Received: 7 February 2018

Revised: 10 December 2018

Accepted: 11 December 2018

DOI: 10.1002/asl.880

RESEARCH ARTICLE

Maintenance condition of back-building squall-line in a numericalsimulation of a heavy rainfall event in July 2010 in Western
Japan
Ryuji Yoshida1,2,3,4| Seiya Nishizawa4| Hisashi Yashiro4 | Sachiho A. Adachi4|

Tsuyoshi Yamaura3,4 | Hirofumi Tomita4,5 | Yoshiyuki Kajikawa3,4
1
CIRES, University of Colorado Boulder, Boulder,
Colorado
2

NOAA Earth System Research Laboratory,
Boulder, Colorado
3
Research Center for Urban Safety and Security,
Kobe University, Kobe, Japan
4
Computational Climate Science Research Team,RIKEN Center for Computational Science, Kobe,
Japan
5
Japan Agency for Marine-Earth Science andTechnology, Yokohama, Japan

Correspondence
Ryuji Yoshida, CIRES/University of Colorado,
NOAA/ESRL, 325 Broadway, Boulder, CO
80305.
Email: ryuji.yoshida@noaa.gov
Funding information
HPCI Junior Researcher Promotion Project, Grant/Award Number: hp170164; Japan Science andTechnology Agency (JST) Core Research for
Evolutional Science and Technology (CREST),
Grant/Award Number: JPMJCR1312; Foundation
for Computational Science (FOCUS) Establishing
Supercomputing Center of ExcellenceBack-building squall-lines (BBSs) cause local heavy rainfalls and lead to natural
disasters. Therefore, better understanding BBSs would have a significant impact on
disaster prevention and mitigation. Although a conceptual maintenance conditionof BBSs is proposed by previous studies, the concept stands on a case study. We
clarified that a mesoscale moist area in water vapor front localizes the continuous
forming of new cells in the BBS and reinforced the concept with another case. A
numerical simulation of the heavy rainfall during July 11–12, 2010, in westernJapan was used to confirmation of the conceptual maintenance condition of the
BBS. We found that the new cells of BBS are continually formed at the confluencezone of the synoptic-scale cold front and the mesoscale area with large water vapor
amount at low-level. It supports the conceptual maintenance condition, and themesoscale moist area in water vapor front is a distinct characteristic in this case.
The position where new cells are formed has been anchored by the mesoscale moist
area for 2 hr.
KEYWORDS

back-building squall-line, mesoscale, mesoscale convective systems, regional
mesoscale modeling, severe weather

1 | INTRODUCTION
Quasi-stationary mesoscale convective systems (MCSs) socalled as back-building squall-lines (BBSs) cause heavy
rainfalls over Japan and lead to natural disasters. In BBSs,
the convective cells are arranged in a line along the midupper wind direction. Newer cells appear on the upwindside, which is the opposite side of the cell motion (Bluesteinand Jain, 1985; Parker and Johnson, 2000). Environment ofrich low-level moisture is necessary for heavy rainfall
including BBSs, and mid-level moisture is also suitable condition (Chuda and Niino, 2005; Tsuguti and Kato, 2014;

Unuma and Takemi, 2016). These environments tend todevelop in the early summer season from June to July called
as Baiu season. In Baiu season, BBSs are observed around a
stationary front, namely Baiu front (Yoshizaki et al., 2000;
Kato, 2006; Shinoda et al., 2009), because the southern partof Baiu front has a rich low-level moisture. The north–south
gradient of water vapor (WV) amount is introduced as WV
front in Moteki et al. (2004a).
The large-scale environments of BBSs have been investigated, however, the maintenance condition of BBSs is still
not clearly understood. Formation of new cells at upwindside is essential to maintain back-building structure, and aThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium,provided the original work is properly cited.
© 2018 The Authors. Atmospheric Science Letters published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society.
Atmos Sci Lett. 2019;20:e880.
https://doi.org/10.1002/asl.880

wileyonlinelibrary.com/journal/asl2

1 of 9

YOSHIDA ET AL.

2 of 9

(a)

37°N

37°N

36°N

36°N

35°N

35°N

34°N

34°N

33°N

33°N

32°N

32°N

03JST 12 JUL 2010

31°N
127°E

129°E

131°E

133°E

135°E

[mm/h] 1.0

12.8

24.6

36.4

48.2

(c)

37°N

127°E

129°E

131°E

133°E

135°E

[mm/h] 1.0

12.8

24.6

36.4

48.2

37°N

36°N

36°N

35°N

35°N

34°N

34°N

33°N

33°N

32°N

32°N

03JST 12 JUL 2010

31°N
127°E

129°E

131°E

133°E

135°E

[mm/h] 1.0

12.8

24.6

36.4

48.2

137°E
60.0

04JST 12 JUL 2010

31°N

137°E
60.0

(b)

137°E
60.0

(d)

ACCUMULATED

31°N
127°E
[mm]

129°E

131°E

133°E

135°E

137°E

15

25

35

45

55

5

Radar/Raingauge-Analyzed Precipitation data provided by the JMA for the period from 03:00 JST to 05:00 JST on July 12, 2010 for (a-c). (d) isan accumulated precipitation in the same period from 03:00 JST to 06:00 JST. A linear precipitation area with a south-west to north-east orientation is observed.
The broken line in (a) shows the surface location of the synoptic-scale cold front, on a weather map provided by the JMA, at 03:00 JST on July 12, 2010

FIGURE 1

condition to maintain continuous updrafts is necessary toform new cells. In this study, the location where new cells
are formed is referred to as an initiation point. The initiation
point would have some mesoscale characteristics to localize
continuous updrafts. In some of the BBSs formed overJapan, continuous updrafts are formed due to the effect of
topographic barriers (Yoshizaki et al., 2000). When theeffect of topography on BBS is not dominant, a convergenceaffected by the temperature contrast over the synoptic-scale
front is a possible reason for the formation of an updraft(Kato and Goda, 2001). Synoptic-scale front may cause
wind shear. Magnitude of wind shear over deep layer of troposphere is significantly stronger for mature MCSs than for
weakening MCSs (Coniglio et al., 2007). However, thesynoptic-scale front is too large to specify the initiation
TABLE 1point. For enhancement of initiated convections, thesynoptic-scale front is not often sufficient and low-level
warm air advection is fundamental feature in growing MCSs
(Jirak and Cotton, 2007). Peters and Schumacher (2016) distinguished an internal (smaller-scale) environmental effectfrom an external (larger-scale) environmental effect. They
found that the internal environments affect to become along-lasting MCS. Moteki et al. (2004a) proposed a conceptual maintenance condition of BBS in Japan, which is a confluence zone of Baiu front and WV front. Similar conditions
TABLE 2

Summary of physics scheme used in the simulation

Items

Settings

Microphysics

Six-category one-moment bulk scheme (Tomita, 2008)

Radiation

A correlated k-distribution scheme (Sekiguchi and
Nakajima, 2008)

PBL turbulence

MYNN level 2.5 scheme (Nakanishi and Niino, 2009)

Computational domain settings

Items

Domain 1

Domain 2

Domain 3

Domain arrangement

OutermostIntermediate

InnermostLand and sea
surface

A similarity scheme (Beljaars and Holtslag, 1991;
Wilson, 2001)

Horizontal grid-spacing (km)

7.5

2.5

0.5

Land process

A diffusion and bucket model

Vertical levels

48

60

80

Sea process

Slab Ocean model

Model top height (km)

30

28

24

Urban process

A single layer urban canopy model (Kusaka et al., 2001)

YOSHIDA ET AL.

3 of 9are found in BBSs over the northern Kyusyu area over Japan(Kato, 2006). A mesoscale cold pool locally strengthens atemperature gradient on Baiu front, and the temperature gradient localizes the continuous updraft on the confluencezone (Moteki et al., 2004b). However, this conceptual

Domain 2 (Δx = 2.5km)
37°N

maintenance condition is suggested based on a case study. A
cause for localization of continuous updraft has not been
fully understood yet. Therefore, a confirmation of conditionsin another BBS case is necessary to consolidate the conceptual maintenance condition.

Domain 3 (Δx = 500m)

50 m/s(a)

34.8°N

36°N

34.6°N35°N

34.4°N

34°N
33°N
12 July 2010
00:30 JST

31°N

127°E 129°E 131°E 133°E 135°E 137°E

36.4

34.0°N24.6(b)130.8°E

35°N

34.4°N

33°N[mm/h]
60.0

(f)

48.2

34.2°N

36.4

34.0°N

24.6
12.8

33.6°N
01:00 JST

31°N

127°E 129°E 131°E 133°E 135°E 137°E(c)

130.8°E

36°N

34.6°N35°N

34.4°N33°N

01:00 JST

33.4°N

34.8°N

34°N[mm/h]
60.0

(g)

48.2

34.2°N

36.4

34.0°N

24.6
12.8

33.6°N31°N

01:30 JST

127°E 129°E 131°E 133°E 135°E 137°E(d)130.8°E

36°N

34.6°N35°N

34.4°N33°N

01:30 JST

33.4°N

34.8°N

34°N[mm/h]
60.0

(h)

48.2

34.2°N

36.4

34.0°N

24.6
12.8

33.6°NACCUMULATED

127°E 129°E 131°E 133°E 135°E 137°E

1.0

131.4°E 132.0°E 132.6°E 133.2°E

33.8°N32°N

1.0

131.4°E 132.0°E 132.6°E 133.2°E

33.8°N32°N

1.0

131.4°E 132.0°E 132.6°E 133.2°E

33.8°N

32°N

31°N

12 July 2010
00:30 JST

33.4°N

34.8°N

34°N

12.8

33.6°N

34.6°N

37°N

48.2

34.2°N

36°N

37°N[mm/h]
60.0(e)

33.8°N

32°N

37°N

25 m/s33.4°N
130.8°E

ACCUMULATED

1.0

131.4°E 132.0°E 132.6°E 133.2°E

Horizontal distribution of simulated precipitation from 00:30 JST to 01:30 JST, July 12, 2010 for (a–c) and (e–g). (d) and (h) are accumulated
precipitation in the period from 00:00 JST to 03:00 JST. (a–d) are results in Domain 2; and (d–f ) are results in Domain 3. Vectors represent horizontal wind at
2 km height, but the vectors in (e–g) are horizontal wind anomalies relative to the area average of Domain 2 representing, a synoptic-scale environment. A
linear precipitation area that is similar to the observations is simulated. In (a) and (e), the red arrow indicates the location of the initiation point of new cells

FIGURE 2

YOSHIDA ET AL.

4 of 9We selected a heavy rainfall event in western Japan in
July 2010. The typical back-building structure was found in
this event, while the precipitation magnitude of this event is
weaker than that in Moteki et al. (2004a). Figure 1 shows
the horizontal distribution of precipitation using Radar/Raingauge-Analyzed Precipitation data provided by Japan Meteorological Agency (JMA). A quasi-stationary rainband witha south-west to north-east orientation was observed, and the
[km]
14
10
6
2

JUL12 / 00:10 JST

rainband consisted of several BBSs. The rainband caused an
extreme precipitation rate of over 60 mm/hr and an accumulated precipitation of more than 50 mm in 3 hr from 03:00
Japan Standard Time (JST) to 06:00 JST on July 12, 2010.
A Baiu front merged with a synoptic scale cold front was
analyzed on the weather map; it was located on the north of
the rainband over the Japan Sea. Southwesterly wind wasobserved over the south of the Baiu front. These are
[km]
14
10
6
2

34.12°N / 130.70°E

14
10
6
2

JUL12 / 00:20 JST

14
10
6
2

34.09°N / 130.70°E

14
10
6
2

JUL12 / 00:30 JST

14
10
6
2

34.04°N / 130.70°E

14
10
6
2

JUL12 / 00:40 JST

14
10
6
2

33.98°N / 130.70°E

14
10
6
2

JUL12 / 00:50 JST

14
10
6
2

33.92°N / 130.70°E

14
10
6
2

JUL12 / 01:00 JST

14
10
6
2

33.88°N / 130.70°E

14
10
6
2

JUL12 / 01:10 JST

14
10
6
2

33.82°N / 130.70°E

14
10
6
2

JUL12 / 01:20 JST

14
10
6
2

33.80°N / 130.70°E

14
10
6
2

JUL12 / 01:30 JST

14
10
6
2

33.74°N / 130.70°E

14
10
6
2

JUL12 / 01:40 JST

14
10
6
2

33.74°N / 130.70°E

14
10
6
2

JUL12 / 01:50 JST

14
10
6
2

33.67°N / 130.70°E

30

60

0.01

0.2

0

90

120

0.4

150

0.6

180

210

0.8

240

270 [km]–2

1.0 ×10 [kg/kg]

0

30

60

90

120

–10 –8 –6 –4 –2

0

150

180

2

4

210

6

8

240

270 [km]

10 [m/s]

Vertical-longitudinal cross section for hydrometeors amounts on the left and vertical wind speed on the right following the squall-line.
Hydrometeors are defined as the sum of the mixing ratios of cloud water, rain droplets, cloud ice, snow, and graupel at each level. Each cross section is along
the linear shape of squall-line, and the squall-line is determined subjectively by observing the precipitation for the period from 00:10 JST to 01:50 JST on July12, 2010. Latitude and Longitude information are locations on the map in the left edge of each panel. Black arrows trace the movement of each cell

FIGURE 3

YOSHIDA ET AL.

5 of 9spacing with 30 levels, 3hr) were used as an initial condition
for all the domains and as a boundary condition for Domain
1. The boundary conditions for Domain 2 and Domain
3 were created by using the cost-effective online nesting procedure (CONeP; Yoshida et al., 2017). The integration
period was from 09:00 JST on July 10, 2010 to 09:00 JST
on July 12, 2010. A spin-up period of 24 hr was taken
before the occurrence of the heavy rainfall in order to organize the BBS in the model.

consistent with synoptic-scale environments reported in
Moteki et al. (2004a). It was determined from 1 hr interval
snapshots of radar images that intense convections werelocated on the south-western side of the BBS.

2 | NU M E R IC A L S I M UL AT I O N S E T T I N GS
We performed a numerical simulation of the BBS by using
one-way triple nested computational domains (Figure S1,
Supporting Information). The Scalable Computing for
Advanced Library Environment-Regional Model (SCALERM; Nishizawa et al., 2015; Sato et al., 2015) was used forthe numerical simulation. SCALE-RM is well optimized touse on massive parallel supercomputers, and can provide
possibilities for higher resolution simulation. Tables 1 and 2
show summaries of the computational domain settings andthe physics schemes, respectively.
The grid point value of the global spectral model provided by the JMA (GSM-GPV) for Japan area (1 grid
34.8°N

3 | O U T L I N E O F T H E SI M U L A T E D B AC K BU IL DING SQUA LL -L IN E
Figure 2 shows the horizontal distribution of precipitation in
Domain 2 and Domain 3 from 00:30 JST to 01:30 JST on
July 12, 2010. Although the simulated rainband is formed alittle earlier and weaker than the observation, a distributionof accumulated precipitation for 3 hr from 00:00 JST to
03:00 JST is similar to that observed in Figure 1d. The linear

(a)

(d)

(b)

(e)

(c)

(f)

34.6°N
34.4°N
34.2°N
34.0°N
33.8°N
33.6°N
34.8°N
34.6°N
34.4°N
34.2°N
34.0°N
33.8°N
33.6°N
33.8°N
34.6°N
34.4°N
34.2°N
34.0°N
33.8°N
33.6°N
130.8°E 131.1°E 131.4°E 131.7°E 132.0°E 132.3°E 132.6°E 132.9°E 133.2°E 130.8°E 131.1°E 131.4°E 131.7°E 132.0°E 132.3°E 132.6°E 132.9°E 133.2°E

Precip. [20 mm/h]
T at 750 mm[K]

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

–3
1.9 2.0 ×10 [kg/kg]

Positional relationship between cold fronts and moist air. Areas within green contours have a precipitation rate of 20 mm/hr (inner area ishatched) and the red contours represent the temperature at 750 m height. A horizontal spatial filter of 100 km scale is applied to the temperature
measurements in order to clearly show the synoptic-scale cold front. The color scale shows the residual specific humidity (Qv) relative to the average specifichumidity of the area (0.016 kg/kg). Only positive residual specific humidity is visualized; shaded areas have higher moisture content than the average specifichumidity of the area

FIGURE 4

YOSHIDA ET AL.

6 of 9shaped structure and the quasi-stationary characteristic of therainband are simulated. In the simulation, an extreme precipitation rate of over 60 mm/hr was observed in Domain 3. A
thermodynamic profile and a wind profile are well simulatedcomparing with the radio-sonde observation and the JMA
GSM-GPV data at the Fukuoka observation station(Figure S2). As shown in Figure 2a–c, the weak northernwind over the Japan Sea and the southwesterly wind in thesouth area of the simulated rainband are found. At mid- andupper-levels, the westerly wind prevails. The simulated rainband is consists of multiple BBSs similarly to the observed
rainband. In this study, we focused on the BBS locates at
upwind side in the rainband to avoid contaminations among
BBSs (indicated by a red arrow in Figure 2).
To investigate the detailed structure of the BBS, we drew
vertical cross section images for hydrometeors (Qhyd) andvertical wind speed as shown in Figure 3. The Qhyd is thesum of the mixing ratios of cloud water, rain droplets, cloud
ice, snow, and graupel. Cross sections are drawn along amajor axis of the linear shape of the BBS by subjectively
following the BBS based on precipitation distributions. It isclear that the BBS is composed of multiple cells. The lifetime of each cell is approximately 60 to 80 min. Qhyd on thewestern side of the BBS are concentrated at the mid-level
while those on the eastern side are concentrated at the upperlevel. An updraft from the low-level to upper-level isobserved on the western side. However, less significant
updraft is observed on the eastern side. Thus, new cells areformed at the western end of the BBS and move eastwards
during its life; the cells located on the eastern side are older
than those on the western side. Therefore, we defined thewestern end of the BBS as the initiation point. These cellcharacteristics are consistent with those of the typical BBS
in Bluestein and Jain (1985).

Figure 4, and it corresponds to the MMA. The WV isadvected from the south-west by the low-level jet reported in
Nagata and Ogura (1991) and concentrated at the MMA.
That is, the initiation point is located over the mesoscale
confluence zone of the synoptic-scale cold fronts and the
MMA. The confluence zone is maintained from 00:10 JST
to 02:10 JST and the initiation point is located over the confluence zone. In the confluence zone, a forced ascent would
be induced by the convergence of the synoptic-scale cold
fronts. The ascent flow would be accelerated by the diabatic
heating in the convection with rich WV. The confluencezone might provide essential conditions for triggering andacceleration of the updraft, and thus the updraft can be maintained at the confluence zone. Since the synoptic-scale cold
front moves south-eastward slowly and the MMA is quasistationary, the confluence zone is almost stationary. Therefore, the updraft occurs at the same point. The confluencezone found in this simulation is similar to the conceptual
(a)

(b)

4 | A C O N D I T I O N F OR M A I N TA I N IN GU P D R A F T AT TH E I N I T I A T IO N P O IN T
The numerical model simulated a BBS, and updraft at theinitiation point was maintained. Some specific features triggering and maintaining updrafts should be found around theinitiation point. First, we investigated the low-level thermodynamic environment. Figure 4 shows the horizontal distribution of WV and temperature at 750 m height in Domain
3. The initiation point is indicated by the yellow circle in
Figure 4a. The synoptic-scale cold front is found as two cold
fronts at 01:20 JST on July 12, 2010, as indicated by theconcentration of temperature contours in Figure 4a. The cold
fronts are found at temperatures around 293 and 295 K.
Over the south-westerly wind area in Figure 2e–g, WV
amount (specific humidity) is larger than 0.014 kg/kg. Furthermore, the meso-scale moist area (MMA) is found in theWV front. The area with WV amounts higher than the mean
WV amounts of the Domain 3 (0.016 kg/kg) is shaded in

Temporal composited images for specific humidity (color
scale), vertical velocity [m/s] (red contours), and temperature [K] (bluecontours). (a) is the parallel component and (b) is the orthogonal component
relative to the direction of the linear shape of the squall-line. Vectors show
flow fields on each plane. Temperature data are shown as anomalies relative
to the domain average. The composite center indicated as 0 km is defined as
the point at which the maximum vertical velocity is observed at the time inthe simulated squall-line represented by precipitation. Each center isdetermined subjectively by observing the squall-line for the period from00:10 JST to 02:00 JST on July 12, 2010 (12 snapshots)

FIGURE 5

YOSHIDA ET AL.

36°N

(a)

34.8°N

35°N

34°N[kg/kg]×10–2(b)34.6°N

1.9

34.4°N

1.8

34.2°N

1.7
34.0°N33°N

1.6

33.8°N32°N

1.5

33.6°N31°N

33.4°N
129°E

131°E

133°E

135°E

1.4

130.8°E

131.4°E

132.0°E

[mm]

(c)

55

34.8°N45
34.6°N35
34.4°N25
1534.2°N534.0°N
130.8°E

131.4°E

132.0°E

132.6°E

133.2°E

[mm]

(d)

55

34.8°N45
34.6°N35
34.4°N25
1534.2°N534.0°N
130.8°E

131.4°E

132.0°E

10–2[kg/kg]
1.85

6

(e)

5

1.80

1.70
1.65
1.60
1.55
0:10

0:30

0:50

QV-CTL20
QV-LWV20

1:10

1:30

1:50

QV-CTL100
QV-LWV100

133.2°E

[m/s]

(f)

1.2
1.0

4

0.8

3

0.6

2

0.4

1

0.2

0

0.0

–1

–0.2

Horizontal covergence

1.75

132.6°E

10–4 [s–1]

–2
0:10

Vertical velocity

maintenance condition in Moteki et al. (2004a). In detail,however, a mesoscale cold pool in the synoptic-scale front is
not found in this case, and the MMA is found in the WV
front. An interface between a synoptic-scale cold front and alow-level warm air advection is emphasized as a significant
environment for MCSs development over the North America
by Augustine and Caracena (1994). This environmental condition is similar to the confluence zone found in this study.
Next, we drew the vertical cross sections shown in
Figure 5 to confirm relationship between the updraft and themesoscale confluence zone. These are divided into aparallel- and an orthogonal-component relative to the major
axis of the BBS linear shape. The direction of the major axis
was determined by the precipitation contour lines. Figure 5
is a temporal composite of 12 snapshots taken during theperiod from 00:10 JST to 02:00 JST on July 12, 2010, centered on the location at which the maximum vertical velocity
was observed in the air column on the BBS. The location ofthis center is determined subjectively. In the parallel component (Figure 5a), the updraft is observed above the synopticscale cold front. Although the maximum vertical velocity is
located at a horizontal distance of 0 km, the low-level
updraft can be found 20 km to the west. While a broad
updraft is observed in the parallel component, the updraft isfound at a particular area around the 0 km point in theorthogonal component. This implies the BBS has narrow
structure in the north–south direction. Interestingly, WV isalso concentrated at the point of −20 km, where the MMA
is found. The maximum WV amount is 0.018 kg/kg for the
specific area between 0 and 20 km. Inflow from the south(west) to the BBS is dominant than from the north (east).
The inflow passes through the MMA before reaches the
BBS. Furthermore, at slightly higher level than the inflow,
the synoptic-scale cold front is significant at a horizontal distance of 20 km with a large temperature gradient of approximately 2 K per 0.1 . These amounts are similar to thatobserved over the Baiu front in Moteki et al. (2004a). The
confluence zone is also clearly found in the vertical crosssections in the area between 0 km and 20 km in
Figure 5b. These structures imply that the updraft is initiatedat the confluence zone. This is maintained for the period
from 00:10 JST to 02:00 JST on July 12, 2010. Since thestandard deviations of the composited samples are approximately 10% of the averaged value, the composited fields represent the mean structure of the simulated BBS. The images
imply that the process continuously occurs for this period
and reinforces the maintenance condition discussed above.
In this case, the MMA is newly found as the mesoscalecharacteristic of the maintenance condition. We confirmed
the necessity of the MMA to maintain the BBS by a restartexperiment with a less WV condition (LWV) than the realcase. We call the original simulation as a control experiment (CTL). The restart time was 00:00 JST on July12, 2010, when the BBS was just organized. The mass of

7 of 9–0.4
0:30

LC-CTL20
LC-LWV20

0:50

1:10

1:30

LC-CTL100
LC-LWV100

1:50
W-CTL
W-LWV

Result of a less water vapor experiment (LWV). (a) and (b) areconditions of a restart simulation for Domain 2 and Domain 3, respectively.
A location of the BBS which is focused on is indicated by a rectangle in (b).
Color shade shows QVCTL at 750 m height. Contour is difference ofspecific humidity (QV) at 750 m height at value of 0.001 kg/kg, andhatched areas are larger difference than 0.002 kg/kg. (c) and (d) are
accumulated precipitation in the period from 00:20 JST to 01:40 JST on
July 12, 2010; (c) is control run (CTL), and (d) is LWV. A location of the
BBS which is focused on is indicated by a rectangle in (c), and the same
location is indicated in (d). (e) and (f ) are area averaged time series for QV
at 500 m height, low-level convergence (LC) at 500 m height, and W(vertical velocity) at 3,000 m height. For lines colored by blue or red, solidlines and dashed-lines are area averaged values in the area of 20 km radiusand 100 km radius, respectively. W is area averaged in the area of 20 km
radius, and black solid-line and black dashed-line are for CTL and LWV,
respectively. The center of area averaging is decided by the BBS tracking ina same manner of Figure 5
FIGURE 6

YOSHIDA ET AL.

8 of 9WV is defined as ρQv kg/m3 (ρ is density, and Qv is WV
in specific humidity) and was decreased to the area meanvalue, which is averaged in each computational domain at
the all levels in 0–4 km height. The difference of Qv from
LWV to CTL at 750 m height is shown in Figure 6a,b. Qv
is decreased in the limited area with high Qv in Domain
3 (Figure 6b). The east–west contrast of WV disappearedby the decreasing process, but the north–south contrast of
WV was not eliminated. It is clear that a precipitation fromthe BBS is not simulated in LWV shown in Figure 6dcomparing with Figure 6c. Vertical velocity is also weakin LWV (Figure 6f ). It is confirmed by 10 min interval
snapshots that the BBS is rapidly decayed by 00:30 JST on
July 12 in LWV. The difference of Qv and in low-level
convergence between LWV and CTL is significantly larger
in 20 km area averaging values (solid-lines) than that in
100 km area averaging values (dashed-lines). Therefore,
the smaller-scale condition was altered by reducing WVamount, but the larger-scale condition was not much modified. Peters and Schumacher (2016) also claimed that thesmaller-scale condition has larger effect to maintain the
MCS than larger-scale condition. To recognize the roleof Qv, we diagnosed acceleration terms of verticalvelocity (w) at the same location as Figure 6 by following
Fovell and Tan (1998). The governing equation is
0

∂w
1 ∂p
∂t ¼ − V  rw − ρ ∂z

0+ B , same as Equation 1 in Fovell

and Tan (1998). The first term and the second term in theright-hand side are vertical velocity advection (WADV) andvertical pressure gradient (VPGA), respectively. ρ is thebase-state density defined as a domain average value. B0 is
perturbation buoyancy (BUOY) defined as gððρ − ρ0 Þ=ρ0 Þ,
00where g is gravitational acceleration. p and ρ are perturbation pressure and perturbation density defined as 20 km area
averaged values. To see the general feature, the values aretemporary and vertically averaged in the same time period as
Figure 6 and in updraft levels (from 3.25 to 7.75 km height).
BUOY terms are 1.9 × 10−2 ms−2 in CTL and8.2 × 10−3 ms−2 in LWV, whereas VPGA terms are
2.1 × 10−2 ms−2 in CTL and 4.1 × 10−2 ms−2 in LWV.
BUOY term in LWV is one order smaller than that in CTL.
WADV terms are three orders less than other terms. Therefore, the less Qv condition in LWV leads to less verticalacceleration by buoyancy than CTL and the updraft would
not be enough accelerated in LWV to maintain the BBS.
This implies that the MMA, that is, local contrast of WV isnecessary to maintain the BBS in this case.

5 | SUMMARY
Moteki et al. (2004a) suggested a conceptual maintenancecondition for the BBSs, which is a confluence zone between
the Baiu front (temperature gradient) and the water vapor
front. However, the conceptual maintenance condition isground on the case study, and the updraft localization has notbeen fully understood. We confirmed the conceptual maintenance condition in another BBS case during June 11–12,
2010, in western Japan by carrying out a numerical simulation. The updraft at the initiation point of the simulated BBS
is maintained over the confluence zone of the synoptic-scale
cold fronts and the mesoscale distinctive moist area at lowlevel. In a sensitivity experiment with locally decreased water
vapor, the simulated BBS was not maintained. This suggeststhat a mesoscale distinctive moist area is essential to maintainthe simulated BBS in this case. This simulated BBS and its
condition support the conceptual maintenance condition of
Moteki et al. (2004a). A cold pool, that is, a local minimumof temperature in a cold front emphasized in Moteki et al.
(2004b) is not observed in this case.

ACKNOWLEDGEMENTS

We are grateful to the editor of ASL and anonymous
reviewers for their useful and critical comments. We are
grateful to Prof. Satoru Oishi in RCUSS Kobe University
for valuable insights. We appreciate Mr Kazuto Ando fortechnical supports. The Radar/Raingauge-Analyzed Precipitation data and the GSM-GPV data were provided by
JMA through the Japan Meteorological Business Support
Center. This research used computational resources of the
K computer provided by RIKEN through HPCI JuniorResearcher Promotion Project (ID: hp170164). This work
was supported by JST CREST grant number
JPMJCR1312, Japan and FOCUS Establishing Supercomputing Center of Excellence.
OR CID
Ryuji Yoshidahttps://orcid.org/0000-0003-3238-2595

Seiya Nishizawa

https://orcid.org/0000-0001-9457-7457

Sachiho A. Adachi

https://orcid.org/0000-0002-3066-6175

Yoshiyuki Kajikawa

https://orcid.org/0000-0001-6863-1720

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SUPPORTING I NFORMATION

Additional supporting information may be found online in
the Supporting Information section at the end of the article.
How to cite this article: Yoshida R, Nishizawa S,
Yashiro H, et al. Maintenance condition of backbuilding squall-line in a numerical simulation of aheavy rainfall event in July 2010 in Western Japan.
Atmos Sci Lett. 2019;20:e880. https://doi.org/10.1002/
asl.880