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The Role of Typhoon Songda (2004) in Producing Distantly LocatedHeavy Rainfall in Japan*
YONGQING WANG
Pacific Typhoon Research Center, KLME, Nanjing University of Information Science and Technology,
Nanjing, China, and International Pacific Research Center and Department of Meteorology,
University of Hawaii at Manoa, Honolulu, Hawaii

YUQING WANG
International Pacific Research Center and Department of Meteorology, University of Hawaii at Manoa,
Honolulu, Hawaii, and Pacific Typhoon Research Center, KLME, Nanjing Universityof Information Science and Technology, Nanjing, China

HIRONORI FUDEYASUInternational Pacific Research Center and Department of Meteorology, University of Hawaii at Manoa,
Honolulu, Hawaii(Manuscript received 5 January 2009, in final form 20 May 2009)
ABSTRACT
When Typhoon Songda (2004) was located southeast of Okinawa over the western North Pacific during
2–4 September 2004, a heavy rainfall event occurred over southern central Japan and its adjacent seas, morethan 1200 km from the typhoon center. The Advanced Research version of the Weather Research andForecast (WRF-ARW) model was used to investigate the possible remote effects of Typhoon Songda on thisheavy precipitation event in Japan. The National Centers for Environmental Prediction (NCEP) global final(FNL) analysis was used to provide both the initial and lateral boundary conditions for the WRF model. The
model was initialized at 1800 UTC 2 September and integrated until 1800 UTC 6 September 2004, during
which Songda was a supertyphoon. Two primary numerical experiments were performed. In the control
experiment, a bogus vortex was inserted into the FNL analysis to enhance the initial storm intensity such thatthe model typhoon had an intensity that was similar to that observed at the initial time. In the no-typhoon
experiment, the vortex associated with Typhoon Songda in the FNL analysis was removed by a smoothingalgorithm such that the typhoon signal did not appear at the initial time. As verified against various observations, the control experiment captured reasonably well the evolution of the storm and the spatial distribution and evolution of the precipitation, whereas the remote precipitation in Japan was largely suppressed in
the no-typhoon experiment, indicting the significant far-reaching effects of Typhoon Songda. Songda enhanced the remote precipitation in Japan mainly through northward moisture transport into the preconditioned precipitation region by its outer circulation. The orographic forcing of the central mountains in Japan
played a small role compared with Typhoon Songda in this extreme precipitation event.

1. Introduction* School of Ocean and Earth Science and Technology Publication Number 7802 and International Pacific Research Center
Publication Number 630.

Corresponding author address: Dr. Yuqing Wang, IPRC/SOEST,
University of Hawaii at Manoa, Post Bldg. 409G, 1680 East–West
Rd., Honolulu, HI 96822.
E-mail: yuqing@hawaii.edu
DOI: 10.1175/2009MWR2933.1
Ó 2009 American Meteorological Society

Tropical cyclones (TCs, or typhoons in the western
North Pacific, hurricanes in the Atlantic or eastern North
Pacific) are often heavy rain producers. In particular,
landfalling TCs usually cause fatalities and economic
losses due to not only strong winds but also torrential
rainfall that often induces flash flooding after their landfall (e.g., Kunkel et al. 1999). A TC can produce

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tremendous rainfall in its eyewall and as well as in both
its inner and outer spiral rainbands, which may be several tens to hundred kilometers from the TC center(Takahashi and Kawano 1998; Lonfat et al. 2007; Kimball2008; Yokoyama and Takayabu 2008). This is generally
referred to as direct effect of a TC on precipitation, or
precipitation induced by the TC itself. On the other
hand, when a strong interaction with other synoptic
systems occurs, a TC can induce heavy rainfall far away,for example, beyond a thousand kilometers from its
center. Such an effect of a TC on precipitation in a remote area is generally referred to as indirect or remote
effect of a TC on precipitation (Bosart and Carr 1978;
Ross and Kurihara 1995; Chen et al. 2006).
Although most TCs mainly produce heavy rainfall
directly by themselves (direct effect), some TCs can
induce heavy rainfall in remote areas when they interact
with other synoptic systems, such as upper-level troughand cold frontal systems in the midlatitudes (indirect
effect). This latter effect, which does not occur very often, usually causes an even greater disaster when it does
happen because special attention is mostly given toprecipitation directly caused by the TC itself (Chen et al.
2006). Quantitative precipitation forecasts (QPF) arevery important whereas they are quite challenging tooperational forecasters. Current skill in QPF, including
the forecast of TC precipitation (Elsberry 2002), is quite
low. In particular, the forecast skill for TC precipitationis largely affected by the prediction accuracy of the TC
track (Chen et al. 2006; Marchok et al. 2007).
Previous studies have mostly focused on precipitation
directly caused by a TC either over the open ocean (e.g.,
Wang and Holland 1996a,b; Rogers et al. 2003; Atallah
et al. 2007) or approaching–making landfall (e.g., Li
et al. 2007; Kimball 2008; Chen et al. 2006). Thesestudies have shown that over the open ocean, the rainfall
distribution in a TC is largely controlled by the vertical
shear in the large-scale environmental flow and thetranslation speed of the storm itself; whoever, after
landfall, both the rainfall intensity and the radial extent
of the outer rainfall are also affected by the soil moistureof the underlying land surface.
Some studies have also investigated the indirect effects
of a TC on remote precipitation. Bosart and Carr (1978)were among the first to document a case study for an
excessive rainfall event due to the indirect effects of
Hurricane Agnes (1972). They showed that a weak shortwave in the mid- to upper troposphere provided the initial triggering mechanism for the growth of the rain area
while Agnes’s circulation transported plentiful moisturefrom the western Atlantic to the rain area, enhancing therainfall greatly. Murata (Chen et al. 2006) studied the
Typhoon Meari (2004) case. Typhoon Meari made landfall

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in Kyushu in Japan and resulted in heavy rainfall along theKii Peninsula, more than 500 km to the east of the typhoon
center. He performed a series of sensitivity numericalexperiments and demonstrated that the moisture supplied by the typhoon’s outer circulation and the interaction with topography played critical roles in theheavy rainfall. Farfán and Fogel (2007) investigated theinfluence of eastern Pacific TC circulations on the distribution of humidity and convection over northwest
Mexico. They suggested that TC activities serve as asource of humid tropical air masses, providing conditions that support deep convection and that, thus, cause
heavy rainfall over northwest Mexico. Another categoryof the indirect effect is the downstream impacts of TCs
that experience extratropical transition. As was recently
demonstrated by Harr et al. (2008), a recurving TC undergoing extratropical transition over the northwest Pacific could perturb the upper-level westerly jet stream in
the midlatitude, exciting Rossby waves in the westerlies
and impacting the midlatitude circulation and weather
far downstream over the North America.
One more category of the indirect effect of a TC on
remote precipitation is the remote heavy rainfall resulting from the interaction between a TC over the open
ocean and a weather system (such as a cold frontal system) far away from the TC center. This type of remote
effect from a typhoon over the northwest Pacific on thecirculation and precipitation in Japan was previously
studied by Kawamura and Ogasawara (2006) and Yamada
and Kawamura (2007) with the use of reanalysis data.
They found that one or two typhoons, which might serveas synoptic convective heating sources over the northwest Pacific, could induce the quasi-stationary barotropic
Rossby wave train emanating northeastward and significantly affect the anomalous weather in the vicinity ofJapan. This teleconnection was called the Pacific–Japan(PJ) pattern. They suggested that the establishment of
the PJ pattern intensifies the east–west pressure gradientat low levels between a typhoon and an anomalous anticyclone east of Japan, which enhances the warm advection and moisture supply from low latitudes to the vicinityof Japan, thus causing remote precipitation in Japan.
A case study for a similar scenario including the indirect effects of a typhoon on remote precipitation in
Japan is investigated numerically here. The case chosen
in our study was Typhoon Songda (2004), which caused
heavy rainfall in southern central Japan and its adjacentseas when it was far away from the heavy precipitation
region. The Advanced Research version of the Weather
Research and Forecasting (WRF) model developed at
the National Center for Atmospheric Research (NCAR)
is used to perform two major simulations, one with and
one without the typhoon circulation, to highlight the

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FIG. 1. Total precipitation (mm) between 1800 UTC 2 Sep and 0000 UTC 5 Sep 2004 and thebest track of Typhoon Songda (2004) from JTWC (information online at http://metocph.nmci.
navy.mil/jtwc). The TRMM 3B42 V6 3-hourly merged rainfall dataset, which was obtained fromthe Asia–Pacific Data Research Center (APDRC) at the University of Hawaii, was used. The
same precipitation dataset was used as the observed precipitation in Fig. 10 as well. The small box
in D3 shows the area of interest that will be used for area averaging in Fig. 10.

effects of Typhoon Songda (2004) on remote precipitation and to investigate the involved physical mechanisms.
Several supplementary experiments are performed to
examine the potential effects of topography over Japanand the radial wind profile of the typhoon itself on thesimulated remote precipitation.
The rest of the paper is organized as follows. The next
section provides a brief overview of Typhoon Songda(2004) and its possible contributions to the heavy rainfall
in Japan. Section 3 describes the basic features of the
WRF model and the experimental design, including
methodologies to enhance the initial typhoon vortex or
remove the typhoon from the initial conditions. Resultsfrom the control simulation are verified with various
available datasets in section 4. The effects of Typhoon
Songda on remote precipitation in Japan and the physical mechanisms involved are discussed in section 5.
Major findings are summarized in section 6.

2. An overview of Typhoon Songda
Typhoon Songda (2004) was 1 of the 10 named typhoons that made landfall on the main islands of Japan

in 2004 and brought extensive damage to Japan due toboth strong winds and heavy rainfall (Nakazawa andRajendran 2007). Te storm formed near the MarshallIslands on 27 August 2004 and moved northwestward over
the western North Pacific. After passing the Okinawa
Islands, it recurved northeastward over the East China
Sea on 1200 UTC 6 September and then made landfall
on Kyushu Island, south of the main island of Japan, at
0000 UTC 7 September (Fig. 1). Songda reached its
lifetime peak intensity with a maximum sustained nearsurface wind speed of 64 m s21 and a minimum central
sea level pressure of 916 hPa.
Similar to many other typhoons, Songda produced
heavy rainfall along its track under its eyewall and in its
major spiral rainbands (Fig. 1). When Typhoon Songda
was located more than 1500 km south of the main islands of Japan over the western North Pacific during
1800 UTC 2 September–0000 UTC 5 September, a rainband with several strong convective cores occurred overthe southern central Japan and the adjacent sea (328–388N,
1358–1458E; see the black box in Fig. 1). It is our hypothesisthat in addition to the heavy rainfall directly associatedwith deep convection in the inner core, Typhoon Songda

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FIG. 2. The geopotential height (10 gpm contours) and wind field (barbs) at (top) 500 and (bottom) 850 hPa with a contour interval of
40 gpm from the FNL analysis for (left) 1800 UTC 2 Sep and (right) 0000 UTC 5 Sep 2004. One flag represents 50 kt and one full barb
represents 10 kt.

contributed significantly to the heavy rainfall over southern central Japan and the adjacent sea because its outercirculation to the east transported moisture northward to
the preconditioned local precipitation system. This can beconsidered to be the indirect effect of Typhoon Songda on
precipitation in a remote area as discussed in section 1.
Although the topography in the central Japan might playa role in enhancing the heavy precipitation in Japan, thenumerical results in section 5 demonstrate that the orographic effect is insignificant in the Songda case.
Figure 2 shows the synoptic patterns at 1800 UTC
2 September and 0000 UTC 5 September 2004 at 500 and
850 hPa. Typhoon Songda was located south of the east–west-elongated subtropical ridge and embedded in thesoutheasterly flow on the southwestern edge of the western Pacific subtropical anticyclone at 1800 UTC 2 Sep-

tember (Figs. 2a and 2c). To the north in the midlatitudes
was a short-wave trough embedded in the westerly jet
stream at 500 hPa over the Japan Sea. In addition to themain subtropical anticyclone to the east around 308N,
there were subtropical anticyclonic cells at both 500 and
850 hPa over southeastern China (Figs. 2a and 2c). This
gave rise to a northeast–southwest-elongated region withconsiderable cyclonic shear and low-level convergence
between the two anticyclonic circulations over the mainislands of Japan. This situation favored the upward motion
and moisture convergence and served as a triggering
mechanism for precipitation in the region in the coming
2–3 days (see further discussion below). By 0000 UTC
5 September, the short-wave trough in the westerlies moved
eastward out of the domain and the jet stream in the
midlatitude weakened considerably with the development

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of a short-wave trough and ridge pair farther to the northof the Japan Sea at 500 hPa (Fig. 2b). In the lower troposphere, the cyclonic shear with horizontal wind convergence was still over Japan but it had weakened
considerably, and now Typhoon Songda was approaching
the main islands of Japan from the southwest (Fig. 2d).
Accompanied by the large-scale circulation given in
Fig. 2 was a northeast–southwest-elongated region withcyclonic shear at the surface off the southeast coast of
the main islands of Japan (Fig. 3a) and a short-wave
trough farther to the north over the Japan Sea. The
short-wave trough moved eastward in the following 12 h
and arrived at the eastern coast of northern Japan, andthe cyclonic shear line moved to the southeast (Fig. 3b).
In the next 12 h, the short-wave trough moved eastward
slowly, whereas the cyclonic shear line shifted northwestward toward southern Japan and aligned itself withthe short-wave trough to the north by 1800 UTC 3 September (Fig. 3c). Corresponding to the surface windfields was a northeast–southwest-elongated cyclonic vorticity zone in the lower troposphere over southern central
Japan at 1800 UTC 2 September (Fig. 3d) with thedevelopment of low-level convergence by 0600 UTC
3 September (Fig. 3e). By 1800 UTC 3 September, astrong convergence zone at 850 hPa with upward motion(not shown) developed over southern central Japan andthe adjacent seas. This is the time when significant rainfall
started in the region. Therefore, the low-level cyclonicshear zone with strong horizontal wind convergence was
the key to the vertical motion and thus was the preconditioning and initiation of precipitation over southern
central Japan and the adjacent seas, where the troposphere was already conditionally unstable to cumulus
convection (not shown). We will show in section 5 that thenorthward moisture transport by the low-level southerly–
southeasterly flow of Songda’s outer circulation was
crucial to the subsequent heavy rainfall over Japan.
Figure 4 shows the satellite images over East Asia andthe western North Pacific. There were two major cloudsystems in the region: one to the southeast of the
Okinawa Islands associated with Typhoon Songda andthe other associated with the surface shear line acrossthe main islands of Japan during 1200 UTC 3 September
and 0600 UTC 4 September. Although interactions might
exist between the two cloud systems, they were definitely separate systems during this period. After 0000
UTC 5 September, Typhoon Songda headed northeastward, approaching the main islands of Japan, and followedby the direct effects of the typhoon itself on precipitationin Japan. Since the focus of this study is on the indirect
effects of Typhoon Songda on precipitation in Japan,
our attention would thus be given to the period during
1800 UTC 2 September–0000 UTC 5 September 2004.

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3. Model configuration and experimental design
a. Model configuration
The model used in this study is version 2.2.1 of theAdvanced Research WRF (ARW) model developed at
NCAR (Skamarock et al. 2008). WRF-ARW is a threedimensional, fully compressible, nonhydrostatic model
formulated within a terrain-following mass coordinate
in the vertical. The model physics include the grid-scale
cloud and precipitation scheme used in the new Eta
Model, namely, the Ferrier cloud microphysics scheme(Ferrier 1994), the Rapid Radiative Transfer Model(RRTM) for longwave radiation (Mlawer et al. 1997),
the Goddard shortwave radiation scheme (Chou andSuarez 1994), the Monin–Obukhov surface flux calculation over the ocean, the Rapid Update Cycle (RUC)
land surface model (Smirnova et al. 1997, 2000), theYonsei University (YSU) planetary boundary layer
scheme (Hong et al. 2006), and the Kain–Fritsch cumulus parameterization scheme (Kain and Fritsch 1990)
for subgrid-scale deep convection.
Three interactive nesting domains (Fig. 1) are used in
all of the numerical experiments performed in this study.
The outermost domain (D1) had 166 3 166 grid points,
centered at 308N, 1358E, with horizontal grid spacing of
27 km. The second domain (D2) had 370 3 370 grid
points with horizontal grid spacing of 9 km. The innermost domain (D3) had 673 3 379 grid points withhorizontal grid spacing of 3 km, which covers the precipitation region in Japan and the adjacent seas. The
second domain (D2) was used to explicitly resolve thestructure of the typhoon and the mesoscale precipitation
systems, as was briefly discussed in section 2 above. The
innermost domain (D3) was used to better resolve thefine structure of the frontal system near Japan. Note thatthe cumulus parameterization was not activated in D3.
The model was run with 28 unevenly distributed verticallevels with higher resolution in the planetary boundary
layer and with the model top at 10 hPa. The NCARarchived National Centers for Environmental Prediction(NCEP) global final (FNL) analysis dataset at 18 3 18
latitude–longitude grids and 6-h intervals was used toprovide both the initial and lateral boundary conditions
to the WRF-ARW model. Sea surface temperature (SST)
used in the model simulation was taken from the NCEP
real-time objective SST analysis with 0.58 3 0.58 horizontal grid spacing at the initial time and did not change
during the model simulation. This could be a limitation
for the realistic simulation of the typhoon intensity.

b. Experimental design
Four numerical experiments were performed using
the model configuration above in this study. The control

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FIG. 3. (left) The 10-m wind (wind barbs) and sea level pressure (contours interval 1 hPa) fields and (right) the
horizontal divergence (shading in 1025 s21) and relative vertical vorticity (contour interval 0.1 3 1024 s21) at(a),(d) 1800 UTC 2 Sep, (b),(e) 0600 UTC 3 Sep, and (c),(f) 1800 UTC 3 Sep 2004 from the FNL analysis. One flag
represents 50 kt and one full barb represents 10 kt. Thick dashed line segments in (a)–(c) show either a short-wave
trough in the midlatitude westerlies or a cyclonic shear line in the surface wind fields.

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FIG. 4. Satellite images over East Asia and western North Pacific (obtained from the Korean
Meteorological Administration), showing the evolution of Typhoon Songda (2004) and theprecipitation system over southern central Japan and its adjacent seas with the time given at the
top of each panel.

experiment (CNTRL) was designed to reproduce theobserved nature discussed in section 2. Typhoon Songda(2004) from the FNL analysis (Fig. 5a) was too weakcompared with the best track and observations, partly
because of the small number of conventional observations over the open ocean and partly because of thecoarse-resolution global model used in the data assimilation for the FNL analysis. Indeed, we could not expect
the FNL analysis to depict the maximum inner-core intensity of a typhoon because of its coarse resolution andthe representativeness of the grid-box mean quantities.
Therefore, in order to realistically represent the typhoonin the simulation, the initial model typhoon needs to beenhanced in some way to match the observed intensity
and structure. Although different methods could be
used to achieve this, such as bogus data assimilation(DBA; e.g., Zou and Xiao 2000; Xiao et al. 2000), we
chose a simple bogussing scheme in this study (Wang

1998). Following Wang (1998), the tangential flow (VT)of the bogus vortex is defined by
V T (r, s) 5 V m   rrp
exp 1 2sin s , (1)rmrm
2where r denotes the radial distance from the observed
typhoon center, Vm (55 m s21) is the maximum azimuthal wind at the radius of rm (60 km), and s 5 ( p 2 pt)/
( ps 2 pt), with p being the pressure and ps and pt being
the unperturbed pressure at the sea surface and at p 5
100 hPa, respectively. The geopotential height was calculated by a gradient wind balance on constant pressure
surfaces and the temperature anomaly was obtained
from the thermal wind balance. This axisymmetric vortex was inserted into the FNL analysis fields without anyother special treatment, which seemed to work well forthis case.

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FIG. 5. Geopotential height (10 gpm contours with 40 gpm interval) and wind (barbs) fields
at 850 hPa at 1800 UTC 2 Sep 2004. (a) The FNL analysis, (b) the initial conditions in thecontrol experiment with the enhanced typhoon vortex in the FNL analysis using a bogus vortex
scheme, (c) the typhoon vortex removed from the FNL analysis, and (d) the initial conditions in
the No-Typhoon experiment with the typhoon vortex removed. One flag represents 50 kt and
one full barb represents 10 kt.

After inserting the bogus vortex into the initial analysis, the initial model typhoon (Fig. 5b) was muchstronger than that in the FNL analysis (Fig. 5a). The
initial maximum wind at the 1000-hPa pressure level wasincreased from 20 m s21 in the FNL analysis to 53 m s21(not shown), which is very close to the 56 m s21 result of
the Joint Typhoon Warning Center (JTWC) best track at
the given time. The minimum sea level pressure wasdecreased from 991 hPa in the FNL analysis to 934 hPa(not shown), which is about 7 hPa higher than that givenin the JTWC best track. The bogus vortex scheme enhances the inner-core structure and intensity of the initial typhoon vortex but has little effect beyond about
300 km in the FNL analysis (Fig. 6). Therefore, the initial field in the control experiment improved the representation of the typhoon vortex considerably and issuitable for this study because we mainly focus on theeffects of the vortex-scale cyclonic circulation on precipitation in the remote areas.

The indirect effects of Typhoon Songda on the remoteprecipitation in southern central Japan in the control
experiment were isolated by conducting a sensitivity
experiment (No-Typhoon), in which the typhoon circulation was removed from the FNL analysis, which
provided the lateral boundary conditions to the WRF
model. The typhoon vortex removal algorithm used by
Ross and Kurihara (1995) was adopted in this study. In
our application, the successive application of a simple
smoothing operator of Kurihara et al. (1990) was applied to the winds, geopotential height, temperature,and specific humidity fields of the FNL analysis centeredat the observed typhoon center at a given time. Figure 5cshows the typhoon component in the FNL analysis field,
and Fig. 5d shows the initial fields with the typhoonvortex removed at 850 hPa. Therefore, we can expectthat the difference between the CNTRL and No-Typhoon
experiments can be considered to be the effect of thetyphoon on its environment.

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FIG. 6. Radial profiles of azimuthal mean tangential wind (m s21) at 850 hPa in the control
experiment with a vortex bogus scheme (dashed) and in the No-Bogus experiment with nobogus vortex (solid) at the initial time (1800 UTC 2 Sep 2004).

Two other sensitivity experiments were also performed. The No-Bogus experiment is the same as the
CNTRL experiment but without the application of thebogus scheme described above. This experiment was
designed to see whether the outer circulation of themodel typhoon was artificially enhanced by the bogus
scheme, thus overestimating the effects of the typhoonon the remote precipitation in Japan in the CNTRL
experiment. The last experiment (No-Terrain) is thesame as the CNTRL experiment but with the terrain
height over the main islands of Japan set to 1 m if it ishigher than 1 m so that the possible effects of topography on precipitation over Japan were artificially eliminated. This experiment was intended to address whether
the interaction between the outer circulation of Typhoon
Songda and the orography in central Japan played any
role in the heavy precipitation over Japan.

4. Verification of the control simulation
Figure 7a compares the observations and the CNTRL
experiment simulated maximum 10-m height surface
wind speed and minimum sea level pressure. Overall,
the model simulated the intensity evolution of Typhoon
Songda reasonably well, with the exception that thesimulated typhoon was stronger than the observed after
about 48 h of integration (Fig. 7a) due to the considerable underestimation of the weakening of Typhoon
Songda. Figure 7b shows the track of Typhoon Songda
from both the observations and the model simulation.
Although the anticyclonic track was reproduced well by
the model, the simulated track had a systematic deflection to the left of the best track. This discrepancy inthe simulated track could be attributed to either the

relatively large size of the model typhoon (Wang andHolland 1996a,b), or the bias in the simulated large-scale
environmental flow due to problems with the model
physics, the initial data, or both. Despite the bias in thesimulated track, the large-scale outer circulation of Typhoon Songda is likely similar to that observed. Therefore, the effects of Typhoon Songda on precipitation in
Japan should be reasonably captured by the model inthis case (see next section).
Verification of the simulated typhoon structure ischallenging due to the lack of observations over the
open ocean. However, the inner-core structure of thesimulated storm can be compared with the TropicalRainfall Measuring Mission (TRMM) Precipitation Radar (PR) observations. Owing to the narrow swath of the
TRMM satellite, PR data are limited by sampling. What
we did here was to pick up the model times that wereclose to the times of the PR data available to us andcompare the surface rain rate from the model simulation
with the PR observation.
Figure 8 shows three examples of such a comparison
during 3–4 September 2004. The TRMM PR data reveal
the fine structure and evolution of precipitation in the
inner-core region of Typhoon Songda. In particular,
Songda seemed to evolve a concentric eyewall cycle(Willoughby et al. 1982) during the given time period(left panels in Fig. 8). The model failed to reproduce theconcentric eyewall cycle, however. Instead, the model
simulated a relatively large eye and eyewall structurewith stronger surface rain rates in the eyewall (rightpanels in Fig. 8). This is not surprising since the initialmodel typhoon did not include detailed information
about the preconditioning of the concentric eyewall
development and the simulation of a concentric eyewall

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FIG. 7. (a) Evolution of the minimum central sea level pressure (hPa) and the maximum
sustained 10-m wind speed (m s21) and (b) the storm track from the JTWC best track data
(dashed) and the WRF model simulation in the control experiment (solid).

is still a challenging task for most numerical models(e.g., Houze et al. 2007).
Although the model did not capture the inner eyewallof Typhoon Songda, it did produce an eyewall size equivalent to the size of the outer eyewall from the TRMM PR
observations. This may imply that the overall inner-coresize of the model typhoon is not unrealistic comparedwith the real typhoon. The stronger rain rates in thesimulation could be partially due to the discrepancies in
the cloud microphysics scheme used in the model, in theinitial conditions, or the model resolution. Again, since
the focus of this study is on the effects of the outer circulation of Typhoon Songda on remote precipitation far
to the north of central Japan, the lack of concentriceyewall structure in the simulation is not expected toalter our qualitative conclusions regarding Typhoon
Songda’s distant effects on precipitation over Japan,
which will be discussed in the next section.
The total precipitation between 1800 UTC 2 September
and 0000 UTC 5 September 2004 in the control simulation

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FIG. 8. Precipitation rates (mm h21) from (left) the TRMM PR data at (top) 0229:50–0233:09
UTC 3 Sep, (middle) 0311:24–0314:53 UTC 4 Sep, and (bottom) 1942:46–1946:05 UTC 4 Sep
2004 and (right) from the control experiment at (top) 0300 UTC 3 Sep, (middle) 0300 UTC
4 Sep, and (bottom) 2000 UTC 4 Sep 2004.

is shown in Fig. 9a. Compared with the total precipitation
results during the same time period from the TRMM
Microwave Imager (TMI) products (Fig. 1), the control
experiment simulated reasonably well the observed spatial pattern of the precipitation (Fig. 9a). However, as
mentioned earlier, the model overestimated the rainfall
in the inner-core region along the typhoon track. The
precipitation to the north over the main islands of Japanwas underestimated, however. The spatial distributionof the simulated precipitation is much smoother than
was observed. This may be partially due to the relatively
coarse model resolution in domain 3 in this study. How-ever, we expect that most of the discrepancies in thecontrol simulation would not prohibit us from a qualitative assessment of the possible effects of Typhoon Songda
on the remote precipitation in Japan as discussed in thenext section.

5. Effects of Typhoon Songda on remoteprecipitation in Japan
Figure 9b shows the total precipitation from the NoTyphoon experiment during the same period as shown in
Fig. 9a from the CNTRL experiment. As expected, with

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FIG. 10. Temporal evolution of 3-hourly rainfall (mm) from2100 UTC 2 Sep to 1800 UTC 5 Sep averaged over 328–388N, 1358–
1458E (i.e., the boxes shown in Figs. 1 and 9) from the CNTRL, NoTyphoon, No-Terrain, No-Bogus experiments, and the TRMM
observations.

FIG. 9. Total precipitation (mm) between 1800 UTC 2 Sep and0000 UTC 5 Sep 2004 in (a) the control experiment and (b) the NoTyphoon experiment. The typhoon track during this time period,
together with the minimum central sea level pressure from thecontrol experiment, are also shown in (a). The small box in eachpanel shows the area of interest, (the same as in Fig. 1) that will be
used for the area means in Figs. 10 and 14.

the typhoon removed in the No-Typhoon experiment,
the precipitation originally associated with Typhoon
Songda over the western North Pacific almost disappeared, indicating the removal of Typhoon Songda’s
direct effects on precipitation along its path. In addition
to the direct effects, Songda had an indirect effect on
precipitation far to the north over the main island ofJapan. Comparing Figs. 9a and 9b, with the typhoon removed in the No-Typhoon experiment, precipitation
over the main islands of Japan and the adjacent seas was
substantially reduced, suggesting that Typhoon Songda
contributed positively to precipitation in Japan (rectan-

gular box in Fig. 9). In addition, the precipitation over
the North Pacific to the east of Japan and in the lower
reaches of the Yangtze River was increased slightly in
the No-Typhoon experiment, implying that Typhoon
Songda might play a role in suppressing precipitationin the two areas. This indirect effect could be related
to the modification of the large-scale flow pattern by
the typhoon outer circulation. Compared to the change
in precipitation over Japan and its adjacent seas, thechanges in other regions are modest. Therefore, ourfollowing analysis will focus on the effects of Typhoon
Songda on remote precipitation over Japan and its adjacent seas.
Figure 10 shows the temporal evolution of the areaaveraged 6-hourly rainfall in a box covering 328–388N,
1358–1458E (rectangular box in Fig. 9) from the TRMM
TMI observations, and the CNTRL and No-Typhoon
experiments. Although the CNTRL experiment underestimated the total precipitation in the given region(Fig. 9), the underestimation occurred mainly during theperiod from 0600 UTC 4 September to 0000 UTC
5 September 2004, namely, the period with heavy rainfall (Fig. 10). Nevertheless, the reduction in the precipitation in the region due to the removal of the typhoon
is dramatic, indicating that Typhoon Songda contributed
over 90% to the precipitation over Japan and its adjacent seas during 1200 UTC 3 September–0000 UTC
5 September 2005. Note that the large-scale circulation
fields, in particular the short-wave trough north of 308Nin the mid- to lower troposphere in the CNTRL and NoTyphoon experiments, are similar (Figs. 11a and 11d),

NOVEMBER 2009

WANG ET AL.

3711

FIG. 11. Geopotential height (10 gpm contours with 40 gpm interval), specific humidity(shading, g kg21), and wind (barbs) fields at 850 hPa from (left) the control and (right)
No-Typhoon experiments, respectively, at (top) 0000 UTC 3 Sep, (middle) 0000 UTC 4 Sep,and (bottom) 0000 UTC 5 Sep 2004. One flag represents 50 kt and one full barb represents
10 kt.

indicating that the preconditioned midlatitude circulation system was not substantially altered by Typhoon
Songda. However, as we can see from Fig. 11, themoisture fields changed dramatically in the precipitation
region over the main islands of Japan and the adjacentseas south of Japan due to the removal of Typhoon
Songda. To investigate how the typhoon affected theremote precipitation in Japan, we performed a diagnostic
analysis on moisture transport and two additional numerical experiments (namely, No-Bogus and No-Terrain)
as discussed below.

a. Moisture transport
Typhoon Songda might significantly transport warm
and moist air to the north in its northeast quadrant due
to the southerly winds of its outer cyclonic circulationand the high moisture content in its circulation (leftpanels in Fig. 11). The southerly winds in the control
experiment were also enhanced due to the increased
pressure gradient between the typhoon and the subtropical high to the east (as suggested by Kawamura andOgasawara 2006), contributing to a moisture convergence3712

MONTHLY WEATHER REVIEWVOLUME 137

FIG. 12. Meridional moisture flux (103 g m22 h21) across 308N, (a),(b) at 1200 UTC 4 Sep
2004 (c),(d) averaged between 1800 UTC 2 Sep and 0000 UTC 5 Sep 2004. The control experiment is the source for (a) and (c) and the No-Typhoon experiment for (b) and (d). The
contour interval is 15 3 103 g m22 h21 and areas with values .15 3 103 g m22 h21 are shaded.
The area of 1358–1458E (for the boxes in Figs. 1 and 9) is marked with vertical dashed lines.

zone to the north in the preconditioned precipitation
region over Japan. The vertical cross section of the meridional moisture fluxes (rqyy, where qy is the water vapor mixing ratio and y the meridional wind speed) along
308N for a snapshot at 1200 UTC 4 September and for an
average between 1800 UTC 2 September and 0000 UTC
5 September are shown in Fig. 12. Comparing Fig. 12 (leftpanels) with Fig. 10, one can see that the deep layer ofnorthward moisture transport from the CNTRL experiment contributed significantly to the precipitation in thecentral region of Japan. In contrast, with the typhoon
removed in the No-Typhoon experiment, the southerly
winds to the west of the subtropical high were much
weaker than those in the CNTRL experiment, leading
to a large reduction of the northward moisture transport,
in particular, in the longitude band with strong precipitation in Japan.

The column-integrated moisture flux divergence isa good measure of whether the northward-transported
moisture contributed to the precipitation (Banacos andSchultz 2005) in the CNTRL experiment. At a given
time it can be expressed as
ðz

t

M5
0

$  (rqy V) dz,(2)

where V is the horizontal wind vector, z is the height,
and zt is the height of the model top.
Figure 13 shows an example of the column-integrated
moisture flux divergence at 1200 UTC 4 September,
namely, at the time close to the observed maximum
rainfall (Fig. 10), from the CNTRL and No-Typhoon
experiments, respectively. In the CNTRL experiment,
the areas with large moisture convergence are collocatedNOVEMBER 2009

WANG ET AL.

3713

FIG. 14. The temporal evolution of the moisture flux divergence(103 g m22 h21) averaged in the box given in Fig. 9 during 2100 UTC
2 Sep–1800 UTC 5 Sep 2004 from the control, No-Typhoon, NoTerrain, and No-Bogus experiments.

2004 from all four experiments. Comparing Figs. 14 and10, one can find that precipitation is well correlated with,but lags, the column-integrated moisture flux convergence by about 3–6 h. Consistent with the precipitation
shown in Fig. 10, the column-integrated moisture flux
convergence is quite small in the No-Typhoon experiment, indicating that Typhoon Songda not only contributed to the northward moisture transport but also
the moisture flux convergence in the precipitation region in Japan and the adjacent seas.

b. Effects of the bogus vortex
FIG. 13. Column-integrated moisture flux divergence (103g m22 h21) at 1200 UTC 4 Sep 2004 from the (a) control and(b) No-Typhoon experiments.

with the regions with heavy rainfall shown in Fig. 9a.
The large moisture flux convergence exists in the innercore region of the typhoon. Additionally, a northeast–
southwest-elongated moisture flux convergence belt is
associated with the rainfall over the main islands ofJapan. With the typhoon vortex removed in the NoTyphoon experiment, the moisture flux convergenceassociated with the typhoon disappeared as expected,
and the moisture flux convergence belt along Japan’s
main islands was largely suppressed, consistent with thelargely reduced precipitation in the region as shown in
Fig. 9b.
Figure 14 gives the temporal evolution of the areaaveraged column-integrated moisture flux divergencefrom 1800 UTC 2 September to 1800 UTC 5 September

The indirect effects of Typhoon Songda on remoteprecipitation in Japan were through its outer cyclonic
circulation, which significantly enhanced the northward
moisture transport and contributed to the moisture convergence in the remote precipitation region. Questions
arise as to whether the effects of Typhoon Songda on
remote precipitation over Japan were artificially overestimated due to the introduction of a bogus vortex usedin the CNTRL experiment. To address this issue, anexperiment (No-Bogus) initialized with the FNL analysis without the application of the bogus vortex scheme
was conducted. Although the FNL analysis cannot resolve the inner-core structure of the typhoon, it can depict
reasonably well the outer circulation of the typhoon since
it includes observations from a large-scale observational
network and the assimilation of satellite data.
Although the simulated storm in the No-Bogus experiment was much weaker than that in the CNTRL
experiment (Figs. 9a and 15a), the precipitation overJapan and its adjacent seas in the No-Bogus experiment

3714

MONTHLY WEATHER REVIEWVOLUME 137

warm, moist air from the southeast that could enhance
precipitation along the southern slope over the mountains of central Japan. This may be the case since precipitation over Japan was to the south (windward side)of the mountainous areas in central Japan (Fig. 1). To
examine this possibility, a No-Terrain experiment was
conducted in which the terrain height over the main island of Japan was set to 1 m if it is higher than 1 m sothat the orographic effects on precipitation over Japan
would be eliminated.
The total precipitation between 1800 UTC 2 September and 0000 UTC 5 September 2004 in the NoTerrain experiment (Fig. 15b) has a spatial distribution
very similar to that in the CNTRL experiment (Fig. 9a)but the precipitation amount was slightly reduced in theformer. This can also be clearly seen from the temporal
evolution of 6-hourly rainfall in the boxed area in Fig. 1
from the No-Terrain experiment (Fig. 10). There wasno significant difference in the moisture flux divergence
between the No-Terrain and CNTRL experiments (Fig. 14),indicating that the orographic lifting enhanced the precipitation over central Japan. For the case studied, theorographic forcing contributed to precipitation overJapan by about 10% and thus it is secondary to theoverall precipitation process compared to the effects of
Typhoon Songda.

6. Conclusions

FIG. 15. Same as in Fig. 9, but for the (a) No-Bogus and(b) No-Terrain experiments.

was generally similar to that in the CNTRL experimentin both the spatial distribution in the time mean (Fig. 15a)and the temporal evolution of the area-averaged rainfall(Fig. 10). This is consistent with the similar moisture flux
convergence in the No-Bogus and CNTRL experiments
as shown in Fig. 14. The overall feature of the indirect
effects of the model typhoon on the remote precipitation
over Japan and its adjacent seas is only slightly altered
even though no bogus vortex is used in the initial conditions, indicating that the initial vortex bogus mainly
improved the simulation of the inner-core intensity andhad little effect on the outer circulation of the storm inthis case.

c. Topographic effects
Another seemingly important factor that might affectthe precipitation over Japan is the orographic lifting of

The Advanced Research WRF model has been used toinvestigate the possible effects of Typhoon Songda (2004)
over the western North Pacific on precipitation in a remote area to its north over central Japan and the involved
physical mechanisms. In this case, when Typhoon Songda
was located southeast of Okinawa, heavy rainfall occurred far to the north over southern central Japan and its
adjacent seas. In a control experiment with an initialvortex enhancement algorithm, the model reasonably
reproduced the major features of Typhoon Songda andthe precipitation over Japan. To explicitly demonstrate
the contribution of Typhoon Songda to the remote precipitation in Japan, a no-typhoon experiment was performed in which the typhoon vortex was removed fromthe NCEP FNL analysis fields that provided both theinitial and lateral boundary conditions to the WRF-ARW
model. As a result of the removal of the model typhoon,
precipitation over central Japan, and the adjacent seas
associated with preconditional cyclonic shear, the convergence zone was largely suppressed, indicating thedominant control of Typhoon Songda on the remoteprecipitation for the case studied.
The major process involved in the remote effects of
Typhoon Songda is through the enhanced northward

NOVEMBER 2009

WANG ET AL.

moisture transport into the preconditioned precipitation
region by its outer circulation. As a result, the presenceof Typhoon Songda was critical to the observed heavy
rainfall in central Japan and the adjacent seas eventhough the typhoon was more than 1200 km to the south
over the western North Pacific. In another experimentwith the terrain removed over Japan, the overall precipitation pattern was very similar to that from thecontrol simulation except for a 10% underestimation of
the precipitation, indicating that the local orographic
forcing played a secondary role in the case studied.
Acknowledgments. The authors are grateful to Prof.
David Schultz and Dr. Ron McTaggart-Cowan for theircritical and constructive comments, which helped improve the quality of this work. This study has been
supported by NSF Grants ATM-0427128 and ATM0754029, as well as ONR Grant 000-14-06-10303. Additional support has been provided by JAMSTEC ofJapan, NASA, and NOAA through their sponsorships
in the International Pacific Research Center at the
School of Ocean and Earth Science and Technology at
the University of Hawaii.
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