Journal of the Meteorological Society of Japan, Vol. 87, No. 1, pp. 139--156, 2009.

139

DOI:10.2151/jmsj.87.139

Relationship between High-Impact Weather Events in Japan and
Propagation of Rossby Waves along the Asian Jet in July 2004

Takeshi ENOMOTO
Earth Simulator Center, Japan Agency for Marine-Earth Science and Technology

Hirokazu ENDO
Meteorological Research Institute, Japan Meteorological Agency

Yayoi HARADA
Climate Prediction Division, Japan Meteorological Agency

andWataru OHFUCHI
Earth Simulator Center, Japan Agency for Marine-Earth Science and Technology(Manuscript received 5 April 2007, in ﬁnal form 12 November 2008)

Abstract
In July 2004, torrential rainfalls caused signiﬁcant damages in parts of Japan, followed by heat waves. Ourdata analysis shows that both rainfall and heat wave events in late Baiu season were caused by the intensiﬁcationof the subtropical anticyclone near Japan (Bonin high) and that intensity of the Bonin high was signiﬁcantly inﬂuenced by propagation of Rossby waves along the subtropical jet. Hindcast experiments from 15 July wereconducted to study the mechanisms and predictability of these high-impact weather events. On 17–18 July, localized rainfalls at a few locations along the coast of the Sea of Japan including Sakata and Fukui were successfully
simulated in a high-resolution (21-km mesh) global hindcast simulation. These rainfall events were found to occur
near the leading edge of a ﬁlament of moist and warm air advected clockwise. On 20 July, anomalously high temperature was reproduced in the high-resolution hindcast simulation. With a moderate resolution of 83 km, theintensiﬁcation of the subtropical anticyclone was reproduced although the föhn was much weaker. This result indicates that temperature distribution associated with föhn requires a resolution high enough to resolve major
mountains. In order to investigate the predictability of propagation of Rossby waves and intensiﬁcation of theBonin high, 25-member ensemble experiments from 1 July 2004 were conducted using the moderate-resolution
model. It is shown that the region along the Asian jet has twice as long predictability as the entire Northern
Hemisphere. This case study suggests that the intensiﬁcation of the Bonin high associated with the propagationof Rossby waves along the Asian jet could be predicted a few weeks in advance with an ensemble forecast at amoderate resolution.

Corresponding author: Earth Simulator Center, Japan
Agency for Marine-Earth Science and Technology,
3173-25 Showa-machi, Kanazawa-ku, Yokohama,
Kanagawa, 236-0001, Japan.
E-mail: eno@jamstec.go.jp( 2009, Meteorological Society of Japan

1. High-impact weather events in July 2004
In July 2004, many parts of Japan were a¤ectedby heavy precipitation. For example, the monthly
precipitation at Sakata in July (Fig. 1, Letter A in

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Fig. 1. Daily and accumulated precipitation(mm) in Sakata, in bars and a curve, respectively. The location of Sakata ismarked with letter A in Fig. 3. Produced
from the JMA AMeDAS data.

Fig. 2. Daily maximum temperature ( C) in
Tokyo. The solid and dashed curves represent values for 2004 and for the 30-year
average between 1971 and 2000. The location of Tokyo is marked as D in Fig. 3.
Produced from the JMA AMeDAS data.

Fig. 3) was 330.0 mm, 177.3% of the climatologicalvalue of 186.1 mm according to the AutomatedMeteorological Data Acquisition System (AMeDAS) data of the Japan Meteorological Agency(JMA). During 12–13 July, the Baiu front was activated and caused torrential rainfall in Niigata andFukushima. Another event occurred during 17–18
July in Fukui and Sakata due to the activated Baiu
front.
High temperature was another feature of thatmonth. Figure 2 shows the daily maximum temperature in Tokyo (Letter D in Fig. 3) in July 2004.

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Fig. 3. Distribution of the 2-m temperature( C) near Japan at 6 UTC on 20 July,
2004. Contours are drawn every 2 degrees
between 21 C and 35 C and regions withtemperature higher than 31 C are hatched.
Letters A–J represent observational stations (See Table 1). Produced from the
JMA MSM data.

Everyday except for 29 July, the observed maxima
exceeded the climatological values. On 20 July, themaximum temperature was a record-breaking valueof 39.5 C, 10.1 C above normal. Anomalously hightemperature was not limited to Tokyo. In fact, thehighest temperature of 40.2 C was recorded at theAMeDAS station in Ushiku (Letter E in Fig. 3).
Figure 3, produced from the meso-scale analysis
for the JMA Mesoscale Model (MSM), shows the
distribution of the 2-m temperature at 6 UTC (15JST). The horizontal resolution of MSM is 10 km
and the resolution of distributed MSM data at thesurface level is 0.125 and 0.1 in zonal and meridional directions. Although analyzed temperature
is cooler by 1.3 to 6.4 C than that from AMeDAS
observations at selected stations (Table 1), its distribution clearly shows that regions with high temperature are located along the southern coast ofJapanese islands and the east coast of the Korean
Peninsula. The distributions of temperature (Fig.
3), winds (Fig. 4) and relative humidity (not shown)
imply föhn. In fact, the regions with high temperature (hatched in Fig. 3) coincide well with thosewith low relative humidity (not shown). Possible
causes of anomalously high temperature will be discussed further in Subsection 3.4 with hindcasts withtwo di¤erent horizontal resolutions.
In July 2004, the Bonin high was anomalously
enhanced as represented by large values (>240 K)

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141

Fig. 4. Distribution of winds (m s1 ) at the
925 hPa surface and sea-level pressurenear Japan at 6 UTC on 20 July, 2004.
Contours are drawn every 2 hPa. Letter H
denotes the approximate centre of the anticyclone. Produced from the JMA MSM
data.

of out-going long-wave radiation (OLR) in thesouthwestern Japan (Fig. 5a). In the climatology(Fig. 5b), the convection along the Baiu frontal
zone (BFZ) resulted in relatively low OLR. Convection was inactive along BFZ and in the Philippine Sea and active in southern China near 110 E,
20 N (Fig. 5c). Nitta (1987) found that the convection along the BFZ was suppressed when the convection in the Phillipine Sea was enhanced (the PJ
pattern). Obviously it was not the case for thismonth. Recently, a mechanism of intensiﬁcation of
the Bonin high by a mid-latitude process was proposed by Enomoto et al. (2003). They argue thatas a result of propagation of Rossby waves along
the subtropical jet over Eurasia (the Silk Road
pattern), an equivalent barotropic anticyclone isformed over Japan.
There are two wave-guides over Eurasia during
the Northern Hemisphere summer: the subpolarand subtropical jets. Intensity and frequency of
propagation of Rossby wave-packets along thesejets have large impacts on the summer climate near
Japan. Rossby waves along the subpolar and subtropical jets intensify the Okhotsk (Nakamura andFukamachi 2004) and Bonin (Enomoto et al. 2003;
Enomoto 2004; Sato and Takahashi 2006) anticyclones, respectively. In 2003, the Rossby-wave

Fig. 5. Distribution of the Out-going Longwave Radiation (W m2 ) of (a) July 2004,(b) the monthly climatology (1979–1995),
and (c) the di¤erence ((a)–(b)). Contours
are drawn every 10 (Panels a, b) and 5(Panel c) W m2 . Negative contours arebroken. Hatched and dotted regions indicate values larger than 300 W m2 andsmaller than 200 W m2 , respectively for
Panels a and b. For Panel c, positive andnegative out-going long-wave radiation(OLR) anomalies larger than 10 W m2
and smaller than 10 W m2 are hatchedand dotted, respectively. Produced fromthe National Oceanic and Atmospheric
Administration(NOAA) interpolated
OLR dataset.

propagation along the subpolar jet was very active
and the propagation along the subtropical jet was
weak to the east of 90 E. As a result, the Okhotsk
anticyclone was intense while the Bonin anticyclone
did not develop well. In 2004, by contrast, the subpolar propagation was very weak and the Okhotsk
high did not develop well (Harada et al. 2006).

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Fig. 6. Longitude-Time cross-section of zonally asymmetric component of the 200 hPa
height (gpm) averaged between 35–50 N
in July 2004. Anomalies smaller than100 gpm and larger than 100 gpm are
dotted and hatched, respectively. Produced
from the NCEP/NCAR reanalysis.

The climatological Rossby wave sources of the
Silk Road pattern suggested by Enomoto et al.
(2003) are due to the descent caused by the monsoon heating (the monsoon-desert mechanism,
Rodwell and Hoskins 1996). According to thismechanism, the contrast between monsoon anddesert represents the strength of the monsoon. In
July 2004, there was a sharper gradient of OLR
near 70 E (Fig. 5a) due to positive anomalies extending from the Arabian Peninsula to Afghanistanwith a maximum near 70 E, 30 N and negative
anomalies extending from India to southern China
with a minimum near 110 E, 20 N (Fig. 5c). Thisenhanced contrast might have acted as a favourable
basic ﬁeld for the Silk Road pattern and intensiﬁcation of the Bonin high without active convection in
the Philippine Sea.
A time-longitude plot of the geopotential heightat 200 hPa (Fig. 6) produced from the National
Centers for Environmental Prediction/National
Center for Atmospheric Research (NCEP/NCAR)
reanalysis (Kalnay et al. 1996) indicates that there
were a few events of Rossby wave-packet propagation in July 2004. One of those packets propagatedaround the globe and intensiﬁed the Bonin high inearly and late July. Another event initiating on 16
July from 25 E caused the intensiﬁcation of theBonin high during 19–21 July near 120 E.

Fig. 7. Distribution of the meridional gradient of the potential vorticity at the 200hPa (106 PVU m1 ¼ 1012 K m kg1 s1 )of (a) July 2004, (b) the monthly climatology (1968–1996). Produced from the
NCEP/NCAR reanalysis.

The basic ﬂows provide insights into the reasons
why Rossby-wave propagation took place along
the subtropical jet rather than the subpolar jet in
July 2004. There is a well-deﬁned wave-guide represented by the large meridional gradient of potential
vorticty (PV) near 40 N over Eurasia (Fig. 7). Its
intensity was not particularly di¤erent from theFebruary 2009

T. ENOMOTO et al.

climatological value in terms of the meridional gradient of the monthly-mean PV. However, there area few distinctive features. 1) The North Atlantic
wave-guide is connected to the subtropical jet overEurasia, as indicated by a larger meridional PV
gradient near 10 W, 45 N. In the climatology, it isconnected to the subpolar wave-guide. In someyears such as in 2003, Rossby-wave propagation
was active along the subpolar jet. In July 2004,
due to an easterly region at the 300-hPa surface
over northern Europe, the energy propagation of
Rossby waves along the subpolar jet was suppressed (not shown). 2) The subtropical jet is split
near 90 E as indicated by an enhanced PV gradientextending northeastward. These split jets imply large
amplitudes of Rossby waves. 3) A maximum of the
PV gradient near Japan is formed. This anomaly
can be interpreted as the results of breaking of
Rossby waves since anticyclonic breakings bringlow PV poleward and reinforce the meridional gradient to their north.
Figure 8 shows the time-averaged upper andlower tropospheric ﬂow patterns during an event in
17–21 July. The undulation of the northern edge of143

the Tibetan high and eastward wave activity ﬂux(Takaya and Nakamura 2001) indicate the propagation of Rossby waves (Fig. 8a). In the calculationof the wave activity ﬂux, the 31-day running average is used as the zonally-varying basic ﬂow. In addition to the two centres within the Tibetan high,another anticyclonic centre is created over the East
China Sea in the upper troposphere. The wave
activity downstream of this high implies a reﬂection (or an over-reﬂection). This anticyclone ismainly equivalent-barotropic, although the surface
centre is located slightly southeast of the uppertropospheric centre. Convection is suppressed inside the high but activated along its fringe (Fig. 8b).
The main purpose of this paper is to study
the mechanisms and predictability of high-impact
weather events that a¤ected Japan in July 2004, described above, as global to regional phenomena.
Hindcast simulations at high and moderate resolutions and ensemble experiments at a moderate
resolution were conducted using an atmosphericgeneral circulation model (AGCM) describedin Section 2. In Section 3, the results from thehigh-resolution hindcast are examined. The highresolution simulation seems to capture the precipitation in Sakata and Fukui and temperature distributions associated with the föhn in many parts
along the Paciﬁc coast of Japan. Dependency on
horizontal resolution, representation of the föhn
in particular, is compared between high- andmoderate-resolution hindcasts. In Section 4, to investigate the predictability of the subtropical jet in
July 2004, the results of ensemble experimentsfrom 1 July for one month with di¤erent surface
boundary conditions are examined. Finally, thesummary and remarks are given in Section 5.
2. Model description

Fig. 8. Distributions of (a) the stream function (10 6 m 2 s, contour) and wave activity ﬂux (m 2 s2 , arrows) at the 200-hPa
surface (b) the stream function (10 6 m 2 s,
contour) at the 850-hPa surface and outgoing long-wave radiation (W m2 , shade)averaged in 17–21 July 2004. Produced
from the NCEP/NCAR reanalysis and
NOAA interpolated OLR.

AFES (AGCM for the Earth Simulator) is usedin this study to enable e‰cient high-resolution
global simulations on the Earth Simulator. It is a
global atmospheric model based on Numaguti et al.
(1997) but rewritten in Fortran 90 and highly optimized for the Earth Simulator (Shingu et al. 2002,
2003). AFES enables global simulations at highresolution simulations; up to a 10-km mesh has
been tested (Ohfuchi et al. 2004). AFES’s dynamical framework employs the pseudo-spectral methodwith a triangular truncation for the horizontal discretization (Hoskins and Simmons 1975) exceptthat Arakawa and Suarez (1983) discretization isused in the vertical. It has full physical packages in-144

Journal of the Meteorological Society of Japancluding statistical grid condensation (Le Treut andLi 1991) and precipitation (Sundqvist 1978), radiation (Nakajima and Tanaka 1986; Nakajima et al.
2000), level-2 moist vertical di¤usion (Mellor and
Yamada 1982; Smith 1990), and surface ﬂux (Loius
1979; Uno et al. 1995) inherited from Numaguti
et al. (1997). There are some changes to the model
physics from Numaguti et al. (1997). One of themost signiﬁcant di¤erences is the use of Emanuelconvective scheme (Emanuel 1991; Emanuel andZ̆ivković-Rothman 1999; Peng et al. 2004). The
cloud model of the Emanuel scheme parametrizes
O(100 m) drafts rather than ensemble of clouds
and does not have parameters that depend explicitly on the horizontal resolution. This feature motivated us to incorporate it in AFES (Enomoto et al.
2008).
AFES is used for both hindcast and ensemble
simulations. In hindcast simulations from 15 July,high (T639L48) and moderate (T159L48) horizontal resolutions are used to compare the representation of high impact weather events between thetwo resolutions. The ﬁgure after letter ‘T’ denotes
the total truncation wave number and the ﬁgure
after letter ‘L’ denotes the number of the model levels. Horizontal grid intervals for high and moderate
resolutions are about 21 and 83 km, respectively. In
25-member ensemble experiments, T159L48 is usedsince propagation of the Rossby waves and enhancement of the Bonin high are su‰ciently reproduced in hindcast simulations with that resolution
as will be shown in Section 3. The details of the experimental design are given in each section showing
simulation results.
3. Hindcast simulations
In this section, hindcast simulations from 15 July
2004 are described. Representation of observed features, such as propagation of Rossby waves andintensiﬁcation of the Bonin high, heavy rain falls
in Sakata and Fukui on 17–18 July and heat waves
on 20 July 2004 are examined.
3.1 Experimental design
Hindcast simulations were conducted for sevendays from 0 UTC, 15 July 2004, 5-days prior tothe heat wave in Tokyo. The initial conditions areprepared from those for JMA GSM (Global Spectral Model). The grid-point values (GPV) of GSM
is provided with the horizontal resolution of 1.25
at 16 pressure levels (1000, 925, 850, 700, 500, 400,
300, 250, 200, 150, 100, 70, 50, 30, 20, 10 hPa). The

Vol. 87, No. 1

horizontal interpolation was conducted in the
spectral space using SPHEREPACK (Adams andSwarztrauber 1997). The surface pressure ð ps Þ in
GPV is not used but recalculated from the geopotential height ðzÞ, temperature ðTÞ in GPV andthe surface height ðzs Þ of AFES in order to avoid
unwanted waves due to the di¤erences in the orography between GPV and AFES. Using the recalculated ps , the horizontal winds ðu; vÞ, and temperature ðTÞ are interpolated vertically to the modellevels. The speciﬁc humidity ðqÞ is calculated fromthe relative humidity (RH), T and pressure ðpÞ atpressure levels and interpolated vertically to themodel levels. The initial values of the cloud andground water are set to naught.
The distribution of the sea-surface temperature(SST) and sea-ice concentration are prepared fromthe NCEP RTG (Real-time global) SST (Thébaux
et al. 2003) and MMAB (the Marine Modelingand Analysis Branch) sea-ice analyses (Grumbine
1997), respectively. These SSTs and sea-ice data
are provided daily at 0.5 in the horizontal. They
are interpolated to the model grid points after asimple quality check for sea-ice.
3.2 The Silk Road pattern and intensiﬁcation on
the Bonin highAt the initial time, a wave packet is located near30 E (Fig. 6) and propagates eastward during thenext several days. The major features of the propagation of a Rossby wave packet in the analysis(Fig. 8) are reproduced in the hindcast (Fig. 9),
in particular, the simulated location of theequivalent-barotropic anticyclone (125 E, 35 N) isvery close to the analysis. Undulation of the northern margin of the Tibetan high implies the propagation of Rossby waves along the Asian jet in
the upper-troposphere (Fig. 9a). Indeed the wave
activity ﬂux is reproduced well although the overreﬂection process at jet entrance and exit is somewhat exaggerated as evidenced by larger arrows
near 60 E and 170 E. In the calculation of the
wave activity ﬂux, the simulated perturbation components include model bias since the climatology of
the NCEP/NCAR reanalysis is used as the basic
ﬁelds in place of a long-term integration of themodel due to prohibitive computational costs toobtain model climatology at a high resolution.
Lower-tropospheric anticyclonic ﬂows are correctly
reproduced in the southwestern Japan and contribute to a realistic distribution of the out-going longwave radiation (OLR, Fig. 9b).

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Fig. 9. Distributions of (a) the streamfunction (10 6 m 2 s, contour) and wave activity ﬂux (m 2 s2 , arrows) at the 200-hPa
surface (b) the stream function (10 6 m 2 s,
contour) at the 850-hPa surface and outgoing long-wave radiation (W m2 , shade)averaged in 17–21 July 2004 simulated by
AFES using the T639L48 resolution from0 UTC, 15 July.

3.3 Heavy rainfall in Sakata and Fukui
In this subsection, intense precipitation events
in Sakata (A in Fig. 3) and Fukui (136 E, 36 N),
which occurred during 17–18 July about a few
days prior to the heat wave event, are investigatedin relation to the intensiﬁcation of the Bonin high.
Figure 10 shows daily precipitation on 17 (Fig.
10a, c, e) and 18 July (Fig. 10b, d, f ) produced
from radar and surface (AMeDAS) observations(Fig. 10a, 10b), and moderate- (Fig. 10c, 10d) andhigh-resolution (Fig. 10e, 10f ) simulations. In thissection the local time (JST, UTF þ 9) is used to
examine changes in precipitation patterns between
the two days. On 17 July, rainfalls are found overTohoku and Hokuriku (between 36 N and 40 N)
including Sakata. As the Baiu frontal zone migrates
southward, rainfalls are concentrated near Fukui
on 18 July. These observed precipitation patterns
are reproduced in simulations with both moderate
and high resolutions. Similarities of precipitation
patterns imply the role of clockwise synoptic-scale
ﬂows that bring moist air at low levels to maintaininstabilities.
Simulated precipitation is, however, much less145

than that from observation. Note that the contour
intervals are halved for simulations. This is partly
due to a rather simple precipitation scheme and an
insu‰cient resolution to resolve cloud scales. The
observed rainfalls in Fig. 10a, 10b are band-shaped(Kato and Aranami 2005), probably caused bylatent instability (Yamasaki 2008) and maintainedby the back-building mechanism (Taniwatari2006). Such cloud- and meso-scale mechanisms are
not represented in GCMs. Simulated precipitation
is caused primarily in the convective parametrization at each column. It is speculated that thedi¤erences in precipitation mechanisms cause thedi¤erent shapes of heavy precipitation.
On 17 July, counterparts of a few centres of precipitation in Tohoku (Fig. 10a) are also found in
the high-resolution simulation (Fig. 10e) but not in
the moderate resolution-simulation (Fig. 10c) although the overall pattern is slightly shifted northward in both simulations. On 18 July, although the
Fukui rainfall (Fig. 10b) is reproduced at bothresolutions (Fig. 10d, f ), the simulated precipitationin the moderate-resolution simulation is very weak.
These di¤erences may be caused partly by the
representation of orography and partly by the representation of ﬂows, ﬁlaments of the potential temperature at low levels, in particular. Although theJapanese Alps near 138 E, 36 N are represented inboth resolutions, they are represented as a single
peak at the moderate-resolution (about 83-kmmesh). At the high-resolution (about 21-km mesh),several major mountain ranges are resolved (notshown). Figure 2 of Kitagawa (2008) compares orography of previous and current global numerical
weather forecast models at JMA. Their horizontal
resolutions (20 and 60 km) are similar to the onesin this paper.
Figure 11 presents the time evolution of the 850hPa equivalent potential temperature at every 12
hour between 12 UTC 16 to 0 UTC 18 July. Figure
11a, c, e, g are produced from the analysis for the
JMA’s regional spectral model (RSM). The horizontal resolution of RSM is 20 km and that of the
distributed data at pressure levels is 0.5 and 0.4 inthe zonal and meridional directions. At 12 UTC on
16 July, a tongue with high potential temperature isextended eastward from southern China toward
Japan. This moist tongue is associated with the
Baiu/Meiyu frontal zone and the low-level ﬂows of
the subtropical anticyclone contribute to converging moisture from the south. The front is intensiﬁed
as it migrates southward.

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Fig. 10. Observed (Radar-AMeDAS, a, b) and simulated (moderate resolution, c, d; high resolution, e, f )daily precipitation (mm d1 ) on 17 (a, c, e) and 18 (b, d, f ) July 2004. Note that the shading intervals arehalved for simulated precipitation.

The high-resolution simulation (Fig. 11b, d, f, h)
reproduces these observed features. At 12 UTC on
17 and 0 UTC on 18 July, large equivalent potential temperature is advected eastward reaching thecoast of the island of Honshu along the Sea ofJapan. The frontogenesis is somewhat more intensein the simulation. Although the simulated moist

tongue is more convex northward than that of theanalysis, the large equivalent potential temperature
comes from the upstream of the anticyclonic ﬂows
in both analysis and simulation. The moderateresolution simulation reproduces similar synoptic
distributions but ﬁne anomalous peaks are absent(not shown).

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Fig. 11. Time evolution of the equivalent potential temperature (K) at 850 hPa from 12 UTC on 16 to 0
UTC 18 July 2004 drawn every 12 hours. Left and right columns are produced from the analysis (RSM)and AFES 21-km mesh simulation, respectively.

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Journal of the Meteorological Society of Japan

148

Fig. 12. Distribution of the 2-m temperature( C) near Japan at 6 UTC on 20 July, 2004
in the (a) T159L48 (b) T639L48 hindcast
from 0 UTC, 15 July. Contours are drawn
every 2 degrees between 22 and 40 C andregions with temperature higher than 36 C
are hatched. The solid line represents thetransection in Fig. 13.

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3.4 Heat wave in Tokyo
Figure 12 shows the simulated 2-m temperature
at 6 UTC 20 July. The 2-m temperature is diagnosed from the temperatures on the surface andmodel’s lowest level and bulk Richardson numberin the surface ﬂux calculation. In both moderate(Fig. 12a) and high- (Fig. 12b) simulations, regionswith warm temperature are located along the
Paciﬁc coast, or the lee side of the mountains. Similarities between the two simulations at di¤erent
resolutions imply the dominance of the large-scaleﬂows, such as the Silk Road pattern and intensiﬁcation of the Bonin high, in the formation mechanisms of the heat wave.
In the high-resolution simulation (Fib. 12b) thesimulated warm regions are more localized andreproducing the temperature distribution of themeso-scale analysis (Fig. 3). Note that slightlydi¤erent contour levels are used between Fig. 3
and 12 in order to highlight the very warm regions.
As discussed in Section 1, the meso-scale analysis
underestimates temperature. The high-resolution
simulation overestimates maxima. This warm bias
may partly be attributable to the initial wetness ofnaught.
The temperature at 6 UTC (15 Japan Standardtime) at some arbitrarily selected two lee- and several windward-side observational stations is summarized in Table 1. The values at the closest grid
points are shown for the analysis and simulation.
This is shown so as not to compare values at eachsite quantitatively but to supplement maps (Fig. 3
and 12) and to demonstrate the capability of the 5day high-resolution hindcast to reproduce localized
high temperature.
There are a few factors causing the föhn: 1) condensational heating on the windward side and dry

Table 1. Observed (AMeDAS), analysed (MSM) and simulated (AFES) temperature values at 6 UTC, 20 July 2004.
Letters A–I are marked in Fig. 3.

A
B
C
D
E
F
G
H
I
J

Stationlatitude  Nlongitude  E

AMeDASMSM

AFES

SakataNiigata
Kumagaya
Tokyo
UshikuHamamatsu
ShinguOkayamaNakamura
Miyakonojo

38.91
37.91
36.15
35.69
35.40
34.71
33.69
34.66
33.00
31.73

139.84139.05139.38139.76140.15137.72135.97133.92132.92131.08

25.3
26.9
37.6
37.4
38.0
35.0
34.6
34.9
35.9
34.1

24.0
24.7
32.3
34.5
31.4
31.8
32.8
32.4
34.0
32.5

25.9
27.1
40.9
41.2
41.2
40.9
40.2
38.9
37.7
39.2

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T. ENOMOTO et al.

descent on the lee side of the mountains, 2) radiative heating, 3) folding down of isentropes (Ikawa
and Nagasawa 1989). The Föhn due to the folded
isentropes is adiabatic and often caused by montainwaves (Scorer and Klieforth 1959). The convective
activity associated with the Baiu front produced
diabatic heating on the windward side of the mountains. In fact, there is a notable contrast of humidity between the windward and lee sides of themountains. The relative humidity is low (50–60%)
where high temperature occurs. In spite of this, themoist process seems to be less important than the
adiabatic process. Even if a parcel near the surface
with 26 C were lifted to 1000 m with lapse rate of
4 K km1 and brought to the sea level with lapse
rate of 10 K, the temperature would be 32 C. Thisis about 5 K less than observed values near Tokyo.
The transsections of the potential temperature(Fig. 13) indicate the primary role of the adiabatic
process. The contours below 800 hPa are violently
meandered at both resolutions. Meanders are lesssigniﬁcant in the moderate-resolution simulation(Fig. 13a) but higher temperature would be reproduced if the altitude on the lee side were lower. Undulation at upper levels in the high-resolution simulation (Fig. 13b) implies mountain waves. Almost
vertical contours imply a well-mixed layer. Presumably radiative heating contributes to heating thesurface to enhance mixing. The near-surface temperature is overestimated probably due to a lack ofinitial soil moisture. As a result, the dry adjustment
might be overreacting. Undulation of the potential
temperature surface is also found in the meso-scale
analysis but it is less signiﬁcant (not shown) in accordance with underestimated temperature maxima(Fig. 3 and Table 1).
Representation of the orography would explain
the di¤erences in localization of high temperature
as in that of rainfalls (cf. Subsection 3.3). The peakof the mountain is lower and less localized in
the moderate-resolution model (shades in Fig. 13).
Moreover, the Kyushu, Shikoku, and Chugoku
mountain ranges are replaced with a single mountain range with reduced height at the peaks (e.g.
796.8 and 246.3 m at the peak in the Kyushu
mountain range in the high and moderateresolution models, repectively). Thus a local maximum near Okayama (Letter ‘H’ in Fig. 3) cannot
be reproduced with the moderate resolution. The
föhn regions are indeed realistically localized along
the lee side of each mountain range in the highresolution simulation (Fig. 12b).

149

Fig. 13. Transsection of potential temperature (K) between 137 E, 38 N and 141 E,
35 N used in (a) the moderate (T159,
about 83-km mesh), (b) high (T639, about
21-km mesh) resolution runs. Contours are
drawn every 1 K. The ﬁlled regions at thebottom represent the shape of the mountains.

4. Ensemble experiments
Deterministic hindcast simulations imply alonger predictability of the Rossby-wave propagation and associated local weather events. In orderto study the predictability of the propagation of
Rossby-wave packets and monsoon variability, ensemble experiments were conducted.
4.1 Experimental settings
A moderate horizontal resolution of T159L48(about 83-km mesh) allows representation of the
Silk Road pattern and intensiﬁcation of the Bonin

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Table 2. Di¤erences in the sea-surface boundary conditions used in three ensemble experiments.
experimentsea-surface boundary conditions

control
hindcast
forecastclimatology
daily analysis
climatology þ anomalies on 1 Julyhigh (cf. Subsection 3.2). Although regional eventsand their feedback are not su‰ciently resolved,
potential for local high-impact events could be
estimated from a moderate-resolution ensemble tosome extent.
Initial ensemble perturbations of the JMA
weekly ensemble forecast are used. Perturbations
are generated using the breeding vector method(Toth and Kalnay 1993, Kyouda 2006). There are
25 members composed of the unperturbed control
run, 12 modes and their sign reversed. The GPV ofweekly forecast is provided at three pressure levels(850, 500, 300 hPa) for u, v, z, T, RH at 850 hPa
and surface variables at the horizontal resolutionof 2.5 . Perturbations from the control are added
to the initial condition as follows. The horizontal
interpolation was performed in spectral space using
SPHEREPACK. Perturbations at 850 hPa are also
used for 1000 and 925 hPa. Perturbations at 700
and 400 hPa are interpolated from adjacent levels
in log pressure coordinates. The perturbations between 250 and 70 hPa are damped towards 0 at
70 hPa.
Three ensemble experiments with di¤erent seasurface boundary conditions were conducted to
identify their impacts (Table 2). The control run
uses the climatological values averaged between
1982 and 2004, prepared from the weekly NOAA
Optimum Interpolation (OI) SST and sea-ice data
(Reynolds et al. 2002). The daily values are interpolated in time from the weekly averages. The
hindcast run uses the same RTG SST and MMAB
sea-ice data as the hindcast simulations but for theperiod from 1 to 31 July 2004. The forecast run employs a technique commonly used in operational
seasonal forecasts with AGCM. The anomalies on
1 July 2004 are ﬁxed throughout the integration
period and superimposed on the climatologicaltime evolution in July. Thus SST and sea-ice distribution of the hindcast and forecast runs are identical at the initial time and gradually become di¤erent.

Vol. 87, No. 1

4.2 Results
In order to examine the predictability of thevariability of the Tibetan anticyclone, attentions
are focused upon the geopotential height at the
200-hPa level. One of the measures of forecast
skills is the correlation of the forecast and analysis
anomalies (anomaly correlation, AC) used inmany previous studies (e.g. Miyakoda et al. 1986).
The correlation is deﬁned asxy
AC ¼ pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ ;x2y2
where x and y are the deviations. Figure 14 shows
the 200 hPa geopotential height anomalies of theensemble mean of the forecast run and NCEP/
NCAR reanalysis from the climatology of the
NCEP/NCAR reanalysis on 4, 7, 10 July 2004.
The ensemble mean of n members is deﬁned as
z¼

n
1X
zi :
n i¼1

On 4 July, the analysis (Fig. 14a) and forecast (Fig.
14b) are very similar. The anomaly correlations in
the Northern Hemisphere (NH) and the Silk Road
region (SR, 30 E–120 E, 30 N–50 N) are 0.84 and
0.83, respectively. On 7 July, there are more discrepancies between the analysis (Fig. 14c) and(Fig. 14d). As a result, AC in NH reduces to be
0.59, marginally useful. In SR, by contrast, anomalies are still quite similar (AC ¼ 0.70). On 10 July,
there are not many similarities outside the Silk
Road region (AC ¼ 0.50 in NH and AC ¼ 0.91 in
SR). Although the amplitude is smaller than theanalysis, a wave-like pattern in SR (Fig. 14e) isreproduced in the forecast (Fig. 14f ).
The time evolution of the anomaly correlationsof the ensemble mean of the three simulations isshown in Fig. 15, where anomalies are deﬁned tobe the di¤erence from the long-time (1968–1996)
average of the NCEP/NCAR reanalysis. The thickand thin curves are the averages in the Northern
Hemisphere and in the Silk Road region. Anomaly
correlations are larger than 0.6, i.e. accurateenough to be a useful forecast, for about 7 and 13
days in NH and SR, respectively. That is, SR has
a longer predictability by 6 days. The maintenance,or even increase of the anomaly correlation around
10 July, might be partly due to the propagation of
Rossby waves, whose signal is present at the initialtime (Fig. 6 and 16a).
Di¤erences in the boundary conditions do notFebruary 2009

T. ENOMOTO et al.

Fig. 14. Anomalies of the 200 hPa geopotential height (gpm) from the climatology of the NCEP/NCAR reanalysis on 4 (a, b), 7 (c, d), 10 (e, f ) July 2004 in the NCEP/NCAR analysis (a, c, e) and ensemble mean of
the AFES forecast (b, d, f ). Anomalies smaller than 100 gpm and and larger than 100 gpm are light- anddark-shaded, respectively. Negative contours are broken. The Silk Road region (30 E–120 E, 30 N–50 N)
is designated by a rectangle.

151

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Vol. 87, No. 1

Fig. 15. Anomaly correlations of the 200hPa geopotential height in the Northern
Hemisphere in thick curves and in the Silk
Road region (30 E–120 E, 30 N–50 N)
in thin curves. The solid, broken and dotdashed curves correspond to the forecast,control and hindcast experiments.

seem to be signiﬁcant in the ﬁrst two weeks but
become gradually important in the latter half.
However, imposing the analysed SST does nothelp the hindcast to outperform the other experiments. The di¤erences among the experiments arefurther examined later in this section.
Figure 16a shows the longitude-time crosssection of the ensemble-mean geopotential heightaveraged between 35 N and 50 N at the 200-hPa
level in the forecast experiment. The zonal averageis removed for clarity. A Rossby-wave packet propagating from 90 E on 1 July to 15 W on 25 July(Fig. 6) is reproduced in the ensemble mean of theforecast. In addition, a few other wave packets arealso discernible although they are not as clear as
the ﬁrst one. At 120 E, the positive anomaly that
corresponds to the equivalent barotropic anticyclone near Japan modulates in time and has
maxima in the middle and end of the month. It issuspected that the Rossby-wave propagation a¤ects
these modulations of the high.
These features found in the second and third
week, even near the end of the month imply somepredictability. Figure 16b shows the ensemble
spread of the geopotential height at the 200-hPa
level. Here the ensemble spread is deﬁned as the ‘biased’ standard deviation (dividing the squared sumof anomalies by the number of ensemble members n
in place of n  1),
sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
n1X
s¼
ðzi  zÞ 2 ;n i¼1

Fig. 16. Longitude-time cross-sections of (a)the ensemble-mean geopotential heightanomaly from the zonal mean and (b) the
ensemble spread of the geopotential heightat the 200-hPa level average in 35 N and
50 N. Values smaller than 80 m (Panela) and 70 m (Panel b) are dotted. Values
larger than 80 m (Panel a) and 90 m (Panelb) are hatched.

where z is the ensemble mean. In general, the ensemble members diverge less over the Eurasian andNorth American continent than over the Paciﬁcand Atlantic. The smaller ensemble spread overthe continent implies less error and hence a longer
predictability. This asymmetry could be due to thedi¤erence in the initial error, growth between cy-

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T. ENOMOTO et al.

153

Fig. 17. Distributions of the di¤erence ofthe geopotential height (contours) at the
200 hPa surface between experiments: (a)forecast–control on 6 July, (b) forecast–control on 16 July and (c) forecast–hindcast 17 July. The conﬁdence level (%)
is shaded.

clone and anticyclone, relative importance between
diabatic and adiabatic processes and boundary conditions (land and ocean). In order to clarify causesof this asymmetry, a number of ensemble forecast
experiments are required, which would be exploredelsewhere.
It is interesting to note that the ensemble spreaddecreases as the ridge near Japan enhances. The
smaller ensemble spread indicates more conﬁdence
on the enhancement of this ridge. It is implied thatsome events that were a¤ected by Rossby waves,such as the Silk Road pattern and the enhancementof the equivalent-barotropic high near Japan, might
have predictability beyond week two.
Finally, the di¤erences between the ensemblemeans are examined in order to obtain insights
into how the forecast error would propagate. Fig-ure 17a and 17b show the di¤erences of the ensemble mean geopotential height at the 200 hPa level
between the forecast and control runs in contours.
The anomalies with darker shades have larger conﬁdence.
On 6 July, a train of errors is found between thewestern Paciﬁc and Japan (Fig. 17a). This pattern
resembles the Paciﬁc-Japan pattern (Nitta 1987).
Anomalies found near Japan and its east can be interpreted as propagation of Rossby waves. Negative (about 1 K) and positive (about 1–2 K, notshown) SST anomalies in the Philippine and East
China Seas are probably responsible for this pattern. On 16 July, anomalies reminiscent of the Silk
Road pattern are found over the Eurasian continent (Fig. 17b). This pattern emerges in the decaying phase of an intensiﬁed anticyclonic anomaly

154

Journal of the Meteorological Society of Japanover the east Mediterranean Sea. These cases exemplify how the error is communicated over the globe.
The di¤erence between the forecast and hindcast
runs is small in the ﬁrst two weeks. These two runs
diverge in the latter half of the month. On 17 July,a train of errors, reminiscent of the propagation
pattern causing the intensiﬁcation of the Okhotsk
anticyclone (Nakamura and Fukamachi 2004), is
found over the northern Eurasia (Maeda et al.
2006). These di¤erences appear to be emanatingfrom the Barents Sea, where no signiﬁcant SST
anomaly from the climatology is found on 1 Julybut a positive SST anomaly (>2 K) emerges on 16
July (not shown). Other locations with signiﬁcant
SST changes include the Mediterranean sea andthe western Paciﬁc to the east of Japan, where thewarm anomalies existent at the initial time disappear about two weeks later.
Similarities between propagation of errors andRossby waves suggest that the activity of Rossby
waves is one of the important source of errors(Palmer and Tibaldi 1988). At the same time, itsuggests the importance of these three propagation
patterns upon the summer climate of East Asia.
5. Summary and discussion
Hindcast simulations were conducted to study
these high-impact weather events in Japan in July
2004 (Fig. 1, 2, 3). Although these high-impact
weather events themselves are usually in mesoscales, they are closely related to the larger scales(Fig. 5–7).
In the precipitation events in Sakata and Fukui(Fig. 10), an unstable region is created at the tip ofthe moist tongue (Fig. 11). Although the bandshaped precipitation cannot be resolved and theamount of precipitation is much less, the model
has succeeded in reproducing almost the correct
timings and locations of the environment thatcaused the events a few days in advance.
The heat wave in Tokyo on 20 July is reproduced
surprisingly well in the 21-km mesh global hindcast
simulation. The 5-day simulation not only captures
the hemispheric propagation of Rossby waves (Fig.
8–9), but also the anomalous temperature distribution over Japan and the Korean Peninsula (Fig. 3,
12, Table 1). In addition to the accurate representation of the large-scale ﬂow, which can be also reproduced at a lower resolution, the detailed orography in the high-resolution has allowed accurate
temperature distribution of the föhn (Fig. 13).
Ensemble forecast experiments were performed

Vol. 87, No. 1

to investigate predictability of propagation of
Rossby waves and associated weather events. It isfound that the Silk Road region has as twice aslong predictability as the entire Northern Hemisphere in the ensemble simulations from 1 July
2004 (Fig. 14, 15). It appears that the propagationof Rossby waves acts to maintain the accuracy.
The modulation of the Bonin high seems to havesome predictability beyond the second week (Fig.
16). No apparent evidence is found that indicates
better performance of the hindcast using the analysed SST over the other two, the control run withthe climatological SST and forecast with the initialanomaly superimposed on the climatological SST
in the ﬁrst two weeks (Fig. 15). However, the distribution of the anomalous SST can be quite di¤erent
even within the week and the error due to thewrong SST anomalies propagates rapidly over the
globe along the wave-guides (Fig. 17).
The case studies here suggest a few methods
that might improve the prediction of high-impact
weather. First, the use of a high resolution of 10–
20 km is rewarding and promising since it has acapability of reproducing some meso-scale systems
and a phenomenon a¤ected by detailed topography. The output from the high-resolution global
model would be better initial and boundary conditions for regional models. Dynamical downscaling
with a regional model seems to be a practical tool
to reproduce local phenomena that cannot be resolved by the global model. Second, consideringthat the propagation of Rossby waves tends tohave a better predictability, it might be able toidentify possible severe weather events. Basically,
the propagation of a signiﬁcant wave packet with asmall ensemble spread implies some severe weather
to occur. Sensitivity analysis using ensemble forecast may be conducted to obtain the initial perturbations that cause the high-impact weather in theveriﬁcation region. Then, a forecast using a higherresolution global or regional model with these perturbations is conducted to predict ‘‘the worst casescenario’’ (e.g. Enomoto et al. 2007).
In general, high-impact weather is likely to occur
at the leading edge of the ﬁlamentary structureof near-surface equivalent potential temperature(‘‘moist tongue’’) or potential vorticity near the
tropopause (‘‘PV streamer’’). The ﬁlamentary structure represents the link among a wide range of horizontal scales. Its width is narrow enough to be in
the meso-scales and its extent is long enough to bein the large-scales. These ﬁlaments are often theFebruary 2009

T. ENOMOTO et al.

result of the propagation and breaking of Rossby
waves at the large-scales and sometimes give birth
to meso-scale systems. The high-impact weather
events that occurred in July 2004 in Japan can be
regarded as such global-to-regional phenomena.
Acknowledgements
Useful comments from the two anonymous
reviewers and editor Mr Hosaka (MRI, JMA)
helped us improving the manuscript. Dr Wakatsuki(FRCGC, JAMSTEC) and Dr Yamane (Doshisha
University) provided Fortran programs to process
GPV and boundary conditions, respectively. The
NCEP/NCAR reanalysis data is provided by the
NOAA/OAR/ESRL PSD, Boulder, Colorado,
USA, from their OPeNDAP server. The NCAR
Command Language (NCL) was used for dataprocessing and visualization. Computation is performed on the Earth Simulator. This work is partly
supported by KAKENHI 17740317.
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