Climate characteristics and factors behind record-heavy rain in Japanin August 2021
24 September 2021
Tokyo Climate Center (TCC), Japan Meteorological Agency (JMA)
https://ds.data.jma.go.jp/tcc/tcc

Summary⚫
⚫

⚫

In mid-August 2021, areas from western to eastern Japan experienced record-heavyrain.
The conditions observed are mainly attributed to the following:➢ Unusually for mid-summer in eastern and western Japan, like atmospheric flow inthe latter half of the early-summer rainy season, known as the Baiu, a stationary
front was strengthened by a significant north-south gradient of temperature in
the lower troposphere between the Okhotsk High to north of Japan and thesouthward shifted North Pacific Subtropical High (NPSH) expanding to the southof Japan. A continuous confluence of water vapor from continental China andalong the margin of the NPSH also contributed to widespread continuous heavy
rainfall.
➢ The southward shift of the NPSH that caused a large amount of water vapor flow
into western and eastern Japan was related to the subtropical jet stream (STJ) in
the upper troposphere, with an overall southward shift over East Asia.
Furthermore, significant southward meandering of STJ to the west of Japan isconsidered to produce a favorable conditions for updraft occurrence andpersistent rainfall.
➢ The southward shift of the STJ was likely affected by sea surface temperatures(SSTs) in the tropical Indian Ocean and related convective activity over the Asiansummer monsoon region.
On 14th August, the northern part of the Kyushu region (see Figure A1 for thelocations) experienced extremely heavy rainfall associated with stationary linear
mesoscale convective systems. In addition to the formation of conditions conducive toconvective cloud development to the south of the front a small-scale low was also
developed on the same day on the front over the East China Sea. This may havecontributed to the formation of cumulonimbus clouds organized as stationary linear
mesoscale convective systems over sea areas west of the Kyushu region to the east of
the small-scale low.

1. Climate conditions
Wide areas from western to eastern parts of Japan experienced record-heavy rainfalltotaling as much as 1,400 mm from mid- to late August 2021 in association with a highly
active stationary front over the country (Figure 1-1, 1-2, 1-3, 1-4 and Table 1-1). From 12th
to 14th August in particular, stationary linear mesoscale convective systems contributed tothese conditions over the northern Kyushu and Chugoku regions (see Figure A1 for thelocations of these regions), with JMA issuing Emergency Warnings for various locations from13th to 15th August. Monthly precipitation on the Sea of Japan and Pacific sides of westernJapan was the highest on record for August since 1946, when collection of the areaaveraged statistical data referenced here began (Table 1-2).
To identify possible causative factors, JMA, with the TCC Advisory Panel on Extreme
Climatic Events (a JMA body staffed by prominent experts on climate science fromuniversities and research institutes), investigated factors considered to have contributed tothese extreme climatic conditions.
2. Factors
The primary factors contributing to the climate conditions detailed above are illustrated
in Figure 2-1.
The widespread heavy rainfall event was partially attributed to unusual frontal formation
for midsummer1 in eastern and western Japan, like atmospheric flow in the latter half of theearly-summer rainy season, known as the Baiu, relating to a significant gradient of northsouth temperature in the lower troposphere between the Okhotsk High in northern Japanand the NPSH in the south. The cold Okhotsk High on the surface developed from earlyAugust onward and persisted due to an evolved blocking high over eastern Siberia in theupper troposphere. The NPSH shifted farther southward than normal and expanded to theseas south of Japan. Continuous and massive inflow of water vapor into the front fromcontinental China and along the margin of the NPSH also contributed to the event (Figure1-5).
The southward shift of the NPSH that caused a large amount of water vapor flow into
western and eastern Japan was related to the southward shift of the subtropical jet stream(STJ) from East Asia to Japan in the upper troposphere. Furthermore, significant southward
meandering of the jet stream to the west of Japan is considered to have supported upward
airflow and persistent rainfall over areas from eastern to western Japan.
From mid-July to early August, tropical SSTs were higher than normal to the southwestof Sumatra and lower than normal in the western part of the Indian Ocean. In association,
the Asian summer monsoon was less active than normal from the South China Sea to thearea east of the Philippines, and its active convection region was shifted southwest of its
normal position.
1

The North Pacific Subtropical High normally extends northwestward around Japan, bringing hot and sunny conditions tothe country in mid-summer.

Statistical analysis indicates that the inactive and southwestern-biased Asian monsoon
may have caused the STJ over East Asia to shift southward overall and meander southward
to the west of Japan.
3. Characteristics of the record-heavy rain event and related atmospheric conditions
From August 12th to 14th, the Chugoku region and the northern part of the Kyushu
region experienced stationary linear mesoscale convective systems and JMA issued nineinstances of Bulletin on Significant Heavy Rain (Table 3-1). Before dawn and on the morningon 14th August in particular, the northern part of the Kyushu region affected by thestationary linear mesoscale convective systems experienced extreme heavy rainfall (Figure3-1). During this period, large amounts of water vapor flowed into areas around the front
over seas west of the Kyushu region. Along with enhanced updraft near the front, lowerlevel winds were strengthened to the south of the front, supporting local development ofconvective clouds (Figure 3-2). On the front over the East China Sea, a small-scale low may
also have contributed to the formation of the stationary linear mesoscale convective
systems over seas west of the Kyushu region to the east of the small-scale low (Figure 3-3).
The increased precipitation of this event may be linked to a rising long-term trend inairborne water vapor associated with global warming (Figure 2-2 and 2-3), although furtherstudies are needed to evaluate the link quantitatively including probability of heavy rainfall.

Figure 1-1. Total precipitation amounts [mm] for 11 – 26 August, 2021

Figure 1-2. Maximum 72-hour precipitation [mm] for 11– 26 August, 2021

Figure 1-3. Frequency distribution of overall total precipitation at 1029 selected AMeDAS
stations throughout Japan for 10-day periods starting on the 1st, 11th and 21st of themonth since 1982

Figure 1-4. Comparison of maximum overall N-day precipitation amount during each heavy
rain event occurrence at 1,029 selected AMeDAS stations throughout Japan since 1982

Table 1-1 Record overall total precipitation at selected 1029 AMeDAS stations all over Japanfor any 10 days starting from 1st, 11th and 21st of the months since 1982
Rank
123
45
6
7
8
910

PeriodMid-August, 2021
Early July, 2018
Early July, 2020
Late June, 1985
Mid-September, 1990
Late October, 2017
Early August, 2014
Late June, 1999
Early July, 1995
Early September, 1989

All stations (mm)
235,788.5
218,844.0
217,037.5209,016.0
205,925.0
203,475.5
185,173.0
170,692.0
166,014.0
165,200.0

Table 1-2 Years and its ratios of the top three highest events of monthly precipitation totals in
August from 1946 to 2021
Western Japan
1st2021(331%）

Sea of Japan side Pacific side of Eastern Japanof western Japanwestern Japan
2021(371%)
2021(297%)
1982(208%)

2nd3rd2014(246%)
1980(230%)

1980(255%)
1993(228%)

2014(275%)
2004(216%)

2021(205%)
2003(201%)

Pacific side ofeastern Japan
1982(231%)
2021(219%)
2003(218%)

Ratios are deviations from the baseline (the 1991 – 2020 average). The rainfall in the Sea of Japanside of eastern Japan in August 2021 was the ninth highest (152% of normal).

(a) Vertical integrated water vapor flux amount and its convergence(b) Water vapor amounts concentrated in western Japan

Figure 1-5. (a) Vertically integrated horizontal water vapor flux amount (vectors) and its
convergence (shade) averaged over the period from 11th to 17th August 2021 and (b) timeseries representation of vertically integrated water vapor flux amount convergence (the 7day running mean) in the area surrounded by the black lines in (a)) from June to July after1958(a) Unit: kg/m/second for vectors and mm/day for shade.
(b) Unit: mm/day. Data for the period from 1958 to 2021 are overlaid into one calendar year. Thered, blue, orange and gray indicate values for 2021, 2020 (The heavy rain event of July 2020), 2018
(The heavy rain event of July 2018), 2014 (The heavy rain event of August 2014) and others after
1958, respectively. (a) and (b) are generated from JRA-55 data (Kobayashi, 2015), and vertical
integration here represents integration from the surface to 300hPa.

Figure 2-1. Characteristics of atmospheric circulation bringing record-heavy rain from 11th
to 17th AugustFigure 2-2. Annual maximum precipitation ratio in 72 hours from 1976 to 2021 in Japan
This graph is based on precipitation data from 637 AMeDAS stations which have been
continuously operated from 1976 to 2021. Bars indicate the ratio to the baseline (the 1991 – 2020average), while the blue and red lines indicate the related five-year running mean and the longterm linear trend, respectively (statistically significant at a confidence level of 95%). The value for2021 is preliminary as of 31st of August. The red triangle marks the timing of a change in theobservation method for precipitation (observed every hour before 2003 and every 10 minutes
thereafter).

Figure 2-3. Specific humidity ratio at 850 hPa for August from 1981 to 2021 in western andeastern JapanThe data are presented as ratios against the baseline (the 1991 – 2020 average).
Note: The term specific humidity refers to the mass of water vapor in a unit mass of moist air(g/kg). The data used in this analysis were based on radiosonde observations (balloon-borne
instrument platforms with a radio-transmitting device) at 6 upper-air observation stations inwestern and eastern Japan (Wajima, Tateno, Hachijojima, Shionomisaki, Fukuoka and Kagoshima).
The thin black line indicates the averages of the data for the 6 stations. The blue and red lines
indicate the related five-year running mean and the long-term linear trend, respectively(statistically significant at a confidence level of 99%). Data from the period marked by the red
triangles may include biases due to instrument changes.

Table 3-1. Issues regarding bulletin on significant heavy rain in mid-August 2021
Bulletin on significant heavy rain relates to ongoing extreme downpours in particular locations
associated with stationary linear mesoscale convective systems. Against a background of increasing
disaster risk, the content is issued to outline situations with keywords related to such systems when
the following conditions are met:⚫

1: Areas in which Radar/Raingauge-Analyzed Precipitation with a spatial resolution of 5 km
exceeds 100 mm over 500 km2 during the preceding three hours.

⚫

2: The shape of the area extracted by the criterion of 1 is linear (with aspect ratios exceeding
5:2).

⚫

3: Maximum cumulative precipitation amount during the preceding three hours in 1 exceeds
150 mm.

⚫

4: Real-time Landslide Risk Map exceeds the criteria for Landslide Alert Information and reaches
80 percent of the threshold of soil rainfall index for Heavy Rain Emergency Warnings, or Realtime Flood Risk Map significantly exceed warning criteria.

Information is re-issued when the related criteria still apply three hours after initial issuance, orwithin the three-hour period if the target area is expanded.
Time of issuance
Hour

MinuteIssuancenumberPrefecturePrimary subdivision

Year

Month Day2021

8121359

1 Fukuoka

Chikugo2021

8121359

1 Kumamoto

Kumamoto

2021

8130919

1 Hiroshima

Northern, Southern

2021

81402

21

1 SagaSouthern

2021

81402

21

1 Nagasaki

Northern

2021

81402

49

2 Nagasaki

Southern, Northern

2021

81405

00

2 SagaSouthern, Northern

2021

81405

59

3 Nagasaki

Southern, Northern

2021

81406

09

1 Fukuoka

Fukuoka

Figure 3-1. Timing of issuance for bulletin on significant heavy rain and related target areas
on 14th August

Figure 3-2. Left: 500 m winds (vectors), equivalent potential temperature (red), water vapor
flux amount (vectors and shading) and sea level pressure (black) based on JMA meso-scale
analysis at 3 a.m. on 14th August 2021 (front and high/low-pressure values frompreliminary weather maps). Right: Infrared satellite imagery with 500 hPa geopotential
height (yellow) and temperature (red dotted line) for the same time. Lower right: 700-hPa
vertical velocity (shade), 850 hPa isotherms (red) and winds (vectors) from the Global
Spectral Model (GSM) initial (T = 0). Also shown are 5,800 m isotherms for 12 hours before(yellow dotted line), trough data (brown) and the same for 12 hours before (brown dottedline).

Figure 3-3. Left: Time-series representation of radar echo intensity at 20-minute intervals
from 3:20 to 4:40 on 14th August 2021. Center: sea level pressure (black), wind (vectors)
and precipitable water (shading) at 3 a.m. on 14th August 2021 based on JMA meso-scale
analysis. The black dotted line represents the main convergence area. Right: simultaneousvertical profiles of atmospheric temperature (red) and dew point temperature (blue dottedline) averaged for the area within the blue box in the central figure, based on JMA local
analysis.

Figure A1. Climatologicalregions of Japan
JMA’s seven regional divisions
for climate monitoring andforecasting (the Sea of Japanand Pacific sides of northern,eastern and western Japan, andOkinawa/Amami)