Primary Factors behind the Heavy Rain Event of July 2018 and the Subsequent Heat Wave in Japan

An extreme rainfall event occurred over western Japan and the adjacent Tokai region mainly in early July, named “the Heavy Rain Event of July 2018”, which caused widespread havoc. It was followed by heat wave that persisted in many regions over Japan in setting the highest temperature on record since 1946 over eastern Japan as the July and summertime means. The rain event was attributable to two extremely moist airflows of trop - ical origins confluent persistently into western Japan and large-scale ascent along the stationary Baiu front. The heat wave was attributable to the enhanced surface North Pacific Subtropical High and upper-tropospheric Tibetan High, with a prominent barotropic anticyclonic anomaly around the Korean Peninsula. The consecutive occurrence of these extreme events was related to persistent meandering of the upper-level subtropical jet, indicating remote influence from the upstream. The heat wave can also be influenced by enhanced summertime convective activity around the Philippines and possibly by extremely anomalous warmth over the Northern Hemisphere midlatitude in July 2018. The global warming can also influence not only the heat wave but also the rain event, consistent with a long-term increasing trend in intensity of extreme precipitation observed over Japan. Precipitation (GSMaP; Ushio et al. 2009) in near real time (GSMaP_NRT) were also used. Climatological means are defined as averages for the period from 1981 to 2010, and anomalies as deviations from the thus-defined climatologies.


Introduction
Japan experienced an extreme climate event that brought unprecedented amounts of precipitation from 28 June to 8 July 2018, which was officially named "The Heavy Rain Event of July 2018" by the Japan Meteorological Agency (JMA). It caused widespread disastrous conditions, especially over western Japan and Tokai region, in early July in the presence of the stationary Baiu front and Typhoon Prapiroon (T1807). As of 9 January 2019, 237 fatalities were reported during the heavy rain event (Cabinet Office, Japan 2019). Subsequently, extremely high temperatures persisted nationwide except in Hokkaido and Okinawa/Amami regions, in association with the North Pacific Subtropical High (NPSH) that intensified in the vicinity of Japan from mid-July to the end of August. As of 5 February 2019, it was reported that these extremely high temperatures in summer 2018 caused 1469 fatalities due to heat stroke nationwide (Ministry of Health, Labour and Welfare, Japan, 2019).
This article described the overall characteristics of and possible related factors behind the Heavy Rain Event of July 2018 (or "2018 rain event" in short) and the subsequent notable heat wave conditions, based on analysis conducted jointly by JMA and the JMA Advisory Panel on Extreme Climate Events. The latter is a JMA body consisting of several experts on climate science from universities and research institutes in Japan.

Data
In-situ observational station data of precipitation and surface temperature over Japan were obtained from the JMA Automated Meteorological Data Acquisition System (AMeDAS). Various atmospheric quantities (e.g., geopotential heights at 200 hPa) used in this study were from the Japanese 55-year Reanalysis (JRA-55; Kobayashi et al. 2015) since 1958 with spatial resolution of 1.25º in both latitude and longitude. In addition, the Centennial in situ Observation-Based Estimates of the variability of sea surface temperatures (SSTs) (COBE-SST; Ishii et al. 2005), NOAA Interpolated Outgoing Longwave Radiation (OLR) data (Liebmann and Smith 1996) and the Global Satellite Mapping of Precipitation (GSMaP; Ushio et al. 2009) in near real time (GSMaP_NRT) were also used. Climatological means are defined as averages for the period from 1981 to 2010, and anomalies as deviations from the thus-defined climatologies. stationary Baiu front due to re-enhancement of the NPSH. This shift also resulted in the withdrawal of the Baiu period. The withdrawal was significantly earlier than its climatological counterpart from Chugoku to southern Tohoku regions. The Baiu withdrawal in Kanto-Koshin region (around Tokyo) occurred around 29 June, the earliest on record.
In the period from mid-July to late August in 2018, area-averaged temperatures in eastern and western Japan were significantly above normal. The July-mean (Fig. 2a) and seasonal-mean (June-July-August) temperature anomalies in 2018 over eastern Japan were +2.8°C and +1.7°C, respectively, both of which were the highest since the area-averaged statistics for reference began in 1946.
At many weather stations daily maximum temperature often exceeded 30°C or sometimes even 35°C. Several stations reported maximum temperatures exceeding 40°C around the peak of the heat wave, and on 23 July a new national record of maximum temperature of 41.1°C was set at the Kumagaya City north of Tokyo. A total of 202 AMeDAS stations set record maximum temperatures this summer. Figure 2b shows evolution of the cumulative number of AMeDAS stations at which observed daily high temperatures were 35°C or higher in June through September for 2018 and its counterpart for some recent years. The cumulative numbers in 2018 show an unprecedented increase that began as early as in mid-July, and finally the annual total for 2018 well exceeds the previous highest record set in 2010 since 1976.

Primary factors behind heavy rainfall from 5 to 8 July
Here, we focus on the heavy rainfall from 5 to 8 July, which yielded most of the rainfall over western Japan and Tokai region, and discuss its primary factors.

a. Synoptic situation around Japan
Around 3 July (Fig. S2f), the Baiu front was persistent over the northern part of the Sea of Japan and Hokkaido between the intensified NPSH and the developing Okhotsk High. On 4

The Heavy Rain Event of July 2018
Most of the regions in Japan experienced significant rainfall during the 2018 rain event from 28 June to 8 July with unprecedented amounts of precipitation recorded at many of the AMeDAS stations. Specifically, some of those stations in Shikoku and Tokai regions recorded more than 1,800 and 1,200 mm, respectively, during the event ( Fig. 1a; see Fig. S1 for the regions referred to in this article), and some areas experienced as much as two to four times the precipitation of the monthly climatology for July. Overall precipitation observed at 966 AMeDAS stations selected throughout Japan in early July 2018 reached 208,035.5 mm (215.4 mm per station), which was the highest among any 10-day periods starting from the 1st, 11th and 21st of the months since 1982, highlighting the nationwide significance of this event.
In comparison with past heavy rainfall events caused by frontal systems and typhoons, a prominent characteristic of the 2018 rain event is that the record-breaking local precipitation, particularly within 48 to 72 hours, was observed extensively over western Japan and Tokai region, including the Seto Inland Sea region, where monthly precipitation climatologies are lower than in the surroundings (Fig. 1b). Total precipitation at the selected AMeDAS stations throughout Japan for the period from 5 to 7 July 2018 was 140,567.0 mm (equivalent to 145.5 mm per station), which was the highest among any three-day periods since 1982. The three-day total during the event was the highest ever for Chugoku region.

Heat wave from mid-July to August
The 2018 rain event terminated with the northward shift of the  July (Fig. S2g), the frontal system shifted further northward as Typhoon Prapiroon moved northeastward over the Sea of Japan. The typhoon subsequently transformed into an extratropical cyclone and approached Hokkaido on 5 July (Fig. S2h). In association with the further development of the Okhotsk High, the Baiu front shifted southward to the vicinity of western Japan, initiating heavy rainfall there.
As the NPSH southeast of Japan gradually re-intensified from 6 July onward, the Baiu front stagnated in the vicinity of western Japan, continuously yielding heavy rainfall (Fig. S2i). On 7 July, an intensifying upper-level trough approached the western part of the Sea of Japan from the west, generating a meso-scale lowpressure system to its east on the Baiu front (Fig. S2j). Enhancing moisture inflow from the south into western Japan to an extreme level (Takemura et al. 2019 (submitted); Sekizawa et al. 2019), this eastward-moving system yielded torrential rainfall over the Seto Inland Sea side of Chugoku and Shikoku regions. By the evening of 7 July the large-scale precipitation area associated with the upper-level trough and surface low-pressure system moved away from western Japan. Still, the low-level warm moist inflow persisted into the following day, to organize several meso-scale convective systems on the Pacific side of western Japan.

b. Large-scale circulation anomalies and associated remote influence
During the heavy rainfall event discussed above, the anomalous intensification of the surface NPSH just southeast of Japan (Figs. 3b and S3) was associated with a persistent northward meander of the upper-level subtropical jet (STJ) east of Japan with a quasi-stationary anticyclonic anomaly (Fig. 3a). Meanwhile, the development of the surface Okhotsk High (Figs. 3b and S3) was associated with a persistent marked meander of the upper-level polar-front jet (PFJ) with the consequent development of a prominent blocking ridge over eastern Siberia (Fig. 3a;Nakamura and Fukamachi 2004). The concomitant meanders of these two jetstreams thus led to the amplification of the NPSH and the Okhotsk High, acting to strengthen temperature and moisture contrasts across the stationary Baiu front during the rain event. In addition, the STJ meander with the upper-level trough lingering around the Korean Peninsula acted to induce large-scale ascent along the Baiu front, thereby favoring the formation of convective rainband (Sampe and Xie 2010). The meanders of the STJ over Eurasia in summer are observed in association with quasi-stationary Rossby wave propagation (teleconnection), whose primary pattern is known as the Silk Road pattern (Enomoto et al. 2003;Kosaka et al. 2009). Even more prominent meanders of the STJ as the Silk Road teleconnection occurred in late June, bringing the earliestever withdrawal of the Baiu rainy season to Kanto-Koshin region (around Tokyo; Figs. S2b and S3).

c. Characteristic climate conditions causing the rain event (1) Confluence of two extremely moist airflows into western Japan
During the rainfall event, enhanced convective activity over the southern East China Sea (Fig. 4a) moistened the lower-and mid-tropospheric air, which was then transported into western Japan by the southwesterlies. As the surface NPSH intensified southeast of Japan, the surface southerlies strengthened south of Japan and thereby transported a huge amount of moisture into western Japan (Figs. 3b and 4a). As demonstrated by Takemura et al. (2019, submitted), the confluence of these two moist airflows brought an unprecedented amount of moisture into western Japan from 5 to 7 July 2018, based on the JRA-55 since 1958. They also argue that the enhanced convection over the East China Sea acted to reinforce the moist southwesterlies by inducing low-level cyclonic anomalies.
(2) Persistent ascent associated with the stationary Baiu front Around 5 July, low-level cool air was transported southwestward over the Sea of Japan due to the prominent Okhotsk High, which in combination with the concomitant intensification of the low-level southerlies, strengthened meridional temperature contrast across the Baiu front. Through these frontogenetic processes, ascent was enhanced on the warmer side of the front, which favored the organization of convective systems over western Japan and its vicinity, leading to heavy precipitation there. This continuous ascent was attributable to the developing upper-level trough (e.g., Takemura

(3) Occurrence of line-shaped precipitation systems
Some areas affected by line-shaped convective precipitation systems experienced extended periods of torrential rainfall, resulting in record-breaking precipitation totals. Some of those systems exhibited a sequential back-building formation of convective clouds, typified by those observed in the evening of 6 July in Hiroshima Prefecture (Tsuguti et al. 2018).

Primary factors behind the heat wave from mid-July to August a. Synoptic situation around Japan and associated large-scale atmospheric circulation
After the early July rain event, both the surface NPSH and the upper-tropospheric Tibetan High (or the South Asian High), which exert substantial influence on Japan's summer climate, persistently extended toward the main islands of Japan (Figs. 3c, 3d, and S3). These high-pressure systems contributed to extremely high surface air temperature (SAT) observed over mainland Japan, through prevailing sunny conditions and adiabatic warming by anomalous descent. The extension of the Tibetan High toward Japan was associated with an equivalent barotropic anticyclonic anomaly and the northward-meandering STJ around the Korean Peninsula (Fig. 3c). The northward meander was enhanced repeatedly due to the intensification of the anticyclonic anomaly with incoming wave trains across Eurasia similar to the Silk Road pattern (Fig.  5a). Three events of such wave-train propagation occurred in the middle through late July, which were more frequent and stronger than those in August. The deep anticyclonic anomaly was also concomitant with the strengthening of large-scale low-level cyclonic circulation around Southeast Asia and the Philippines (i.e., the monsoon trough) and the associated enhancement of convective activity around the Philippines (Figs. 4b and 5b). The enhanced convection persisted from July through August, peaking in mid-July, around 10 August and in late August. The cyclonic anomaly around the Philippines and the anticyclonic anomaly around Japan are a manifestation of the Pacific-Japan (PJ) pattern (Nitta 1987;Kosaka and Nakamura 2010). These atmospheric conditions causing extreme positive SAT anomalies over Japan were marked in the middle and late July and similar conditions were also observed in August. The northward STJ meander became most pronounced in late July, influencing the untypical track of Typhoon Jongdari (T1812), which moved westward after making landfall on mainland Japan. At that time, the STJ axis was located around 50°N with weak easterlies south of 40°N (not shown).

b. Persistent warm anomalies in the Northern Hemisphere midlatitude
In addition to the aforementioned effects by the anomalous circulation, factors considered to act as a background to the heat wave over Japan include marked tropospheric warmness over the Northern Hemisphere (NH) midlatitude since March 2018. In fact, zonally averaged tropospheric temperature in the NH midlatitude (e.g., 40°N−60°N) was the highest in 2018 for July since 1958 (Fig. 6a). Though more investigation is needed into possible contribution from decadal variability and/or global warming, this warmness might be attributable, at least in part, to enhanced convective activity in the 10°N−20°N band over the western and central North Pacific including the vicinity of the Philippines, in combination with suppressed convective activity in the tropical South Pacific. This equatorial asymmetry in anomalous convective activities may be attributable to the corresponding asymmetry in the SST field with positive anomalies mostly over the tropical North Pacific and negative anomalies largely over the tropical South Pacific (Fig. 6b).

Summary and discussion
This article offers an overview of the two extreme events that occurred in Japan during 2018 summer, the Heavy Rain Event of July 2018 and the pronounced heat wave from mid-July to the end  of August. As illustrated in Fig. 7a, our analysis of the 2018 rain event, with primary focus on the heavy rainfall from 5 to 8 July, has revealed the three primary atmospheric factors as follows that could contribute to the event: (A) Persistent confluence of two extremely moist airflows with tropical origins into western Japan; (B) Persistent ascent in a large-scale rain band along the stationary Baiu front; and (C) Formation of line-shaped convective systems.
The factors (A) and (B) were contributed to by the intensified surface NPSH and Okhotsk High in association with persistent pronounced meanders of the upper-level STJ and PFJ, respectively (Fig. 7b). Overall, (A) and (B) appear to be the dominant factors behind the event, while (C) played a significant role in torrential meso-scale precipitation over some regions around western Japan.
As illustrated in Fig. 7c, the primary factors for the heat wave was the persistent extension of the surface NPSH and the uppertropospheric Tibetan High toward mainland Japan, through the PJ-like teleconnection from convective activity enhanced persistently around the Philippines and a persisted poleward meander of the STJ in association with the teleconnection over Eurasia similar to the "Silk Road pattern". Although Fig. 7c illustrates the conditions in the middle and late July, similar conditions were observed in August.
The consecutive occurrence of these two extreme climate events was attributable in part to pronounced meanders of the STJ (Fig. 5), including a manifestation of the Silk-Road-pattern-like teleconnection over Eurasia. This wave teleconnection gave rise to consecutive occurrence of abnormal weather conditions across Eurasia, including anomalous high temperatures over central Asia. Another contributor to the heat wave can be above-normal zonalmean tropospheric temperature over the NH midlatitude that persisted since March 2018.
Recent studies have focused on possible contribution to extreme events from global warming (e.g., Imada et al. 2018). JMA (2018) reported that an upward long-term trend in SAT over Japan is superimposed on its interannual and decadal fluctuations. The warming trend was likely to act as a background to the heat wave in 2018 summer. In fact, Imada et al. (2019) argued that the extremely high temperatures over Japan in July would never have happened without anthropogenic global warming, based on large-ensemble simulations through an event attribution approach. Furthermore, it has been revealed from observations at AMeDAS stations all across Japan that the nationwide average of the annual 72-hour maximum precipitation has increased by about 10% over the last 30 years (Fig. S4). This statistic provides certain evidence for an increasing trend in intensity of extreme local precipitation events observed recently over Japan. According to IPCC (2013),  an increasing trend in tropospheric water vapor is very likely almost globally since the 1980s in association with the observed increase in atmospheric temperature. It is widely accepted that saturated water vapor amount increases approximately by 7% for air temperature increase by 1°C. In fact, JMA (2015) reported an increasing trend in lower-tropospheric water vapor over Japan since the 1980s based on radiosonde observations (Fig. S5). These trends suggest a possibility that the 2018 rain event may be influenced by the global warming.
Although additional analyses are presented in companion papers (e.g., Takemura et al. 2019 (submitted); Sekizawa et al. 2019), further investigations are required in future to deepen our understanding of the aforementioned extreme events, especially their mechanisms and predictability as well as specific contributions from global warming and decadal climate variability (e.g., Urabe and Maeda 2014). It is also instructive to perform event attribution studies targeting global warming impacts on these extreme events (e.g., Imada et al. 2019) as well as other extreme events occurred in Japan (e.g., Imada et al. 2018).