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Numerical Study of the Local Downslope Wind “Hirodo-Kaze” in JapanHIRONORI FUDEYASU,* TSUNEO KUWAGATA,⫹ YUKITAKA OHASHI,# SHIN-ICHI SUZUKI,@YASUTOMO KIYOHARA,& AND YU HOZUMI**,⫹⫹* Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan⫹ National Institute for Agro-Environmental Sciences, Tsukuba, Japan# Okayama University of Science, Okayama, Japan@ National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan& Graduate School of Science and Technology, Kobe University, Kobe, Japan** Acid Deposition and Oxidant Research Center, Nishi-ku, Japan(Manuscript received 10 October 2006, in final form 16 April 2007)
ABSTRACTThe “Hirodo-kaze,” a local strong wind accompanying the downslope winds in Japan, is examined usinga mesoscale numerical model. The model successfully reproduces the major features of the observed
Hirodo-kaze that occurred in association with Typhoon Pabuk. During the Hirodo-kaze, the severe downslope winds in the transitional flow develop in the lower troposphere below the mean-state critical layer.
The Hirodo-kaze is closely linked to the strong wind region accompanying the severe downslope winds.
After the cessation of the Hirodo-kaze, distinct mountain waves dominate in the lower troposphere where
the Scorer parameter l2 decreases with height. The region of strong wind retreats windward as the Hirodokaze ceases. Temporal changes in the characteristics of mountain waves in the lee of Mt. Nagi are primarily
attributed to the changes in the large-scale environmental winds due to the movement of the intense
cyclone.
Environmental conditions favorable for the occurrence of the Hirodo-kaze include strong northerlies in
the lower troposphere overlain by southerlies in the middle troposphere. The intense cyclone that moves
over the sea southwest of the Kii peninsula creates favorable environmental conditions that support theoccurrence of the Hirodo-kaze.

1. Introduction
The “Hirodo-kaze” is one of the well-known strong
local winds in Japan. Hirodo-kaze occurs at the southern base of Mt. Nagi (1240 m) in the northeastern
Okayama Prefecture, Japan (see Fig. 1), when thelower-troposphere synoptic wind is strong northerly inassociation with an intense cyclone. In a typical case of
the Hirodo-kaze, strong northerlies with a maximum
wind speed Vmax (10-min average) exceeding 15 m s⫺1
persist for several hours and cause severe damage locally in areas south of Mt. Nagi. In late June 1998, for⫹⫹ Deceased.

Corresponding author address: H. Fudeyasu, Japan Agency for
Marine-Earth Science and Technology, Institute of Observational
Research for Global Change, JAMSTEC Yokosuka headquarters, 2-15, Natsushima-Cho, Yokosuka-city, Kanagawa 237-0061,
Japan.
E-mail: fudeyasuh@jamstec.go.jp
DOI: 10.1175/2007MWR2049.1© 2008 American Meteorological Society

example, a Hirodo-kaze with Vmax of 25 m s⫺1 occurredin association with Typhoon Peter and caused damagesof about $1.0 million (U.S. dollars).
Previous studies considered that Hirodo-kaze winds
are related to severe downslope winds on the southernslope of Mt. Nagi that are induced by strong lowertropospheric northerlies accompanying an intense cyclone. Yoshino (1986) suggested that the orographic
configuration of Mt. Nagi, a steep southern slope and a
gradual northern slope (see Fig. 1), is favorable for theoccurrence of a downslope wind on the southern slopeof Mt. Nagi. Sahashi (1988) observed a roll cloud parallel to the ridgeline in the south of Mt. Nagi at the day
when the Hirodo-kaze occurred in association with Typhoon Freda (1987). He speculated that severe downslope winds of the Hirodo-kaze are associated with alarge-amplitude mountain wave with a rotor cloud(Queney et al. 1960; Vosper 2004, 2006). Nakamura etal. (2002) observed two cases of Hirodo-kaze at the
crest and southern base of Mt. Nagi. Observational evidence suggested that there is no significant change in28

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FIG. 1. Model domains of (left) numerical simulations and (right) terrain (contours) around Mt. Nagi. Contour
interval is 100 m. Altitudes of 400–800 m are lightly shaded, while altitudes above 800 m are heavily shaded.
Triangles show the meteorological stations recording surface wind data shown in Fig. 3.

rainfall and surface temperature accompanied by the
Hirodo-kaze, indicating different situation from a boralike windstorm (e.g., Reed 1981; Smith 1987) and foehn
phenomena (e.g., Ikawa and Nagasawa 1989; Furger et
al. 2001). Another interesting observational fact is thatthe Hirodo-kaze ceased, although the northerly at the
crest of Mt. Nagi remained strong. The reason for thisinconsistency at the cessation of the Hirodo-kaze is notclear. Sahashi (1993) statistically investigated the response of a Hirodo-kaze to the location of a cyclone. A
Hirodo-kaze always occurs as an intense cyclone moves
northeastward over the sea southwest (offshore) of theKii peninsula (see Fig. 2). The Hirodo-kaze starts to
weaken as the cyclone makes landfall on the Kii peninsula. This relationship between the behavior of Hirodo-kaze and position of an intense cyclone remains
unclear.
Severe downslope winds have been observed in
mountainous area throughout the world and have beenstudied using several approaches. Observational studies(e.g., Lilly and Zipser 1972; Doyle and Smith 2003)
have described the flow structure of severe downslope
winds associated with large-amplitude mountain waves
in the lee of mountains. Numerical experiments (e.g.,
Klemp and Lilly 1975; Clark and Peltier 1977, 1984;
Peltier and Clark 1979; Zängl 2002) have suggested thatlarge-amplitude mountain waves are highly nonlinear
phenomena, which makes it difficult to apply a linear
theory. These studies (e.g., Peltier and Clark 1979; Dur-ran 1986a) revealed nonlinear effects, such as criticallayer reflection mechanisms at a critical layer (layer ofzero wind or flow reversal) that acts as a boundary toreflect upward propagating waves back toward the

FIG. 2. Track (broken line) of Pabuk from best-track data, compared with simulated Pabuk’s track (solid line) derived from domain 3 between 1800 UTC 20 Aug and 0000 UTC 22 Aug 2001.
Closed circles represent positions of Pabuk at 0000 and 1200 UTC,
while open circles represent positions at 0600 and 1800 UTC.

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ground, producing a resonant wave in the lee of themountain. Smith (1985) solved Long’s (1954) equation for nonlinear flow beneath a well-mixed stagnant
layer induced by a breaking wave. He obtained atransitional flow, manifested as a severe downslope
windstorm, as the mountain flow transitions froman upstream subcritical flow to a downstream supercritical flow. His prediction agreed with numericalresults (e.g., Clark and Peltier 1984) and was essential
to the application of hydraulic theory (Saito 1992).
Results from realistic simulations using a nonhydrostatic mesoscale model have described the details of
severe downslope wind (e.g., Saito 1993, 1994; Jackson and Steyn 1994; Doyle et al. 2000). Saito and Ikawa(1991) and Saito (1993, 1994) studied the strong local
wind “Yamaji-kaze” in Japan. Their realistic simulations accurately reproduced the severe downslope
wind and showed that the Yamaji-kaze was associated with a large-amplitude mountain wave and aninternal hydraulic jump. Saito (1993) also discussed
the effects of three-dimensional orography on the
Yamaji-kaze. Recently, in several case studies, aboundary layer response to wave forcing has been examined by numerical and observational studies (Jiang
and Doyle 2005; Jiang et al. 2006, 2007; Vosper et al.
2006).
The studies noted above have described the mechanisms that force severe downslope winds and largeamplitude mountain waves. However, descriptions of
the effect of temporal changes in the large-scale environment on the severe downslope wind are scarce. For
the case of Hirodo-kaze, a controversial issue includes
the response of a Hirodo-kaze to the location of an
intense cyclone (Sahashi 1993). Temporal changes in
the large-scale environment due to the movement of an
intense cyclone are likely a key factor controlling the
behavior of a Hirodo-kaze.
The purpose of the present study is to examine theforcing mechanisms during a Hirodo-kaze and the effects of an intense cyclone on the occurrence of a Hirodo-kaze. We used the result of a high-resolution numerical model to invest a Hirodo-kaze that occurred on
21 August 2001 in association with Typhoon Pabuk. In
addition, we statistically investigated the effects of theintense cyclone on the occurrence of the Hirodo-kaze.
Section 2 of this paper describes observational data andthe numerical model used in the study. Section 3 compares structures of the Hirodo-kaze as simulated by thenumerical model with observations. Mechanisms forcing the Hirodo-kaze and the dynamic effects of the intense cyclones are discussed in section 4. Finally, results
are summarized in section 5.

29

2. Data and model configurations
a. Data
Data from the Automated Meteorological Data Acquisition System (AMeDAS) were used to identify thespatial and temporal changes in the surface structuresof the Hirodo-kaze. The AMeDAS data include surface
winds, temperature, and rainfall amounts averaged at a
10-min interval. These data have been available since1995. Stations A–D, indicated on the right-hand side ofFig. 1, are AMeDAS stations Nagi, Tsuyama, Tottori,
and Wake, respectively. Large-scale atmospheric conditions over Mt. Nagi were investigated using the 40-yr
European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data. These
data have a resolution of 2.5° at 6-h intervals.
The plotted track of Pabuk was derived from thebest-track archives of the Regional Specialized Meteorological Centers (RSMC) Tokyo–Typhoon Center.
The archived dataset includes a cyclone’s name, positions, surface pressure, and maximum wind speeds,
generally recorded at 6-h intervals.

b. Model configuration
Numerical simulation used the fifth-generation Pennsylvania State University (PSU)–National Center for
Atmospheric Research (NCAR) Mesoscale Model(MM5, version 3.5). MM5 is a nonhydrostatic model(Dudhia 1993; Grell et al. 1995) that solves nonlinear
primitive equations on Cartesian coordinates in thehorizontal and terrain-following sigma coordinates inthe vertical.
The numerical simulation was run in the three domains shown in Fig. 1. The outermost domain (D1),centered at 34°N, 132°E, had 200 ⫻ 180 grid points witha horizontal resolution of 9 km. The intermediate domain (D2) had 301 ⫻ 301 points with a horizontal resolution of 3 km, and the innermost domain (D3) had
700 ⫻ 700 points with a horizontal resolution of 1 km.
All three domains included 30 vertical layers with fineresolution near the surface, and the top of each model
domain was 100 hPa. The model run on D1 produced
initial and boundary conditions for the inner domains.
The model run on D3 explicitly resolved the fine structure of the Hirodo-kaze.
A simple ice scheme (Dudhia 1989) was used in themodel for cloud microphysics. Cloud water and rainwater cooler than 0°C were treated as ice and snow, respectively. The model also included a turbulence parameterization in the planetary boundary layer (Hongand Pan 1996), a five-layer soil model, and a cloud–radiation interaction scheme (Dudhia 1989, 1993; Grell

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et al. 1995). A convective parameterization scheme was
not used in this simulation.
The initial and lateral boundary conditions for simulation on D1 were derived from the Regional Analysis(RANAL) data and daily mean sea surface temperature (SST) produced by the Japan Meteorological
Agency (JMA). The dataset includes the threedimensional analyses over East Asia. The data have ahorizontal resolution of 20 km on 11 vertical layers at
6-h interval.
Simulation on D1 was run for 36 h, starting at 1200
UTC 20 August 2001, after a 12-h model integrationfrom 0000 UTC 20 August using the RANAL that allowed initial spinup. Continuous dynamic assimilation
occurred during the initial spinup by adding forcing
functions to the governing model equation, which
gradually “nudged” the wind components of the model
state toward the RANAL (Grell et al. 1995). Lateralboundary conditions used relaxation (Anthes et al.
1987), in which the model-predicted values were relaxed to those estimated from the large-scale analysis.
Simulations on D2 and D3 began at 1200 UTC 20 August 2001 and ran for 36 h using initial and boundary
conditions interpolated from the results of the outerdomain simulation.

3. Resultsa. Features of the observed Pabuk and
Hirodo-kaze
Typhoon Pabuk strengthened to a typhoon (maximum 10-min sustained wind speeds in excess of 17.2
m s⫺1, or 34 kt) from a tropical depression near the
Marshall Islands on 14 August 2001. It recurved overthe sea to the south of Japan on 20 August and subsequently approached the Kii peninsula (Fig. 2). Pabuk
made landfall on the Kii peninsula at 1200 UTC 21
August with a central pressure of 970 hPa and maximum wind speeds of 30 m s⫺1.
At 0600 UTC 21 August 2001, when Pabuk was located over the sea about 80 km southwest of the Kii
peninsula, strong surface winds related to the cyclonic
circulation of Pabuk were observed in the Shikoku andKinki districts (Fig. 3). Relatively weak northerlies prevailed in the Chugoku district far from Pabuk, but astrong northerly of 16 m s⫺1 was observed at station A
located in the lee of Mt. Nagi, about 5 km south of thecrest of Mt. Nagi. This strong northerly at station A wasa Hirodo-kaze. Winds as strong as the Hirodo-kazewere not observed at stations B, C, and D. The Hirodokaze occurred only locally just south of Mt. Nagi.
Figure 4 shows time series of surface wind speed and

VOLUME 136

FIG. 3. Surface wind (barbs) and terrain (contours) at 0600 UTC
21 Aug 2001 derived from the AMeDAS data. A full wind barb
represents 5 m s⫺1. Contour interval is 300 m. The cyclone symbol
represents the position of Pabuk.

direction observed at each of the four stations. The
Hirodo-kaze was observed only at station A: windspeeds increased between 0100 and 0600 UTC 21 August and then decreased between 1300 and 1800 UTC.
Strong northerlies exceeded 15 m s⫺1 for about 7 h.
Relatively weak northerlies were observed at station B,
which was near station A, as the Hirodo-kaze occurred.
At that time, weak northerlies were observed at stations C and D, which were windward and leeward of
station A, respectively. Southerlies occurred at station
A between 1700 and 2100 UTC after the Hirodo-kazeceased. Southerlies were also observed at station B between 0900 and 1500 UTC. Strong northerlies were recorded at station C between 1500 and 2200 UTC after
the Hirodo-kaze ceased at station A. Counterclockwisechanges in wind directions (veering) occurred at stations C and D as Pabuk passed, because these stations
were to the left of the moving direction of Pabuk.
Counterclockwise changes in wind direction at stations
A and B were not distinct. No significant change insurface temperature was recorded at station A as the
Hirodo-kaze occurred. Weak rain fell at station A during the Hirodo-kaze (not shown). Observational results
confirmed that the Hirodo-kaze accompanying Pabuk
was a typical Hirodo-kaze.

b. Validation of the simulated Pabuk and
Hirodo-kaze
We used observations to validate the simulated results of the Pabuk and the Hirodo-kaze. Figure 2 showsJANUARY 2008

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FIG. 4. Time series of surface wind speed (lines) and direction (dots) recorded at meteorological stations during
the passage of Pabuk on 20–21 Aug 2001. Station locations are indicated in Fig. 1.

the track of the simulated Pabuk in D3, the run with thefinest resolution. The position of the Pabuk’s center
was determined from the area of minimum sea levelpressure in the simulated Pabuk and was in good agreement with the observed track of Pabuk. Central pressures of the simulated Pabuk were about 970 hPa on 21
August. Observed pressures of Pabuk at this time
ranged from 965 to 970 hPa. The MM5 reproduced thetrack and intensity of Pabuk.
Figure 5 shows surface winds at 0600 UTC 21 August
2001 as simulated by D3. A simulated cyclonic circulation prevailed over the Shikoku and Kinki districts.
Relatively weak northerlies appeared over the
Chugoku district, but simulated northerlies exceeding
15 m s⫺1 appeared near Mt. Nagi. The region of strong
winds was 80 km long and 20–30 km wide and extended
southwest to the south of Mt. Nagi. Wind exceeded 20
m s⫺1 at station A; simultaneously, winds were less than
10 m s⫺1 at stations B and D. The limited region ofstrong simulated northerlies south of Mt. Nagi was consistent with observational results (Fig. 3).
Figure 6 shows a time series of simulated surface
wind at 10-min intervals at each station. The following
simulation results compared favorably with the observations (Fig. 4). The Hirodo-kaze was simulated at station A. Wind speeds increased to near 20 m s⫺1 between 0100 and 0700 UTC and then decreased between
1100 and 1500 UTC. Wind speeds of the simulated Hi-

rodo-kaze were slightly higher than the observed Hirodo-kaze. However, temporal changes in the simulated Hirodo-kaze resembled the observed changes.
Relatively weak northerlies appeared at station B as
the Hirodo-kaze occurred at station A. Northerlies

FIG. 5. Simulated surface wind direction and wind speed (barbs)
in the large- and local-scale area derived from the simulation indomain 3 at 0600 UTC 21 Aug 2001. A full wind barb represents
5 m s⫺1. Wind speeds exceeding 15 and 20 m s⫺1 are heavily andlightly shaded, respectively.

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FIG. 6. As in Fig. 4, but for simulated surface wind speed (solid lines), observed wind speed (broken lines), andsimulated wind direction (dots).

weaker than the Hirodo-kaze were simulated at stations C and D as the Hirodo-kaze occurred at station A.
However, the winds at stations C and D were stronger
than observed. Southerlies were simulated at station A
between 1600 and 1900 UTC after the Hirodo-kazeceased. Southerlies were simulated at station B between 0900 and 1300 UTC. Simulated northerlies persisted at station C after the Hirodo-kaze ceased. Counterclockwise changes in simulated wind direction withtime were clear at stations C and D, but less distinct at
stations A and B. Surface winds simulated at each station agreed well with the observations.
The simulation reproduced major features of the observed Hirodo-kaze. The simulated data could therefore be used with confidence to investigate the verticalstructure of the Hirodo-kaze.

c. Structure of the simulated Hirodo-kaze
The structures of the simulated Hirodo-kaze are examined in this section. Figures 7–9 show vertical crosssections of simulated meridional wind, vertical velocity,and potential temperature across station A and Mt.
Nagi (see Fig. 5) at 0200, 0800, and 1600 UTC 21 August 2001.
At 0200 UTC, several hours before the peak of the
Hirodo-kaze, southerlies were present in the upper andmiddle troposphere and northerlies were present in thelower troposphere (Fig. 7a). Wind speeds at the crest of
Mt. Nagi were 10 m s⫺1. Ascent occurred along thenorthern (windward) slope of Mt. Nagi and weak descent occurred from 650 hPa down to the surface along
the southern lee slope of Mt. Nagi, indicating a downslope wind (Fig. 7b). The region of descent extended far
to the lee of Mt. Nagi, but mountain waves were notclearly present in the lee of Mt. Nagi. Thus, althoughisentropes shifted downward in the lee of Mt. Nagi, a
subsequent upward shift in the isentropes to the lee
linked to mountain waves was not conspicuous (Fig.
7c).
At 0800 UTC, as the Hirodo-kaze occurred at stationA, upper- and middle-tropospheric southerlies persisted as lower-tropospheric northerlies became quite
strong (Fig. 8a): wind speeds at the crest of Mt. Nagi
exceeded 30 m s⫺1. Downslope winds developed along
the lee slope of Mt. Nagi (Fig. 8b). Maximum descentexceeded 6 m s⫺1. Isentropes shift downward in the leeof Mt. Nagi until they reached the lower troposphere,
and they extended leeward in the lower troposphere
over the base of Mt. Nagi (Fig. 8c). The region of strong
winds exceeding 30 m s⫺1 extended leeward far from
Mt. Nagi. Weak ascent at 35.0°N was related to the
topography. These simulation results suggested thatduring a Hirodo-kaze, strong downslope winds develop
along the lee slope of Mt. Nagi and the strong horizon-

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FIG. 7. Cross section of simulated (a) meridional
wind speed, (b) vertical velocity, and (c) potential temperature along 134.18°E for 0200 UTC 21 Aug 2001.
Cross-section location is marked as line NS in Fig. 5.
Contour intervals are (a) 3 m s⫺1, (b) 1.0 m s⫺1, and (c)3 K, respectively. Solid lines in (a) and (b) denote 0m s⫺1, while the broken line denotes northerlies in (a).
Negative regions are shaded in (b).

FIG. 8. As in Fig. 7, but for 0800 UTC 21 Aug 2001.

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FIG. 9. As in Fig. 7, but for 1600 UTC 21 Aug 2001.

tal winds accompanying the severe downslope winds
extend far to the lee of Mt. Nagi.
By 1600 UTC, the Hirodo-kaze had ceased at stationA, and middle-tropospheric southerlies changed tonortherlies. At the same time, lower-troposphericnortherlies remained strong (Fig. 9a): wind speeds atthe crest of Mt. Nagi remained above 30 m s⫺1. Strong
northerlies in the lower troposphere allowed severe
downslope winds to persist along the lee slope of Mt.
Nagi (Fig. 9b). These severe downslope winds were accompanied by a wavelike vertical circulation in the leeof Mt. Nagi. Mountain waves were also recognized by
the rapid downward and subsequent upward shifts in
the isentropes (Fig. 9c). The mountain waves dominated in the layer between 600 hPa and the surface.
Subsequent ascent at 35.1°N weakened the horizontal
winds, forcing a windward retreat of the strong windregion that extended to the lee of Mt. Nagi. These results suggest that after the cessation of a Hirodo-kaze,
distinct mountain waves dominate in the lee of Mt.
Nagi.
Characteristics of the mountain wave in the lee of Mt.
Nagi changed at about 1200 UTC. This temporalchange forced the behavior of the Hirodo-kaze. Figure
10 shows time–latitude sections of vertical velocity at
700 hPa and surface meridional wind. Downslope

winds, manifested as a strong descent, appeared at35.15°N between 0400 and 2100 UTC (Fig. 10a). The
region of northerly surface winds exceeding 15 m s⫺1
extended southward (leeward) from 0400 to 0900 UTC(Fig. 10b), and subsequently retreated northward(windward) from 0900 to 1200 UTC. Between 0600 and
1200 UTC, the region of strong winds overlapped thelocation of station A at 35.11°N. This was the peak of
the Hirodo-kaze.
After 1200 UTC, steady ascent at 35.09° and 34.98°N
and steady descent at 35.03°N persisted in the lee of thedownslope winds, indicating distinct mountain waves in
the lee of Mt. Nagi (Fig. 10a). Strong surface winds
were limited to the region between the downslope
winds over Mt. Nagi and subsequent ascent at 35.09°N(Fig. 10b). The subsequent ascent was located windward of station A during this period; therefore, surface
wind speeds at station A were weak. This weakness
corresponded to the cessation of the Hirodo-kaze.
Southerly surface winds appeared in the lee of the subsequent ascent; these winds were consistent with thesoutherlies at station A after the Hirodo-kaze ceased(Figs. 4 and 6). In the shallow layer below 900 hPa overstation A (Fig. 9a), there is the reversal of the horizontal wind with height between the ascent and subsequent
descent of a lee wave, characteristic of mountain-wave-

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35

FIG. 10. Time–latitude section of (a) simulated vertical velocity at 700 hPa (contour interval is 1 m s⫺1 anddescent is shaded) and (b) meridional surface wind speed (contour interval is 5 m s⫺1, speeds greater than 15 m s⫺1
are lightly shaded, and southerlies are heavily shaded) along 134.18°E from 0000 to 2100 UTC 21 Aug 2001.
Cross-section location is marked as line NS in Fig. 5. The solid shaded region at the bottom of both figures denotes
the terrain.

induced rotors (e.g., Doyle and Durran 2002), so thatsoutherly surface winds are regarded as the reversal
flow at the base of the mountain-wave-induced rotor.
Changes in the characteristics of mountain waves in
the lee of Mt. Nagi are primarily attributed to temporalchanges in the large-scale environmental winds due tothe movement of Pabuk. Figure 11 shows time series ofseveral quantities averaged in the region surrounding
station A (35.0°–35.5°N, 134.0°–134.5°E). Until 0300
UTC, as Pabuk moved over the sea far from the Kii
peninsula (Fig. 2), east-northeasterlies prevailed in thelower troposphere and east-southeasterlies prevailed in
the upper and middle troposphere (Fig. 11a). As Pabuk
approached the Kii peninsula during 0300 and 0900
UTC, lower-tropospheric northerlies strengthened(Fig. 11b). The acceleration of the northerlies forced by
the approach of Pabuk caused a leeward extension of

the strong wind region (Fig. 10b). On the other hand,
middle-tropospheric easterlies continued until 1200
UTC (Fig. 11a), as part of the cyclonic circulation in thenorthern part of Pabuk (not shown). At 0900 UTC, themiddle-tropospheric easterlies started to change tonortherlies as Pabuk moved northeast away from Mt.
Nagi. Changes in middle-tropospheric winds fromsoutherly to northerly during 0900 and 1200 UTC (Fig.
11b) are related to the changes in the characteristics
of mountain waves in the lee of Mt. Nagi. Large-scale
environmental winds during the occurrence of the
Hirodo-kaze were characterized by strong lowertropospheric northerlies and middle-troposphericsoutherlies.
Throughout the simulation, a continuous stable layer
prevailed in the middle and lower troposphere (notshown). This stable layer allowed for the propagation

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FIG. 11. Time series of simulated (a) horizontal wind, (b) meridional wind speed, (c) Scorer parameter l2, and (d)first term of Scorer parameter l2 over the averaged region surrounding station A (35.0°–35.5°N, 134.0°–134.5°E)
from 0000 to 2100 UTC 21 Aug 2001. A full wind barb in (a) represents 5 m s⫺1. Contour intervals are (b) 3 m s⫺1
and (c), (d) ⫻10⫺7 m⫺2. The solid line in (b) represents 0.0 m s⫺1. Regions higher than 1.0 ⫻ 10⫺4 m⫺2 in (c), (d)are shaded.

of gravity waves over the region. There is, however, nodrastic change in the atmospheric static stability during
the occurrence of the Hirodo-kaze. Therefore the behavior of the Hirodo-kaze is not affected by thechanges in atmospheric static stability.

4. Discussion
Our simulated results reproduced the behavior of the
Hirodo-kaze as a temporal change in large-scale environmental winds. The simulation results include the following. Severe downslope winds developed along thelee slope of Mt. Nagi and a region of strong winds
accompanying the severe downslope winds extended
far to the lee of Mt. Nagi as the Hirodo-kaze occurred(Fig. 8). Distinct mountain waves dominated in the leeof Mt. Nagi after the Hirodo-kaze ceased (Fig. 9).
Durran and Klemp (1982) used a numerical simulation to investigate a case in which the Scorer parameterl2 decreased with height. The results of their simulations show that a trapped lee wave was caused by partial downward reflection of vertically propagatingwaves, producing large-amplitude mountain waves in
the lee of the mountain. A similar situation arose in ourstudy. Figure 11c shows the Scorer parameter l2 aver-aged in the region surrounding station A. The Scorer
parameter l2 is defined as
l2 ⫽

N2
U

2

⫺

1 d 2U
U dz2

,

共1兲

where N is the Brunt–Väisälä frequency and U is themeridional wind speed. Between 1200 and 2100 UTC,the Scorer parameter l2 decreased with height below
the 600-hPa level, mainly because of an increase in thecurvature of the meridional wind speed rather than because of the first term of the Scorer parameter l2 (Fig.
11d). During this time, distinct mountain waves occurred in the layers below the 600-hPa level in the lee of
Mt. Nagi (Fig. 9). The simulated mountain waves hadno tilt, a characteristic of trapped lee waves (Durran
1986b). The distinct mountain waves in the lee of Mt.
Nagi after 1200 UTC (Fig. 9) were consistent withtrapped lee waves for which the Scorer parameter l2
decreased with height.
Smith’s (1985) theory predicts that transitional flowcauses strong downslope winds beneath a well-mixed
stagnant layer. According to this theory, conditions
above the stagnant layer are such that mountain waves
are negligible above the stagnant layer. The layer above

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37

FIG. 12. Frequency distribution of 6-hourly (a) meridional wind speeds and (b) wind directions in the lower
troposphere (925 hPa) and middle troposphere (average from 500 to 400 hPa) at grid point 35°N, 135°E. Wind data
were obtained from the reanalysis data (ERA-40) between 1989 and 2001. Frequency is counted at each (a) 1 m s⫺1
and (b) 10° intervals. The contour interval is 5 days (13 yr)⫺1. Regions having ⬎1 day (13 yr)⫺1 are shaded. Closed
circles represent Hirodo-kaze events.

the stagnant layer is therefore consistent with thepresence of a critical layer. During the Hirodo-kaze
between 0400 and 1200 UTC, a layer in which the
Scorer parameter l2 is infinite existed between 600 and
300 hPa (Fig. 11c)—this layer is affected by the firstterm of the Scorer parameter (Fig. 11d) and corresponds to the critical layer. This critical layer is present
in environmental conditions characterized by middletropospheric southerlies and lower-tropospheric northerlies, that is, mean-state critical layer (e.g., Clark andPeltier 1984; Tomine 1984, 1987; Kimura 1992). Below
the mean-state critical layer, isentropes began to descend over the crest of Mt. Nagi and continued descending until the flow reached the base of Mt. Nagi
(Fig. 8), resembling transitional flow (Smith 1985). The
downslope winds that developed were therefore comparable to the severe downslope winds in transitional
flow.
Durran and Klemp (1987) studied the relationshipbetween transitional flow and critical-layer height using
results from numerical experiments in which the heightof a preexisting critical layer in the mean flow and themountain both varied. Behavior of the transitional flow
depended on the height of the critical layer. For thecase of Hirodo-kaze, between 0900 and 1200 UTC, theretreat of the region of strong surface winds (Fig. 10b)
was related to the weakening transitional flow, which
was a response to the change in critical-layer heightfrom 5 to 10 km (Fig. 11c). After 1200 UTC, the criticallayer height was above 10 km, and transitional flow was

not clear. Instead, mountain waves related to trapped
lee waves dominated, for which the Scorer parameter l2
decreased with height (Fig. 11c).
The region of strong surface winds extended windward between 0400 and 0900 UTC, the time when the
Hirodo-kaze developed (Fig. 10b). The leeward movement of the leading edge of the region resulted from anincrease in the lower-troposphere northerlies caused by
the approaching Pabuk. In the case of the Yamaji-kaze,
the leading edge of the region of strong surface winds,“the Yamaji-kaze front,” also moves leeward as theenvironmental wind increases (Saito and Ikawa 1991).
This leeward movement of the region of strong winds
caused by increases in environmental winds is similar
for both Yamaji-kaze and Hirodo-kaze.
Environmental conditions favorable for the occurrence of a Hirodo-kaze are characterized by strong
northerlies in the lower troposphere beneath southerlies in the middle troposphere. Favorable environmental conditions such as these occur over Mt. Nagi when
an intense cyclone exists over the sea southwest of theKii peninsula. Figure 12a presents the frequency ofsuch a configuration of lower- and middle-tropospheremeridional winds at the grid point (35°N, 135°E) near
Mt. Nagi (35.1°N, 134.2°E) between 1989 and 2001.
Data for this plot were obtained from the reanalysis
data (ERA-40). According to the several reports of the
Okayama meteorological station, 12 Hirodo-kaze
events occurred during this period (Nakamura et al.
2002). During these events, strong lower-tropospheric

38

MONTHLY WEATHER REVIEW

northerlies exceeding 10 m s ⫺1 were overlain bymiddle-tropospheric southerlies. The configuration described above occurred only during Hirodo-kaze eventsin the 13 yr. Strong lower-tropospheric northerlies
sometimes occurred in winter, but concomitant overlying middle-tropospheric southerlies were not present.
Strong lower-tropospheric northerlies were seldom recorded in summer except within an intense cyclone.
Favorable environmental conditions occurred only as
intense cyclones passed offshore near the Kii peninsula.
Thus, intense cyclones that move over the sea southwest of the Kii peninsula produce the favorable environmental conditions that support the occurrence ofHirodo-kaze.
It seems that the shape of mountain range north of
Mt. Nagi (see Fig. 1) is favorable for the occurrenceof a downslope wind in the case of lower-troposphericnorth-northeasterlies rather than north-northwesterlies.
Figure 12b shows the frequency distribution of winddirections taken from the same date as in Fig. 12a. The
difference in the occurrence of Hirodo-kaze events between north-northeasterly and north-northwesterly
winds at the 925-hPa level is not clear. It is noted thatthe results using wind directions at other lower levels
are essentially the same as those using this level. It isdifficult to discuss statistically the response of Hirodokaze to wind direction as affected by the topography,
using ERA-40 with coarse spatial and temporal resolution. Such an examination on an effect of threedimensional topography to the response of Hirodokaze behavior could be considered for future study.

5. Summary
A mesoscale numerical model was used to simulate a
Hirodo-kaze event that occurred in association with Typhoon Pabuk. The model successfully reproduced the
major features of the observed Hirodo-kaze. Figure 13
illustrates the mechanisms controlling the developmentof the Hirodo-kaze. During a Hirodo-kaze, a meanstate critical layer exists in the middle troposphere (Fig.
13a). Severe downslope winds in the transitional flow
develop along the lee slope of Mt. Nagi. The Hirodokaze is closely linked to the strong wind region accompanying the severe downslope winds.
After the cessation of the Hirodo-kaze, distinct
mountain waves dominate in the lower troposphere
where the Scorer parameter l2 decreases with height(Fig. 13b). The region of strong winds retreats windward as the Hirodo-kaze ceases, although winds at the
crest of Mt. Nagi remained strong. The southerly surface winds, a reversal flow at the base of the mountainwave-induced rotor, appear at station A. Temporalchanges in the characteristics of mountain waves in the

VOLUME 136

FIG. 13. Schematic cross section of mountain waves throughstation A and Mt. Nagi for periods of (a) occurrence and (b)cessation of the Hirodo-kaze. Solid arrows indicate streamlines,
while open arrows indicate the environmental winds in the middle
and lower troposphere. The stippled area indicates a region ofstrong winds.

lee of Mt. Nagi are primarily attributed to changes in
the large-scale environmental winds due to the movement of the intense cyclone.
Environmental conditions favorable for the occurrence of the Hirodo-kaze include strong northerlies in
the lower troposphere overlain by southerlies in the
middle troposphere. These favorable conditions occur
only as an intense cyclone moves over the sea southwest of the Kii peninsula. The intense cyclone thatmoves over the sea southwest of the Kii peninsula thus
creates favorable environmental conditions that support the occurrence of the Hirodo-kaze.
The actual structure of a Hirodo-kaze may be more
complicated. We used only surface observations to detected Hirodo-kaze. High-resolution radar data are required to detect and characterize mesoscale atmospheric structures accompanying the Hirodo-kaze. Suchdata will also enhance our ability to predict Hirodokaze and lead to a better understanding of strong localscale winds.
Acknowledgments. We thank Prof. O. Tsukamoto
and K. Moritou for their thoughtful comments and forJANUARY 2008

FUDEYASU ET AL.

providing observational data. Thanks are also extended
to the staff and students at Okayama University and the
Okayama meteorological station for their help and encouragement. Many thanks to Drs. T. Satomura, F.
Kimura, M. D. Yamanaka, K. Yasunaga, K. Otsuka,
and H. Ueda for fruitful discussions, comments, andcriticisms. The authors are grateful for the efforts ofmany scientists at both PSU and NCAR in maintaining
the MM5 and acknowledge the Meteorological Research Institute and Japan Meteorological Agency forsupplying meteorological data.
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