February
Journal of2017
the Meteorological Society of Japan, Vol.
R.95,
YOSHIDA
No.1, pp.et35-47,
al. 2017
DOI:10.2151/jmsj.2017-001

The Effect of Water Vapor on Tropical Cyclone Genesis:
A Numerical Experiment of a Non-Developing
Disturbance Observed in PALAU2010
Ryuji YOSHIDARIKEN, Advanced Institute for Computational Science, Kobe, Japan
Research Center for Urban Safety and Security, Kobe University, Kobe, Japan

Yoshiaki MIYAMOTO
RIKEN, Advanced Institute for Computational Science, Kobe, Japan
Rosenstiel School of Marine and Atmospheric Science, University of Miami, Florida, USA

Hirofumi TOMITARIKEN, Advanced Institute for Computational Science, Kobe, Japan
Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan

andYoshiyuki KAJIKAWA
RIKEN, Advanced Institute for Computational Science, Kobe, Japan
Research Center for Urban Safety and Security, Kobe University, Kobe, Japan(Manuscript received 29 April 2016, in final from 5 October 2016)

Abstract
The environmental conditions for tropical cyclone genesis are examined by numerical experiment. We focus on
the case of a non-developing disturbance showed features for tropical cyclone genesis in the Pacific Area Long-term
Atmospheric observation for Understanding climate change in 2010 (PALAU2010) observation campaign over thewestern North Pacific. We clarify the importance of the presence of abundant moisture around the disturbance forcontinuous convection and demonstrate that the collocation of a mid-level vortex and a low-level vortex, i.e., thepersistence of an upright structure of vortices, is important in tropical cyclone genesis. We conduct two numericalexperiments using the Weather Research and Forecasting Model Advanced Research WRF model in double nested
domains with a horizontal grid space of 27 km and 9 km for the outer domain and the inner domain, respectively.
The first experiment is based on reanalysis data (a control experiment) and the second includes increased water
vapor content over the northwestern dry area of the disturbance. In the control experiment, the disturbance did not
develop into a tropical cyclone in spite of the existence of the mid-level and low-level vortices. In contrast, thesensitivity experiment shows that a tropical cyclone was formed from the disturbance with increased water vapor
content. The presence of persistent upright vortices was supported by continuous convection until the genesis of theCorresponding author: Ryuji Yoshida, RIKEN, Advanced
Institute for Computational Science, 7-1-26 Minatojima-minami-machi, Chuo-ku, Kobe, Hyogo 650-0047,
Japan
E-mail: ryoshida@riken.jp
J-stage Advance Published Date: 14 October 2016©2017, Meteorological Society of Japan

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tropical cyclone.
Keywords

1.

tropical cyclone genesis; genesis environmental condition; water vapor; mesoscale vortex; nondeveloping disturbance; numerical experimentIntroduction

Tropical cyclones (TCs) are one of the mosthazardous phenomena, and many studies have investigated their structures, development, and decay(Anthes 1982; Chan and Kepert 2010; Emanuel2003). TC genesis (TCG) is one of the most challenging issues in TC studies because of the complicated processes involved. A TC is developed froma convective coupled disturbance over the tropicaloceans (Anthes 1982; Gray 1968, 1975, 1998). The
disturbances are large-scale phenomena with easterly waves and convective systems organized insidethe disturbances. Gray (1975) listed favorable environmental conditions for TCG. These environmental
conditions are 1) a large Coriolis parameter, 2) the
existence of cyclonic relative vorticity at the low-leveltroposphere, 3) weak vertical shear of horizontal
winds, 4) a high sea surface temperature, 5) a verticalgradient of equivalent potential temperature between
the low-level and the mid-level troposphere, and 6)
abundant moisture from the low-level to the mid-leveltroposphere.
The detailed process of TCG is gradually being
understood through remote-sensing observations
and numerical experiments. A mesoscale convective system (MCS) is organized in a convective
coupled disturbance, and long-lasting MCSs sometimes organize mesoscale vortices in their clouds at
the low-level or mid-level. The mesoscale convective
vortex at the mid-level is considered an importantenvironment for the organization of a TC vortex byobservational and numerical studies (Houze et al.
2009; Kieu and Zhang 2008; Ritchie and Holland
1997; Simpson et al. 1997). The vortical hot tower
is mainly a low-level convective scale vortex and issuggested as a possible embryo for the organization of
a TC vortex (Hendricks et al. 2004; Houze et al. 2009;
Montgomery et al. 2006). However, such mesoscalevortices are also found in disturbances that do notdevelop into a TC (a non-developing disturbance) (Fuet al. 2012; McBride and Zehr 1981; Zehr 1992).
It is important to understand the differences
between developing disturbances and non-developing
disturbances. Fu et al. (2012) showed the differencesin environmental conditions between developing
disturbances and non-developing disturbances over
the western North Pacific. Larger meridional gradientsof zonal wind and low-level convergences are foundin developing disturbances. In contrast, the differences
in sea surface temperature, vertical shear of horizontal
wind, and relative vorticity are not significant.
Considering the TCG process, Erickson (1977)
found fewer differences in the convective cloudinessof developing disturbances compared with non-developing disturbances. Meanwhile, Zehr (1992) revealed
that developing disturbances have convective maximums before they become TCs, and the convections
in non-developing disturbances are not persistent.
McBride and Zehr (1981) revealed the similarities and differences between the developing disturbances and the non-developing disturbances over thewestern North Pacific. The common characteristics
are a cyclonic vortex, an upper-level warm core, andan axisymmetric structure for the vortex center. The
developing disturbances have a clearer warm core,a stronger vorticity, and a lower location of peakvorticity compared with the non-developing disturbances. Bessho et al. (2010) also pointed out that thewarm core is well organized in developing disturbances, but the technique to determine the warm core
via satellites is still difficult (Stern and Nolan 2012;
Ohno and Satoh 2015).
The characteristics of the environments and structures in developing disturbances have been gradually revealed by previous studies. To address the
TCG process, we should identify the environmental
conditions and processes that are found in developing disturbances. Tuleya (1991) conducted numerical simulations for two disturbance cases, a developing case and a non-developing case. The environmental conditions in the developing case had moremoisture and a weaker vertical shear of horizontal
wind than the conditions in the non-developing
case. In addition, upper-level warming occurred
continuously only in the developing case. In the nondeveloping case, a drying process at the mid-levelinterrupted the convections and led to insufficient
upper-level warming. The collocation of an upperlevel warming over the low-level vortex was also an

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important feature in the developing case. In contrast,
upper-level warming was located to the east of the
low-level vortex in the non-developing case. Although
Tuleya (1991) conducted a sensitivity experiment by
increasing water vapor content with 10 % relative
humidity, a TC did not develop in the non-developing
case.
Even after these abovementioned studies and theunderstanding of these differences, it is still not clear
if non-developing disturbances have the potential
to develop into a TC. Non-developing disturbancesmight lack a feature crucial for their development
into a TC. Another issue is the process required for
TCG. If non-developing disturbances can develop
into a TC under optimal environmental conditions,
understanding which process is necessary for thedevelopment process of TCG is crucial. Therefore,
we conduct two numerical experiments in this study
to determine: (1) the potential of non-developing
disturbance to develop into a TC and (2) a necessary
process for the development.
We describe a target case and the simulation
settings in Section 2. We explain the control experiment based on reanalysis data in Section 3 and the37experiment with increasing water vapor content in
Section 4. Finally, we provide a summary of thepaper in Section 5. In this paper, a tropical cyclone
is defined as a convective disturbance with the eye ofthe vortex for convenience.
2. A target case and simulation settings
We focus on a convective coupled disturbanceobserved during the Pacific Area Long-term Atmospheric observation for Understanding climate change
in 2010 (PALAU2010) observation campaign operated by the Japan Agency for Marine-Earth Scienceand Technology (JAMSTEC). In the PALAU2010
observation campaign, a disturbance exhibited thepotential to develop into a TC; this disturbance wasobserved via a dropsonde during June 18–22. Herein,
we use the observation data obtained during thisperiod for verifying the numerical simulation. The
disturbance in question is a low of an easterly wave.
Easterly waves are well known as precursors of TCs(Gray 1968, 1975, 1998; Heta 1990, 1991; Yanai
1961), and TCGs with an easterly wave compriseapproximately 20 % of the TCG events over thewestern North Pacific (Lee et al. 2008; Ritchie andFig. 1. Blackbody temperature (TBB) distribution observed by MTSAT-1R (IR1 channel) from 12Z June 18, 2010
to 00Z June 20, 2010. Red circles show the location of the disturbance, and the radius of the circle is 10°.

Fig. 2. Vertical component of relative vorticity at 12Z on June 18, 2010 at the 500 hPa level and the 850 hPa level.
Red circles show the same area with the same size as show in Fig. 1. The relative vorticity was calculated using
NCAP-FNL 1° data.

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Fig. 3. A computational domain setting. Dark rectangles show the domain areas for Domain 1 and Domain 2,
respectively.

Holland 1999; Yoshida and Ishikawa 2013).
Figure 1 shows the infrared images observed by ageostationary satellite MTSAT-1R. The disturbance
moved westward from 150°E along 7°N during theobservation period. At 12Z on June 18, 2010, in the
disturbance highlighted by the red circle, an MCS
was found with a blackbody temperature (TBB) lower
than 208 K on a horizontal scale ranging several kilometers. The MCS did not continue for a long period
and dissipated by 12Z on June 19. This disturbanceis also represented by the National Centers for Environmental Prediction final analysis data (NCEPFNL; NCEP 2000). It can be observed in the verticalvorticity field at 850 hPa around 8°N/160°E at 00Z
June 16, and develops gradually moving westward.

Figure 2 shows a relative vorticity at 12Z June 18,
2010; the red circles show the same location indicatedin Fig. 1 (i.e., the area of the concentrated convection). Positive relative vorticities are present both at
the 500- and 850-hPa levels, indicating that the disturbance has vortices at the mid-level and low-level.
Although these vortices can be identified on June 23
in the NCEP-FNL, the disturbance did not develop
into a TC and dissipated after June 23. From the dropsonde data, the low-level temperature in the cloudy
area in the disturbance is approximately 1 K lower
than that in the clear sky area (not shown). The clear
sky area is located at the northwestern side of thecloudy area. The low temperature implies the existence of a cold pool at the location and supports the

Table 1. The experimental settings for the numerical simulation.

settings

Domain 1

Domain 2

horizontal grid space

27 km

9 km

horizontal grid number (x, y)(200, 140)

(436, 298)

vertical levels

36

36time integration period

12Z June 16, 2010–00Z June 23, 2010

initial/boundary data

NCEP-FNL

model top level

50 hPa levelcloud microphysics scheme

WSM6

cloud parameterization

Kain–Fritsch (5 min interval)

boundary layer scheme

Yonsei University scheme

radiation scheme

RRTM and Dudhia scheme

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identification of the MCS from the satellite data.
In this study, the Weather Research and Forecasting
Model (WRF)-Advanced Research WRF (WRFARW) model version 3.4.1 (Skamarock et al. 2008)is used for numerical simulation. The computational
domains are double nested, as shown in Fig. 3, andthe horizontal grid space is set as 27 km and 9 km forthe outer domain and the inner domain, respectively.
The number of vertical levels (layers) is 36 for both
domains. The one-way online domain nesting system
is used for efficiently preparing the lateral boundary
condition for the inner domain. The experimental
settings are listed in Table 1. The time integration
period is from 00Z June 16, 2010 to 00Z June 23,
2010. To allow adequate spin-up of the atmosphericfield and simulate the genesis process in the model,
the initial time is set as two days before the development of the disturbance at 00Z June 18, 2010.
3.

Control experiment and time evolution of anon-developing disturbanceThe simulated horizontal distribution of the cloudy
area is shown in Fig. 4. Deep convections associatedwith the simulated disturbance spread over an area
wider than the observation area shown in Fig. 1. The
simulated disturbance develops as a circular cloudcluster by June 20, 2010. It did not develop into a TC
by the end of time integration. The temperature in theclear sky is approximately 0.5 K higher than that in
the cloudy area of the disturbance (not shown); this isconsistent with the dropsonde observations.
Figure 5 shows the relative vorticity field toconfirm the simulated vortex structure. Positive
vorticities are found both at the 500- and 850-hPa
levels, same as those reported in the NCEP-FNL
shown in Fig. 2. The positive vorticity areas at the
500 hPa level collocate over the positive vorticity

39

at the 850 hPa level at 00Z on June 19, i.e., thevortices have an upright structure in the convective
coupled disturbance. According to previous studies
on TCG (Ritchie and Holland 1997; Simpson et al.
1997), an upright structure of the vortices is foundin developing disturbances and is attributed to TCG.
However, in this simulation, the positive vorticity
area at the 500 hPa level moves away toward the westof the positive vorticity at 850 hPa by 00Z on June
20. The upright structure of the vortices graduallytilts westward for 24 h. In this simulation, since thevertical shear of horizontal wind is approximately 10m s−1 with almost easterly shear, the structure of thevortices leans out toward downshear. The simulated
disturbance fails to maintain the upright structure of
the vortices and dissipates eventually. A similar tilted
structure has been found in non-developing disturbances. According to Tuleya (1991), upper-level
warming is located just above the low-level vortex
in developing disturbances, while it is located to theeast of the low-level vortex in non-developing disturbances.
One of the possible reasons for the tilting of thevortices is the vertical shear of horizontal wind.
However, from the viewpoint of storm scale, themagnitude of the easterly wind shear between the
850- and 400-hPa levels is approximately 10 m s−1
in the simulation, which is calculated by area-averaging over a rectangular area of one degree by one
degree centering on the disturbance. The magnitude
of vertical wind shear is moderate compared to theaverage value of the vertical shear in TCG (Yoshida
and Ishikawa 2013). McBride and Zehr (1981)
concluded that vertical shear is substantially strong
even in developing cases over the western North
Pacific. Fu et al. (2012) also found that a differencein the strength of vertical shear was small between
developing cases and non-developing cases over the

Fig. 4. TBB distribution of the simulated disturbance in Domain 2 (9 km) from 12Z June 18, 2010 to 00Z June 20.
The TBB is estimated from outgoing longwave radiation based on the Stefan–Boltzmann law. Red circles show
the location of the disturbance, and the radius of the circle is 10°.

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Fig. 5. Vertical component of relative vorticity in Domain 2 from 00Z June 19, 2010 to 00Z June 20. The color
shading shows the relative vorticity at the 850 hPa level, and the blue contours show a relative vorticity of
1.0×10-4[s−1] at the 500 hPa level.

western North Pacific. Therefore, vertical shear would
not be a crucial environmental condition for TCG
over the western North Pacific.
Fu et al. (2012) emphasized that moisture content at
the mid-level and low level are very different between
developing disturbances and non-developing disturbances. Compared with developing disturbances,
the relative humidity is quite low in non-developing
disturbances at the west of the disturbance both atthe low-level and mid-level. The moisture field in thesimulated disturbance is shown in Fig. 6 along withthe horizontal distribution of convective available
potential energy (CAPE) and relative humidity. The
yellow rectangular area indicates the area around the
moisture convergence of the disturbance, which isshown in Fig. 6b. The northeastern area of the low
has much lower CAPE compared with the southernarea of the moisture convergence. Therefore, theconvection tends to be suppressed over the low CAPE
region. It is also clear that dry air exists over thenorthwestern area of the disturbance.
The vertical cross-section of equivalent potential
temperature and vertical mass flux through the AB
path in Fig. 6b is shown in Fig. 7. In Fig. 7, the redarrow indicates the disturbance location at 450 km
from point A. The large upward mass flux is observedin the disturbance location, and the upward mass flux
corresponds to the convection at the disturbance. The
low equivalent potential temperature air is found in
the northwestern area of the disturbance (from 150
to 300 km) at the mid-level of the troposphere. In
the dry-air area, downward mass flux is observed but
upward mass flux is observed in the upper level ofthe dry-air area. In addition, significantly high equiv-

Fig. 6. Horizontal distributions of environmental factors. (a) Convective available potential energy (CAPE), (b)relative humidity at the 850 hPa level, and (c) the difference in relative humidity between QVplus and CTL at
the 850 hPa level. CAPE was calculated by the lift-up of the parcel at a 100-m height. Yellow rectangles are themodification region for increased Qv. The path A-B shows the path of the vertical cross-section.

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41

Fig. 7. Vertical cross-section of equivalent potential temperature (color shade). The contour shows the verticalmass flux [kg s−1 m−2] (contour) in the A-B path in Fig. 6. For the vertical mass flux, the upward flux is a positive
value. A red arrow shows the location of the disturbance.

alent potential temperatures are detected from thesurface up to a 1-km height. Therefore, this dry air
might not have resulted from the large-scale subsidence but could have been created by advection. A
similar feature regarding the existence of dry air was
suggested by Tuleya (1991) in the non-developing
case.
Therefore, we focus on water vapor content as therequired environmental condition for TCG and verify
a hypothesis that the upright structure of the vortices
in the disturbance can be maintained with continuous
convection against the vertical shear of horizontal
wind. We evaluate the effect of water vapor content
on continuing convection and processes toward TC
via an additional numerical experiment by artificially
increasing water vapor content; this is described in thenext section.
4.

Experiment to increase water vapor content

4.1 Developed into a TC
To evaluate the effect of the water vapor content
on continuing convection and allowing the disturbance to progress toward a TC, we initiated a restartcalculation at 12Z on June 18, 2010, when the disturbance shows remarkable convection. Although Tuleya(1991) increased moisture over the whole computational domain, we increase the mixing ratio of water
vapor (Qv) only in the area indicated by the yellow
rectangle in Fig. 6 to increase the low-level moisture
convergence around the disturbance. Lu et al. (2012)used the data assimilation system to modify Qv in thesensitivity test of a TC genesis case, but we modified
Qv by directly adding Qv for the simple instruction.
We assume that the intense convection will lead to an
intensified upward mass flux, which will strengthen

the low-level convergence. The upper limit of Qv is
set as the average value of the computational domain
at each level, and the Qv is increased up to thisupper limit. The rest of the experimental setting wascompletely the same as that for the control experiment (CTL). In this study, we increased water vapor
content in all the model layers. The difference in relative humidity between the CTL and the Qv increasing
experiment (QVplus) is shown in Fig. 6c. A difference
is found only in the yellow rectangular area shown in
Fig. 6b. The maximum difference in relative humidity
is 30 %, which is observed at the northwestern areaof the disturbance. The increasing amount is larger
than that reported by Tuleya (1991), which was only
10 % in relative humidity. The horizontal wind field
is not affected significantly by this modification at 1 h
after the restart of the time integration process (at 13Z
on June 18), but the low-level convergence is intensified, which is the aim of the experimental setting. The
horizontally averaged wind around the disturbanceis easterly wind from 1000 hPa to 300 hPa and weak
westerly wind aloft 200 hPa. From the viewpoint of
the environment scale, the magnitude of the vertical
shear of zonal wind is approximately 6 ms−1, which is
calculated by area-averaging over the rectangular areaof six degrees by six degrees centering on the disturbance. The magnitude of vertical shear in QVplus is
not significantly different from that in CTL during
several hours after the restart calculation.
Figure 8 shows the time evolution of the disturbance in QVplus. Compared with CTL, at 00Z on
June 19, a more intense convection can be foundin QVplus (Fig. 4), and the cloudy area decreases
rapidly in the first 24 h. The disturbance in QVplus
has a larger area of deep convection, as indicated by

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Fig. 8. TBB distribution of QVplus experiment in Domain 2 (9 km) from 00Z June 19, 2010 to 12Z June 21. The
TBB is estimated from outgoing longwave radiation based on the Stefan–Boltzmann law.

a TBB lower than 208 K. Especially, the convections
are intensified eastward of the moisture convergence(12 –13°N, 144 –150°E), and the vertical profile oftemperature difference from the area-averaged value
at the intensified convection is similar to that observedin CTL. Although the disturbance in CTL starts todissipate after 12Z on June 20, the disturbance in
QVplus continues to develop into a TC till 12Z on
June 21 with an eye formation. The disturbance thatdissipates in reality develops into a TC when water
vapor content is increased. Therefore, non-developing
disturbances have the potential to develop into a TC if
the appropriate environment is present.
The temporal evolution of the relative vorticity isshown both for the 500- and 850-hPa levels in Fig. 9.
Although the low-level vortex is located to the east of
the location of the mid-level vortex in CTL (Fig. 5)
at 00Z on June 20, the low-level vortex is collocatedunder the mid-level vortex during the whole time
period in QVplus. Therefore, the vortices persist in
the upright structure in QVplus. At 00Z on June 19,
the high positive vorticity area (< 3 × 10−4 s−1) in
QVplus is larger than that in CTL. This suggests thatthe low-level vortex in QVplus is more intense.
To investigate the difference in temporal evolution

between CTL and QVplus, we track the low-levelvortex for each experiment and calculate thearea-averaged vorticity, potential temperature, andsea-level pressure in a rectangular area of 90 × 90
km2 centering on the low-level vortex location (Fig.
10). The low-level vorticity in QVplus is always
higher than that in CTL after 12Z on June 18, whichis the start time of QVplus. It is confirmed that the
disturbance in QVplus has a more intense low-levelvortex in comparison to the disturbance in CTL. The
mid-level vorticity above the low-level vortex in CTL
is occasionally negative. In contrast, the mid-levelvorticity in QVplus is continuously positive after 12Z
on June 18. Therefore, it is suggested that the upright
structure of the vortices persists in QVplus but not in
CTL. In addition, the potential temperature averaged
from the 400- to 200-hPa levels in QVplus is higher
than that in CTL, and the sea-level pressure in QVplus
decreases more than it does in CTL. These characteristics would result in a more intense convection andits persistence in QVplus than in CTL.
Thus, increased water vapor content intensifies theconvection around the disturbance, and the intensified
convection induces a stronger updraft and horizontal
convergence at the low-level. The stronger low-level

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43

Fig. 9. Vertical component of relative vorticity for QVplus experiment in Domain 2 (9 km) from 00Z June 19,2010 to 00Z June 20. The color shading shows the relative vorticity at the 850 hPa level, and the blue contours
show a relative velocity of 1.0×10-4 [s−1] at the 500 hPa level.

Fig. 10. VTemporal evolution of (a) vertical vorticity (VR) at the 500- and 850-hPa levels and (b) potential
temperature (TH) and sea-level pressure (SLP). Each is a horizontally averaged value in a rectangular area of 90
× 90 km2 centering on the low-level vortex location. The low-level vortex location was tracked for the CTL and
QVplus experiments. Theta is also vertically averaged between the 400- and 200-hPa levels.

and mid-level vortices are organized by continuous
convections through tilting, stretching, and convergence mechanisms, and the upright structure of thevortices persists against the vertical shear of horizontal wind. Although Tuleya (1991) concluded thatthe vertical shear of horizontal wind was stronger in

the non-developing case than in the developing case,the disturbance developed into a TC at a similar
magnitude of vertical shear in this study.
In addition, the continuous convection intensifies and sustains mid- to upper-level warming. The
mid- to upper-level warming results under decreasing

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Journal of the Meteorological Society of Japanpressure at the low-level below the warming area andthe low pressure at the low-level would intensify the
low-level vortex. The mid- to upper-level warm coreis stable upon intensification of the low-level vortex
considering the thermal wind balance. This process
would work efficiently with collocation of upperlevel warming and the low-level vortex, which is
attributed to the continuous convection. Although amore detailed analysis is needed for complete understanding of the temporal evolution of vorticity and therelationship between vorticity and convective activity,
QVplus experiment supports the hypothesis that abundant moisture leads to continuous convection, whichallows the upright structure of vortices to persist, andthat a non-developing disturbance can develop into aTC. Therefore, sufficient water vapor content is oneof the crucial environmental conditions for continuous convection and the intensification of low-levelvortices.
4.2 Sensitivity of development to the increasing Qv
In Section 4.1, the Qv in QVplus is increased up to
the upper limit as the average value of the computational domain at each level. To evaluate the sensitivityof development to the increasing Qv, we performed
three additional experiments by changing the amountand area of the increasing Qv; (1) increasing amountof Qv is the upper limit to 66 % of QVplus (QVp66), (2) increasing amount of Qv is reduced the upper
limit to 83 % of QVplus, but the increasing area is
extended northward to cover the whole area of dry
air (QVp-83ext), and (3) increasing area of Qv issame in QVp-83ext but the upper limit is increased
to 125 % of QVplus (QVp-125ext). QVp-125ext isused to investigate the impact of the extension of theVol. 95, No. 1

increasing area. Figure 11 shows the difference inrelative humidity between CTL and each experiment.
The other experimental settings are completely thesame as those used in QVplus.
The horizontal distribution of clouds and thetrack of the simulated disturbance in three experiments are similar to those in the CTL, but the track
in QVp-125ext tends to shift northward comparedwith the CTL. The temporal evolution of mid-levelvorticity is shown in Fig. 12, calculated using themethod described in Section 4.1. The disturbances in
QVp-66 and QVp-83ext do not develop into a TC.
Mid-level vorticity is not significant in both cases, andlow-level vorticity is also not intensified (not shown).
In comparison to QVplus, the Qv values in both
QVp-66 and QVp-83ext are insufficient to intensify
the convection and maintain the upright structure of
the vortices. Although the increasing area is extended
in QVp-83ext, the disturbance still does not develop
into a TC. The mid-level vorticity in QVp-83ext issimilar to that in CTL. Therefore, the extension of theincreasing area does not have an impact sufficiently
large to intensify convection. By tracking each vortex
at the mid-level and low-level, the upright structureof the vortices is not maintained in both QVp-66 andQVp-83ext.
On the other hand, the disturbance in QVp-125ext
develops into a TC, but the vorticity is not more
intense than that in QVplus during the first 24 h.
In the first 12 h after the restart at 12Z on June 18,
convections are initiated over the area in QVp-125ext,
which is larger than that in QVplus. As a result, theconcentrated low-level convergence is not organizednear the low of the easterly wave, and the weak broad
convergence would not be appropriate to rapidly

Fig. 11. Horizontal distributions of the difference in the relative humidity; (a) between CTL and QVp-66, (b)between CTL and QVp-83ext, and (c) between CTL and QVp-125ext. Yellow rectangles are the modification
regions for increasing Qv.

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45

Fig. 12. Temporal evolution of vertical vorticity at the 850-hPa level. This is a horizontally averaged value in arectangular area of 90 × 90 km2 centering on the low-level vortex location. The low-level vortex location was
tracked for each experiment.

intensify vorticity. However, the convective activity is
sufficient to maintain the upright structure of vortices,and the disturbance can become a TC.
From these additional experiments, it is suggested
that the increasing Qv has a certain threshold todevelop into a TC, and the increasing Qv around thelow of the easterly wave is appropriate rather than thatover the larger area. In Tuleya (1991), the increasing
amount of moisture would be insufficient for the
disturbance to develop into a TC. In addition, moisture was increased over the whole computational
domain in Tuleya (1991); hence, this setting would
not be appropriate to develop the disturbance because
it prevents the organization of concentrated convergence. On the other hand, Lu et al. (2012) examined
the sensitivity of Qv to moisture by reducing the relative humidity to 60 % of the original amount in the
disturbance that develops into a TC in reality, and the
disturbance still developed into the TC. Therefore, thethreshold of moisture required to develop a disturbance into a TC can be different on a case by casebasis.
5.

Summary

We investigated the potential of non-developing
disturbances for their development into a TC anddemonstrated that simulated disturbances dissipated in nature had the potential for developing
into TCs in the presence of abundant moisture. We
also suggested that the persistence of the upright
structure of vortices in a disturbance is a necessary
process for this development. A numerical simulation for a convective coupled disturbance accompanying a mesoscale vortex observed in PALAU2010

was carried out using the WRF-ARW model. The
convective coupled disturbance selected in this study
did not develop into a TC in spite of the existence ofvortices. The numerical simulation provided a structure and temporal evolution similar to the observations. The simulated disturbance had both a low-levelvortex and a mid-level vortex, and the vortices had an
upright structure. These features are similar to thoseobserved in the developing disturbances reported inprevious studies. However, the upright structure gradually leaned out toward the downshear direction, andthe disturbance did not develop into a TC because
the convection was not continued likely because of alow-moisture condition.
To examine the effect of the moisture field, we
conducted an additional experiment (QVplus) that
increased the water vapor content only near the disturbance. In the QVplus experiment, the convectionin the high-moisture area became more intense andpersisted longer than it did in CTL. As a result, theupright structure of the vortices persisted for a longer
period than in the control experiment. In the experiment conducted by Tuleya (1991), a non-developing
disturbance in the observation did not develop into a
TC under an environment with increased moisture
content. However, under the environment createdin this study, a non-developing disturbance from theobservation data developed into a TC.
We found that a non-developing disturbance has
potential to develop into a TC if the necessary environment conditions were satisfied. As summarized
in the illustration in Fig. 13a, one of the possible
obstructions for the development of a TC was a breakin convection due to the low water vapor content. If

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Journal of the Meteorological Society of Japan

Vol. 95, No. 1

Fig. 13. Schematic of the necessary environment conditions and the process to develop a TC compared with adissipation case. In the developing disturbance, abundant environmental moisture leads to continuous convection,
which allows persistence of the upright structure of the vortices.

the convection did not continue, mid- to upper-level
warming and the intensification of the low-level
convergence and vortex were lost. Therefore, theupright structure did not persist. However, as depicted
in Fig. 13b, mid- to upper-level warming occurred justabove the low-level vortex when the upright structure persisted. When the surface pressure decreased,
the low-level vortex intensified again. A continuing
upright structure was the key process found in developing disturbances.
Acknowledgments
The authors are grateful to the editor of Journal of
the Meteorological Society of Japan and their anony-mous reviewers for their useful and critical comments.
The dropsonde observation data were obtained fromthe observation campaign of JAMSTEC PALAU2010(MR10-03). We express our thanks to Dr. Qoosaku
Moteki for the dropsonde observations. An author(Ryuji Yoshida) of this study joined PALAU2010(MR10-03 Leg2) and appreciated the support
provided by the staff of the observation campaign. We
appreciate the members of the JAMSTEC NICAM
group for providing computational resources. We
appreciate Profs. Tetsuya Takemi and Hirohiko
Ishikawa for providing the support and management necessary for joining the observation team,for providing data from MTSAT-1R, and for fruitfulFebruary 2017

R. YOSHIDA et al.

discussions. We are grateful to Drs. Hiroyuki Yamada,
Masaki Katsumata, Ryuichi Shirooka, Tomoe Nasuno,and Masato Sugi for valuable discussions. We also
thank the members of the Computational Climate
Science Research Team, RIKEN AICS, for their input.
This study was supported by CREST, JST.
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