SOLA, 2013, Vol. 9, 89−93, doi:10.2151/sola.2013-020

89

Possible Impact of a Tropical Cyclone on the Northward Migrationof the Baiu Frontal Zone
Tsuyoshi Yamaura1, Yoshiyuki Kajikawa1, Hirofumi Tomita1, 2, and Masaki Satoh2, 3
1
RIKEN Advanced Institute for Computational Science, Kobe, Japan
2
Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan
3
Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Japan

Abstract
The impact of a tropical cyclone on the northward migrationof the Baiu frontal zone (BFZ) is investigated in the case of thetropical cyclone MAWAR (2012) using a global cloud-system
resolving model, called NICAM. From 4 to 6 June in 2012, theBFZ rapidly shifts northward with MAWAR. A simulation with
the initial data of 29 May reproduces the northward migration of
the BFZ and the tropical cyclone. Strong southerlies on the easternside of the tropical cyclone transport moist and high-temperature
air into the BFZ. This horizontal advection affects the northwardmigration of the BFZ. In contrast, the BFZ stagnates to the southof Japan in another simulation with the initial data of 30 May
because the tropical cyclone track is diverted eastward. Thus,
realistic reproducibility of a tropical cyclone is needed for better
simulations and prediction of the BFZ migration.
(Citation: Yamaura, T., Y. Kajikawa, H. Tomita, and M. Satoh,2013: Possible impact of a tropical cyclone on the northwardmigration of the Baiu frontal zone. SOLA, 9, 89−93, doi:10.2151/
sola.2013-020.)

1. Introduction
Baiu (in Japanese; Mei-yu in Chinese; Changma in Korean)
is a typical rainfall event with a quasi-stationary front in thesummertime over East Asia. Many researchers have investigatedheavy rainfall events caused by the stationary Baiu front (e.g.,Nino­miya 2001; Kato 2006) and the abrupt withdrawal of Baiu
(e.g., Nakazawa 1992; Ueda et al. 1995). Recently, it is suggested
that rainfall events near a front are strengthened by a tropical
cyclone, discussed as the predecessor rain events (PRE; Cote
2007; Wang et al. 2009; Galarneau et al. 2010; Bosart et al. 2012).
Tropical cyclones have the radius of gale force winds which isoften greater than 500 km, inducing horizontal advection processes in that region. Horizontal water vapor transport with a tropical
cyclone leads to anomalous precipitation near the Baiu frontal
zone (BFZ; Yamaura and Tomita 2012; Yoshida and Itoh 2012).
Because heavy rainfall events frequently occur near the BFZ
during the Baiu season, detailed understanding of the physicalmechanism of BFZ migration can contribute to disaster prevention
and the prediction of Baiu precipitation. The latitudinal migrationof the BFZ has been kinematically explained by horizontal advection processes with a strong wind. Chen et al. (2006) has notedthat the horizontal advection of potential vorticity associated withthe lower level jet (LLJ) contributes to the northward migration of
the BFZ. A tropical cyclone may also affect the migration of the
BFZ because it can provide strong wind and water vapor for the
BFZ.
Here, we address the following questions: Does a tropical
cyclone cause the northward migration of the BFZ, and if so, whatis the mechanism of this migration? We examine a case of northward BFZ migration in early June 2012. The Baiu front clearly
Corresponding author: Tsuyoshi Yamaura, RIKEN Advanced Institute for
Computational Science, 7-1-26, Minatojima-minami-machi, Chuo-ku,
Kobe, Hyogo 650-0047, Japan. E-mail: tyamaura@riken.jp. ©2013, theMeteorological Society of Japan.

appeared to the south of Japan during this period. The tropical cyclone MAWAR (2012) simultaneously approached the Baiu front
from the south. This case is suitable for investigating the behaviorof the BFZ with a tropical cyclone.

2. Data and model
As an observational evidence, we employed the Japanese 25year Reanalysis (JRA-25)/Japan Meteorological Agency (JMA)
Climate Data Assimilation System (JCDAS) reanalysis (Onogi
et al. 2007) and the Tropical Rainfall Measuring Mission (TRMM;
Huffman et al. 2007). The former is a 6-hourly, 1.25° global griddataset, and the latter is a 3-hourly, 0.25° grid dataset from 50°S
to 50°N.
We also used the Nonhydrostatic Icosahedral Atmospheric
Model (NICAM) (Satoh et al. 2008) for simulations. This modelhas the advantage of reproducing both synoptic-scale phenomena
and the life cycle of cumulus convection. This advantage is crucial
for realistically reproducing the genesis of a tropical cyclone andconvective precipitation. Therefore, the NICAM is useful for the
research on meteorological phenomena in the tropics, such as the
Asian monsoon (Oouchi et al. 2009), tropical cyclones (Emanuelet al. 2010), and the Madden–Julian Oscillation (Miura et al.
2007). To realistically reproduce the Baiu front and the tropical
cyclone MAWAR, numerical experiments were configured witha horizontal resolution equivalent to about a 14 km mesh and 38
vertical levels, with the model top at about 35 km. To produce
the divergence of the tropical cyclone track in the NICAM simulations, this study employed two initial data at 00 UTC on 29
and 30 May 2012 before 18 UTC 1 June when MAWAR grew
into a tropical storm. To initialize the simulations, we applied
the Grid Point Value (GPV) of the JMA Global Spectral Model(GSM) data for atmospheric parameters, the National Centers forEnvironmental Prediction (NCEP) finalized data for the ocean,and the long-term spin up data of the NICAM simulations withlow resolution (equivalent to about 224 km mesh) for the land.
The sea surface temperature (SST) was nudged with the weekly
Reynolds SST (Reynolds and Smith 1994). The relaxation time of
SST was 7 days. We then analyzed the horizontally and verticallyinterpolated output data of the numerical experiments. The horizontal grid interval was 1.25° globally, and the vertical levels were
the standard 17 pressure levels. The time interval of the output
data was 3-hourly for three-dimensional variables and hourly for
two-dimensional variables.

3. Results and discussion
To display the impact of different tropical cyclone tracks on
the BFZ, we first diagnose observations and the two NICAM
simulations in early June 2012. Because the BFZ is recognized asa large horizontal gradient of the equivalent potential temperature(EPT; Tomita et al. 2011), we define the BFZ as the area in whichhorizontal gradient of the EPT is large. The location of the tropical
cyclone MAWAR in the observations is derived from the JMA
best track data. Detection of the tropical cyclone in the simulations is based on the criteria shown in Oouchi et al. (2006). In theobservation (Figs. 1a, b, c), the BFZ approximately corresponds

90

Yamaura et al., Possible Impact of a Tropical Cyclone on the Northward Migration of the Baiu Frontal Zone

to the EPT contours from 330 K to 340 K, which clearly appears
from 18 UTC on 3 June to 18 UTC on 5 June. MAWAR moves
northeastward from (21°N, 126°E) to (30°N, 138°E) during thisperiod. At 18 UTC on 4 June, local maximum rainfall areas appear
around (30°N, 129°E) and in the western Japan (33°N, 132°E)
near the BFZ. These rainfall areas are observed in places 600 km
and 1000 km away from the tropical cyclone center, respectively.
A large amount of water vapor is transported north−northwestward
through 130°E−140°E, which suggests rainfall strengthening.
These characteristics are similar to the PRE. Then, MAWAR comeclose to the BFZ, and the BFZ protrudes northward at 18 UTC on
5 June. The rainfall area of MAWAR can be hardly distinguishedfrom the rainfall area over the western Japan. In the NICAM simulation with the initial data of 29 May (Figs. 1d, e, f; case 0529
hereafter), the BFZ and MAWAR are realistically reproduced until
18 UTC on 4 June. Although the PRE near the BFZ is insignificant, a large amount of water vapor is transported from the east of
MAWAR into the BFZ. MAWAR moves slowly around Okinawa
at 18 UTC on 5 June, which displays the delayed movement of
MAWAR in case 0529 compared with the observation. As seen in
Fig. 1c, the southward kink in the western part of the BFZ doesn’t
appear because westward transport of water vapor is large by thecyclonic circulation. In contrast, another NICAM simulation with
the initial data of 30 May does not reproduce the northward migration of the BFZ (Figs. 1g, h, i; case 0530 hereafter). The BFZstagnates around 26°N during this period. The track of MAWAR
is off to the east side of the observations. Here, it is important thatlarge-scale atmospheric circulations are almost the same between
the two simulations. In the two simulations, the initial dates areclose and the integrated time is shorter than the time scale ofdevelopment of baroclinic waves (about 1 week). The horizontal
wind and geopotential height fields at 500 hPa before 3 June aresimilar between the two simulations (not shown). Under these
atmospheric conditions, if tracks of MAWAR are still different in
the two simulations, a comparison of the two simulations suggests
the impact of different tracks of a tropical cyclone. As such, theseresults suggest that a tropical cyclone track is important for the
BFZ migration.
We then evaluate the time evolution of BFZ migration inearly June 2012 (Fig. 2). In the observation, the BFZ is zonallyelongated along 30°N at 18 UTC on 4 June. The area in which
the horizontal gradient of the EPT is larger than 2 × 10−5 K m−1
migrates northward until 6 June, after expanding to the south on 3
June. The northward BFZ migration coincides with the northward
movement of a strong cyclonic circulation by MAWAR. The
area-averaged precipitation in the BFZ near the Pacific coast ofJapan (30°N−35°N, 130°E−140°E) is strengthened together with
MAWAR approaching to the BFZ. The precipitation peak is displayed at 15 UTC on 5 June. In case 0529, the BFZ and tropical
cyclone simultaneously shift northward from 3 June until 6 June.
Because the tropical cyclone wanders slowly around the south of
the BFZ, carrying water vapor into the BFZ continuously during
this period, the peak of precipitation on the Pacific coast of Japanis delayed about 1 day. In case 0530, the cyclonic circulation shifts
northward until 12 UTC on 3 June and then weakens, indicatingthat the tropical cyclone is off to one side. Little precipitation
occurs along the Pacific coast of Japan during this period.
To ascertain how MAWAR influences the northward migrationof the BFZ, we then examine the local tendency of the EPT. Because the EPT decreases northward in the Northern Hemisphere,
the local tendency of the EPT is positive in the BFZ when the
BFZ shows northward migration. The local tendency of the EPT
is positive to the south of the BFZ when the BFZ is strengthened.
This indicates the importance of the location of the area where theFig. 1. (a‒c) Accumulated rainfall over the past 24 hour (shade; mm), the EPT (contours; K), and water vapor flux (vector; g kg−1 m s−1) at 925 hPa at 18
UTC on 3, 4, and 5 June, respectively. The black circle indicates the location of the tropical cyclone MAWAR at the time. (d‒f ) Same as (a‒c) except forcase 0529. (g‒i) Same as (a‒c) except for case 0530.

SOLA, 2013, Vol. 9, 89−93, doi:10.2151/sola.2013-020
local tendency of the EPT is positive. The equation for the local
tendency of the EPT is defined as∂θe∂θ= −V ⋅∇θe − ω e + R,∂t∂pwhere qe is the EPT, V is the horizontal wind vector, Ñ is the horizontal gradient operator, w is the vertical pressure velocity, and R
is a residual term, such as the effect of sub-grid scale motions or
sensible/latent heat flux from the ocean. The first (second) term on
the right side is the horizontal (vertical) advection term. The physical mechanism of the BFZ migration is established by analyzing
the contribution of each term to the local tendency of the EPT.
The equation is applied to the observational data and output of
the model simulations (Fig. 3). Here, we define the 340 K contourof the EPT as the Baiu front because the contour corresponds tothe southern edge of the BFZ (Fig. 2a). In the observations, thelocal tendency of the EPT is largely positive near the Baiu front
between 130°E and 135°E at 18 UTC on 4 June. Because the hor-

91

izontal advection is comparable to the local tendency of the EPT,
the contribution of horizontal advection dominates the tendencyof the EPT. Northward water vapor fluxes are large over the longitudinal zone (Fig. 1b). These features are also found in case 0529.
In case 0530, the Baiu front between 130°E and 140°E stagnates
near 25°N, where the local tendency of the EPT is small. A positive local tendency of the EPT appears to the east of 140°E. The
difference between cases 0529 and 0530 is related to the locationof area where northward water vapor fluxes are large.
To reveal the contribution to the northward BFZ migration indetail, we display the contribution of horizontal advection, verticaladvection, and residual term to the local tendency of the EPT in
the latitude–height cross section around the BFZ (Fig. 4). At 18
UTC on 4 June, the Baiu front stands around 30°N in the observations. The local tendency and the horizontal advection of the EPT
are significant from the surface to 500 hPa. The vertical advection
has less contribution to the northward BFZ migration. Rather, thevertical advection moves the BFZ southward in the middle−upper
troposphere. These horizontal and vertical advections coordinately

Fig. 2. (a) Horizontal map of the EPT (contour; K), its horizontal gradient (shade; 10−5 K m−1), and the water vapor flux (vector; g kg−1 m s−1) at 925 hPa and18 UTC on 4 June. (b) Latitude–time cross section of 130°E−140°E averaging the horizontal gradient of the EPT (contour; 10−5 K m−1) and 125°E−135°E
averaging relative vorticity (shade; 10−5 s−1) at 925 hPa. The contour interval is 1 × 10−5 K m−1 from 2 × 10−5 K m−1. (c) Time variation of area-averaged precipitation (mm) near Japan (30°N−35°N, 130°E−140°E). (d–f ) Same as (a–c) except for case 0529. (g–i) Same as (a–c) except for case 0530.

Fig. 3. (a) The 340 K contours of the EPT (thick contour; K), local tendency of the EPT (thin contour; K h−1), and horizontal advection of the EPT (shade;
K h−1) at 925 hPa and 18 UTC on 4 June. The thin contour interval is 0.5 K h−1. The zero lines of the thin contour are omitted for clarity. (b) Same as (a)except for case 0529. (c) Same as (c) except for case 0530.

92

Yamaura et al., Possible Impact of a Tropical Cyclone on the Northward Migration of the Baiu Frontal Zone

Fig. 4. (a‒c) Latitude–height cross section of the EPT (thick contour; K), local tendency of the EPT (thin contour; K h−1), meridional wind and verticalp-velocity (vector; m s−1 and hPa h−1) averaged between 130°E−140°E at 18 UTC on 4 June. Shading in each panel indicates horizontal advection (top),
vertical advection (middle), and residual (bottom) term of the EPT (K h−1). The thick (thin) contour interval is 10 K (0.5 K h−1). Zero lines of thin contour
are omitted for clarity. (d‒f ) Same as (a‒c) except for case 0529. (g‒i) Same as (a‒c) except for case 0530.

change northward tilted EPT with the vertically uniform EPT,
implying neutral stratification of the BFZ. The contribution of theresidual is small in this case, although Ninomiya and Yamazaki(1979) pointed out the importance of vertical transport of the EPT
by convection included the residual. The horizontal advection of
the EPT dominates the local tendency of the EPT, with southerlies
in the south of the BFZ, which suggests that the BFZ shifts northward consistently from the surface to 500 hPa. Case 0529 also
shows that positive tendency of the EPT prevails from the surface
to the upper troposphere in the BFZ. This is mainly caused byhorizontal advection with the southerly in the south of the BFZ. In
contrast with the observation, significant horizontal advection appears above 500 hPa. This is caused by zonal advection associatedwith westerly and the upper level trough that is slightly stronger
than the observation (not shown). The vertical advection and theresidual of the EPT are insignificant in the BFZ. The Baiu frontin case 0530 appears around 25°N. The local tendency of the EPT
is insignificant around the BFZ, indicating stagnation of the Baiu
front.

4. Summary and discussions
We investigated the northward migration of the BFZ as atropical cyclone approached. Around the Pacific coast of Japan,
the BFZ was northward-shifted during 48 h from 18 UTC on 3
June. The northward migration of the BFZ was reproduced in case0529. Strong southerlies on the eastern side of the tropical cyclone
carried moist and high-temperature air on the BFZ. This horizontal advection influenced the northward migration of the BFZ.
The tropical cyclone MAWAR to the south of the BFZ induced
the horizontal advection of EPT. In contrast, case 0530 failed toreproduce the northward migration of the BFZ. Because the trackof MAWAR was averted eastward, moist and high-temperature air
was transported far from Japan, and then the BFZ stagnated to thesouth of Japan. The two simulations successfully demonstrated
that the BFZ stagnated to the south of Japan when the track of
MAWAR was displaced to the side.
For the large track error in case 0530, the NICAM’s spin-up
time for convection is one possible reason. MAWAR became atropical depression and a tropical storm in an active convection
area over the Philippine Sea at 18 UTC 31 May (42 hours after the
initial time) and 18 UTC 1 June (66 hours after the initial time),
respectively. It might be too short for the NICAM to produce thisactive convection area in case 0530 simulation due to its modelspin-up issue. Thus the genesis and the subsequent track predictions might have relatively large errors. The accurate prediction oftropical cyclogenesis events is still a challenging issue.
This work demonstrated one of the mechanisms of the BFZ
migration. The BFZ migration is also caused by some otherprocesses, such as convective activity in the Baiu front (Ninomiya
and Yamazaki 1979) and horizontal advection by the LLJ (Chenet al. 2006). The dominance of each BFZ migration process isa challenging issue. Further study is necessary with a statisticalanalysis.

Acknowledgments
The authors acknowledge Dr. Yoshiaki Miyamoto for discussion on tropical cyclone activity and thank two anonymous reviewers for useful and constructive comments. Part of this work has
been funded by MEXT Strategic Programs for Innovative Research (SPIRE). Part of the results is obtained by using the K
computer at the RIKEN Advanced Institute for Computational
Science (Proposal number hp120313).

SOLA, 2013, Vol. 9, 89−93, doi:10.2151/sola.2013-020

References
Bosart, L. F., J. M. Cordeira, T. J. Galarneau, Jr., B. J. Moore, andH. M. Archambault, 2012: An analysis of multiple predecessor rain events ahead of tropical cyclones Ike and Lowell:
10−15 September 2008. Mon. Wea. Rev., 140, 1081−1107.
Chen, G. T.-J., C.-C. Wang, and L.-F. Lin, 2006: A diagnostic
study of a retreating Mei-yu front and the accompanying
low-level jet formation and intensification. Mon. Wea. Rev.,
134, 874−896.
Cote, M. R., 2007: Predecessor rain events in advance of tropical
cyclones. M.S. thesis, Department of Atmospheric and Environmental Sciences, University at Albany, State Universityof New York, 198 pp.
Emanuel, K., K. Oouchi, M. Satoh, H. Tomita, and Y. Yamada,
2010: Comparison of explicitly simulated and downscaled
tropical cyclone activity in a high-resolution global climate
model. J. Adv. Model. Earth Syst., 2, Art. #9, 9 pp., doi:
10.3894/JAMES.2010.2.9.
Galarneau, T. J. Jr., L. F. Bosart, and R. S. Schumacher, 2010: Predecessor rain events ahead of tropical cyclones. Mon. Weat.
Rev., 138, 3272−3297.
Huffman, G. J., and co-authors, 2007: The TRMM Multisatellite
Precipitation Analysis (TMPA): Quasi-global, multiyear,
combined-sensor precipitation estimates at fine scales. J.
Hydrometeor., 8, 38−55.
Kato, T., 2006: Structure of the band-shaped precipitation system
inducing the heavy rainfall observed over northern Kyushu,
Japan on 29 June 1999. J. Meteor. Soc. Japan, 84, 129−153.
Miura, H., M. Satoh, T. Nasuno, A. T. Noda, and K. Oouchi, 2007:
A Madden-Julian Oscillation event realistically simulatedusing a global cloud-resolving model. Science, 318, 1763−
1765.
Nakazawa, T., 1992: Seasonal phase lock of intraseasonal variation during the Asian summer monsoon. J. Meteor. Soc.
Japan, 70, 597−611.
Ninomiya, K., 2001: Large λ-shaped cloud zone formed around
July 6, 1991 with pole-ward moisture transport from intense
rainfall area in Meiyu-Baiu front. J. Meteor. Soc. Japan, 79,805−813.
Ninomiya, K., and K. Yamazaki, 1979: Heavy rainfalls associatedwith frontal depression in Asian subtropical humid region. J.

93

Meteor. Soc. Japan, 57, 399−413.
Onogi, K., and co-authors, 2007: The JRA-25 Reanalysis. J.
Meteor. Soc. Japan, 85, 369−432.
Oouchi, K., J. Yoshimura, H. Yoshimura, R. Mizuta, S. Kusunoki,
and A. Noda, 2006: Tropical cyclone climatology in a global-warming climate as simulated in a 20 km-mesh global
atmospheric model: Frequency and wind intensity analyses.
J. Meteor. Soc. Japan, 84, 259−276.
Oouchi, K., A. T. Noda, M. Satoh, B. Wang, S.-P. Xie, H. G.
Takahashi, and T. Yasunari, 2009: Asian summer monsoon
simulated by a global cloud-system-resolving model: Diurnal
to intra-seasonal variability. Geophys. Res. Lett., 36, L11815,
doi:10.1029/2009GL038271.
Reynolds, R. W., and T. M. Smith, 1994: Improved global sea
surface temperature analyses using optimum interpolation. J.
Climate, 7, 929−948.
Satoh, M., T. Matsuno, H. Tomita, H. Miura, T. Nasuno, and S.
Iga, 2008: Nonhydrostatic Icosahedral Atmospheric Model(NICAM) for global cloud resolving simulations. Journalof Computational Physics, the special issue on Predicting
Weather, Climate and Extreme events, 227, 3486−3514,
doi:10.1016/j.jcp.2007.02.006.
Tomita, T., T. Yamaura, and T. Hashimoto, 2011: Interannual
variability of the Baiu season near Japan evaluated from theequivalent potential temperature. J. Meteor. Soc. Japan, 89,
517−537.
Ueda, H., T. Yasunari, and R. Kawamura, 1995: Abrupt seasonalchange of large-scale convective activity over the western
Pacific in the northern summer. J. Meteor. Soc. Japan, 73,
795−809.
Wang, Y., Y. Wang, and H. Fudeyasu, 2009: The role of typhoon
Songda (2004) in producing distantly located heavy rainfall
in Japan. Mon. Wea. Rev., 137, 3699−3716.
Yamaura, T., and T. Tomita, 2012: Covariability between the Baiu
precipitation and tropical cyclone activity through largescale atmospheric circulations. J. Meteor. Soc. Japan, 90,449−465.
Yoshida, K., and H. Itoh, 2012: Indirect effects of tropical cyclones on heavy rainfall events in Kyushu, Japan, during the
Baiu season. J. Meteor. Soc. Japan, 90, 377−401.
Manuscript received 7 March 2013, accepted 28 May 2013
SOLA: http://www. jstage. jst. go. jp/browse/sola