Journal
August 2022of the Meteorological Society of Japan, 100(4),H. FUDEYASU
617−630, 2022.
et al. doi:10.2151/jmsj.2022-031
Special Edition on Typhoons in 2018–2019

617

Contributions of the Large-Scale Environment to the Typhoon Genesis of Faxai (2019)
Hironori FUDEYASU
Graduate School of Education, Yokohama National University, Yokohama, Japan
Typhoon Science and Technology Research Center, Yokohama National University, Yokohama, Japan

Udai SHIMADA
Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, Japan
Typhoon Science and Technology Research Center, Yokohama National University, Yokohama, Japan

Yoshinori OIKAWA
Japan Meteorological Agency, Tokyo, Japan

Hisaki EITO, Akiyoshi WADA
Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, Japan

Ryuji YOSHIDA1
CIRES, University of Colorado Boulder, Colorado, USA
NOAA Earth System Research Laboratory, Colorado, USA
RIKEN Center for Computational Science, Kobe, Japan
Research Center for Urban Safety and Security, Kobe University, Kobe, Japan

andTakeshi HORINOUCHI
Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japan
Typhoon Science and Technology Research Center, Yokohama National University, Yokohama, Japan(Manuscript received 21 April 2021, in final form 8 March 2022)

Abstract
This study investigated the atmospheric and oceanic contributions to the genesis of Typhoon Faxai in 2019.
Our statistical analysis using the tropical cyclone genesis score (TGS) attributed the tropical disturbance thatdeveloped into Faxai (Pre-Faxai) to easterly waves (EWs). The EW score evaluated by a grid version of the TGS(Grid-EW) averaged around the occurrence of Pre-Faxai was approximately twice as large as the climatologicalmean, and it was the second largest value in the past 38 years. The Pre-Faxai area with high Grid-EW scores
Corresponding author: Hironori Fudeyasu, Faculty of Education, Yokohama National University, 79-2 Tokiwadai,
Hodogaya-ku, Yokohama, 240-8501, Japan
E-mail: fude@ynu.ac.jp
1 Present affiliation: Yokohama National University, Yokohama, Japan
J-stage Advance Published Date: 21 April 2022
©The Author(s) 2022. This is an open access article published by the Meteorological Society of Japan undera Creative Commons Attribution 4.0 International (CC BY 4.0) license (https://creativecommons.org/licenses/by/4.0).

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could be traced back to the eastern North Pacific (ENP) around August 25, 2019. The lower-troposphere environment characterized by high Grid-EW scores was favorable for vortex formation because it provided a containment area for moisture entrained by the developing circulation or lofted by the deep convection therein. The
Pre-Faxai area with high Grid-EW scores moved westward due to the background easterly flow over the ENP andthen entered the western North Pacific (WNP). The Typhoon Intensity Forecast Scheme (TIFS) showed that theimportant environments for its genesis were ocean conditions and the vertical wind shear. The oceanic conditions
contributed to the development of Pre-Faxai as it traveled over the WNP. The enhancement of vertical wind shear
and subsequent suppression of the development of Pre-Faxai were caused by the lower-troposphere easterly winds
associated with high EW scores. They were also caused by upper-troposphere westerly winds associated withan upper cold low northwest of Pre-Faxai. When the vertical shear decreased with weakening of the upper cold
low, Pre-Faxai reached tropical storm intensity on September 4. Therefore, TGS and TIFS detected Pre-Faxai
10 days before the typhoon arose, which indicates that monitoring environmental factors such as EW and verticalwind shear are important for disaster prevention.
Keywords

typhoon genesis; easterly waves; vertical wind shear; upper cold low

Citation Fudeyasu, H., U. Shimada, Y. Oikawa, H. Eito, A. Wada, R. Yoshida, and T. Horinouchi, 2022: Contribu­
tions of the large-scale environment to the typhoon genesis of Faxai (2019). J. Meteor. Soc. Japan, 100, 617–630,
doi:10.2151/jmsj.2022-031.

1.

Introduction

Typhoon Faxai made landfall in Chiba, Japan, at 21
UTC September 8, 2019, causing enormous damage tothe Kanto region. The winds were the strongest everrecorded in Japan, with instantaneous wind speeds of
57.5 m s−1 in Chiba City. According to the best-track(BT) data of the Regional Specialized MeteorologicalCenter Tokyo–Typhoon Center, Faxai first reachedtropical storm intensity (maximum sustained windspeeds greater than 17 m s−1) at 18 UTC September 4,
2019. We refer to this as the typhoon genesis in thisstudy. There were only 4 days for notification prior
to landfall after typhoon genesis (Fig. 1). However,tropical cyclones (TCs) are generally first observed as
tropical disturbances or lows several days before thetyphoon genesis stage, which is defined as the period
from the appearance of a tropical disturbance to thetyphoon genesis itself.
The Japan Meteorological Agency (JMA) estimatesthe intensity and position of typhoons over the ocean
mainly using the Dvorak method (Dvorak 1984), basedon an analysis of Japanese Geostationary Meteorological Satellite cloud images. Because the organization
period of clouds associated with a tropical disturbanceduring the genesis stage is short, the data are insufficient for analysis purposes. The Dvorak method isapplied only a few days before the typhoon genesis.
Therefore, the JMA has developed what has been referred to as “Early Dvorak Analysis” (EDA) to determine the position of tropical disturbance objectively

based on Geostationary Meteorological Satellite cloud
images (Tsuchiya et al. 2001). Improvements to the
EDA (Kishimoto et al. 2007; Kishimoto 2009) haveallowed its use in forecasting operations since 2007.
According to the EDA, the tropical disturbance thatdeveloped into Typhoon Faxai (hereafter, Pre-Faxai)
was first detected over the eastern North Pacific (ENP),
immediately east of the International Date Line, at 12
UTC August 29, 2019 (Fig. 1). The genesis stage ofFaxai, from the first detection of Pre-Faxai to typhoon
genesis, lasted 6 days. The average duration of the TC
genesis stage is 1.4 days, and only 3 % of all cases
exceed 6 days, according to Fudeyasu et al. (2020)
who analyzed the genesis stages of 476 TCs (including
cases that dissipated before reaching tropical storm intensity) using EDA data for the period 2009 – 2017. Incomparison, Pre-Faxai had an unusually long genesisstage, which was affected by the surrounding environment.
Ritchie and Holland (1999) examined large-scale
flow patterns as the environmental factor contributing to typhoon genesis over the western North
Pacific (WNP), i.e., the shear line, confluence region,
monsoon gyre, easterly waves (EWs), and Rossby
wave energy dispersion from a preexisting TC. The
shear line and confluence region are enhanced by
the easterly trade winds and westerly winds over the
WNP. Typhoon genesis in the monsoon shear line
occurs when the disturbance is located in a region of
low mean sea-level pressure (Chen and Weng 1998).
The confluence region between easterly and westerlyAugust 2022

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flows contains a range of scale interactions that cancontribute to enhanced typhoon genesis conditions(Zhang and Webster 1989; Chang and Webster 1990).
The monsoon gyre is a synoptic-scale gyre embedded
within a developed Asian monsoon trough (Lander
1994; Chen et al. 1996). EWs occur in synoptic-scaleeasterly trade winds, with the trough of EWs providing an environment that is favorable for the genesisof a tropical disturbance (Heta 1990, 1991). A mature
typhoon disperses its energy as a Rossby wave in asoutheastward direction, and low-pressure areas of the
wave train sometimes develop into another typhoon(McDonald 1998; Li and Fu 2006; Li et al. 2006).
Yoshida and Ishikawa (2013) developed a TC genesisscore (TGS) as an objective index to determine thecontribution of five environmental factors to theoccurrence of a tropical disturbance. This study used
the TGS to identify the main environmental factors
contributing to the occurrence of Pre-Faxai.
Furthermore, the developments in the main environmental factor of Pre-Faxai were investigated usinga gridded TGS (Grid-TGS). Yoshida and Fudeyasu(2020) developed the Grid-TGS, which obtains thegrid point values of five selected environmental
factors. It is possible to investigate the temporal andspatial changes in each environmental factor, namely,
the preconditioning of the environment leading totyphoon genesis. A detailed understanding of thepreconditioning for typhoon genesis is important fordisaster prevention and predictions of typhoon genesis.
This study also used the Typhoon Intensity Forecast
Scheme (TIFS) based on the statistical hurricane
intensity prediction scheme (Yamaguchi et al. 2018;
Shimada et al. 2018) to assess the environmental contributions to the development of Pre-Faxai’s intensity
during the genesis stage. The statistical hurricane
intensity prediction scheme is a statistical model thatpredicts the changes in intensity of hurricanes follow­
ing the initial prediction using a multiple linear
regression equation (DeMaria and Kaplan 1994, 1999;
DeMaria et al. 2005). The advantage of the statisticalhurricane intensity prediction scheme for typhoon
intensity prediction is that the contribution of eachpredictor variable to the total intensity change can bequantified.
This study aimed to investigate the environmental
factors responsible for the occurrence of Pre-Faxai,
using the TGS and reanalysis data. Furthermore, thecontribution of the surrounding environment to thegenesis of Typhoon Faxai was quantitatively determined using the TIFS. The rest of this paper isorganized as follows. Section 2 introduces the meth-Fig. 1. Track (solid line) and daily locations at 00
UTC (circles) of Faxai in 2019 derived from the
BT and EDA data. The blue line denotes the track
detected only by the EDA, the green line denotes
the track from the first detection of the BT data
to typhoon genesis, and the red line denotes the
track of the typhoon after it had reached tropicalstorm intensity.

odology and data. Section 3 describes the main environmental factors and contribution of the surrounding
environment to Faxai’s genesis. Section 4 discusses
the contribution of the surrounding environment toFaxai’s genesis compared with other cases. Finally,
Section 5 summarizes the study.
2.

Methodology and data

This study uses a modified version of the TGS by
the JMA, which we hereafter refer to as JMA-TGS.
Similar to the conventional TGS, the JMA-TGS calculates scores for five previously proposed environmental factors. As an example, the EW score is calculated
as follows:
SCREW = A ⋅( ∂∂vx )⋅ exp( B ⋅ distmin _ EW),

(1)

where v is the meridional wind at 850 hPa, anddistmin_EW is the distance between the nearest troughgrid and the genesis location (hereafter, TGS location). We searched for a trough grid of the easterly
wave, which was determined as the location where
the meridional wind was northward to the east andsouthward to the west. A and B are arbitrary constants,

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where A = 2.0 ´ 10−1 and B = −1.0 ´ 10−2. To determine the main contributor to Pre-Faxai, the five scores
were normalized by their maximum and minimum
values such that they were all between zero and one.
The details are provided in Yoshida and Ishikawa(2013).
As an improvement on the conventional TGS, the
JMA-TGS uses the location and time first detected by
the EDA as the TGS location and time. Briegel andFrank (1997) showed that the occurrence of tropicaldisturbances was affected by large-scale flows fora period of about 3 days. For a conventional TGS
time, the scores for the shear line, confluence region,
monsoon gyre, and EW were obtained by analyzing
data collected 66 h and 72 h before the first detection
according to the BT data, while the score of Rossby
wave energy dispersion from a preexisting TC was
obtained from the first detection time. For the TGS
time of JMA-TGS, scores for the four environmental
factors were obtained using data collected 24, 48,
and 72 h before the first detection according to the
EDA (hereafter referred to as 24-h JMA-TGS andsimilar terms); the score of Rossby wave energy
dispersion from a preexisting TC was obtained by the
first detection time to the EDA (00-h JMA-TGS). The
factor with the highest score was considered the main
contributor to the occurrence of Pre-Faxai.
This study used the Grid-TGS developed by Yoshida
and Fudeyasu (2020) to investigate the temporal andspatial changes in the environmental factors around
Pre-Faxai. While the JMA-TGS was calculated forthe genesis location of Faxai with reference to the BT
data, the Grid-TGS was calculated on each grid pointat each time for the input data in a similar manner to
the JMA-TGS. For example, the grid score for EW iscalculated as follows:
Grid -SCREW =( ∂∂vx )⋅ exp( −1⋅ distmin _ EW).

(2)

The details are provided in Yoshida and Fudeyasu(2020). Note that the Grid-TGS becomes high when
the distance is short. Because each grid point of the
input data is considered a candidate for the genesis location at each time in the calculation of the grid score,
information reading a real genesis location is not
required. The grid score gives a spatial distribution of
the environmental conditions. For simplicity, the gridscore is not normalized, and the grid score was notapplied to determine the main contributor. The gridscore used the Japanese 55-year Reanalysis Project(JRA55) (Kobayashi et al. 2015) as the input data. The
JRA55 dataset has a horizontal resolution of 1.25° forVol. 100, No. 4

both longitude and latitude, with a 6-h time interval.
To compare the grid score values, this study calculated
the climatological mean of scores averaged over a 38year period from 1979 to 2016.
The contributions of the surrounding environment
to changes in the intensity of Pre-Faxai during Faxai’s
genesis stage were quantified using the TIFS, whichis essentially a modified version of the statisticalhurricane intensity prediction scheme (DeMaria andKaplan 1994, 1999; DeMaria et al. 2005). It predicts
the change in maximum wind speed of a typhoon,
as well as the central pressure. Because the BT data
include the TC central pressure before genesis, but not
the maximum wind speed of a typhoon, the centralpressures predicted by the TIFS at a forecast time of
6 h were compared to pressure derived from the BT
data. The atmospheric data and central pressures at theinitial time of each TIFS forecast in this study were
derived from JRA55 and BT data. There are 26 predictors in the TIFS, which are predictors already usedin the statistical hurricane intensity prediction scheme
plus meteorological satellite image data (Shimada
et al. 2018).
This study used merged infrared satellite images
obtained by the Global Precipitation Measurement
mission (Hou et al. 2014; Skofronick-Jackson et al.
2017). Merged data for the Global Precipitation
Measurement multisatellite, including the Geostationary Meteorological Satellite, provide a blackbody
temperature (TBB) over the WNP and the ENP, with atemporal resolution of 30 min and spatial resolution as
small as 0.1° × 0.1°.
3.

Results

3.1  Environmental factors during the genesis stageof Faxai
Figure 2 shows the scores for the five environmental factors of the JMA-TGS, corresponding to the firstdetection of Pre-Faxai according to the EDA. With the
exception of the score of the monsoon gyre for 48-h
JMA-TGS, only the EW score exceeded zero. The
score of the Rossby wave energy dispersion from apreexisting TC for 00-h JMA-TGS was zero becauseof the absence of a preexisting TC near Pre-Faxai.
Therefore, the EW pattern was the environmental
factor associated with the genesis of Pre-Faxai.
Figure 3 shows the distribution of the Grid-TGS
score for EW (hereafter, Grid-EW) and the climatological mean for Grid-EW (Clim-Grid-EW). Grid-EW
scores were averaged over the 3-day period from 12
UTC August 26 to 12 UTC August 29, 2019, before
the first detection of Pre-Faxai by the EDA. The

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Fig. 2. Scores for the environmental factors of
Faxai at the 00, 24, 48, and 72-h JMA-TGS time
points, based on the time and position of Faxai at
12 UTC August 29, 2019, according to the EDA.
SL, shear line; CR, confluence region; GY, monsoon gyre; PTC, Rossby wave energy dispersion
from a preexisting TC.

621

Grid-EW extends east to west in a belt shape within
5 – 15°N in the North Pacific. Compared with ClimGrid-EW averaged for the same period, the distribution of the Grid-EW scores was similar to that of theclimatological mean, but the value of the Grid-EW
score associated with Pre-Faxai was higher than thatof Clim-Grid-EW. Figure 4 shows the time series ofdaily values for each Grid-EW score and the ClimGrid-EW score, which were averaged over the region
around Pre-Faxai bounded by 5 – 20°N and 160°E –160°W. Daily Clim-Grid-EW scores changed seasonally and reached a maximum in mid-August. The
Grid-EW value around Pre-Faxai was approximately
twice as large as Clim-Grid-EW with the second
largest value in the past 38 years.
Figure 5 shows the horizontal winds and the relative
vorticity at 850 hPa, as well as Grid-EW scores. The
area with high Grid-EW scores (greater than 0.12) as-

Fig. 3. The distribution of (a) Grid-EW averaged over the 3-day period from 12 UTC August 26 to 12 UTC August29, 2019, and (b) Clim-Grid-EW averaged over the 3-day period from 12 UTC August 26 to 12 UTC August 29
from 1979 to 2016. Areas with values greater than 0.03 are shaded. The cross represents the position of Pre-Faxai
at 12 UTC August 29, 2019.

Fig. 4. Daily values of Grid-EW (black) in 2019, average of Clim-Grid-EW (red), 75 % confidence interval (green),and 25 % confidence interval (blue) of all cases of Grid-EW averaged over the region bounded by 5 – 20°N and
160°E – 160°W.

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Fig. 5. Horizontal winds (m s−1; vectors) and relative vorticity (s−1; contour) at 850 hPa and Grid-EW values (shaded).
The contour area indicates a relative vorticity greater than 4.0 × 10−5 s−1, and the contour interval is 2.0 × 10−5 s−1.
Areas of Grid-EW values greater than 0.03 are shaded. The cross represents the position of Pre-Faxai (black) andTyphoon Podul (green).

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sociated with Pre-Faxai could be traced to August 25,
when it was around 158°W, 10°N. This suggested thatthe environmental preconditioning leading to the genesis stage of Faxai started over the ENP and moved
westward to the WNP. The environmental conditions
built up diabatically due to the convergence associatedwith the intertropical convergence zone in the ENP,
which began to be restored on August 23.
Within the lower-troposphere environment characterized by high Grid-EW scores, three cyclonic vortices with a relative vorticity greater than 4.0 × 10−5 s−1
developed around 165°W (Pre-Faxai), 157°W, and145°W on August 26. In infrared images derived fromthe Global Precipitation Measurement (Fig. 6), theclouds that developed into Pre-Faxai could be traced
as band-like clouds on August 25. Although the origin
of Pre-Faxai in the band-like clouds developed dueto the horizontal shear in the lower troposphere (Fig.
5), an interval of 10 days was necessary for one of thevortices to develop into a typhoon, unlike previously
studied conspicuous intertropical convergence zone
breakdowns (Hack et al. 1989). Easterly waves represent a broad category of disturbances in the easterly
wind environment in the tropics. They include largescale traveling waves with respect to backgroundflows (e.g., Serra et al. 2008), as well as wavy
disturbances arising from baroclinic and barotropic instabilities, such as the well-known African EWs (e.g.,
Burpee 1972; Thorncroft et al. 2008). The disturbanceassociated with the present vortices could be regarded
as an EW. Whereas its wavelength of ~ 1000 km was
smaller, it is consistent with the results of a previous
statistical study (Fudeyasu and Yoshida 2018).
The vortices moved westward at 5 – 6 m s−1 becauseof the background easterly flow (Fig. 5), which was
4 – 8 m s−1 between 950 hPa and 500 hPa (not shown).
Figure 7 shows the 850-hPa streamlines under the comoving frame for Pre-Faxai. The streamlines aroundthe vortices that developed into Pre-Faxai were closed
circulations under the co-moving frame. The closed
circulation in the co-moving frame served to containmoisture and cyclonic vorticity, providing favorable
conditions for tropical cyclogenesis (Dunkerton et al.
2009). This will be discussed in the next section.
The mid- and upper-troposphere cyclonic vortex,
which was placed over the lower-troposphere vortex,
could not be traced backward in time before August 29(not shown), implying that the disturbance did not previously have a rigid vertical structure. Figure 8 shows
the winds and geopotential heights at 200 hPa and thevertical wind shear between 850 hPa and 200 hPa. On
August 26, a deep trough to the north extending to the623

southwest was observed (Fig. 8a). On the south sideof the trough, the vertical wind shear was enhanced by
the westerly winds in the upper troposphere and theeasterly winds in the lower troposphere. The strong
vertical shear of more than 15 m s−1 would suppress
the vortex developing into Pre-Faxai. Although thelower-troposphere environment associated with highGrid-EW scores was favorable for vortex formation,
the strong vertical shear due to the deep trough slowed
the development of the vortex over the ENP.
3.2 Contributions of the large-scale environment to
the genesis of Faxai
According to the EDA and BT data, Pre-Faxai
moved westward in the WNP during Faxai’s genesisstage (Fig. 1). From the temporal changes in Grid-EW
scores (Fig. 5), it was apparent that the area with highGrid-EW scores associated with Pre-Faxai continued
to move westward and entered the WNP. This suggeststhat the lower-troposphere environment continued to
be favorable for vortex formation over the WNP.
Figure 9 shows the time series of sea-level centralpressure for Faxai from the BT data and the TIFS
model at a forecast time of 6 h. Because the TIFS
forecasts are determined by multiple linear regression,
the accuracy of TIFS forecasts decreases with increasing forecast time. The accuracies of the changes
in intensity up to a forecast time of 6 h, as predicted
by TIFS, are generally high. This is an importantconsideration for statistical evaluation of the contributions of large-scale environmental conditions to thedevelopment of Pre-Faxai. According to the BT data,
Pre-Faxai was first detected at 00 UTC September 2,
3.5 days after the first detection of Pre-Faxai by the
EDA. No significant changes were found in Faxai’s
intensity up to 18 UTC September 3. The central pressure decreased gradually thereafter: typhoon genesis
was observed at 18 UTC September 4. These trends inthe central pressure change were captured by the TIFS
forecasts: the mean decrease in central pressure from
12 UTC September 2 to 06 UTC September 4 (from12 UTC September 4 to 00 UTC September 5) was 1.9
hPa (6 h)−1 in the early genesis stage [3.0 hPa (6 h)−1
in the late genesis stage]. These data allowed evaluation of the contributions of large-scale environmental
conditions to changes in the intensity of Pre-Faxai,
although TIFS forecasts tend to over-forecast slightly
in the early genesis stage.
As shown in Fig. 10, the magnitude of the centralpressure decreased in Pre-Faxai with fractions foreach TIFS predictor at a forecast time of 6 h, over interval of 6 h. The main predictors with a contribution

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Fig. 6. The TBB distribution from the infrared images obtained by the GPM mission. The circles represent the position of Pre-Faxai (red) and Typhoon Podul (green).

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625

Fig. 7. The 850-hPa co-moving streamlines around Pre-Faxai at (a) 00 UTC August 26, (b) 00 UTC August 28, (c)00 UTC September 1, and (d) 00 UTC September 4, 2019. Shading indicates a vertical relative vorticity (10−5 s−1).
Purple curves indicate zero relative zonal flow. The co-moving frame removes 5 m s−1 west–northwestward propagation speed. The relative vorticity is not impacted by the co-moving translation.

of a linear effects more than 1.0 hPa (6 h)−1 were as
follows: the difference between the maximum potential intensity (Emanuel 1986) and the maximum windspeed of a typhoon (POT), ocean heat content (OHC),
and the magnitude of vertical wind shear (SHDC).
Here, a positive value indicates a pressure decrease.
With an average value of +2.8 hPa (6 h)−1, the predictor of POT related to the maximum potential intensity
contributed to Pre-Faxai’s intensity throughout thegenesis stage. The averaged OHC was +2.6 hPa (6 h)−1throughout the genesis stage, significantly contributing
to the development of Pre-Faxai’s intensity. The sea
surface temperature and OHC in the central North Pacific during the genesis stage were both very high (seasurface temperature more than 30°C), and Pre-Faxai
passed over this warm ocean (not shown). The SHDC
had an average value of approximately −1.1 hPa (6 h)−1
in the early genesis stage from 12 UTC September 2
to 18 UTC September 3 and +1.8 hPa (6 h)−1 in thelate genesis stage from 00 UTC to 18 UTC September
4. The change in the contribution of SHDC in the lategenesis stage presumably promoted the developmentof Pre-Faxai.
At 12 UTC August 29, the time Pre-Faxai was firstdetected by the EDA, a deep trough developed in theupper troposphere (Fig. 8b). On September 1, twoupper cold lows were generated and separated fromthe upper-troposphere trough. In this study, an upper
cold low is defined as a depression with a cold air
center in the upper troposphere, and it is related to acutoff low in either midlatitude westerlies (Molinari
and Vollaro 1989; Postel and Hitchman 1999; Sakamoto and Takahashi 2005) or a tropical upper tropospheric trough cell (Sadler 1976; McTaggart-Cowan
et al. 2013). Pre-Faxai moved westward, approaching
one of the upper cold lows around September 3 (Fig.
8c). The TIFS showed that SHDC made a negative
contribution to the development of Pre-Faxai (Fig.
10); the strong vertical shear south of this upper cold
low continued to provide an unfavorable environment
for the development of Pre-Faxai.
Figure 11 shows averaged infrared images derived
from the Global Precipitation Measurement and thevertical wind shear between 200 hPa and 850 hPa
during the genesis stage. The infrared images areaveraged radially from the Pre-Faxai center to the
100-km radius. On September 1, clouds were distributed over a wide range under a weak vertical shear

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Fig. 8. Winds (m s−1; vectors), geopotential height (contours) at 200 hPa, and the magnitude of vertical wind shear
between 850 hPa and 200 hPa (shaded) at (a) 00 UTC August 26, (b) 12 UTC August 29, (c) 00 UTC September 3,and (d) 18 UTC September 4, 2019. The regions with a magnitude of vertical wind shear greater than 10 m s−1 areshaded. The contour interval is 50 m. The black circle represents the position of Pre-Faxai.

Fig. 9. Time series of sea-level central pressure for Faxai from the BT data (blue) and TIFS at a forecast time of 6 h(red).

environment. The clouds then developed mode deeply
within a 100-km radius of the Pre-Faxai center on
September 2 under a strong westerly shear environment. They mainly distributed east of the center of
Pre-Faxai (0 – 180°). The cloud area was swept to theeastern side. Although the environment characterized
by the warm ocean provided favorable conditions
for the development of Pre-Faxai, the strong vertical
shear enhanced by the upper cold low presumably
suppressed the deep clouds. At 18 UTC September

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Fig. 10. Time series (6-h intervals) of the contribution of central pressure changes of Pre-Faxai, divided by threemain predictors, derived from the TIFS at a forecast time of 6 h. Note that positive (negative) values indicate thedevelopment (decline) of Pre-Faxai’s intensity.

Fig. 11. (a) The vertical wind shear between 200 hPa and 850 hPa averaged within a 500-km radius of the TC center(arrows; scale at bottom left). (b) TBB distribution (shaded; K) radially averaged along a 100-km radius from theTC center. Images are obtained from the Global Precipitation Measurement merged infrared.

4, westerlies in the upper troposphere decreased dueto the weakening upper cold low (Fig. 8d), resulting
in weak vertical shear around Pre-Faxai (Figs. 10,11a). Clouds developed more deeply within a 100-km
radius of the center of Pre-Faxai (Fig. 11b). Pre-Faxai
developed and eventually reached tropical storm intensity, resulting in typhoon genesis.
4.

Discussion

Our results using JMA-TGS showed that the initial vortex that developed into Pre-Faxai within thelarge-scale flow pattern was mainly associated with
EWs over the North Pacific. Dunkerton et al. (2009)

estimated the center of a closed cyclone, which wastermed a recirculating Kelvin cat’s eye, based on theoccurrence of tropical disturbances associated with
EWs over the ENP and North Atlantic. The Kelvincat’s eye within the critical layer represents the optimal location for the occurrence of tropical disturbances because it provides a containment area. The area
provides a containment effect inside the vortex formoisture, entrained by the developing circulation and/or lofted by the deep convection therein.
As shown in Fig. 7, the large-scale flows of highGrid-EW scores associated with Pre-Faxai consisted ofvortices that moved due to the mean flow. The initial628

Journal of the Meteorological Society of Japanvortex developed a closed circulation around them.
A Kelvin cat’s eye presumably developed, which
satisfied the conditions suggested by Dunkerton et al.
(2009). This is likely to be one of the reasons that thevortices persisted for long periods prior to dissipation.
Regarding the typhoon genesis of Faxai, the key environmental condition that resulted in the achievementof tropical storm intensity was a decrease in the vertical wind shear. In the early genesis period, an upper
cold low was present to the northwest of Pre-Faxai,
which inhibited the development of Pre-Faxai becauseof strong vertical wind shear (Figs. 10, 11). However,the favorable environment around Pre-Faxai throughout the genesis stage was characterized by a warm
ocean and the strong vertical wind shear.
A statistical analysis (Fudeyasu and Yoshida 2019)of the genesis of TCs during the 38 years from 1979
to 2016 was performed to determine whether an upper
cold low existed within the northwestern quadrant
1,500 km from the TC center, at the time of the firstdetection according to the BT data. In approximately
9 % of all TC cases, typhoon genesis was associatedwith an upper cold low, which provided favorable
conditions for typhoon genesis because of upper-level
divergence and convective available potential energy.
When Pre-Faxai approached the upper cold low, thevalue of the TIFS predictor related to the upper-level
divergence was −0.1 hPa (6 h)−1. This had a minimal
effect on typhoon genesis. Pre-Faxai was generated inassociation with the large-scale flows of EWs, which
dominated in the lower troposphere. Along with EWs
in the lower troposphere, the vertical wind shear wasenhanced by westerly winds in the upper tropospherecaused by the upper cold low. Therefore, the upper
cold low negatively contributed to the developmentof the TC. The contribution of upper cold lows to thedevelopment of Pre-Faxai may have differed according to environmental factors proposed by Ritchie andHolland (1999). Such contributions should be a research target for future studies.
5.

Conclusion

This study investigated the contribution of thelarge-scale environment for the typhoon genesis of
Faxai. Using the JMA-TGS, environmental factors
proposed by Ritchie and Holland (1999) contributing
to the occurrence of Pre-Faxai were revealed. In thisregard, the large-scale flows of EWs were important.
The Grid-EW score averaged for the time and the location where Pre-Faxai was first detected by the EDA
was approximately twice as large as the climatologicalmean for Grid-EW, and it was the second largest valueVol. 100, No. 4

in the past 38 years. The area with high Grid-EW
scores that developed into Pre-Faxai can be traced
back to 158°W on August 25, 2019.
The results of the TIFS forecast showed that theocean conditions significantly contributed to the development of Pre-Faxai throughout the genesis stage,
whereas strong vertical wind shear would suppress
the development of Pre-Faxai. The vertical wind shear
was enhanced by a combination of upper-troposphere
westerly winds associated with an upper cold low
and lower-troposphere easterly winds associated with
EWs. In the late genesis period, the contribution of thevertical wind shear increased when the vertical wind
shear decreased due to the weakening upper cold low.
Pre-Faxai eventually reached tropical storm intensity.
The key factors for typhoon genesis were the ocean
conditions and the temporal changes in the verticalwind shear caused by the weakening upper cold low.
The organized clouds of Pre-Faxai in the area withhigh Grid-EW scores could be traced backward foran extended period from the ENP to the central North
Pacific. There remains no specific explanation for thelong genesis stage of Faxai. In addition, the mechanism of cloud organization leading to Pre-Faxai in the
EW remains an open question. However, this study
showed that the environmental factors associatedwith the genesis of Faxai formed over the ENP 10
days before typhoon genesis. These results imply thata tropical disturbance associated with EWs can bepredicted over a long period before typhoon genesis.
Monitoring the contributions of large-scale environmental conditions associated with initial tropicaldisturbances over the ENP through the Grid-TGS andTIFS is important for TC disaster prevention over the
WNP.
Data Availability Statement
The BT can be derived from the JMA website available at https://www.jma.go.jp/jma/jma-eng/jma-center/rsmc-hp-pub-eg/trackarchives.html. The datasets of
EDA and TIFS analyzed in this study are not publicly
available.
Acknowledgments
The authors wish to thank the editor and two anonymous reviewers for their careful peer review, important comments, and useful suggestions that improved
this article. This research was conducted in collaboration with the JMA and Meteorological ResearchInstitute of JMA and utilized the dataset of JMATGS, EDA, TIFS, and JRA55 provided by the JMA.
This work is supported by the Ministry of Education,

August 2022

H. FUDEYASU et al.

Culture, Sports, Science and Technology (MEXT)
KAKENHI Grant 19K24677, 19H00705, 19H05696,
and 21K03658. This work also is supported by thejoint research program of CEReS, Chiba University(CJ20-23), and Integrated Research Program for Advancing Climate Models (TOUGOU program) from
the MEXT, Japan.
Appendix: Abbreviations
Abbreviation
BT

Definition
The best-track data of the Regional
Specialized Meteorological CenterTokyo–Typhoon Center

Clim-Grid-EW The climatological mean for GridEW
EDA

Early Dvorak Analysis

ENP

The eastern North Pacific

EWs

Easterly waves

Grid-EW

Grid-TGS score for EW

JMAJapan Meteorological Agency

JMA-TGS

A modified version of the TGS by
the JMAJRA55

The Japanese 55-year Reanalysis
Project

OHC

Ocean heat content

Pre-Faxai

The tropical disturbance that developed to Typhoon Faxai

POD

The predictor related to the maximum potential intensity

SDHC

The magnitude of vertical wind
shear

TBB

A blackbody temperature

TGS

TC genesis score

TIFS

Typhoon Intensity Forecast Scheme

WNP

The western North Pacific
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