trends in landfalling tropical cyclone–induced
TRANSCRIPT
Trends in Landfalling Tropical Cyclone–Induced Precipitation over China
LU LIU
State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China
YUQING WANG
International Pacific Research Center and Department of Atmospheric Sciences, School of Ocean and Earth Science and
Technology, University of Hawai‘i at M�anoa, Honolulu, Hawaii, and State Key Laboratory of Severe Weather, Chinese
Academy of Meteorological Sciences, Beijing, China
(Manuscript received 17 September 2019, in final form 6 January 2020)
ABSTRACT
In this study, trends in landfalling tropical cyclone (TC)-induced precipitation over China during 1980–2017
and the involved possible mechanisms are analyzed. Consistent with previous studies, it is found that the total
annual TC precipitation shows a distinct spatial distribution with a significant increasing trend in southeastern
China but a decreasing trend in southern China. This characteristic is found to be related to the increase in
both the annual TC precipitation frequency and the precipitation intensity per TC over southeastern China
but to the decrease in the annual TC precipitation frequency over southern China. A noticeable northward
shift of total landfalling TC-induced annual precipitation has been identified. It is shown that the precipitation
induced by strong TCs (STCs) significantly increased in southern China, whereas that induced by weak TCs
(WTCs) increased in southeastern China, with the latter dominating the northward shift of total landfalling
TC-induced precipitation over mainland China. The increasing trend of STC-induced precipitation in
southern China is found to be closely related to sufficient water vapor supply and the increase in average
duration and intensity of STCs after landfall. The increasing trend of WTC-induced precipitation in south-
eastern China is related to the northward shift of the average landfalling position ofWTCs and changes in the
environmental conditions that are more favorable for TC maintenance and precipitation.
1. Introduction
In response to global warming, precipitation has
shown significant regional dependent trends in the
globe. For more than half of the global land, an in-
creasing probability of intense precipitation events has
been documented by Groisman et al. (2005). A number
of weather systems can produce intensive precipitation
over land. The tropical cyclone (TC) is one of them. A
strong TC can produce torrential rainfall, leading to
floods and landslides after their landfall, which could
cause loss of life and severe damage of properties (Karl
and Easterling 1999; Zhang et al. 2009). Many studies
have shown an increasing trend in the destructiveness of
TCs due to the increasing trend in TC intensity and
lifetime from observations and climate model simula-
tions (Knutson and Tuleya 2004; Emanuel 2005;
Webster et al. 2005; Klotzbach 2006; Knutson et al. 2007;
Knutson et al. 2010; Park et al. 2014; Zhang et al. 2016;
Liu et al. 2020). Some studies have also shown that a
positive relationship exists between TC-induced rainfall
rate and TC intensity (Rodgers et al. 2001; Lonfat et al.
2004; Jiang 2012; Yu et al. 2017; M. Liu et al. 2019).
Other studies have reported significant contributions by
landfalling TCs to both the annual total precipitation
and extreme precipitation events in the United States
(Knight and Davis 2009; Shepherd et al. 2007; Kunkel
et al. 2010).
East Asia is one of the regions that suffer the most
severe damages from landfalling TCs. Several studies
have investigated the changes in spatial distribution and
variability of heavy rainfall induced by landfalling TCs
(Ren et al. 2006, 2007; Kim et al. 2006; Wu et al. 2007;
Ying 2011a,b; Zhang et al. 2013). Kim et al. (2006) found
an abrupt increase in heavy rainfall ($100mmday21)
related to TCs over the Korean peninsula during
August–September after the late 1970s. Ren et al. (2006,
2007) investigated TC precipitation over China andCorresponding author: Prof. Yuqing Wang, [email protected]
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DOI: 10.1175/JCLI-D-19-0693.1
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found a significant downward trend in annual TC precip-
itation volume. Wu et al. (2007) studied the effect of TCs
on the total and extremeTCprecipitation inHainan Island
and found that both the number of TCs affecting Hainan
Island and the TC-induced precipitation and its contribu-
tion to the total precipitation decreased. However, both
the total amount of precipitation from the extreme rainfall
events and the number of extreme heavy precipitation
days affected by TCs increased significantly. On average,
both the number of extreme rainfall days and the precip-
itation amount induced by each TC also increased signifi-
cantly over Hainan Island. Based on a quantile regression
analysis of station data, Ying et al. (2011a) estimated the
TC-induced precipitation trends over mainland China and
found that although the landfalling TC frequency de-
creased over China, especially over South China, both
precipitation per TC and maximum hourly precipitation
induced by a TC increased significantly at stations in the
southeast coastal region. They also found that the location
of significant precipitation trends was related tomountains
and coastline in southeastern China. However, in another
study, Ying et al. (2011b) found that there were insignifi-
cant trends in extreme TC rainfall, such as the maximum
TC precipitation and the maximum hourly precipitation
over China during 1955–2006.
Zhang et al. (2013) further investigated the landfalling
TC precipitation features over mainland China and the
contribution of TC rainfall to changes in the precipita-
tion climate and the average rainfall per TC in the peak
TC season (July–September) during 1965–2009. They
found that the TC rainfall accounted for more than 10%
of the summer rainfall in South and Southeast China,
even more than 40% along the southeast coastline, and
the average rainfall per TC significantly increased in
Southeast China south of the Yangtze River and east of
1108E, which was consistent with the results of Ying
et al. (2011a). Zhang et al. (2013) also found that the
increasing trends in the average rainfall per TC were not
accompanied by enhanced TC intensity, nor did they
result from the slowdown of TC movement. Instead,
they argued that the increasing trends in the average
rainfall per TC in China might be associated with
changes in the East Asian summer monsoon activity
through modulating the moisture transport and other
synoptic forcing conditions.
Although previous studies have revealed many char-
acteristics of TC-induced precipitation over mainland
China, including the long-term increasing trend in the
average rainfall per TC, the detailed spatial patterns and
the contributions by different category TCs have not
been examined, and the involved factors responsible for
the observed trends have not been well understood. In
this study, we explore the spatial distribution of trend
in landfalling TC-induced precipitation over mainland
China and the involved possiblemechanisms. The rest of
the paper is organized as follows. Section 2 describes the
data and analysis methods. The temporal and spatial
characteristics of trends in the observed landfalling TC
precipitation over mainland China are presented in
section 3. Section 4 examines the trend characteristics of
TC precipitation in different categories and the involved
possible mechanisms. The primary findings are sum-
marized in the last section.
2. Data and analysis methods
The best-track TC dataset used in this study is ac-
quired from the Shanghai Typhoon Institute of China
Meteorological Administration (STI/CMA), which in-
cludes latitude and longitude of the TC center, TC in-
tensity in terms of the maximum sustained 10-m wind
speed, and central sea level pressure at 6-h intervals. TCs,
including tropical depressions (TD), that made landfall
over mainland China, including Hainan Island, during
1980–2017 are included in our analyses. In this study, the
STI/CMA best-track dataset is used as the primary TC
data because relatively more observational data were
available over mainland China when the postseason TC
analysis was conducted to generate the best-track TC
data. Actually, the annual postseason analysis of TC data
is performed by STI/CMA to reduce uncertainties and
improve the accuracy using all available data, including
station observations, ship weather reports, automatic
surface observations, synoptic charts, radiosonde data,
aircraft reconnaissance, satellite, coastal radar observa-
tions, and the real-time TC warming advice from various
agencies (Ying et al. 2014). Note that TCs whose centers
remained offshore and did not make landfall over main-
land China were not considered in our analyses.
The daily precipitation dataset used in this study is
obtained from STI/CMA as well. Previous studies sim-
ply defined the TC precipitation as precipitation within a
radius of 550 km from the center of a TC (Englehart and
Douglas 2001) or 500km from the center of a TC (Dare
et al. 2012; Zhang et al. 2019). In this study, the objective
synoptic analysis technique (OSAT) developed by Ren
et al. (2006, 2007) is adopted to isolate the TC-induced
rainfall from the total daily precipitation; a similar
technique was also used in Zhang et al. (2013). TC
precipitation can result from the TC eyewall and spiral
rainbands, or from the interactions between the TC
circulation and other local weather systems. TC rain-
bands are generally asymmetric about the TC center.
Therefore, the simple circle method is not good enough.
The OSAT method imitates the process by which a
weather forecaster manually analyzes a synoptic map.
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There are two primary steps: first, separating the daily
precipitation into several independent rainbands, and
second, according to the distance function between the
TC center and the distribution of the rainbands, dis-
tinguishing which rainbands are related to the TC. The
details of the method can be found in Ren et al. (2006,
2007). Note that because having too few stations with
precipitation is not representative, the only cases that
are considered in our analyses are those for which there
were more than 30 stations at which daily precipitation
was observed in a TC day. The TC precipitation is de-
termined when the TC’s rainbands are over mainland
China, including Hainan Island, even if the center of the
TC was still over the ocean but made landfall later.
The European Centre for Medium-Range Weather
Forecasts (ECMWF) interim reanalysis (ERA-Interim)
data at the horizontal resolution of 0.758 3 0.758 (Dee
et al. 2011) are used to examine changes in large-scale
environmental conditions responsible for the observed
changes in the precipitation characteristics of landfalling
TCs over mainland China. The sea surface temperature
(SST) data are obtained from NOAA’s Optimum
Interpolation Sea Surface temperature (OISST), which
provide a series of global analysis products including the
daily SST at the resolution of 0.258 3 0.258. The soil
moisture is obtained from the NOAA Gridded Climate
Dataset, which has a horizontal resolution of 0.58 3 0.58.
3. Characteristics of landfalling TC precipitation
Figure 1a shows the spatial distribution of the average
annual precipitation induced by landfalling TCs over
mainland China during 1980–2017. Consistent with pre-
vious studies (Ren et al. 2006; Zhang et al. 2013), large
TC precipitation is mainly located in the southern and
southeastern China, including Guangxi, Guangdong, and
Fujian provinces, and Hainan Island. The time series of
total annual TC precipitation over mainland China as a
whole presents a slightly decreasing linear trend (Fig. 1b),
which is consistent with the results of Ren et al. (2006),
who found that the total volume of landfalling TC pre-
cipitation over China was slightly decreasing. However,
the trend of the annual landfalling TC precipitation over
mainland China shows a distinct spatial distribution
(Fig. 1c). The annual TC precipitation increased signifi-
cantly over southeastern China and decreased notably
over southern China, including Hainan Island (Fig. 1c).
Comparing Figs. 1a and 1c, we can see that the maximum
increasing trend in the annual TC precipitation occurs in
the northern area and the maximum decreasing trend
occurs in the southern area of the large precipitation re-
gion, indicating a northward shift in landfalling TC-
induced precipitation over mainland China during the
study period. The frequency of landfalling TCs shows a
significant decreasing trend (Fig. 1d); that is, the number
of TCsmaking landfall overmainlandChina decreased in
recent decades. Further, we calculated the frequency
of landfalling TCs in the southern area and southeast-
ern area (with 248N as the dividing line; Figs. 1e and 1f).
The results show that the landfalling TC frequency in
the southeastern area was stable, while the frequency in
the southern area experienced a significant decreasing
trend during recent decades. Therefore, the decrease in
total landfalling TC frequency over mainland China was
contributed primarily by the decrease in frequency in the
southern area. This suggests that the total annual pre-
cipitation decrease could be primarily due to the signifi-
cant decrease in landfalling TCs over mainland China,
especially in southern China.
Figure 2 shows the time series and trend in the average
precipitation per landfalling TC. There is a significant
increasing linear trend of the average precipitation per
landfalling TC over mainland China as a whole (Fig. 2a)
with statistical significance over the 95% confidence
level. This means that the precipitation intensity per
landfalling TC increased significantly during 1980–2017,
especially in the southeastern coastal region over
mainland China (Fig. 2b). However, overHainan Island,
the precipitation per landfalling TC shows a significant
decreasing trend. We also analyzed the trend of the TC
precipitation per landfalling TC per day, which shows a
significant increasing trend over mainland China as a
whole as well (Fig. 3a). The trend also displays a spatial
distribution with the largest increasing trend in the
southeastern coastal area (Fig. 3b), which contributes to
the increase in the average precipitation per landfalling
TC in the region (Fig. 2b). The trend in annual TC
rainfall days (frequency) shows a clear transition near
248–258N—namely, an increasing trend over southeast-
ern China and a decreasing trend over southern China
(Fig. 3c). Therefore, both the annual precipitation fre-
quency and the average precipitation per TC per day
increased over southeastern China, leading to the large
increasing trend in total TC precipitation over south-
eastern China (Fig. 1c). The decreasing trend in the
frequency of landfalling TCs (Fig. 1d) offsets the in-
creasing trend of the average precipitation per TC per
day over southern China (Fig. 3a). This leads to the
decreasing trend in the total TC precipitation over
southern China as seen in Fig. 1c. Wu et al. (2005)
found a considerable decreasing trend of TCs in the
South China Sea, leading to fewer rainfall days of
landfalling TCs in the southern coastal area over main-
land China. In addition, the deceasing trends in both the
average precipitation per TC per day (Fig. 3b) and the
annual frequency of TC precipitation days (Fig. 3c) give
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FIG. 1. (a) Spatial distribution of the landfalling TC-induced annual precipitation averaged during 1980–2017
over China (mmyr21). (b) Time series and trend of the landfalling TC-induced total annual precipitation averaged
over China during 1980–2017 (mmyr21). (c) Spatial distribution of trends in the landfalling TC-induced annual
precipitation over China during 1980–2017 (mmyr21). Also shown are time series and trend of the landfalling TC
frequency over (d) China mainland, (e) the southern area, and (f) the southeastern area during 1980–2017. The
dashed, solid, and blue curves in (b), (d), (e), and (f) indicate the raw data, 5-yr running mean, and the corre-
sponding linear trends, respectively. Regions with trends that are significant over 95% confidence level are shown
with dots in (c).
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rise to a significant decrease in the total TC precipitation
over Hainan Island (Fig. 1c). This is consistent with the
results ofWu et al. (2007), who found that the number of
TCs impacting Hainan Island significantly decreased.
4. Characteristics of strong and weak TCprecipitation
We further classify landfalling TCs into strong TCs
(STCs; with sustained maximum wind speed$ 32m s21,
including typhoons, severe typhoons, and super-
typhoons) and weak TCs (WTCs; with sustained maxi-
mum wind speed , 32m s21, including tropical storms,
severe tropical storms, and tropical depressions) at
landfall. The TDs are included here because, on one
hand, TDs in the STI/CMA best-track dataset were, to
some extent, reliable based on the description of the
STI/CMA best-track data as mentioned in section 2; on
the other hand, the precipitation induced by TDs could
be very large when they interacted with some favor-
able weather systems. Thus, the precipitation induced
by TDs is also important. The spatial distributions of
precipitation induced by STCs andWTCs are shown in
Figs. 4a and 4b, respectively. Precipitation induced by
STCs is large in the southern coastal area, such as
Guangxi, Guangdong, and Hainan provinces (Fig. 4a),
and slightly south of the maximum in the total TC pre-
cipitation shown in Fig. 1a. Precipitation induced by
WTCs is generally large in the southern and southeastern
coastal area, including eastern Guangdong and Fujian
provinces (Fig. 4b) and is to the northeast of that induced
by STCs. Note that precipitation induced by WTCs is
generally weaker than that induced by STCs.
Since the spatial distributions of STC and WTC pre-
cipitation are different, it is interesting to examine the
spatial pattern of their respective trends in the study pe-
riod. Figure 5 presents the time series and the trends of
average precipitation per STC and average precipitation
perWTC, respectively, during 1980–2017. Consistent with
the average precipitation per TC (Fig. 2a), STC precipi-
tation also shows a significant increasing trend with sta-
tistical significance over 95% confidence level (Fig. 5a),
indicating that precipitation induced by a STC increased
in the study period. The trends in STC precipitation are
positive over mainland China but negative over Hainan
Island (Fig. 5b). The most significant increase in STC
precipitation occurs in the southern coastal area (mainly
inGuangdong andGuangxi provinces), namely the region
with the maximum rainfall per STC shown in Fig. 4a. This
indicates that the precipitation induced by STCs signifi-
cantly increased over mainland China, especially in the
southern coastal area, but decreased over Hainan Island.
The average precipitation per WTC shows no signifi-
cant trend over mainland China (Fig. 5c). This is pri-
marily due to the cancellation of the decreasing trend in
the southern coastal area and the increasing trend in the
southeastern coastal area of mainland China (Fig. 5d). It
is interesting to note that significant increasing trends
primarily occurred in Fujian and Zhejiang provinces,
north of the peak WTC precipitation (Fig. 4b). This
means that WTC-induced precipitation shifted north-
ward in the study period, resulting in an increase inWTC
FIG. 2. (a) Time series and trend of precipitation (mmyr21) per landfalling TC averaged over China during 1980–
2017. (b) Spatial distribution of trend in total precipitation (mmyr21) per landfalling TC over China during 1980–
2017. The dashed, solid, and blue curves in (a) indicate the raw data, the 5-yr running mean, and the corresponding
linear trend, respectively. Regions with trends that are significant over 95% confidence level are shown with dots
in (b).
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precipitation in the southeastern coastal area and a de-
crease in the southern coastal area (Fig. 5d). This is in
sharp contrast to the STC-induced precipitation, which
shows that the maximum increasing trend almost in the
region coincided with peak STC precipitation. This in-
dicates that the northward shift of total landfalling TC
precipitation over mainland China (Fig. 1c) is mainly
contributed by the northward shift of WTC-induced
precipitation.
To understand the possible mechanisms responsible for
the overall increasing trend in STC precipitation, we ex-
amined the large-scale environmental conditions and their
corresponding trends. It is found that in addition to some
other favorable dynamic and thermodynamic conditions
(not shown), the atmospheric moisture supply seems to
play a key role in enhancing STC precipitation. Figures 6a
and 6b display the average water vapor and vapor flux
vertically integrated between 1000 and 700hPa and their
trends during 1980–2017. The southern coastal area of
mainland China, where large STC precipitation occurred
(Fig. 4a), is perennially covered by high low-level water
vapor content and large moisture flux convergence be-
tween the southwesterly flow with high moisture content
from theBay of Bengal and southeasterly flowwith plenty
of water vapor from the western North Pacific. The area is
very conducive to precipitation induced by STCs. The
water vapor in this area increased significantly in the study
period (Fig. 6b), which favored the increase in STC-
induced precipitation. In addition, an increasing trend in
divergence in the upper troposphere occurred in the
southern area and neighboring seas (Fig. 6c) and an in-
creasing trend in ascending motion (Fig. 6d) appeared at
FIG. 3. (a) Time series and trend of precipitation (mm yr21) per landfalling TC per day averaged over China
during 1980–2017. (b) Spatial distribution of trend in averaged precipitation (mm yr21) per landfalling TC per
day over China during 1980–2017. (c) Trends in TC precipitation frequency (TC precipitation; days yr21). The
dashed, solid, and blue curves in (a) indicate the raw data, the 5-yr running mean, and the corresponding linear
trend, respectively. Regions with trends that are significant over 95% confidence level are shown with dots in
(b) and (c).
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500hPa. Both are conducive to the maintenance of TC
intensity and thus contribute to the decrease in overland
intensity weakening rate and the increase in mean over-
land duration of landfalling TCs (e.g., Park et al. 2011).
Meanwhile, both the landfall intensity (Fig. 6e) and du-
ration after landfall (Fig. 6f) of STCs increased noticeably,
with statistical significance over the 90% confidence level.
This is also conducive to increasing precipitation of STCs.
Previous studies have suggested that there was some re-
lationship between TC-induced rainfall rate and TC in-
tensity (Rodgers et al. 2001; Lonfat et al. 2004; Jiang 2012;
Yu et al. 2017;M. Liu et al. 2019). Yu et al. (2017) analyzed
the rainfall distribution in TCs making landfall over
mainland China and revealed that, on average, axisym-
metric rainfall was closely related toTC intensity. Stronger
TCs produce more averaged total rainfall, larger averaged
rain areas, and higher averaged rain rates. M. Liu et al.
(2019) also showed that TCs with high intensity tended to
produce high rainfall rates, especially toward the storm
center. Therefore, the sufficient water vapor supply and
the increase in both duration after landfall and the increase
in storm intensity at landfall are all responsible for the
increasing trend in STC precipitation in the southern
coastal area ofmainlandChina except overHainan Island.
The northward shift of WTC precipitation is mainly
due to the northward shift of mean landfalling location
of WTCs (Fig. 7a) and changes in favorable environ-
mental conditions, including surface conditions and
low-level vertical wind shear. The average landfalling
latitude ofWTCs shows a clear increasing trend (Fig. 7a)
with statistical significance over 90% confidence level.
The average landfalling latitude increased from about
228N to about 248N in the study period. In addition to
the northward shift of the average landfalling location,
the average intensity of landfalling WTCs also shows a
significant increasing trend in the study period (Fig. 7b).
Therefore, nomatterwhether theywereSTCsorWTCs, the
intensity of TCs at landfall over mainland China increased
significantly in recent decades. This increasing trend is
consistent with the significant warming trend in SST in the
coastal oceans near East Asia, particularly offshore of
mainland China (Fig. 7c). The near-surface air temperature
(Fig. 7d) and soil moisture (Fig. 7e) also show increasing
trends, particularly in the southeastern coastal area of
mainland China, providing more favorable thermodynamic
conditions to slowdown theweakeningofTCsafter landfall.
Previous studies have demonstrated that the soil moisture
and temperature are critical for TC maintenance and TC
precipitation over land (Tuleya and Kurihara 1978; Tuleya
et al. 1984; Tuleya 1994; Shen et al. 2002; Zhang et al. 2011;
L. Liu et al. 2019). As shown in Tuleya (1994), warm land
surface and high soil moisture are favorable for surface
enthalpy flux and thus may slow down the weakening of a
TC after landfall. Zhang et al. (2011) conducted several
sensitivity numerical experiments and found that both the
latent and sensible heat flux sustained the landfalling TCs
and maintained the spiral structure of rainbands.
In addition, the low-level vertical wind shear between
850 and 1000hPa shows a significant decreasing trend over
southeastern China (Fig. 7f), which provides a favorable
dynamical environmental condition conducive to TC in-
tensification and maintenance. Note that the deep-layer
vertical wind shear between 850 and 200hPa was also an-
alyzed and the results showed a decreasing trend to the
north of approximately 338N over mainland China, but the
trend in the region affected by landfalling TCs was not
statistically significant (figure not shown). Wang et al.
(2015) compared the correlation between TC intensity
change and vertical wind shear in various vertical layers
over the WNP. They found that the low-level shear
FIG. 4. Spatial distributions of the averaged annual precipitation (mmyr21) induced by (a) strong landfalling TCs
and (b) weak landfalling TCs over China during 1980–2017 (see text for more details).
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between 850 (or 700) and 1000hPa wasmore destructive to
TCs than the commonly used deep-layer vertical wind
shear between 200 and 850hPa in the active typhoon sea-
son over the WNP. As a result, the combination of the
northward shift of the average landfalling position ofWTCs
and the more favorable dynamic and thermodynamic
conditions led to the increasingWTC-induced precipitation
in the southeastern coastal area of mainland China. This
also explains why the increasing trend of WTC precipita-
tion occurred north of the maximum WTC precipitation
and the dominant contribution by WTCs to the northward
shift of the total landfalling TC precipitation.
5. Conclusions
In this study, the daily TC precipitation dataset pro-
duced using the OSAT method (Ren et al. 2006, 2007)
and the best-track TC data obtained from STI/CMA are
used to explore the characteristics of the landfalling TC-
induced precipitation and the precipitation per land-
falling TC after landfall over China during 1980–2017.
The total annual TC precipitation over China as a whole
shows a decreasing trend primarily due to the decreasing
trend in the frequency of landfalling TCs during 1980–
2017, which is consistent with the results of Ren et al.
(2006). It is found that the total annual TC precipitation
shows a distinct spatial distribution with a significant
increasing trend in southeastern China but a decreasing
trend in southern China. This increasing to the north and
decreasing to the south are found to be related to the
increase in both the annual TC precipitation frequency
(annual average TC precipitation days) and the average
precipitation per TC per day, over southeastern China,
and to the decrease in the annual TC precipitation
FIG. 5. Time series and trends (mmyr21) of (a) strong TC–induced precipitation per TC and (c) weak TC–
induced precipitation per 1980–2017 averaged over China. Also shown are spatial distributions of trends (mmyr21)
of (b) STC precipitation per TC and (d) WTC precipitation per TC during 1980–2017. The dashed, solid, and blue
curves in (a) and (c) indicate the raw data, 5-yr running mean, and the corresponding linear trend, respectively.
Regions with trends that are significant over 95% confidence level are shown with dots in (b) and (d).
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FIG. 6. Spatial distributions of (a) water vapor mixing ratio (shading; kg kg21) and vapor flux (vectors; kg
m kg21 s21) integrated from 1000 to 700 hPa, (b) trends in water vapor mixing ratio (shading; 1022 g kg21 yr21)
integrated from 1000 to 700 hPa, (c) upper-tropospheric divergence (1026 s21 yr21) at 200 hPa, and (d) vertical
motion at 500 hPa (Pa s21 yr21) during 1980–2017. Also shown are time series and trends of (e) averaged landfalling
intensity (m s21) and (f) duration after landfall (h) of STCs over China during 1980–2017. Regions with trends that
are significant over 90% confidence level are shown with dots in (b)–(d). The dashed, solid, and blue curves in
(e) and (f) indicate the raw data, the 5-yr running mean, and the corresponding linear trend, respectively.
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FIG. 7. Time series and trends of (a) averaged landfalling latitude (8N) and (b) averaged landfalling intensity
(m s21) of WTCs over China during 1980–2017; spatial distributions of trends in (c) SST (K yr21), (d) air tem-
perature at 950 hPa (K yr21), (e) soil moisture (g kg21 yr21), and (f) vertical wind shear between 1000 and 850 hPa
(m s21 yr21) during 1980–2017. The dashed, solid, and blue curves in (a) and (b) indicate the raw data, the 5-yr
running mean, and the corresponding linear trend, respectively. Regions with trends that are significant over 90%
confidence level are shown with dots in (c)–(f).
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frequency over southern China. In addition, a northward
shift in precipitation induced by landfalling TCs over
mainlandChina during recent decades has been identified.
Wehave further classified landfallingTCs into STCs and
WTCs to examine the differences in their precipitation
characteristics overmainlandChina. Some interesting new
findings include a noticeable northward shift in the average
landfalling location of WTCs and their associated precip-
itation, and an overall increasing trend in landfalling in-
tensity of both STCs and WTCs. It is shown that the
precipitation induced by STCs mainly concentrated in the
southern coastal area, slightly south of the maximum total
TCprecipitation, while the precipitation induced byWTCs
is mainly located in the southeastern coastal area over
mainland China, coinciding with the region of total TC
precipitation. Moreover, the STC precipitation shows an
increasing trend in most regions with large STC precipi-
tation over mainland China, especially in southern China.
The location of WTC-induced precipitation is east and
north of that of STC-induced precipitation and shows a
decreasing trend in the southern coastal area and an in-
creasing trend in the southeastern coastal area ofmainland
China. The most significant increasing trend occurred
north of the peak precipitation area induced by WTCs,
indicating a noticeable northward shift of the WTC-
induced precipitation in the study period. These results
demonstrate that the northward shift of total landfalling
TC precipitation is mainly contributed by the WTC pre-
cipitation primarily due to the northward shift of land-
falling location of WTCs.
Results from further analyses indicate that both the
intensity at landfall and the duration after landfall of
STCs increased noticeably. The region of STC pre-
cipitation is perennially located in the convergence
zone with high humidity in the lower troposphere.
Water vapor in the area increased significantly in the
study period, providing more water vapor supply for
TC intensification/maintenance and precipitation. In
contrast, WTCs show a significant increasing trend in
their average landfall latitude, leading to a northward
shift of WTC-induced precipitation. Moreover, the
increasing intensity of WTCs at landfall and the in-
creasing SST in the coastal oceans, the increase in land
surface soil moisture and temperature, and the de-
crease in low-level vertical wind shear over south-
eastern and eastern China provide more favorable
dynamic and thermodynamic conditions for WTCs to
survive longer after landfall, leading to the increase in
WTC precipitation.
Note that there was a remarkable decreasing trend
in landfallingTC-induced precipitation overHainan Island,
regardless of STCs or WTCs. We found that this is mainly
due to the significant decreasing trend in both the average
precipitation per TC and the annual frequency of TC
precipitation days over the past decades. Wu et al.
(2007) found that the number of TCs impacting Hainan
Island significantly decreased. Therefore, the decreas-
ing trend in landfalling induced precipitation over
Hainan Island is most likely a result of the decreasing
landfilling TC frequency. In addition, the decreasing
trend in the average precipitation per TC over Hainan
Island might be partially due to the decrease in TC
intensity at landfall and unfavorable environmental
conditions. For example, Wang et al. (2013) found that
due to the increase in the tropical Indian Ocean SST,
the lower-level anomalous anticyclone occurred over
South China Sea, which was unfavorable for TC genesis
and intensification over the South China Sea. This is ev-
ident by a trend of decreasing ascending motion in the
western South China Sea seen in Fig. 6d. Nevertheless,
because of the small area of the island and the sensitive
regional climate patterns over Hainan Island, a more
detailed analysis on the possible mechanism responsible
for the observed decreasing trend of landfalling TC-
induced precipitation over Hainan Island is reserved
for a future study.
Note that this study has focused on the analysis of long-
term trends in the observed precipitation induced by
landfalling TCs over mainland China based on data be-
tween 1980 and 2017. Because the limited period was an-
alyzed due to TC precipitation data quality, the northward
shift in TC precipitation could be partly contributed by the
decadal/interdecadal variations, which need further in-
vestigations. However, a recent study by Liu et al. (2020)
pointed out that the landfalling TC activity over mainland
China seems not to be significantly modulated by the
decadal/interdecadal variations because several in-
terdecadal shifts identified in other climate parameters
did not appear in landfalling TC characteristics over
mainland China. Nevertheless, results from this study im-
ply an increasing stress of potential landfalling TC-induced
precipitation in the populated southeastern coastal region
ofmainlandChina if the trend continues in the near future.
Acknowledgments. The authors thank three anony-
mous reviewers for helpful review comments and Dr.
Fumin Ren for providing the TC-induced precipitation
dataset over China in this study. This study has been
supported in part by the National Key R&D Program of
China under Grants 2017YFC1501602 and in part by
National Natural Science Foundation of China under
Grants 41675044, 41705027, 41730960, and 41875114.
The TC best-track data were obtained from STI/CMA
(http://tcdata.typhoon.org.cn). The ECMWF data used
in this study were obtained online (http://apps.ecmwf.int/
datasets/).The soil moisture dataset used in this study was
15 MARCH 2020 L IU AND WANG 2233
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obtained online (https://www.esrl.noaa.gov/psd/data/
gridded/data.cpcsoil.html). All figures were produced using
the NCAR Command Language (NCL).
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