January
Journal of
2016
the Meteorological Society of Japan, Vol.
T. MURAMATSU
94A, pp. 105−120,
et al.2016
DOI:10.2151/jmsj.2015-053

105

Future Change of Tornadogenesis-Favorable Environmental Conditions inJapan Estimated by a 20-km-Mesh Atmospheric General Circulation Model
Takanari MURAMATSU
Hakodate Airport Branch, Hakodate, Japan

Teruyuki KATO
Meteorological Research Institute, Tsukuba, Japan

Masahisa NAKAZATO
Japan Meteorological Agency, Chiyoda, Japan

Hirokazu ENDO
Meteorological Research Institute, Tsukuba, JapanandAkio KITOH
Faculty of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Japan(Manuscript received 30 October 2014, in final form 24 August 2015)

Abstract
Because tornadoes cause enormous damage to humans and societies, projecting plausible future changes in tornado
activity is an important research focus. We used the results of climate experiments under the A1B emissions scenario
with a 20-km-mesh and a high-resolution atmospheric global circulation model to project future changes in thefrequency of conditions conducive to the generation of strong tornadoes (F2 or greater on the Fujita scale) in Japan.
We found that this frequency is likely to double in future in almost all areas of the Japanese Islands in spring and on
the Japan Sea side of the Japanese Islands in summer due to intensification of atmospheric instability caused by anincrease in the water-vapor mixing ratio and a temperature rise in the lower troposphere. In contrast, the frequency ofstrong vertical wind shear, which is conducive to tornadogenesis, was projected to hardly change or decrease slightly.
Comparison with climate fields generated by a 60-km-mesh 12-member ensemble experiment showed that futurechanges in tornadogenesis-favorable environmental conditions projected by the 20-km-mesh experiment were highly
reliable. Moreover, we found that the predicted future changes were robust when we used other thresholds for theenvironmental parameters that created conditions conducive to strong tornadoes. Our results indicate that there will
be a significant increase in the frequency of strong tornadoes in Japan in the future.
Keywords

tornado; global warming; general circulation model (GCM)

Corresponding author: Takanari Muramatsu, Hakodate
Airport Branch, Japan Meteorological Agency, 511 Takamatsu-cho, Hakodate, Hokkaido 042-0952, Japan
E-mail: takanari_muramatsu@met.kishou.go.jp©2016, Meteorological Society of Japan

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1.

Journal of the Meteorological Society of Japan

Introduction

Tornadoes cannot be directly simulated by current
climate models because the typical scales of tornadoes(horizontal spatial and time scales are approximately
100 m and 10 min, respectively) are far smaller
than the resolution of climate models. In contrast,
the occurrence probability of a parent cloud systemcausing strong tornadoes (i.e., a supercell storm)
can be estimated from upper air soundings related
to convective instability and vertical wind shear(Rasmussen and Blanchard 1998; Thompson et al.
2003). Strong tornadoes occur in atmospheric environmental fields with significant convective instability
and adequate vertical wind shear. Several studies
have projected an increase in the frequency of severe
storms that generate tornadoes, hail, and gusts over alarge portion of the United States in the future basedon atmospheric environmental indexes (Trapp et al.
2007, 2009).
Tornadoes often occur in association with synoptic-scale extratropical cyclones (Johns et al. 1993;
Niino et al. 1997), which can be reproduced by
low-horizontal-resolution climate models. Lee (2012)
estimated future changes of synoptic patterns favorable for tornadoes and predicted that the peak season
for tornados in the United States will shift from
May to April; the frequency will increase primarily
between February and April but will decrease slightly
between May and August. It might therefore be
possible to apply synoptic climatological methods
with low-horizontal-resolution climate models to
project future changes in the likelihood of tornadogenesis. However, before synoptic methods canbe applied to the projection of future changes, thecorrelation between tornadogenesis and synoptic
patterns must be a priori established in the present-day climate by analyzing tornado case studies. To
obtain an adequate number of tornado observations,the target areas must be enlarged, but considerably
fewer tornadoes are recorded in Japan than in the
United States. Furthermore, local environmental fields
are strongly affected by complicated terrain such asthat which characterizes the Japanese Islands. Therefore, high horizontal resolution is necessary to project
future changes in conditions favoring tornadogenesis
in Japan.
The archived results of a climate experiment witha 20-km-mesh atmospheric general circulation model(MRI-AGCM3.2; Mizuta et al. 2012) are very suitable for examining future changes in the occurrence
possibility of strong tornadoes. The 20-km experimentVol. 94A

was deterministically conducted only once with theSpecial Report on Emission Scenarios (SRES) A1B
scenario (control run), whereas an ensemble experiment with 12 members that included multiple physics
and multiple sea surface temperature (SST) scenarios
was also conducted to address the uncertainty of
projected future changes in environmental conditions
favoring tornadogenesis (Endo et al. 2012).
This is the first study to assess future changes in
the occurrence of environmental conditions favorable
for tornadoes in Japan using the results of the 20-kmmesh experiment. U.S. tornado statistics and theNorth American Regional Reanalysis (NARR) dataset(Mesinger et al. 2006) were first compared to determine the threshold values of environmental parameters for the occurrence of strong (F2 or greater on the
Fujita scale) tornado events, and future changes in
the United States were also investigated to verify thesuitability of our analysis method by comparing futurechanges with those of previous studies. To verify theperformance of the 20-km-mesh general circulation
model, the NARR dataset was also used as a reference
dataset for the present-day experiment.
This paper is organized in the following manner.
In Section 2, the data, environmental parameters,
and analysis method used in this study are described.
Reproducibility by the numerical model of the present-day climate is described in Section 3. In Section 4,
future changes in the occurrence possibility of strong
tornadoes are examined. In Section 5, the results ofthe 20-km-horizontal-resolution model are comparedwith those of ensemble experiments with a horizontal
resolution of 60 km. A summary and discussion arepresented in the last section.
2.

Methods

2.1 Climate experiment dataset
The analysis period for the present-day (future)climate was the 25-year period of 1979–2003 (2075–
2099). Output from the global warming experiment
was archived at intervals of 6 h (0000, 0600, 1200,
and 1800 UTC) with a horizontal resolution of about
20 km (TL959) at the surface and at 16 pressure levels(975, 950, 925, 900, 850, 800, 700, 600, 500, 400,
300, 250, 200, 150, 100, and 50 hPa) in the vertical.
The original model grid data were unfortunately
converted to standard pressure levels, which resulted
in the loss of vertical resolution near the ground, especially in high-elevation regions.
2.2 Environmental parameters
In this study, we use convective available poten-

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T. MURAMATSU et al.

tial energy (CAPE) and storm-relative environmental
helicity (SREH) as indicators of tornadogenesis-favorable environmental conditions. CAPE is a metric ofatmospheric instability and is calculated as follows:
CAPE = g ∫Z LNB Tvp − Tve
Z LFC

Tve

dz [ J kg−1 ],where Tvp and Tve are virtual temperatures of a lifted
air parcel and the environment, respectively, g is theacceleration of gravity, ZLFC is the height of the levelof free convection (LFC), and ZLNB is the level ofneutral buoyancy (LNB), where the virtual temperature excess first becomes zero above the LFC. An air
parcel with the largest equivalent potential temperature in the lowest 150 hPa is lifted for calculating
CAPE.
SREH is a measure of the tendency of a supercell torotate, and is calculated as follows:
 
h 
SREH = ∫ (V − C ) ⋅ ω dz [m 2 s−2 ],0  ∂Vω = k×,∂zwhere V is the horizontal wind vector, C is the storm
motion vector estimated from the vertical wind profile
following the method of Bunkers et al. (2000), ω isthe horizontal vorticity vector associated with verticalwind shear, and the storm inflow depth h is chosen to
be 3000 m AGL (above ground level).
Although high CAPE and SREH values arecommonly observed near strong tornadoes, previous
studies have shown that CAPE is not a good discriminator between a tornadogenesis-favorable environment and a non-tornadic (significant hail and wind)environment (Thompson et al. 2003; Grams et al.
2012). Environmental parameters related to low-levelvertical wind shear, however, can discriminate well
between the two environments. Grams et al. (2012)
have shown that the 25th percentile value of the 0–1
km bulk wind difference, which is the magnitude of
the vector difference between the horizontal winds atthe surface and at 1 km AGL, is greater for tornado
events than the median or the 75th percentile values
for significant hail and wind events in the UnitedStates (See Fig. 15b in Grams et al. 2012). They
have also shown that the SREH and the 0–1 km bulk
wind difference follow the same trend. In this study,
we therefore considered that conditions suitable forthe occurrence of strong tornadoes can be generally
discriminated with the combined use of CAPE and
SREH.

2.3

107

Definition of threshold values for the environmental parameters
The horizontal resolution of the NARR data is
about 32 km, which is coarser than that of the futureclimate experiments (20 km). However, the 32-km
horizontal resolution of NARR could be adequate
because favorable environmental fields for tornado
occurrence and their corresponding index values are
distributed over a distance of 40–80 km from the siteof the tornado (Potvin et al. 2010). The NARR data
used in this study were available every 6 h at thesame times (0000, 0600, 1200, and 1800 UTC) as
the climate experiment archive. The calculation of
the SREH depends on the vertical resolution, because
the vertical profile of horizontal winds in the lower
troposphere exhibits rather unique characteristics
when tornadoes occur (Esterheld and Giuliano 2008).
Only NARR data corresponding to data levels in the
climate experiment archive were therefore used to
calculate CAPE and SREH. It is an additional merit ofour study that the climate experiment dataset included
several pressure levels at relatively low altitudes,
because a tornadogenesis-favorable environment
could not be properly simulated with low-vertical-resolution datasets.
Grams et al. (2012) defined a tornado event as
the occurrence of one or multiple tornadoes within
a 300-km radius of the occurrence site of a tornado
within a timeframe of 6 h before and after its occurrence. The time and location of the event are determined from those of the tornado with the maximum
Fujita Scale (F-scale) value. Figure 1 explains theprocedure for defining the occurrence of tornado
events and shows four examples. For the values of
CAPE and SREH in the environmental field, we used
the maximum values among the NARR grid points
within an 80-km radius from the site of the tornado
event and within 2 h before or after its occurrence.
Figure 2 presents box plots of the maximum CAPE
and SREH in the environmental field for tornado
events with different F-scales observed in the UnitedStates in March–April–May (MAM) and June–July–
August (JJA). The values of CAPE indicate that theatmosphere was more unstable during relatively large
F-scale tornado events and that the instability waslarger in JJA than in MAM for the same F-scale. Even
for F2 tornado events observed in MAM, the 25th
percentile of CAPE reached approximately 500 J kg–1.
In contrast, for the same F-scale, the SREH was larger
in MAM than in JJA. Most of the strong tornado
events with F2 or stronger occurred in an environmental field with the SREH exceeding 150 m2 s–2,

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TIME

ex.1

F2 2 hours F0
100km

ex.2

F1 4 hours F2
100km

ex.3

F1 7 hours F2
100km

ex.4

F2 4 hours F0
100km

3 hours
200km

5 hours
200km

3 hours
200km

3 hours
400km

F2 1 event
F2 1 event
F3 2 events
F1 2 events

Fig. 1. Four examples of schematic diagrams for
deciding whether a tornado event has occurred.
A circle indicates a tornado, and tornadoes
enclosed with dashed lines indicate a tornado
event. The F-scale of each tornado is shown ina circle, and the occurrence time interval anddistance between nearby tornadoes are indicatedunder the double-headed arrows. The tornados
indicated by gray circles are the reference
tornados for the occurrence time and site in eachtornado event. The earlier tornado becomes thereference tornado if two or more tornadoes in an
event have the same F-scale.

although the SREH was slightly smaller in JJA (25thpercentile value: 129.8 m2 s–2). In this study, therefore, the combination of CAPE ≥ 500 J kg–1 and
SREH ≥ 150 m2 s–2 defined conditions suitable for
tornadogenesis. Using these conditions, we examined
the projected changes in environmental conditions
favorable for tornadogenesis in MAM and JJA.
3.

Reproducibility in the present-day climate

Figure 3 depicts the geographical distribution of
the “appearance frequency of conditions suitable
for tornadogenesis” averaged in MAM and JJA for
25 years in the present-day climate, calculated fromthe present-day climate experimental results and the
NARR data. Hereafter, the “appearance frequency
of conditions suitable for tornadogenesis” is abbreviated to FSC. The black box in Fig. 3a is “Tornado
Alley,” a high-tornado-frequency area in the Great
Plains (Brooks et al. 2003). In MAM (Figs. 3a, b), the
FSC in the present-day climate experiment peaked

Vol. 94A

in Tornado Alley, although it was underestimated byabout 25 % in the southern part. This underestimation
was caused by the fact that the frequency of CAPE ≥500 J kg–1 was also underestimated by approximately
20 %. In JJA (Figs. 3c, d), the present-day climate
experiment successfully reproduced the geographicaldistribution of the FSC, especially around Tornado
Alley. These results indicate that the model results canbe used to project future changes in the FSC.
The fact that an area of high FSC (exceeding 10 %in Fig. 3) is distributed in Tornado Alley corresponds
well with the distribution of strong tornadoes causedby supercell storms (Ashley 2007; Grams et al. 2012).
In contrast, areas of low FSC (< 2 %) are prevalentaround the Florida peninsula, where many tornadoes
also occur, though most of these are of strength F1 or
F0 (Brooks et al. 2003). Around the Florida peninsula,some processes that cannot be taken into account by
CAPE and SREH may contribute significantly to theoccurrence of the tornadoes, because most tornadoes
there occur in association with precipitation systems
such as non-supercell storms or hurricanes (McCaul
1991; Smith et al. 2012). This study, therefore,
focused on future changes in the occurrence of strong
tornadoes (F-scale ≥ 2) such as those caused by supercell storms (i.e., supercell tornadoes).
For the Japanese region, we assumed that the same
threshold values used for the United States simulations could be used to project future changes in the
FSC, because no analytical data with a high horizontal
resolution such as the NARR data were available fora long enough period to compare with present-day
climate experimental simulations. To verify the reproducibility of the present-day climate over the Japaneseregion, low-level equivalent potential temperature and
SREH were calculated in seasonal mean fields fromthe Japanese 55-year Reanalysis (JRA-55) (Kobayashi
et al. 2015) with a grid spacing of 0.5° × 0.5°1. The
mean values in the present-day experiment agreed
well with those in JRA-55, with the exception of the
low-level equivalent potential temperature in JJA(Figs. 4, 5). The larger mean 950-hPa equivalent
1Dataset of 0.5° × 0.5° is produced from original data
(TL319L60) with a grid spacing less than 0.5625° and 60
σ-p hybrid vertical levels as follows. The data are simply
interpolated using four neighborhood points in the horizontal, and spline interpolation is applied in the vertical and 32
atmospheric pressure levels (1000, 975, 950, 925, 900, 875,
850, 825, 800, 775, 750, 700, 650, 600, 550, 500, 450, 400,
350, 300, 250, 225, 200, 175, 150, 125, 100, 70, 50, 30, 20,
and 10 hPa) are prepared in addition to the surface.

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T. MURAMATSU et al.
51

3000
2000
1000

332

156

51

500
400
300
200
100

F0

c 5000 JJA

1822

F1

F2

F3

F4−5

1104

326

90

28

4000
3000
2000
1000
0

859
704
b 600 MAM

SREH [ m 2 s −2 ]

156

4000

0

CAPE [ J kg −1 ]

332

0

F1

F2

F3

F4−5

1822

1104

326

90

28

F0

F1

F2

F3

F4−5

F0

d 600 JJA
SREH [ m 2 s −2 ]CAPE [ J kg −1 ]859
704
a 5000 MAM

500
400
300
200
100

F0

F1

F2

F3

F4−5

0

Fig. 2. Box plots of the environmental field indices for the F-scales of tornado events observed in the United States. (a)Maximum CAPE in MAM, (b) maximum SREH in MAM, (c) maximum CAPE in JJA, and (d) maximum SREH in
JJA. Top (bottom) edges of boxes represent 75th (25th) percentile values. Top (bottom) whiskers represent 90th (10th)
percentile values. The horizontal line within each box represents the median value. White circles indicate the meanvalues, and the numbers above each panel indicate the number of tornado events for each F-scale.

Fig. 3. Geographical distributions of the FSC in the present-day climate. (a) Present-day climate experiment in
MAM, (b) NARR in MAM, (c) present-day climate experiment in JJA, and (d) NARR in JJA.

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Vol. 94A

Fig. 4. Geographical distributions of equivalent potential temperature (θe) at 950 hPa in the present-day climate. (a)Present-day climate experiment in MAM, (b) JRA-55 in MAM, (c) present-day climate experiment in JJA, and (d)JRA-55 in JJA.

potential temperature in JJA (Figs. 4c, d) in the present-day experiment, compared with that in JRA-55,
was caused by both the larger mean temperatureand specific humidity at 950 hPa. In the present-day
experiment, CAPE was therefore overestimated in
JJA, the implication being that the FSC may also havebeen overestimated compared to JRA-55 in JJA.
4.

Future changes

4.1 The United States
Figure 6 illustrates the future/present FSC ratios
in the United States in MAM and JJA based on thepresent-day and future climate experiments. In bothseasons, the differences revealed a statistically significant increase (p = 0.10 based on a two-tailed t-test)of the FSC over a wide region east of 105°W (Figs.

6a, c). In particular, in Tornado Alley during MAM
and JJA, the increase in the FSC reached 5 % (Figs.
6a, c). The fact that the FSC is approximately 10 %in the present-day climate (Figs. 3a, c) indicates thata future/present ratio of the FSCs exceeding 1.5 canbe expected (Figs. 6b, d). The ratio of change is also
large in and around the Atlantic coastal region, except
for the Florida peninsula, where the future change is
not statistically significant. The region with increased
FSC in JJA shifts northward by about 10° comparedwith the analogous region in MAM (Figs. 6a, c). In
MAM and JJA in the United States, the increase in
the appearance frequency of CAPE ≥ 500 J kg–1 in the
future was statistically significant (p < 0.10) except insome regions of the Pacific Ocean coast. The increasein the appearance frequency of CAPE ≥ 500 J kg–1

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T. MURAMATSU et al.

Fig. 5.

111

Same as Fig. 4, but for SREH.

was caused by an increase in the water-vapor mixing
ratio and temperature in the lower troposphere. In
contrast, the appearance frequency of SREH ≥ 150
m2 s–2 did not change significantly (p > 0.10) in MAM
and JJA.
Previous studies based on the SRES-A1B scenario
suggest a future increase of 20–60 % in severe thunderstorms during the warm season in Tornado Alley(Trapp et al. 2007, 2009). Another study based on the
SRES-A2 scenario projected a 20–70 % increase intornado days over the United States between Februaryand April (Lee 2012). Diffenbaugh et al. (2013), who
projected future changes of severe thunderstorms
in the United States based on the fifth phase of the
Coupled Model Intercomparison Project (CMIP5), a
global climate model ensemble experiment, showed
an increase in the frequency of severe thunderstorms
exceeding 100 % (more than twice the frequency ofthe present-day climate) in Tornado Alley and theAtlantic coastal region during MAM and JJA. In
contrast, according to Lee (2012) and Diffenbaugh
et al. (2013), some ensemble members indicate adecrease of severe thunderstorm frequency in thesouthern part of Tornado Alley during JJA. However,
our result for the distribution of the future/present
ratios of FSCs exceeding 1.5 in tornadogenesis possibilities in Tornado Alley and the Atlantic coastal
region (Fig. 6) is within the range of their projections.
We therefore consider our 20-km-mesh climate experimental data and the method of analysis to be also
applicable to the projection of future conditions inJapan.
4.2 Japan
In the case of Japan, the FSC was greater in JJA
than in MAM in the present-day climate (Figs. 7a, d).

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Vol. 94A

Fig. 6. Future changes in the FSC in the United States. (a) Difference (F−P) between the present-day and futureclimate experiments in MAM, and (c) that in JJA. (b) Change ratio (F/P) in MAM, and (d) that in JJA. Areas
with a FSC less than 0.1 % in the future climate are not shaded in (c) and (d). Areas with change ratios ≥ 3.0 and
a FSC less than 0.1 % in the present-day climate are shaded purple in (c) and (d). Areas where the differencesof FSC between present-day and future climate are statistically significant at p < 0.10 by a two-tailed t-test aremarked with diagonal lines.

In the future climate, we found a significant increase(p < 0.10) of the FSC over almost all areas of theJapanese Islands in MAM and on the Japan Sea sideof the Japanese Islands in JJA (Figs. 7b, e). The difference in the FSC (Figs. 7b, e) was considerably smallerin Japan than in the United States (Figs. 6a, c), but it
was comparable to the FSC in the present-day climate(Figs. 7a, d). Therefore, strong tornadoes may become
twice as frequent over almost all of the JapaneseIslands in MAM and on the Japan Sea side of theJapanese Islands in JJA (Figs. 7c, f).
In the future climate, the appearance frequency of
CAPE ≥ 500 J kg–1 is projected to increase significantly (p < 0.10) over almost all areas of the JapaneseIslands in MAM and JJA (not shown). In MAM, agreater increase in FSC was found in southern Japan(Fig. 7b), whereas a high future/present ratio (> 3) of
FSCs was found in western Japan and in some areasof eastern Japan (Fig. 7c). The high ratios in westernand eastern Japan was calculated because in the pres-ent-day climate the appearance frequency of CAPE ≥500 J kg–1 is very low over Japan, except for southern
Japan, and the appearance frequency increases in the
future climate, even in northern Japan. This increasein the appearance frequency over Japan suggests thatin the future climate, strong tornadoes may occur evenin areas where they have never yet been observedin MAM (areas shaded purple in Fig. 7c). In JJA, asignificant (p < 0.10) increase in high CAPE conditions (CAPE ≥ 500 J kg–1) was predicted over andaround the Sea of Japan. This increase was caused
mainly by the increase in sea-surface temperature
over the Sea of Japan and the increase in saturated
vapor pressure of the atmosphere due to the temperature rise in the lower troposphere. On the Pacific
Ocean side of the Japanese Islands to the south of
35°N, the appearance frequency of SREH ≥ 150
m s–2 decreased significantly (p < 0.10) in JJA. Thisdecrease can be explained by a delay in the northward
movement of the Baiu front in the future (Kusunoki

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T. MURAMATSU et al.

113

Fig. 7. Future changes in FSC in Japan. (a) Distribution of FSC in the present-day climate experiment in MAM,
and (d) that in JJA. (b) Difference (F−P) between the present-day and future climate experiments in MAM, and (e)that in JJA. (c) Change ratio (F/P) in MAM, and (f) that in JJA. Areas with a FSC less than 0.1 % in the futureclimate are not shaded in (c) and (f). Areas with a change ratio ≥ 3.0 and a FSC less than 0.1 % in the present-day
climate are shaded purple in (c) and (f). In (b), (c), (e), and (f), areas where the differences of FSC between
present-day and future climate are statistically significant at p < 0.10 by a two-tailed t-test are marked withdiagonal lines.

et al. 2006). The Baiu front is a quasi-stationary front
that causes the rainy season from May to late July in
East Asia.
Southerly winds composed of warm moist air typically flow into the lowest 1 km of the Baiu frontal
region. Strong westerly winds occur at approximately
3-km altitude in the Baiu frontal region because thesoutherly winds veer clockwise with height owing to
the Coriolis force when the low-level air is lifted byconvective activity. The SREH tends to be larger on
the south side of the Baiu front, because the southerly
winds and the upper westerly winds create a largevertical-wind-shear environment (Kato 2015).
In Japan, a considerable percentage of tornadoes is
associated with typhoons that frequently approach theJapanese Islands and make landfall in autumn. Consequently, among the four seasons, tornados occur withmaximum frequency in autumn, primarily the monthsof September–October–November (SON) (Niino et al.
1997). The FSC increases slightly, even in SON in the
future (Fig. 8), but the increase is not as significant as
in MAM and JJA (Fig. 7). This slight increase might
be caused by a decrease in the approach and landfallof typhoons in Japan, although strong typhoons areprojected to increase in the future (Murakami et al.
2012b). In fact, Murakami et al. (2012b), who studied
potential future changes in tropical cyclone activity
by using the same 20-km-mesh climate experiment
dataset as in the present study, have shown that theannual mean frequency of tropical cyclone landfalls
or land approaches (within 200 km of the coast) will
decrease by 35 % in the western North Pacific (seetext in Murakami et al. 2012b for the domain definition). They have also shown that the annual mean
instantaneous maximum surface wind speed of a tropical cyclone within 200 km of the coast of the western

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Journal of the Meteorological Society of Japan

Fig. 8.

Same as Fig. 7, but for SON.

North Pacific will increase by 6.5 m s–1. However,additional investigations focusing on the structure oftropical cyclones, especially as they relate to tornadogenesis, will be required to project future changesof tornadoes associated with tropical cyclones.
5.

Vol. 94A

Comparison with an ensemble experiment

To verify the accuracy of the projected experimental results deterministically calculated with one
control run (hereafter, the deterministic experiment),
an ensemble experiment was also conducted using an
atmospheric general circulation model with a 60-km
horizontal resolution (TL319) (Endo et al. 2012). In
the ensemble experiment, three different cumulus
convective parameterizations, namely the Yoshimura
scheme (Yukimoto et al. 2011), the improved versionof the Arakawa–Schubert scheme (Arakawa andSchubert 1974; Randall and Pan 1993), and the Kain–
Fritch scheme (Kain and Fritsch 1990, 1993) wereused. Moreover, four different SSTs were specified. A
total of 12 ensemble runs were therefore performed.
In the deterministic experiment, only the Yoshimura
scheme was used, and the SST was averaged from thepredictions of 18 coupled atmosphere-ocean models
that participated in the third phase of the Coupled
Model Intercomparison Project (CMIP3). Based on
the predictions of the coupled atmosphere-ocean
models, the SSTs used in the ensemble experiment
had a deviation pattern of El Niño-like SSTs, which
exhibit a larger increment in the east-central tropical
Pacific Ocean than in the western tropical Pacific
Ocean.
Figure 9 illustrates the differences in sea-level pressure (SLP) and the 850 hPa meridional wind (V850)
in the United States between the present-day andfuture climates in JJA. In both the deterministic andensemble experiments, positive anomalies of SLP
were commonly found in the Gulf of Mexico andrelatively lower values were found over the Great
Plains (Figs. 9a, c). The anomaly pattern caused anincrease in the northwest–southeast pressure gradientin the lower troposphere that intensified the V850 in anorth-south direction around 100°W (Figs. 9b, d). The
advection of moist, warm air associated with V850
intensification increased the appearance frequencyof high CAPE and SREH, which increased the FSC
in Tornado Alley in JJA (Figs. 6c, d). These common
features indicate that future changes in the FSC in
the deterministic experiment are highly credible. In
contrast, in Tornado Alley at latitudes of 35–40°N,
although all ensemble members in the 60-km experiment projected decreases in SLP, the deterministic
experiment projected slight increases. The 60-km
ensemble member, which involved the same experimental design as the deterministic experiment,
produced results similar to the ensemble mean (Fig.
9c). This similarity may be attributable to the difference of the reproducibility of atmospheric phenomena
as a function of horizontal resolution. Some of thesedifferences were caused by terrain effects. Althoughsome uncertainty related to model resolution exists,
the common increase in V850 supports the high probability of an increase in the FSC in Tornado Alley.
Figure 10 illustrates the differences in SLP and
V850 in Japan between the present-day and futureclimates during MAM, when the FSC is projected toincrease over almost all areas of the Japanese Islandsin the future (Figs. 7b, c). In the deterministic experiment and in all ensemble members, the largest negative anomaly of SLP was commonly found at thenortheastern edge of the Japanese region, whereas
positive anomalies were found over the ocean south of

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Fig. 9. Future changes in sea-level pressure (SLP) and 850-hPa meridional wind (V850) in the United States
in JJA. (a) Future changes (shadings and white contours) and the present-day climate (black contours) of SLP
for the 20-km model, and (b) those in V850 and 850-hPa wind (vector). (c) Same as (a), but for 60-km model
ensemble experiment with 12 members, and (d) same as (b), but for the 60-km model ensemble experiment. For (a)and (b), areas where the differences were significant at p < 0.10 determined by a two-tailed t-test are marked withdiagonal lines. For (c) and (d), areas where the sign of the future change was matched by all (more than 10) of 12
members are marked with heavy (light) diagonal lines. For (a) and (c), areas above 1000 m are masked in gray.

the Japanese Islands (Figs. 10a, c). The result that thepressure gradient force produced from this anomaly
pattern intensified the southwesterly winds in westernJapan can be confirmed by the V850 (Figs. 10b, d).
This intensification of the southwesterly winds wasconsistent among all four ensemble members thatused the Yoshimura scheme. This intensification andits associated advection of moist, warm air increased
the frequency of high CAPE. These common features
also support the results of the deterministic experiment.
For the United States, in both the deterministic and
11 of 12 ensemble experiments, positive anomalies
in V850 were commonly projected in Tornado Alleyin MAM. Furthermore, the fact that the anomalies
were more pronounced in the ensemble experimentthan in the deterministic experiment indicates that the
FSC may increase more in Tornado Alley in MAM
in the climate of the future than was predicted by thedeterministic experiment (Figs. 6a, b). The ensemble

members that used the SST pattern with a slightincrement of SST over the central Pacific (Cluster1
in Murakami et. al. 2012a; see their Fig. 2) projected
relatively large increases of V850 in Tornado Alley in
MAM.
For Japan, the fact that negative anomalies in SLP
extending in an east–west direction were commonly
projected in JJA of both the deterministic andensemble experiments indicates a delay in the northward movement of the Baiu front. The deterministic experiment projected V850 to intensify on theJapan Sea side of the Japanese Islands in JJA. Thisintensification led to an increase in CAPE due to theincrease in low-level specific humidity. The intensification in V850 was common among all four ensemble
members that used the Yoshimura scheme. In the
East China Sea south of 30°N, however, most of theensemble members that used the Kain–Fritch schemeor the SST pattern with a large increment of SST in
the tropical Western Pacific (Cluster3 in Murakami

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Fig. 10.

Vol. 94A

Same as Fig. 9, but for the Japan region in MAM.

et. al. 2012a; see their Fig. 2) projected a decrease in
V850. Negative anomalies of SLP were projected to
expand further southwest by these ensemble members
than by the other members, and the climatologicalpressure-gradient force toward the west was weakened at low levels over the East China Sea south of
30°N. The decrease in the inflow of warm, moist air
due to weakening of southerly winds might depress
convective activities over the Baiu front zone. Future
increases of the FSC might therefore be smaller thanprojected by the deterministic experiment (Figs. 7e,f).

6.

Summary and discussion

This study has shown that the FSC will increasein Japan between MAM and JJA. Future changes in
the United States were also investigated to verify thesuitability of our method of analysis by comparing
the simulated future changes with those of previous
studies. In the United States, the predicted distributions of the future/present ratio of FSCs (Fig. 6) werein almost all cases consistent with those of previous
studies, an example being the prediction that the ratio
would exceed 1.5 in Tornado Alley.
Nearly a doubling of the FSC was projected inalmost all areas of the Japanese Islands in MAM andJanuary 2016

T. MURAMATSU et al.

on the Japan Sea side of the Japanese Islands in JJA.
This increase was caused by a large increase in thefrequency of appearance of high CAPE during bothtime intervals, although some decrease was found to
be statistically significant (p < 0.10) in areas of high
SREH in and around the Japanese Islands in JJA.
Whereas CAPE ≥ 500 J kg–1 and SREH ≥ 1502
m s–2 were defined as suitable thresholds for thegenesis of strong tornadoes in this study, they are
only necessary conditions for tornadogenesis. In fact,
the frequency of occurrence of CAPE ≥ 500 J kg–1
and SREH ≥ 150 m2 s–2 (Fig. 3) is much higher thanthat of tornadoes. This suitable condition is therefore
a diagnostic guide for supercell storm generation.
Moreover, previous studies have shown that only 26% (Trapp et al. 2005) or as few as 3 % (Jones et al.
2004) of radar-detected supercell storms are accompanied by tornadoes.
In addition to CAPE and SREH, we also investigated convective inhibition (CIN) as a predictorof tornadogenesis, because large CIN suppresses
the development of moist convection. On the basisof a statistical survey of CIN when tornado eventsoccurred in the United States (the same procedure wasused in the cases of CAPE and SREH, as describedat Section 2.3), we added 30 ≤ CIN ≤ 400 J kg–1 tothe conditions of CAPE ≥ 500 J kg–1 and SREH ≥150 m2 s–2. We then estimated the future changes ofthese conditions. The lower threshold of CIN (30 J
kg–1) was the 10th percentile value, and the higher
threshold (400 J kg–1) was the 90th percentile value
in all tornado events (not shown). The lower thresholdvalue was applied as well, because tornado events
almost never occurred when the CIN was very small,probably because low-level dry air, which reinforces
downdrafts and cold outflow associated with supercell storms, is an important factor that contributes totornadogenesis (Markowski and Richardson 2010;
Noda and Niino 2010). The results showed that thecharacteristics of the distributions of FSC future/present ratios were very similar to those without theCIN condition (Figs. 6b, d), although FSC future/present ratios tended to increase slightly by adding theCIN condition.
In this study, future changes of tornadogenesis
in Japan were estimated with the thresholds (CAPE≥ 500 J kg–1 and SREH ≥ 150 m2 s–2) based on thestatistics of U.S. tornado events (Figs. 7, 8). However,
the results might differ if other suitable conditions for
tornadogenesis are adopted. Moreover, it is importantto verify that the threshold based on U.S. tornado
events is applicable to future changes of the FSC in

117

Fig. 11. The domain (shaded gray) used in theinvestigation with the JRA-55. The bias corrections in the appearance frequency of CAPE and
SREH between JRA-55 and the present-day
experiment were conducted in this domain.

Japan. Other thresholds of CAPE and SREH based ona statistical survey of tornado events (F-scale ≥ 2) that
occurred in Japan from January 1961 to December
2012 were therefore examined. This survey of theJapanese region was conducted using JRA-55 witha grid spacing of 0.5° × 0.5°. Bias corrections in theappearance frequency between JRA-55 and the present-day experiment were conducted because the 0.5°
× 0.5° horizontal resolution was not enough to represent conditions suitable for tornadogenesis (Potvin
et al. 2010). These procedures were as follows:(1) The 25th percentile value of the maximum
CAPE (CAPE-25th) when tornado events occurred
was calculated with JRA-55. The definition of a
tornado event was the same as that used in the UnitedStates (described in Section 2.3).
(2) The area-averaged appearance frequency of
CAPE ≥ CAPE-25th was calculated over the area
shown in Fig. 11 from January 1979 to December
2003. This area was selected to conduct bias corrections around the Japanese islands.
(3) In the present-day experiment in the same area,
the threshold that produced an area-averaged appearance frequency of CAPE equal to that calculated in(2) from JRA-55 was estimated.
(4) Steps (1)–(3) were also done for SREH.
The new suitable conditions for tornadogenesis are

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Vol. 94A

Table 1. Thresholds for tornadogenesis. The values are the 25th percentile values of
CAPE or SREH when tornado events occurred in Japan from January 1961 to December 2012. The revised thresholds of CAPE and SREH in each season were calculated
based on the bias correction between JRA-55 and the present-day experiment using
data from January 1979 to December 2003.
Parameter

SeasonThreshold in JRA-55

Revised threshold for Japan

CAPE [J kg–1]

DJF
MAM
JJA
SON

20
20
250
190

30
50
810
700

SREH [m2 –2

DJF
MAM
JJA
SON

130
130
180
220

160
150
190
270

s ]

Fig. 12. As in Fig. 7c, but for future changes in FSC based on a statistical survey of tornadoes (F-scale ≥ 2) that
occurred in Japan during January 1961 to December 2012. (a) Change ratio (F/P) in MAM, (b) that in JJA, and (c)that in SON. The conditions suitable for tornadogenesis were as follows: CAPE ≥ 50 and SREH ≥ 150 for MAM;
CAPE ≥ 810 and SREH ≥ 190 for JJA; and CAPE ≥ 700 and SREH ≥ 270 for SON. Units of CAPE and SREH are
J kg–1 and m2 s–2, respectively.

shown in Table 1: CAPE ≥ 50 J kg–1 and SREH ≥ 150
m2 s–2 for MAM; CAPE ≥ 810 J kg–1 and SREH ≥ 190
m2 s–2 for JJA; and CAPE ≥ 700 J kg–1 and SREH ≥270 m2 s–2 for SON. In this survey with JRA-55, for
December–January–February (DJF) and MAM, the25th percentile value was calculated from 17 tornado
events that occurred in both seasons (9 in DJF, 8 inMAM) because the mean differences of CAPE and
SREH when tornado events occurred were not statistically significant (p > 0.10, two-tailed Student’s t test)
between the two seasons.
Figures 12a and 12b show that the patterns of
future changes in MAM and JJA are similar to
Figs. 7c and 7f, respectively, which project nearly adoubling or more over much of the area of the Japa-

nese Islands. In both time intervals, therefore, theincreased occurrence of tornadoes appears to behighly robust when compared with the use of other
CAPE and SREH thresholds. In SON (Fig. 12c);
however, the area with a significant increase wasprojected to become smaller than that shown in Fig.
8c.
The present results are based on a climate experiment that was conducted deterministically with one
20-km-mesh control run; it was not an ensemble
experiment with multiple runs. There is likely to be,
therefore, some uncertainty in the conclusions of thepresent study about future changes in the environmental fields associated with the occurrence of tornadoes. Fortunately, the ensemble experiment with aJanuary 2016

T. MURAMATSU et al.

60-km resolution model supports the robustness ofthe deterministic projection with a 20-km model. The
common projection is that there will be a significant
increase in the frequency of strong tornadoes in Japanin the future.
Acknowledgments
The authors would like to express their heartfelt thanks to Dr. Tomoaki Ose of the Meteorological Research Institute of the Japan Meteorological
Agency and two anonymous reviewers for theirvaluable comments and discussions. This work was
supported by the KAKUSHIN and SOUSEI Programsof the Ministry of Education, Culture, Sports, Science,
and Technology (MEXT) of Japan. The calculations
were performed on the Earth Simulator. We obtained
observation data of the tornadoes in the United Statesfrom the web site http://www.spc.noaa.gov/climo/
historical.html.
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