scale sur- face pressure ridge derived from the meridional surface pressure gradients, respectively.

(Fig. 5). On 4 July (Fig. 5b), cold air advection showing a baro- and another is deepened along the track of Typhoon Prapiroon tropic structure below 500 hPa in the ascending area of Typhoon between 125°E−130°E (indicated by the brown bold line in Fig. Prapiroon appeared, and a cold air mass less than 295 K formed 6a). The two distinguish troughs merge on 4 July (Fig. 6b) and a over the Sea of Japan. For 5−7 July (Figs. 5c, 5d, and 5e), the very deep trough remains over the Korean Peninsula. Considering thickness of the cold air mass increased due to descending cold the formation of the large-scale upper-level trough, the passage air advection, with its baroclinic structure below 700 hPa. Strong of Typhoon Prapiroon could partly affect the trough deepening ascending warm air (−0.5–1 Pa/s) moving over thick cold air between 125°E−130°E. After 5 July, the westerly jet axis was could induce continuous precipitation. On 8 July (Fig. 5f), the located over the Japanese islands to the east of the merged trough cold air mass began to decay due to enhanced warm air advection, (Figs. 6c, 6d, 6e, and 6f). The westerly jet passed over the isentro- and the Baiu frontal rainfall weakened (Fig. 1f). pic upslope: it passed over the contours of potential temperature, For the process of a cold air mass, the strong southward PGF and a broad ascending area was distributed along the jet axis north of Typhoon Prapiroon broke down the convergence line of (Sampe and Xie 2010). Thus, the passage of Typhoon Prapiroon is the Baiu front remaining at approximately 45°N before 12Z on suggested to influence the enhancement of the upper-level trough 3 July and was a triggering factor of the cold air advection over and the formation of the ascending westerly jet located over the the Sea of Japan after 18Z on 3 July. Cold air advection north of Japanese islands. Typhoon Prapiroon showing a barotropic structure below 500 hPa On 5−6 July, a broad band of heavy precipitation (more than on 4 July contributed to the formation of the thick cold air mass. 200 mm/day) was visible (Figs. 1c and 1d), and strong ascending In addition, the northerly winds were maintained by the broad and motions of −1–1.5 Pa/s were widely distributed across the Japa- continuous southward PGF between the surface pressure trough nese islands. The adiabatic component of the ascending motion and ridge extending in the west-east direction, even after Typhoon was calculated as −0.01–0.1 Pa/s from the wind speeds of the jet Prapiroon became extratropical and moved away from the Sea of and slope angle of the isentropic surface. The adiabatic ascending Japan to the western North Pacific. Due to the enhancement of jet could be a favorable condition for sustaining convective activi- cold air advection associated with northerly winds, a cold high- ties in the Baiu frontal zone. pressure system was maintained over the Sea of Japan, which allowed the Baiu front to remain stationary along the northern coast of western Japan. 4. Summary JMA (2018) also pointed out that an upper-level trough was located to the east of the Baiu front during the latter period and Heavy rain in western Japan was broadly induced by the stag- an upper-level westerly jet was accelerated over western Japan. nation of the Baiu front during the period of 5−7 July 2018. This Figure 6 shows that there are two upper-level troughs which study focused on a role of Typhoon Prapiroon on the formation are derived from the zonal gradient of the geopotential height process of the Baiu front over western Japan. The passage of (Supplement 3). One is originated from an eastward traveling Typhoon Prapiroon (Typhoon No. 7) was found to play a role in baroclinic wave (indicated by the green bold line in Fig. 6a) triggering cold air advection over the Sea of Japan from the Sea 40 Moteki, Role of Typhoon Prapiroon on the Formation Process of the Baiu Front

Fig. 3. The meridional pressure gradient force (1 × 10−3 m/s2; colored) Fig. 4. Time-latitude cross sections of (a) the meridional surface pressure and meridional wind speed (m/s; magenta contours) with the JRA-55 at gradient force (where the northward direction is positive, 1 × 10−3 m/s2) (a) 12Z and (b) 18Z on 3 July 2018. The vectors of the pressure gradient and (b) the SLP anomaly from the mean SLP of 32°N−35°N averaged force are shown. The 12-hours forward trajectories were calculated from from 130°E−140°E (hPa; colored). The meridional surface wind speed is the 91 cross marks in the area of 135°E−150°E and 42.5°N−50°N at every contoured every 2 m/s. The open circles denote the positions of Typhoon 1.25° on the isentropic surface of 300 K using the 3 hourly winds linearly Prapiroon from the JMA best track. The light green triangles show the po- interpolated from the 6 hourly JRA-55. The triangles indicate the terminal sitions of the Baiu front from the JMA weather chart. point of each trajectory. The terminal points intruding into the area to the south of 42.5°N and west of 140°E are shown by blue and the others are shown by purple. mass was formed along the northern coast of western Japan. The southward PGF from the Okhotsk High, which expanded over the Sea of Japan, was balanced with the northward PGF from of Okhotsk and the drastic southward expansion of the Okhotsk the Pacific High, and the Baiu front was meridionally stationary. High. During the period of 27 June−4 July before the typhoon pas- The cold air advection north of Typhoon Prapiroon had a deep sage, the Baiu front remained at approximately 45°N. The strong barotropic structure and the thick cold air mass was enhanced in southward PGF north of Typhoon Prapiroon passing over the Sea the lower layer below 700 hPa. Such thick cold air advection was of Japan on 4−5 July broke down the convergence line of the Baiu maintained after the passage of Typhoon Prapiroon, and strong front, which remained at approximately 45°N, and triggered the ascending motion moving over the thick cold air along the Baiu cold air intrusion over the Sea of Japan. As a result, the thick cold front continued to create strong rainfall. This cold air advection air originating from the Okhotsk High intruded over the whole was dominant over the whole Sea of Japan, and the Baiu front was Sea of Japan and the Baiu front was formed over western Japan. stagnant in western Japan on 5−7 July. The Okhotsk High expanding toward the Sea of Japan generated In addition, the upper-level trough was deepened along the a broad and continuous southward PGF over western Japan. The typhoon track, and the westerly jet axis was fixed over the Japa- PGF across the Japanese islands was almost balanced during the nese islands. The jet was associated with the adiabatic component 3-day stagnation of the Baiu front. In addition, the fact that there of the ascending motion over the isentropic upslope and was sug- was no passage of eastward traveling extratropical cyclones after gested to contribute to the maintenance of Baiu frontal convection. the typhoon passage as an external forcing to break the balanced Regarding future issues, past cases that had similar patterns meridional PGF field could be a favorable condition for the stag- for the stagnation of the Baiu front after typhoon passages should nation of the Baiu front. be reviewed. For example, the heavy rainfall events in western Typhoon Prapiroon triggered strong cold air advection over Japan in 2014 (15−20 August) and northern Kyushu in 1997 the Sea of Japan, and the Baiu front ahead of a thick cold air (12−13 August) occurred just after the passages of SOLA, 2019, Vol. 15A, 37−42, doi:10.2151/sola.15A-007 41

Fig. 5. Meridional vertical cross section for the daily averaged meridional advection of potential temperature (1 × 10−5 K/s; colored), potential temperature (K; black contours) and upward pressure velocity (scaled by a factor of 100; blue contours) averaged from 130°E−135°E with JRA-55 from 3−8 July 2018. The vectors of the meridional wind speed and negative vertical pressure velocity are shown. The orange and light blue stars represent the latitudinal positions of the center of Typhoon Prapiroon and those of the center of Typhoon Prapiroon as it became extratropical, respectively. The light green triangles represent the position of the Baiu front from the JMA weather chart. The latitudinal zones represented by the green and brown bars at the bottom of each panel repre- sent ocean and land, respectively.

Fig. 6. Same as Fig. 1 but for the daily averaged vertical upward pressure velocity (Pa/s; colored) and potential temperature (K; contours) with JRA-55 from 3−8 July 2018 at 500 hPa. The black and gray wind vectors represent the northeastward winds that have adiabatic upward motion on sloping isen- tropic surfaces across potential temperature contours and the others, respectively. The green, brown, and magenta bold lines represent the synoptic-scale troughs traveling eastward, deepened along “Prapiroon” track, and after merging, which are derived from the zonal gradient of geopotential height (Fig. S3), and the blue bold arrows indicate the jet axis, with wind speeds > 15 m/s. 42 Moteki, Role of Typhoon Prapiroon on the Formation Process of the Baiu Front

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