Wind and Structures, Vol. 24, No. 6 (2017) 637-656
637

DOI: https://doi.org/10.12989/was.2017.24.6.637

Review of downslope windstorms in Japan

Hiroyuki Kusaka 1 and Hironori Fudeyasu2
1

Center for Computational Sciences, University of Tsukuba, 305-8577, 1-1-1 Tennodai, Tsukuba, Japan
2
College of Education, Yokohama National University,
240-8501, 79-2 Tokiwadai, Hodogaya-ku, Yokohama, Japan(Received March 3, 2017, Revised April 15, 2017, Accepted April 18, 2017)
Abstract. In Japan, at least 28 local winds are known by name, most of them associated with downslope

windstorms and gap winds. To review these windstorms, we categorize them based largely on theatmospheric conditions and formation mechanisms, and then focus on representative examples. These
representative cases include the “Yamaji­kaze”, a typical downslope windstorm, the “Hirodo­kaze”, adownslope windstorm induced by a nearby typhoon (intense tropical cyclone), and the “Karak-kaze”, adownslope wind with a clear diurnal variation. Other downslope winds such as the “Inami-kaze” and the gap
wind “Kiyokawa­dashi” are also described. Among these winds, the “Yamaji-kaze”, “Hirodo-kaze”, and“Kiyokawa-dashi” are considered the three most notorious due to their destructive power. After describing
and comparing these winds, we discuss remaining issues to be considered in future studies.
downslope windstorm; gap wind; local wind; Yamaji-kaze; Hirodo-kaze; Karak-kaze;
Kiyokawa-dashi; numerical simulation; complex terrain
Keywords:

1. Introduction
A downslope windstorm blows down the lee slope of a mountain range, often reaching the footof the mountains. The downslope windstorm has been explained as an analog to hydraulicsupercritical flow (e.g., Durran and Klemp 1987, Smith 1985). Arakawa and Oobayashi (1968), as
well as Houghton and Kasahara (1968), used a shallow-water-equation model with two divided
layers and could simulate strong downslope wind as a hydraulic supercritical flow. Smith (1985)
extended the conventional hydraulic theory to the local hydraulic theory with an atmosphere ofconstant stratification, and showed that mountain-wave breaking and stagnant region play
important roles in forming the supercritical flow on a leeward mountain slope. Another theory ofthe downslope windstorm is amplification of a vertically propagating wave, with the reflection off
a pre-existing or self-induced critical layer (the wave-resonance theory) (e.g., Clark and Peltier
1984, Peltier and Clark 1983). Mountain-wave breaking must be important in this theory as well as
Smith‟s theory because it produces self-induced critical layer. Later, Durran and Klemp (1987)
describe that Smith‟s theory may be considered as an extension of Peltier and Clark‟s analysis tononlinear steady-state problems and conclude that the strong wind on a leeward mountain slope isfundamentally explained by the local hydraulic theory presuming the presence of mountain-wave
Corresponding author, Professor, E-mail: kusaka@ccs.tsukuba.ac.jp
Copyright © 2017 Techno-Press, Ltd.
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ISSN: 1226-6116 (Print), 1598-6225 (Online)

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breaking.
Here, we consider the mountain and atmospheric conditions that help to create downslope
windstorms. Downslope windstorms require a mountain typically range about 1-km high (or more)
and a steep leeside slope (American Meteorological Society, 2016). It also helps if they have asaddle area (col) with a lower-angled slope on the upwind mountain range (e.g., Lily and Klemp
1979, Pitts and Lyons 1989, 1990, Saito 1993, Smith 1985). Favorable atmospheric conditions
include an intense pressure gradient across the mountain range (moderate to strong windcomponent in the across-mountain-range direction) and stable stratification. An inversion layer
above the mountaintop and a pre-existing critical layer are supportive conditions (e.g., Klemp andDurran 1987).
A non-dimensional mountain heightε= NH/U is a widely used parameter to describe thesefavorable conditions. Here, N is the reference buoyancy (Brunt-Väisälä) frequency, H is themountain ridge height, and U is a reference windspeed. As defined, ε is the inverse of themountain Froude number. For a downslope windstorm to occur, ε should be nearly 1 or larger.
For instance, Lin and Wang (1996) found that downslope windstorms occur when the mountain
Froude number Fr is between 0.3 and 1.12. This condition agrees with the criteria for wave
breaking, which supports the theories for downslope windstorms. More details on downslope
windstorm theories are well described in Lin (2010), Markowski and Richardson (2010), andJackson et al. (2012).
Downslope windstorms have been observed in many places of the world and given a variety of
local names, including the “Foehn” in the foothills of the Alps (e.g., Brinkmann 1971, Jaubert andStein 2003, Seibert 1990), the “Bora” in the Adriatic Sea (e.g., Grisogono and Belusic 2009, Lepriet al. 2014, Smith 1987), the “Chinook” in the foothills of the Rocky Mountains (e.g., Cook andTopil 1952, Oard 1993), the “Santa Ana” in the Los Angeles Basin (e.g., Raphael 2003, Sommers
1978), and the “Zonda” in the foothills of the Andes (e.g., Maximiliano and Nunez 2003, Norte etal. 2008). The “Bora” is both a cold downslope wind and a gap wind flowing between the Alpsand Dinaric Alps.
For engineering applications focusing on downslope wind effects on structures and vehicles it
is necessary to further elucidate characteristics of downslope windstorms in more details, as someother transient winds proved to have a strong impact on structures, e.g., Solari (2014), Aboshosha
and El Damatty (2015), Solari et al. (2015), Yang and Zhang (2016), Lou et al. (2016).
Japan is a mountainous country of complex topography (Fig. 1(a)) and thus has a variety of
local wind types such as the downslope windstorm and gap wind. The review here is based
primarily on recent results of studies in meteorology and geography. For the relationships between
these local winds, the physical geography, and the livelihoods of people, we refer readers to theearlier study of Yoshino (1975).

2. Local winds in JapanAs described in the section 1, there is a variety of local winds in Japan. Twenty-eight of thesewinds are mapped in Fig. 1(b). Comparison of Figs. 1(a) and 1(b) show that these local winds
occur at the foot of the mountain ranges on the four big islands (Main Island, Hokkaido Island,
Shikoku Island, and Kyushu Island). The names and characteristics of these winds are listed inTable 1. The names of local winds have a suffix of “kaze”, “oroshi”, or “dashi”. The Japanese“kaze” means wind, “oroshi” means downslope windstorm or fall wind, and “dashi” suggests an

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639

(a)

(b)

Fig. 1 (a) Topography of the four largest islands of Japan. The enlarged regional maps are in thelabeled figures and (b) Distribution of local strong winds in Japan. Their name and associatedweather event are listed in Table 1. (Adapted from Yoshino, 1975)

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easterly wind when the boats start to sail from the shore to the sea. Most of “dashi” type isgenerally considered a gap wind that comes blowing from a channel between two mountain ranges
or a gap in a mountain barrier (e.g., Colle and Mass 2000, Overland and Walter 1981, Reed 1931).
The front part of the names comes from the local place or mountain name. These windstorms canseverely damage buildings, trees, and other structures, the most notorious being the Yamaji-kaze,
Hirodo-kaze, and Kiyokawa-dashi. In addition to these three, the Karak-kaze and Inami-kaze aregood examples of Japanese local winds that are discussed in the present review paper.

Table 1 Name and synoptic conditions associated with local winds in Japan. The numbers refer to theirlocation on the map in Fig. 1(b)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28

Name
Hikata-kaze
Tokachi-kaze
Rausu-dashi
Suttsu-dashi-kaze
Hidaka-shino-kaze
Kiyokawa-dashi
Arakawa-dashi
Tainai-dashi
Yasuda-dashi
Nasu-oroshi
Akagi-oroshi
Haruna-oroshi
Tsukuba-oroshi
Karak-kaze
Dashi-no-kaze
Dashi-no-kaze
Shougawa-arashi
Inami-kaze
Masita-kazeHira-hakko
Suzuka-oroshi
Hirano-kaze
Oroshi
Rokko-oroshi
Hirodo-kaze
Yamaji-kaze
Hijikawa-arashi
Matsubori-kaze

Weather Chart
CycloneCycloneWinter monsoon
CycloneToropical cyclone
CycloneCycloneCycloneCycloneWinter monsoon
Winter monsoon
Winter monsoon
Winter monsoon
Winter monsoon
Toropical cyclone
Anticyclone
Anticyclone
CycloneWinter monsoon
Winter monsoon
Winter monsoon
CycloneWinter monsoon
Winter monsoon
Toropical cyclone
Toropical cyclone
Anticyclone
Cyclone

Review of downslope windstorms in Japan641

3. Downslope windstorms in Japan
3.1 Yamaji-kaze
The Yamaji-kaze is a typical downslope windstorm in Japan (No. 26 in Fig. 1(b) and Table 1).
It is a strong southerly wind that occurs on the northern side of the Shikoku Mountains with amountain width of 40 km and the Niihama Plain facing the Setouchi Sea (Fig. 2). This wind had
been investigated by the Japan Meteorological Agency (JMA) (e.g., Akiyama 1954, 1956).
According to these studies, the Yamaji-kaze often occurs in spring and autumn, with particularly
strong events happening when a well-developed extratropical cyclone passes over the Sea of Japanor a typhoon passes just west, to the northwest side of the Shikoku region. The Yamaji-kazeseverely damages housings and trees. According to Yoshino (1975), wind velocities of over 15 m
s-1 are observed 2-5 times annually. As one of the most serious disaster cases, the collapse of thesteel tower (transmission line tower) in 1991 is known. On the day the tower collapsed, windspeed of 73.2 m s-1 was observed at the 27.5 m above ground level during the passage of Typhoon
Mireille.
The Yamaji-kaze develops as follows: Before the Yamaji-kaze occurs, a northerly wind called
the Sasoi-kaze (literally 'inviting wind') develops on the north side of the Shikoku Mountains.
After the Sasoi-kaze blows, the southerly downslope windstorm Yamaji-kaze begins to blow in thenortheast section of the Shikoku Mountains. As the Yamaji-kaze strengthens, a local front, called
the “Yamaji-kaze front" forms (due to an internal hydraulic jump). The area of highest windspeed
is limited to this front. Later, after the Yamaji-kaze weakens, westerly winds appear.

Fig. 2 Topography surrounding the Yamaji-kaze area on Shikoku (see Fig. 1(a) for location)

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Fig. 3 Vertical cross-section of the horizontal wind over a bell-shaped mountain range with a col.
Simulated results (a) in the vicinity of the peak of the mountain and (b) in the vicinity of the col. In the
diagrams, the wind is reported to blow from left to right. (Saito 1993)

A notable inversion layer is found at a level near the mountaintop for the case of the
Yamaji-kaze observed on 21 April 1987. Saito and Ikawa (1991) conducted numerical experiments
and confirmed that the surface wind strengthens in the presence of the inversion when compared tothat without the inversion.
In a theoretical study of the Yamaji-kaze structure, Saito and Ikawa (1991) compared ananalytical solution of two-dimensional flows for symmetric mountains against asymmetricmountains. The asymmetry involved mild slopes on the upwind side and steep slopes on the
downwind side. They showed that asymmetric mountains tend to strengthen the mountain waves
on the downwind side, with the wave-breaking occurring on the downwind slopes. Their solution
supported the results from previous observational studies (e.g., Lilly and Klemp 1979, Pitts andLyons 1989) and theoretical studies (e.g., Lily and Klemp 1979, Pitts and Lyons 1990, Smith
1985).
Saddle areas between mountain peaks, including ridgeline depressions and cols, also contributeto the formation of the Yamaji-kaze. For example, Saito (1993), in running a numerical simulation(JMA non-hydrostatic model; NHM) of the Yamaji-kaze, found that the Yamaji-kaze needs saddles(col) (Fig. 3). Moreover, the strong-wind area tends to spread forward downwind of the saddles. In
this case, height of mountain and saddle area were 1050 m and 250 m, respectively. The
non-dimensional mountain height ε was 2.6. Later, Saito (1994) used the same model to reproduce
the Yamaji-kaze with a hydraulic jump, but was successful only when the NHM had a 2.5-km
horizontal grid spacing, not a larger 10-km spacing. This series of studies confirmed that the local

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643

characteristics of the mountain range, specifically the saddle area and mountain-slope asymmetry,
have major influences on downslope windstorms. Additionally, the studies demonstrate that anumerical model with 2.5-km horizontal grid spacing is a very powerful tool for the study ofdownslope windstorms.
3.2 Hirodo-kaze
The Hirodo-kaze blows just south of the base of Mt. Nagi, which is located on the western
main island of Japan (Fig. 4 and No. 25 in Fig. 1(b) and Table 1). It blows in an area of 10 × 10 km,
but the most serious damages concentrated in an area of 5 × 2 km (Yoshino 1975). This windmainly occurs when a typhoon approaches the region in September and October (Yoshino 1975).
The Hirodo-kaze severely damages buildings, trees, and other structures. In late June 1998, forexample, the Hirodo-kaze with surface wind speed over 25 m s-1 occurred along with Typhoon
Peter and caused damages of about US $1.0 million.

Fig. 4 Terrain around Mt. Nagi, location of the Hirodo-kaze. Contour interval is 100 m. Altitudes of 400–
800 m are lightly shaded, while altitudes above 800 m are heavily shaded. Filled circles connected by aline are the locations every 3 hr of Typhoon Pabuk from JMA Best Track data. Gray zone indicates the
Hirodo-kaze zone. Dashed line N–S is a cross-section used for Fig. 5

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Fig. 5 Cross-section of simulated Hirodo-kaze event on 21 August 2001 at 06 UTC for the top row, 15
UTC for the bottom row. (a) and (b) are the results for meridional wind. Solid lines denote 0 m s -1, broken
line denotes southerlies. Contour intervals are for 5 m s-1. The shaded areas indicate northerlies over 25 m
s-1 and southerlies. (c) and (d) are the results for vertical velocity. Contour intervals are 1.0 m s-1. Negative
regions are shaded. (e) and (f) are the results for potential temperature. Contour intervals are 3 K. Contour
longitude 134.18E. In the diagrams, the wind is reported to blow from right to left. (Fudeyasu et al. 2008)

Yoshino (1975) suggested that the orographic configuration of Mt. Nagi, with its steep southernslope and a mild northern slope, is favourable for downslope windstorms. Sahashi (1988), after
observing a roll cloud (e.g., Vosper 2004, Vosper et al. 2006) parallel to the ridgeline south of Mt.
Nagi during a Hirodo-kaze event, speculated that its severe winds are associated with alarge-amplitude mountain wave with a rotor cloud. He also showed that the Hirodo-kaze tends tooccur as a typhoon that moves northeastward over the sea southwest (offshore) of the Kii
peninsula, determining a region called the “Hirodo-kaze zone” (see Fig. 4).
Fudeyasu et al. (2008) examined the effects of a typhoon on the occurrence of the Hirodo-kazeusing the PSU/NCAR-fifth-generation Mesoscale Model (MM5) with 1-km horizontal grid
spacing and 30 vertical levels. Their results, reproduced in Fig. 5, show vertical cross-sections ofsimulated meridional wind, vertical velocity, and potential temperature across station A at Mt.
Nagi.
As the Hirodo-kaze occurred at station A, downslope windstorms developed along the lee slopeof Mt. Nagi with maximum vertical velocity exceeding 6 m s-1 (Fig. 5(c)). Isentropes shifted
downward in the lee of Mt. Nagi until they reached the lower troposphere, and they extended
leeward over the base of Mt. Nagi (Fig. 5(e)), with the pre-existing critical layer near 3000 mheight and the region of strong winds exceeding 25 m s-1 extending leeward far from Mt. Nagi (Fig.

Review of downslope windstorms in Japan6455(a)). Strong northerlies in the lower troposphere and southerlies in the middle troposphere
allowed the severe downslope windstorm to persist along the lee slope of Mt. Nagi. As the
middle-tropospheric southerlies changed to northerlies, the pre-existing critical layer near 700 hPa
level disappeared and the Hirodo-kaze ceased (Fig. 5(b)). Favourable atmospheric conditions forthis strong-wind event arise when a typhoon passes over the sea southwest of the Kii peninsula in
the Hirodo-kaze zone (see Fig. 4) and forms a pre-existing critical layer (See Fig. 5(a)).
3.3 Kiyokawa-dashi
The Kiyokawa-dashi is a strong easterly wind that blows around the Mogami Gorge in the
Shonai Plain located in northern Honshu (No. 6 in Fig. 1(b)). Its strong-wind area fans out fromthe Gorge, but it is limited within radius of ten km (Fig. 6(a)). The Kiyokawa-dashi damages ricecultivation in this area. On the other hand, it has been also used for wind power generation.
The Kiyokawa-dashi is recognized as a gap wind, but most of the downslope winds in Japan
also has features of the gap wind (Arakawa 2002) and thus it is introduced here. It occurs when an
anticyclone develops over the Sea of Okhotsk or the Chishima Islands (Kuril Islands) and an
extratropical cyclone moves over the Sea of Japan (Yoshino 1975).
Sasaki et al. (2004) investigated the vertical structure of the atmosphere during
Kiyokawa-dashi events. They found that this wind occurred under a strong atmospheric stablelayer (inversion layer) at a height near the mountain top. Such a stable layer is well known as afavourable atmospheric condition for gap winds. Sasaki et al. (2004) also found out that strong
easterly winds blow at an altitude of up to 500 m, with a weaker wind above. Later, using a
Coherent Doppler Lidar, Ishii et al. (2007) found the critical layer during a Kiyokawa-dashi event.
Recent studies have used numerical simulations to better understand the Kiyokawa-dashi.
Sasaki et al. (2010) used the NHM to reproduce the Kiyokawa-dashi. They found good agreementwith observations when the horizontal grid-spacing was respectively set to 1-km and 85 verticallevels (Fig. 6(b)). But the agreement was poor when using the model with a 5-km-horizontal grid
spacing and 35 vertical atmospheric levels. The results indicated that the detailed topography ofboth the valley and mountains on the Kiyokawa-dashi were important.
3.4 Karak-kaze
The Karak-kaze is a dry and strong northwesterly wind frequently observed in the Kanto Plain
during a typical winter monsoon (Fig. 7 and No. 14 in Fig. 1(b) and Table 1). According to Kusaka
et al. (2011), the Karak-kaze is stronger when there is a larger northwest–southeast pressure
gradient over Japan, and also when the low-pressure centre lies northwest of Hokkaido. However,
the typical wind speeds of the Karak-kaze are around 10 m s-1 even under a large pressure gradient(Fig. 2) and lower than those of other downslope windstorms in Japan, including the Yamaji-kazeand the Hirodo-kaze. This study also showed that the Karak-kaze has a clear diurnal variation withmaximum wind speed in the daytime (Fig. 8), which indicates that a significant influence on thediurnal variation is the downward momentum flux associated with a developing dry convective
boundary layer. Kusaka et al. (2011) conducted the numerical simulations and showed that thedaytime downward momentum flux and nocturnal surface inversion layer is a necessary condition
for the diurnal variation of the Karak-kaze.

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(a)

(b)

Fig. 6 (a) Horizontal distribution of surface winds during a Kiyokawa-dashi event at 10 LT 18
January 1950. (Yoshino 1975) and (b) Vertical distribution of the Kiyokawa-dashi observed by a
Coherent Doppler Lidar (CDL) and simulated by the NHM with 1-km horizontal grid spacing
and 85 vertical levels (L85 all). R1 indicates the observation at the surface station. Figs. 6(b) and6(b) present wind direction and horizontal wind speed, respectively. (Sasaki et al. 2010)

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647

Fig. 7 Distribution of surface winds at 19 local time on February 2002 during a Karak-kaze event. Solidand broken lines indicate coastal line and terrain height, respectively. (Kusaka et al. 2011)

Fig. 8 Diurnal variation of wind speed from MRI tower during Karak-kaze events. (a) Average of 74
strong wind days, (b) Average of 194 medium wind days and (c) Average of 75 weak wind days. The lines
indicate the observation height above ground level (Kusaka et al. 2011)

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Hiroyuki Kusaka and Hironori Fudeyasu

On the other hand, Yoshino (1975) suspected that the Karak-kaze is a kind of a Bora-type
downslope windstorm from the central mountains. However, it is known that there are several
types of Karak-kaze. At the moment, there is no established conceptual model of the formation
mechanisms of Karak-kaze.
3.5 Foehn winds
Downslope winds can produce a large rapid temperature increase on the lee-side plain of
mountains, a phenomenon well-known as a foehn. In Japan, research on foehn winds has focusedon the high-temperature aspect rather than the strong winds (e.g., Arakawa et al. 1982, Ikawa andNagasawa 1989, Ishizaki and Takayabu 2009, Mori and Sato 2014, Takane and Kusaka 2011,
Takane et al. 2016). Studies revealed influences of the foehn winds on the extreme temperature
events of Hokkaido Island (Mori and Sato 2014), Hokuriku region (Ishizaki and Takayabu 2009),
and Kanto region (Takane and Kusaka 2011, Takane et al. 2016).

Fig. 9 The mean energy variation (J kg-1) along the trajectories for course high (a) and course low (b). The
solid lines indicate geopotential height gZ, dotted lines show enthalpy cpT, the white circles show dry
static energy s, and broken lines with dots show height of particle zi. Shades represent terrain height along
the trajectories. (Takane and Kusaka 2011)

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Downslope winds can receive some sensible heat from the leeward mountain surface. Crookand Tucker (2005) theoretically and numerically examined effects of such surface heating on
airflow when it passes over such a heated mountain. Ishizaki and Takayabu (2009) showed that thedry static energy increased as airflow passed over the mountain range of mainland Japan. Later,using numerical simulations, Takane and Kusaka (2011) argued that the record-breaking extreme
high temperature event of 40.9°C observed in the inland Kanto Plain was due to foehn winds withdiabatic heating from a heated mountain surface. Fig. 9 shows the Lagrangian mean energy budget
along the trajectories for courses high and low. Dry static energy for course High, estimated fromless particles (30% of total particles), is almost constant. On the other hand, for course Low,
estimated from more particles (57% of total particles), dry static energy increases to about +5000 J
kg-1. This indicates the importance of diabatic heating due to surface sensible heat flux andboundary layer turbulence on the dry static energy increase, resulting in record-breaking extreme
temperature. Such simulations should be tested against observations in a future study.

4. Conclusions
Numerical studies confirmed that asymmetric mountains and saddle areas (col) between
mountain peaks contribute to the formation of the strong Yamaji-kaze. Additionally, it was
revealed that the Hirodo-kaze occurs due to the existence of a pre-existing critical layer whichforms when typhoons approach. Concerning the two most notorious downslope windstorms ofJapan, the Yamaji-kaze and the Hirodo-kaze, we have a basic understanding of their essential
features and formation mechanism. But for other windstorms such as the Karak-kaze, there is noestablished conceptual model of their formation mechanisms. In the case of some downslope
windstorms such as the Inami-kaze, their small sizes (within several square kilometres), indicatethat very small valleys and mountains significantly influence their formation. However, thecontribution from small gaps in the mountains remains unclear. As Arakawa (2002) has pointed
out, the complex topography of Japan makes it difficult to assign an individual local wind as being
simply a downslope windstorm or being a gap wind. To help resolve this difficulty, numericalmodels with very high spatial resolution, as well as observations using remote sensing systems
such as Lidar, will play important roles in explaining the mechanisms of downslope windstorms.
As spatial resolution increases, smaller-scale and steeper terrain features become more
explicitly resolved. This raises challenges regarding coordinate systems and parameterization
schemes when studying downslope windstorms in Japan. Many mesoscale models use aterrain-following coordinate system where only the vertical coordinate is transformed to fit thelower boundary. While this system has the advantage of having a vertical coordinate axis thatuniquely determines the grid by providing topographical information, thus making it easier todescribe gravity-related and boundary-layer processes, it has the disadvantage that it cannot beapplied to terrain with very steep slope, for instance a slope over 45 degrees due to numericalinstability and large error (Fig. 10). This is caused by the nonorthogonality of the grid system (e.g.,Janjic 1989, Pielke 2001, Steppeler et al. 2006, Sundqvist 1976, Zängl 2003). Alternative
coordinate system for the steep slope may be a generalized curvilinear coordinate system (e.g.,Kajishima et al. 1998, Zhang et al. 1994) or an advanced Cartesian-grid method, for example an
immersed boundary method (e.g., Fadlun et al. 2000, Tseng and Ferziger 2003) and a cut-cell
method (a shaved-cell method) (e.g., Adcroft et al. 1997, Yamazaki and Satomura 2010, Ye et al.
1999).

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Hiroyuki Kusaka and Hironori Fudeyasu

Fig. 10 Contours of vertical velocity component (interval 1 m s-1, negative values dashed) for flow overthe steep slope mountain. The averaged slope angle is 59°. (a) Simulated results from the local windmodel with terrain-following (z*) coordinate system and (b) simulated results from the same model but
using generalized curvilinear coordinate system. In the diagrams, the wind is reported to blow from left toright. (Ikeda et al. 2017)

Numerical simulations of flow over mountains show that the above mentioned coordinate
systems may be appropriate for simulating downslope windstorms over the steep slope (e.g., Fang
and Porté-Agel 2016, Lock et al. 2012, Lundquist et al. 2010, Satomura 1989, Steppeler et al.
2002, Yamazaki and Satomura, 2010, 2016). A remaining issue with the use of the cut-cell method
is the incorporation of physical process models and other components, such as a land-surface
model because many of the physical parameterization schemes are based on a single column
model (e.g., Yamazaki et al. 2016). The same issue remains for generalized curvilinear coordinate
system. Numerical simulation of downslope windstorms is also a subject for future studies.
As the spatial resolution increases, turbulence parameterization faces challenging issues
because mesoscale models traditionally use the Reynolds-Averaged Navier-Stokes (RANS)
equations. Wyngaard (2004) describes that it is not clear how to apply conventional mesoscalemodels within the “terra incognita” region, where spatial-filter length scale (proportional to model
grid interval) approaches to energy-containing turbulence scale (that may be ~100-1000 m). At thisscale, it becomes theoretically inappropriate to use turbulence parameterization. Based on our
experience, however, merits attained by increased resolution (by which complex terrain features
are better represented in the model) well surpasses the possible downside of ignoring theReview of downslope windstorms in Japan

651

theoretical presumption for using turbulence parameterization in the “terra incognita” grey zone.
Here, we will introduce a part of experimental results in our laboratory. Fig. 11 shows simulated
results of a very strong downslope windstorm “Inami-kaze” (No.18 in Fig. 1(b)) from the WRF
model with 300-m horizontal resolution. Using an interview survey of the local citizens, thestrongest wind areas are confined to a few square kilometers, with such areas appearing several
kilometres apart, generally in very small valley-like channels on the lee slope of the mountain
range. Comparing the simulated results between the 50-m and 1-km mesh topography data
indicates that the downslope wind blowing in a very small area requires very detailed topography
data. This implies merit attended by increased resolution. However, further studies are necessary
for appropriately balancing the model resolution and turbulence parameterization.
Several different types of the planetary boundary layer (PBL) schemes have been recently
improved for the grey zone resolution (e.g., Ito et al. 2015, Kitamura 2015, Ramachandran and
Wyngaard 2011, Shin and Hong 2014). Such a scheme will be adopted to community mesoscalemodels and used for very high resolution downslope windstorm simulations in the near future.
Large Eddy Simulation (LES) models may provide a practical approach for very highresolution simulation that can resolve energy-containing turbulence. LES model has beensuccessfully applied to real atmospheric flow over complex terrain (e.g., Chow and Street 2009),
but the applied examples are still rather sparse compared to flat terrain cases. Iizuka and Kondo(2004) investigated performance of various sub- grid scale (SGS) models in LES for atmosphericflow over idealized hills. As a result, a hybrid static/dynamic SGS model provided very accurate
predictions and produced the best results of the four SGS models compared in their study.
Appropriate LES model configuration for flow over complex terrain is a subject for future studies.

Fig. 11 Distribution of wind speed during a simulated Inami-kaze event. Simulations performed using the
WRF model with 300-m horizontal grid-spacing. Color shading and contour lines indicate the surface
wind speed and terrain height, respectively. (a) 50-m resolution topography data and (b) 1-km resolution
topography data. (Koyanagi and Kusaka 2017)

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Hiroyuki Kusaka and Hironori Fudeyasu

Coupling RANS and LES models may be still a challenging issue because LES requires
boundary conditions with appropriate turbulence components and update intervals. Despite thechallenge, several works have been made with the RANS and LES coupling approach (e.g.,
Moeng et al. 2007, Talbot et al. 2012, Wyszogrodzki et al. 2012). However, Michioka and Chow
(2008) show that simulated results are sensitive to lateral boundary update interval. Not all studies
in this field are necessarily complete.
The studies cited here have focused on dynamical processes. However, as Japan is within the
Asian monsoon region, moist atmospheric physics must also be essential to understand themechanism of downslope windstorms in Japan. Additionally, surface fluxes and boundary-layer
processes play an important role of forming the Karak-kaze and Foehn winds. Thus, future studies
of Japanese downslope windstorms should focus on the atmospheric physics as well as dynamics.
As it is well known, simulated results of mesoscale models are very sensitive to lateralboundary conditions (LBCs). Many of the Japanese downslope windstorms occur when a typhoon
or a mid-latitude cyclone approaches. Therefore, accurate locations of typhoon or mid-latitude
cyclone must be provided by LBC in order to accurately predict timing and location of downslope
windstorms. In this sense, ensemble technique with different LBCs will be valuable in improving
the downslope windstorm simulation.
As described earlier, pre-existing critical layer and inversion layer are often observed during the
Hirodo-kaze and Kiyokawa-dashi events, which indicates importance of accurate initial conditions(ICs) resulting from data assimilation. On the other hand, Reinecke and Durran (2009) report thatsensitivity to ICs is very large in the downslope wind predictions. In this sense, ensemble
technique with different ICs will be appropriate.
It is considered that an ensemble prediction of wind speed using mesoscale models withdifferent ICs/LBCs produces more spread than the models with different PBL schemes in small
areas (e.g., Deppe et al. 2013). However, simulated results of mesoscale models depend on physics
scheme selections as well. Yang et al. (2013) investigated the sensitivity of the WRF model‟s
performance to the choice among three PBL schemes for simulating rapid wind speed changes(RAMP events) over a complex terrain. Their results showed that the University of Washington
scheme has the best overall performance in RAMP event prediction, but no single PBL scheme isclearly superior to the others when all atmospheric conditions are considered. In this sense,
PBL-scheme ensemble technique may be valuable for improving the downslope windstorm
simulation as well as RAMP event simulations. Understanding the biases and strengths ofmesoscale models and PBL schemes will help to improve downslope windstorm simulations.

Acknowledgments
This work was supported by Council for Science, Technology and Innovation (CSTI),
Cross-ministerial Strategic Innovation Promotion Program (SIP), “Technologies for creatingnext-generation agriculture, forestry and fisheries” (funding agency: Bio-oriented Technology
Research Advancement Institution, NARO). We would like to thank Mr. Akifumi Nishi and Mr.
Takuma Koyanagi of Graduate School of Life and Environmental Sciences, University of Tsukuba,
for making figures.

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653

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