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Observed Link of Extreme Hourly Precipitation Changes to Urbanization over Coastal South China

  • Institute of Meteorological Sciences, Zhejiang Meteorological Bureau


Understanding changes in subdaily rainfall extremes is critical to urban planners for building more sustainable and resilient cities. In this study, the hourly precipitation data in 1971–2016 from 61 rain gauges are combined with historical land-use change data to investigate changes in extreme hourly precipitation (EXHP) in the Pearl River delta (PRD) region of South China. Also, 120 extreme rainfall events (EXREs) during 2011–16 are analyzed using observations collected at densely distributed automatic weather stations and radar network. Statistically significant increase of hourly precipitation intensity leads to higher annual amounts of both total and extreme precipitation over the PRD urban cluster in the rapid urbanization period (about 1994–2016) than during the preurbanization era (1971 to about 1993), suggesting a possible link between the enhanced rainfall and the rapid urbanization. Those urbanization-related positive trends are closely related to more frequent occurrence of abrupt rainfall events with short duration (≤6 h) than the continuous or growing rainfall events with longer duration. The 120 EXREs in 2011–16 are categorized into six types according to the originating location and movement of the extreme-rain-producing storms. Despite the wide range of synoptic backgrounds and seasons, rainfall intensification by the strong urban heat island (UHI) effect is a clear signal in all the six types, especially over the inland urban cluster with prominent UHIs. The UHI thermal perturbation probably plays an important role in the convective initiation and intensification of the locally developed extreme-rain-producing storms during the daytime.
Observed Link of Extreme Hourly Precipitation Changes to Urbanization
over Coastal South China
State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, and Institute of Meteorological
Sciences, Zhejiang Meteorological Bureau, Hangzhou, and University of Chinese Academy of Sciences, Beijing, China
State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, and Collaborative
Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information
Science and Technology, Nanjing, China
State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China, and
National Center for Atmospheric Research, Boulder, Colorado
Hong Kong Observatory, Hong Kong, China
(Manuscript received 22 October 2018, in final form 7 April 2019)
Understanding changes in subdaily rainfall extremes is critical to urban planners for building more sus-
tainable and resilient cities. In this study, the hourly precipitation data in 1971–2016 from 61 rain gauges are
combined with historical land-use change data to investigate changes in extreme hourly precipitation (EXHP)
in the Pearl River delta (PRD) region of South China. Also, 120 extreme rainfall events (EXREs) during
2011–16 are analyzed using observations collected at densely distributed automatic weather stations and radar
network. Statistically significant increase of hourly precipitation intensity leads to higher annual amounts of both total
and extreme precipitation over the PRD urban cluster in the rapid urbanization period (about 1994–2016) than
during the preurbanization era (1971 to about 1993), suggesting a possible link between the enhanced rainfall and the
rapid urbanization. Those urbanization-related positive trends are closely related to more frequent occurrence of
abrupt rainfall events with short duration (#6 h) than the continuous or growing rainfall events with longer duration.
The 120 EXREs in 2011–16 are categorized into six types according to the originating location and movement of the
extreme-rain-producing storms. Despite the wide range of synoptic backgrounds and seasons, rainfall intensification
by the strong urban heat island (UHI) effect is a clear signal in all the six types, especially over the inland urban cluster
with prominent UHIs. The UHI thermal perturbation probably plays an important role in the convective initiation
and intensification of the locally developed extreme-rain-producing storms during the daytime.
1. Introduction
More than half of the global population now resides in
urban areas (Grimm et al. 2008), and that number is
expected to increase to 60% by 2030 and 70% by 2050
(UN Department of Economic and Social Affairs,
Population Division 2011). Urban areas, especially
coastal cities, are vulnerable to heavy rainfall-induced
flood exposure that is increasing in the changing climate
Denotes content that is immediately available upon publica-
tion as open access.
Corresponding author: Dr. Yali Luo,
Publisher’s Note: This article was revised on 14 August 2019 to
correct the first author’s affiliations, which were not correctly
presented when originally published.
AUGUST 2019 W U E T A L . 1799
DOI: 10.1175/JAMC-D-18-0284.1
Ó2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright
Policy (
(Hallegatte et al. 2013). The majority of previous studies
suggested that daily extreme rainfall has increased in
more regions than it has decreased in a warming world
(Minetal.2011;Seneviratne et al. 2012;Westra et al.
2013;Donatetal.2016;Seneviratne et al. 2016). How-
ever, engineering practices and urban infrastructure de-
sign demand information about changes in subdaily (e.g.,
hourly or shorter) rainfall.
Subdaily precipitation extremes are often pro-
duced by convective events that are affected by
complex interactions among mesoscale dynamics,
cloud microphysical processes, and underlying sur-
face forcing. Current understanding of the changes
in subdaily extreme precipitation is very limited
(Zhang et al. 2017) because of lack of high-resolution,
long-term observations and limited knowledge about
the physical mechanisms that govern the evolution of
convective events. Climate models are not able to sim-
ulate such events well (Siler and Roe 2014;Z. H. Jiang et
al. 2017;Pfahl et al. 2017), making it difficult to attribute
past changes and to assess future changes in short-
duration precipitation extremes. Moreover, factors
affecting changes in short-duration precipitation ex-
tremes over urban environments are even less known
because of complex and sometimes compensating effects
of cities, namely, the destabilization caused by urban
heat island (UHI)-induced thermal perturbation and its
downstream rainfall enhancements translation (Huff and
Vogel 1978;Hjelmfelt 1982;BornsteinandLin2000;Craig
and Bornstein 2002;Niyogi et al. 2011), the building-
barrier and thermal effects of urban canyons (Bornstein
and Lin 2000;Guo et al. 2006;Zhang et al. 2009;Miao et al.
2011), and the anthropogenic aerosol emissions for cloud
condensation nuclei (CCN) sources (Rosenfeld 2000;Bell
et al. 2009;Jin and Shepherd 2008;Ntelekos et al. 2009).
Guangdong province, located in coastal South
China (Fig. 1a), is exposed to high flood-induced risk
(Hallegatte et al. 2013) because of the high frequency
of heavy rainfall at hourly (Luo et al. 2016) to longer
scales (Zheng et al. 2016). The heavy rainfall in South
FIG. 1. (a) The urban region (color shading) in Guangdong province identified by the
DMSP/OLS nighttime lights data in five different years. The bold black line represents the
PRD urban region in 2013, which is also used in most of the following figures. The dashed
rectangle denotes the key region defined in this study (21.78–23.78N, 112.68–114.68E). The
locations of the major cities in the PRD are labeled out. (b) Evolution of urbanization re-
flected by time series of the population density (km
) and built-up area (10
Guangdong and the urban ratio (%) within the key region based on the land-use data.
China is influenced by large-scale to synoptic-scale
weather systems including the Asian summer mon-
soon (Ding 1994), subtropical ridge of high pressure
in western North Pacific and heat low over southern
and southwestern parts of China (Ramage 1952),
cyclonic vortex anomalies moving from downstream
of the Tibetan Plateau (Huang et al. 2018), and
tropical cyclones (Li and Zhou 2015), as well as
mesoscale forcing associated with an underlying
surface (such as land–sea contrast and coastal
mountains) cold pool generated by significant con-
vective storms (Wang et al. 2014;Wu and Luo 2016;
Liu et al. 2018).
The rapid expansion of urban areas in Guangdong
province during the past two decades (Fig. 1b) led to
the formation of city clusters in the Pearl River delta
(PRD) (Fig. 1a). In 2016, the PRD covered an area of
about 42 200 km
with a population over 57 million,
accounting for 52.0% of the total population in
Guangdong (Guangdong Statistical Bureau 2016). It
encompasses a cluster of big cities, including the cap-
ital city of Guangdong province (Guangzhou) at its
northern apex, Hong Kong and the Special Economic
Zone (SEZ) of Shenzhen at the southeast, while Ma-
cao and Zhuhai SEZ over the southwestern part of
PRD. Based on satellite data in 1998–2009, Li et al.
(2011) suggest that the urban areas in PRD experience
more (less) occurrences of heavy (light) precipitation
compared to surrounding suburban regions. Under-
standing changes in subdaily precipitation extremes
FIG. 2. (a) Land-use map over Guangdong in 2015, overlaid with the PRD urban region
boundary based on the DMSP/OLS nighttime lights data in 2013 (solid black). (b) Spatial dis-
tributions of 61 national-level stations (red dots) in the analysis domain. Each station is assigned a
number, which is also used in many of the following figures. The key region is outlined by dotted
rectangle. Dots of smaller sizes represent AWSs within the key region, with orange/lavender ones
denoting stations inside/outside of the PRD urban region. Shading represents the topography.
AUGUST 2019 W U E T A L . 1801
and their relationship with urbanization is critical to
urban policy makers for developing more sustainable
and resilient cities in this region.
This study aims to investigate changes of extreme
hourly precipitation in coastal South China with a
focus on its possible relationship with the urbaniza-
tion in PRD. To achieve this objective, the long-term
changes in extreme hourly precipitation (EXHP; de-
fined by the 95th percentile) are analyzed using hourly
precipitation observations at 61 national-level surface
stations from 1971 to 2016. Then a total of 120 extreme
rainfall events (EXREs) (with at least one record of
hourly rainfall at 60 mm or more in the PRD region)
during 2011–16 are further examined using observa-
tions collected by densely distributed automatic
paper is organized as follows: The next section de-
scribes the data and analysis methods. Section 3
presents the changes of hourly precipitation extremes
during 1971–2016. Section 4 classifies the EXREs
characteristics in the movement of rainstorms, and
compares between the strong- and weak-UHI sub-
groups for each type. A summary and conclusions are
provided in section 5.
2. Data and analysis methods
a. Estimating the PRD urbanization
This study combines historical land-use data,
satellite-based nighttime lights dataset, and statisti-
cal data of population density and built-up area
from the Guangdong Statistical Bureau (2016;http:// to assess the urbanization in
PRD. The historical land-use data of horizontal reso-
lution at 1 km are available from the Resources
and Environment Scientific Data Center, Chinese
Academy of Sciences (
DATAID598) for 1980, 1990, 1995, 2000, 2005, and
2015basedonvisualinterpretation and digitalization
of the 30-m Landsat TM/ETM satellite images (Gong
et al. 2013). They include six land-use types: crop-
lands, forest, grasslands, water bodies, built-up lands,
and others. Because of its high accuracy in monitoring
land-use change in China, this dataset has been ex-
tensively used in the fields of land resource surveys,
hydrology, and ecology (e.g., Li et al. 2015;Liu et al.
2014). Moreover, the fourth version composite satellite-
based nighttime lights data gathered during 1992–2013
derived from the Defense Meteorological Satellite
Program’s Operational Linescan System (DMSP/OLS)
of the United States (
downloadV4composites.html) are utilized. The digi-
tal numbers (DN) values of 50, 56, and 57 are used
as the urban thresholds for the years of 1992, 1995,
and in and after 2000, respectively, based on Wang
et al. (2013).
The aforementioned three independent data sources
all reveal a rapid urbanization in the PRD region since
the early to mid-1990s, as suggested by the continuous
increases of the built-up area, population density, and
ratio of urban land use, as well as the expansion of
nighttime light (Figs. 1a,b). The boundary of the PRD
urban region is represented hereafter by the con-
tour line derived from the DMSP/OLS data in 2013
(Fig. 1a), which agrees well with the land-use data in
2015 (Fig. 2a).
b. Analysis of the long-term changes in hourly
precipitation extremes
This study utilizes the long-term, gauge-based
hourly precipitation dataset provided by the National
Meteorological Information Center (NMIC) of the
China Meteorological Administration (CMA; http://
TABLE 1. Classification of the hourly extreme precipitation in 1971–2016 according to the temporal evolution of hourly precipitation in the
3 h prior to the hourly extreme.
evolution type
Hourly rainfall rates in 3 h prior to the extreme rainfall hour (R
, respectively) compared with the extreme hourly rainfall (R
Contribution to total occurrence of the
non-TC hourly precipitation extremes (%)
Abrupt type R
and R
and R
Growing type (R
or R
or R
) and at least one of
).10% but ,R
Continuous type At least one of (R
TABLE 2. Number of the categorized 2011–16 extreme
rainfall events.
Type Strong-UHI events Weak-UHI events Total
Local/SW wind 18 20 38
Local/shear line 17 8 25
Migratory-NW 20 14 34
Migratory-SW 3 10 13
Migratory-NE 6 0 6
Migratory-S 4 0 4
Total 68 52 120
long-term changes of hourly extreme precipitation
over the analysis domain (Fig. 2b). A strict quality-
control procedure has been applied to the dataset
by NMIC consisting of a climatological limit value
test, a station extreme value test, an internal consis-
tency test, and a comparison with manually checked
daily rainfall data. This dataset has been extensively
used to investigate the characteristics of subdaily pre-
cipitation over China (e.g., Yu et al. 2007;Li et al. 2008;
Yuan et al. 2012;Luo et al. 2016;Guo et al. 2017). This
study utilizes 60 stations in Guangdong and one station
in Hong Kong that have continuous records from 1971 to
2016 (distribution shown in Fig. 2b). Each of the 61
stations has over 95% of valid hourly data each year.
The EXHP at each station is defined using the 95th
percentile of the distribution function during 1971–2016
as the threshold. The threshold values increase south-
ward from about 6 to 13 mm h
(not shown). The
rainfall amount and occurrence frequency of the EXHP
are calculated as the annual rainfall amount and the
total number of hours of the EXHP, respectively.
The EXHP intensity is calculated as the rainfall amount
divided by its occurrence frequency. The rainfall
amount, occurrence frequency, and intensity of hourly
precipitation ($0.1 mm h
) are defined similarly. Their
long-term trends are estimated using a linear ten-
dency method. The significances of the trends are ex-
amined utilizing the Mann–Kendall nonparametric test
(Mann 1945;Kendall 1975). The test has been applied
extensively including in studies that test for changes
in rainfall extremes (Alexander and Arblaster 2009;
Westra et al. 2013). To make the trends more compa-
rable among the various variables (i.e., precipitation
amount, occurrence frequency, and intensity), relative
percentage changes are calculated using the equation
D510 3S
P(100%), (1)
where Dis the relative change in precipitation
(% decade
), Sis the slope of the linear model, and Pis
the precipitation value in the start year estimated using
the linear regression equation. The significances of the
linear regression equation are examined using the Ftest
(Lomax and Hahs-Vaughn 2007).
Moreover, rainfall produced by tropical cyclones
(TCs) is identified with an objective synoptic analysis
technique (OSAT; Ren et al. 2006,2007,2011). This
method uses the distance from TC center and the
closeness and continuity between neighboring raining
stations to trace TC-influenced rain belts. An extreme
hourly precipitation record in this study is classified
identified TC-influenced rain belts; otherwise it is a
non-TC record. Previous studies have suggested that
rainfall trends over China are complicated by TCs
FIG. 3. (a),(c) Scatterplot of DTand its corresponding DTat each of the 6 h prior to the onset of the 2011–16
EXREs with (a) a strong-UHI and (c) a weak-UHI, respectively. (b),(d) Box-and-whisker plots of UHII for the
2011–16 EXREs with (b) a strong-UHI and (d) a weak-UHI, respectively, showing the interquartile range
(rectangle), outliers (i.e., the 10th and 90th percentile as whiskers), and mean (solid line).
AUGUST 2019 W U E T A L . 1803
(Chang et al. 2012;Li et al. 2017), and the TC-induced
hourly precipitation extremes contribute about 20% to
the total occurrence frequency of the extremes over
coastal South China (Luo et al. 2016). The precipitation
excluding the TC-produced is termed as non-TC pre-
cipitation hereafter. Furthermore, the hourly extreme
precipitation records are classified into the abrupt,
growing, and continuous types according to the tempo-
ral evolution features of the hourly rainfall series
(Table 1;Liang and Ding 2017), and into the short-
(1–6 h), moderate- (7–12 h), and long-duration (.12 h)
types based on the time span of continuous rainfall
(.0.1 mm h
with at most 1-h intermittence).
c. Analysis of the 2011–16 extreme rainfall events
To complement the 46-yr linear tendency analysis,
the possible relationship between a strong UHI ef-
fect and the 2011–16 EXREs are examined. Both the
quality-controlled hourly records at densely distrib-
uted AWSs and 10-min mosaic radar reflectivity in
Guangdong are used to provide detailed information
about the convective events. The EXRE is defined
as a rain system that produces at least one rainfall
record of .60 mm h
observed by the AWSs within
the key region (defined as a 28328region in the
analysis domain; dotted rectangle in Fig. 2)covering
the entire PRD urban cluster and the adjacent forest,
cropland, and water. The 10-min radar images are
adopted to verify the reliability of the extreme
rainfall records, as well as to characterize the life
cycle of the extreme-rain-producing convective sys-
tems. A total of 133 EXREs are found. Among them
13 cases are featured by an extensive cyclonic vortex
at 850 hPa over South China that helps produce a
persistent (duration .1 day), extensive precipitation
region (.1000 km on the radar image). The large-scale
forcing, rather than the UHI-influenced local envi-
ronment, is believed to play a dominant role in mod-
ulating the 13 EXHRs. Contrastingly the other 120
EXREs are mainly produced by MCSs lasting less than
24 h. As the focus of this study is the possible link be-
large-scale vortices are excluded from the following
For the remaining 120 EXREs, the intensity of the
PRD UHI during the onset of each EXRE (UHII) is
calculated using the equations below:
UHII 5DT2DT, (2)
DT5Tu2Tr, and (3)
6311 , (4)
where T
and T
, respectively, are the hourly surface air
temperatures averaged over the densely distributed
AWSs within the PRD urban region (small orange dots
in Fig. 2b) and outside the PRD urban region (small
FIG. 4. The pre-event potential temperature perturbation (Du) for the 2011–16 EXREs with (a) a strong-UHI and
(b) a weak-UHI, respectively. The hourly observations in the 3-h period prior to each EXRE onset are used in the
purple dots in Fig. 2b); thus DTroughly represents the
hourly temperature difference between the PRD urban
cluster and its surrounding areas. To remove the sea-
sonal and diurnal variations of DT, UHII is represented
by the difference between DTand DT. The value of DT
is calculated by averaging DTat the same local solar
time (LST) during 11 days (from 25to15 days) in
2011–16. For example, if the DTvalue is calculated at
1500 LST 12 June 2013, its corresponding DTis the av-
erage of 66 DTvalues at 1500 LST from 7 to 17 June in
2011–16. Thus, UHII .0 means that the UHI intensity
prior to each EXRE onset is stronger than its corre-
sponding multiyear average intensity. The pre-event
hourly UHII
(i51, 2, ... , 24; denoting the hour prior
to the EXRE onset) values are calculated for each
EXRE. Existence of a strong PRD UHI for an EXRE
is defined as at least four out of six hours with UHII
(i51–6); these EXREs are referred to as the strong-
UHI events. The rest EXREs have at most three out of
six hours with UHII
.0(i51–6) and are classified
into the weak-UHI events. Existence (lack) of such a
strongUHIisfoundfor68(52)EXREs(Table 2).
Composites of the UHII for the strong- and weak-
UHI events are shown in Fig. 3. The mostly (99.25%)
positive values of the multiyear average DTindicate
tensities from nearly zero to about 18C(Figs. 3a,c),
probably in association with the seasonal and diur-
nal variations of UHI intensity (Landsberg 1981;
Shepherd 2005;Ren and Zhou 2014). The hourly
UHII values are mostly positive in the 4-h preonset
periods and increases rapidly 6-h before the onset of
the strong-UHI events (Figs. 3c,d). For the weak-
UHI events, about 68.7% of the DThas positive
values, reflecting presence of an UHI prior to the
events’ onset. A certain fraction (31.3%) of negative
DTcan be contributed by rain evaporative cooling
due to previous convection in the urban cluster that
might occur occasionally prior to the EXREs. Mostly
negative values of UHII are observed in the 6-h preonset
periods (Fig. 3d), suggesting UHI intensities (DT) are
weaker than the multiyear average.
To examine the spatial distribution of UHI, the sur-
face potential temperature perturbation (Du) at each
station is calculated as the deviation from the spatial-
average value of uwithin the key region at each hour.
This method largely removes the seasonal and diurnal
variations of surface potential temperature (u). The
distribution of Duaveraged during 1–3 h prior to each
EXRE is compared between the strong- and weak-
UHI EXREs (Fig. 4). A prominent surface warm
center over the inland PRD urban region is noticed in
the average Dufield of the strong-UHI events (Fig. 4a).
Such a prominent warm center does not exist in the
FIG. 5. (a) Annual precipitation amount (mm yr
) averaged during 1971–2016 and (c) its difference between 1994–2016
and 1971–93. (b),(d) As in (a),(c), respectively, but for the extreme hourly precipitation (.95th percentile).
AUGUST 2019 W U E T A L . 1805
weak-UHI events, as smaller positive Duvalues are
widely distributed in the southwest part of the key re-
gion (Fig. 4b).
The 2011–16 EXREs are then categorized into six
types mainly based on the rain storms’ initiation location
and movement observed by the radar network. Specifi-
cally, the EXREs with extreme-rain-producing storms
initiating and developing within the key region are
selected first, and classified as either local/southwest
(SW) wind type or local/shear line type according to
their synoptic patterns on 925 hPa using the ERA-
Interim reanalysis (Dee et al. 2011). The former has
prevailing southwesterly airflows and the latter has
a shear line over the key region. Then, all the other
EXREs with extreme-rain-producing storms moving
from outside the key region are classified into four
types based on the direction the storms move into the
key region, namely, the migratory-northwest (NW),
migratory-northeast (NE), migratory-south (S), and
migratory-southwest (SW) types.
To examine possible UHI impacts, rainfall distribu-
tions of the strong- and weak-UHI EXREs in the same
type are compared. For this purpose, the normalized
rainfall (NR) is calculated for each AWS in the key re-
gion following Eqs. (5)(7):
Rjk(j51, m;k51, n), (5)
m, and (6)
R, (7)
where R
is event-accumulated rainfall amount of
EXRE k(k51, n; where nis the total number of
EXREs in a subgroup) at site j(j51, m; where mis the
total number of AWSs in the key region), R
is rainfall
accumulation of an EXRE subgroup at site j,Ris the
subgroup rainfall accumulation averaged over all the
AWSs in the key region, and NR
is the normalized
rainfall anomaly at site jrelative to the domain average
of the EXRE subgroup. This method is able to highlight
local features of the rainfall distribution (Yu 2007;
Dou et al. 2015).
3. Changes in hourly precipitation extremes
during 1971–2016
Figures 5a and 5b shows the spatial distributions of
total precipitation amount and extreme precipitation
amount over the analysis domain averaged during
1971–2016. The total precipitation amount has three
peaks in the southwest coastal, central, and south-
east coastal Guangdong, respectively, with the annual
maximal precipitation amounts exceeding 2100 mm yr
in the southwest center and of about 1900–50 mm yr
in the other two centers. The inland PRD urban re-
gion, with an annual precipitation amount of about
1800 mm yr
, is located between the southwest
and central precipitation centers. Such a distribution
shows a largely similar pattern to that of the hourly
extremes, except that the southwest and central
FIG. 6. The observed 1971–2016 changes (% decade
hourly precipitation ($0.1 mm h
): (a) amount, (b) occurrence
frequency, and (c) intensity. Dots (circles) denote signifi-
cant (insignificant) trends at the 95% confidence level using
the Mann–Kendall test. Size of the dots and circles represents
the various magnitudes of the changes. Shadings represent the
centers tend to connect crossing the east PRD urban
region while the southeast center is less evident. Of in-
terest is that the difference in the annual precipita-
tion amount between the two 23-yr periods, that is,
1994–2016 minus 1971–93, reveals a prominent increase
over the PRD region and the northern Guangdong re-
gion (Figs. 5c,d).
Figure 6 shows an increasing trend in the precipitation
amount for most stations over the PRD region. Never-
theless, only the Guangzhou station (number 35) passes
the significant test at 95% confidence level. Positive and
negative trends are scattered outside the PRD region,
but they are statistically insignificant. Negative trends in
the occurrence frequency of hourly precipitation are ob-
served at most stations over Guangdong, with only
a few passing the significant test. By contrast, the hourly
precipitation intensity exhibits an increasing trend
at most stations. Importantly, the most significant in-
creasing trends in the hourly precipitation intensity are
mainly confined in the PRD region. These results suggest
that more intense hourly precipitation, which is statisti-
cally significant over the PRD urban region (Fig. 6c),
leads to the higher annual amounts of both total pre-
cipitation and extreme precipitation over the PRD in
1994–2016 than 1971–93 (Figs. 5c,d), suggesting a pos-
sible link to the rapid urbanization since mid-1990s.
FIG. 7. (a)–(c) The observed 1971–2016 changes (% decade
) in the non-TC extreme hourly precipitation:
(a) amount, (b) occurrence frequency, and (c) intensity. (d)–(f) As in (a)–(c), but for all the extreme precipitation
including the TC-induced records. Dots (circles) denote significant (insignificant) trends at the 95% confidence
level using the Mann–Kendall test. Size of the dots and circles denotes different magnitudes of the changes.
Shadings represent the topography.
AUGUST 2019 W U E T A L . 1807
Changes in the amount, frequency, and intensity of
the non-TC extreme hourly precipitation during 1971–
2016 are shown in Figs. 7a–c. Data from 11 stations
reveal statistically significant (95% level) positive
trends in the extreme rainfall amount, with six stations
located in the PRD urban region while the other five
stations scattered in the western, northern and eastern
parts of Guangdong, respectively. The significant pos-
itive trends of the extreme rainfall frequency are ob-
served only at seven stations over the analysis domain,
mostly (five out of seven) in the PRD urban region,
especially its inland portion. The extreme hourly pre-
cipitation intensity also increases at most of the PRD
urban stations. However, the trends are insignificant
at 95% level. These results suggest that the in-
creased amount of non-TC precipitation extremes
may mainly be caused by their more frequent oc-
currence over the PRD urban region. The inclusion
of TC-induced extreme hourly rainfall results in a
smaller number of stations with statistically signifi-
cant positive trends of the amount (8 vs 11) (Fig. 7d
vs Fig. 7a)andfrequency(4vs7)(Fig. 7e vs Fig. 7b),
which is qualitatively consistent with the finding of a
reduction in landfalling TC occurrence over South
China during 1975–2014 by Li et al. (2017). Note that
the significant positive trends in the occurrence fre-
quency and amount of the hourly precipitation ex-
tremes, including the TC-induced ones, are still
concentrated over the PRD urban region, again
suggesting a possible rainfall enhancement by the
urban cluster.
The seven stations with a significant increasing
trend of the EXHP’s occurrence frequency (Fig. 7b) are
selected to further examine their time series and
change rates during 1971–2016. Similar analyses are also
conducted for the pre- and post-rapid-urbanization
FIG. 8. (a)–(g) Time series (gray bars) and linear trends (solid lines) of the non-TC extreme hourly rainfall
occurrence frequency at seven stations, with the station number labeled at the upper-left corner. The locations of
the seven stations could be seen in Fig. 1a. The annual occurrence frequency is normalized by subtracting the mean
value of the study period from the annual value of each year, and then divided by the corresponding standard
deviation. Blue lines denote the linear trends for the period of 1971–2016, while the orange/red lines are for
1971–93/1994–2016. The solid (dashed) line denotes that the linear regression equation is significant (insignificant)
at the 95% confidence level using the Ftest. The corresponding slope values of the linear model for the three
periods are shown in the bottom sequentially.
periods, respectively, to further examine possible in-
fluence of the urban effect on the increasing EXHP.
The comparison between the two periods at each
station remains at least qualitatively unchanged when
any year of the 1991–97 period is used as the de-
marcation point. Therefore, only the results using
year 1994 as the demarcation point are presented
herein. There are five stations (numbered 33, 34, 35,
39, 40) are in the PRD urban region, and the other two
(numbered 04 and 27) are in the north and east
Guangdong, respectively. The five stations in the PRD
urban region have either increase or decrease rates
during the preurbanization era of 1971–2016 (orange
lines; Figs. 8a–e). They all have increase rates during
the rapid urbanization period (1994–2016) (red lines;
Figs. 8a–e). The later 23-yr increase rates are about 1.44
to 2.36 times of the 46-yr (1971–2016) change rates. In
contrast, at the other two stations away from the core
PRD urban region the increase rates in the later 23-yr
period are nearly the same (station 04; Fig. 8f) or even
smaller (station 27; Fig. 8g) compared to their coun-
terparts in the preurbanization era. These results con-
sistently support possible contribution of the PRD
urbanization to more occurrences of hourly precipita-
tion extremes.
Moreover, it is found that the statistically significant
trends in thehourly precipitation extremes over the PRD
urban region are more closely related to the abrupt type
FIG. 9. Spatial distributions of the occurrence frequency (shading, %) and trends (line, decade
) of the cate-
gorized non-TC extreme rainfall events in 1971–2016: (a) abrupt type, (b) growing type, (c) continuous type,
(d) short-duration type, (e) middle-duration type, and (f) long-duration type. Gray dots represent that the trends
are significant at the 95% confidence level using the Mann–Kendall test. The occurrence frequency of an extreme
precipitation type is the fractional contribution of this type to the total number of the extreme rainfall events.
AUGUST 2019 W U E T A L . 1809
extremes than the growing or continuous types (defini-
tions given in Table 1). The abrupt type accounts for 44%
of the total occurrence of the non-TC hourly extremes
over the 61 stations in 1971–2016, the growing type is a
close second (40%), and the continuous type contributes
the least (16%). The spatial distributions of their fractional
contributions (shadings in Figs. 9a–c) show that the abrupt
type contributes the most in west Guangdong but rela-
tively less along the coastline and in north Guangdong,
while both the growing and the continuous types similarly
show a largely opposite pattern to the abrupt type. Sig-
nificant increase trends are observed only in the abrupt
FIG. 10. Composite geopotential height (gpm; green lines) and wind barbs on 925 hPa for the six types of the
2011–16 extreme rainfall events: (a) local/SW wind type, (b) local/shear line type, (c) migratory-NW type,
(d) migratory-SW type, (e) migratory-NE type, and (f) migratory-S type. A full barb is 5 m s
. The gray dots and
bold wind barbs denote the values that are over the 90% confidence level using the Student’s ttest. A typical rain
storm of each extreme rainfall type is illustrated by radar reflectivity (color shadings), overlaid with a black arrow to
denote its movement direction. The contribution of each type to the total number of the 2011–16 extremes (120) is
labeled in parentheses. The gray-shaded regions denote the portions of isobaric surfaces underneath the ground.
type, mainly over the PRD urban region, with only a few
scattered in other places of Guangdong (Fig. 9a).
Furthermore, when the non-TC extremes are clas-
sified by the time span of continuous rainfall (.0.1 mm h
with at most 1-h intervals), the short-, moderate-, and long-
duration types contribute 33.5%, 34.0%, 32.5%, re-
spectively, to the total occurrence of the extremes. Their
corresponding spatial patterns (shadings in Figs. 9d–f)
suggest that the short-duration type contributes more
in west Guangdong (about 40%–48%) than in other
places of Guangdong especially along the coastline
(mostly ,32%); the long-duration ones show a nearly
opposite feature; and the moderate-duration type ac-
counts for about 32%–36% evenly over the entire
analysis domain (Figs. 9d–f). Qualitatively similar
to the abrupt type (Fig. 9a), significant increase
trends of the short-duration events are observed and
concentrated in the PRD urban region (Fig. 9d),
while the longer-duration types show a lack of posi-
tive trends that are significant at the 95% confidence
The abovementioned results collectively suggest that
the statistically significant increasing trends of hourly
extreme precipitation in the PRD urban cluster are
likely attributed to the rapid urbanization. Such trends
are closely related to increasing occurrence frequency of
short-duration, abrupt rainfall events, which may pose a
greater threat to flash floods in the urban cluster and
raise more challenge for accurate prediction.
4. The 2011–16 EXREs
As described in section 2c, totally 120 EXREs
during 2011–16, each with at least one rainfall record
FIG. 11. Seasonality of the 2011–16 extreme rainfall events: (a) local/SW wind type, (b) local/shear line type,
(c) migratory-NW type, (d) migratory-SW type, (e) migratory-NE type, and (f) migratory-S type. Pink and blue
bars, respectively, represent the strong- and weak-UHI events. The number of events within each subtype is shown
in parentheses.
AUGUST 2019 W U E T A L . 1811
of .60 mm h
within the key region, are classified
into six types (Table 2). This section first describes
synoptic circulation and seasonality of each EXRE
type. Then the two subgroups with the strong- and
weak-UHI that belong to the same EXRE type are
compared in terms of the temporal and spatial distri-
butions of the precipitation.
a. Synoptic circulation and seasonality
Among the EXREs, 63 events (52.5%) have locally
developed rain systems, including 38 (31.7%) under the
influence of prevailing southwesterly winds in the lower
troposphere and the planetary boundary layer (PBL)
and 25 (20.8%) accompanied by a synoptic shear line
in the PBL over South China (Figs. 10a,b). These two
types of locally developed rain systems are catego-
rized into the local/SW wind and local/shear line types,
respectively. The other 57 EXREs originated outside of
the key region and moved from the northwest (28.4%),
southwest (10.8%), northeast (5.0%), or south (3.3%) to
influence the PRD urban region. These are categorized
into the migratory-NW, migratory-SW, migratory-NE,
or migratory-S types, respectively. The migratory-NW
type is usually accompanied by a northeast–southwest
oriented shear line in South China (Fig. 10c). In the
migratory-NE type, a significant TC or its remnants
is present around the Taiwan Strait, accompanied by
a large precipitation area from where the migratory
storm is separated (Fig. 10d). The migratory-SW and
migratory-S types take place in prevailing southerly and
southeasterly winds in the PBL (Figs. 10e,f).
Figure 11 shows the monthly occurrence frequency
of the 2011–16 EXRE types, with a strong or weak
PRD UHI, respectively. The two local types occur
FIG. 12. (a) The timing of the local/SW wind type events with a strong UHI. The beginning time of each event is
marked with a cross. The number of events and their average duration are also shown. (b) Composite spatial
distribution of region-normalized rainfall amount (shading). Blue (dark blue) dots denote hourly rainfall records
exceeding 60 mm h
occur once (more than once) in total among this type. (c),(d) As in (a),(b), respectively, but
for the local/SW wind type events with a weak UHI.
more evenly from May to September than the migra-
tory types. However, the local/SW wind type has
about 60% (23 out of 38) in the middle-and-late stage
of the presummer rainy season (May and June), while
the local/shear line type has about 60% (15 out of 25)
in August and September. The migratory-NW type
mostly (85%) takes place in April and May with the
remaining 15% in June–August. The migratory-SW,
migratory-NE, and migratory-S types are completely
found in April-June, July–August, and May-July, re-
spectively. The former four types have both strong- and
weak-UHI events, without a remarkable difference
between the strong and weak UHI events in the sea-
sonality of each EXRE type. The migratory-NE and
migratory-S types of the hourly precipitation extremes
occur only with the strong UHI.
b. Comparison between local EXREs with a strong
and weak UHI
The local/SW wind type with the strong UHI mostly
(13 out of 18) initiates from the midday to afternoon and
ends in the evening (Fig. 12a), producing extreme hourly
rainfall .60 mm h
mainly in the late afternoon. In
contrast, with the weak UHI, this type of rain storms
mostly (14 out of 20) initiates from late evening to
nocturnal hours (Fig. 12c) and lasts significantly longer
by average (11.3 vs 7.0 h). This interesting contrast
suggests possible difference in in large-scale environ-
ments for the two subgroups. Therefore, the key region-
averaged values of 700-hPa vertical velocity, 850-hPa
wind speed, 925-hPa wind convergence, and precipitable
water, are compared among the local/SW wind type of
EXREs (Fig. 13). The results suggest that the strong-UHI
events tend to be associated with relatively slower upward
motion, lower wind speed, weaker PBL wind conver-
gence, and smaller amount of precipitable water (PW).
These indicate relatively less favorable large-scale con-
ditions for persistent convective development and could
partially explain the shorter duration of the strong-UHI
events. These contrasting results between the two sub-
groups suggest that the UHI thermal perturbation (e.g.,
Shepherd et al. 2002;Shepherd and Burian 2003)likely
plays an important role in the convective initiation and
development in the afternoon, when the UHIsare greater
in clear and calm conditions (Landsberg 1981;Oke
1987;Yang et al. 2017) but the PBL southwesterly flows
over South China tend to be weaker than in nocturnal
hours (Du et al. 2014). The extreme hourly rainfall
(.60 mm h
) records of the local/SW wind type
(blue dots in Figs. 12b,d) are observed mainly within
the urban cluster and close (within about 50 km) to its
northern boundary regardless of the UHI strength,
while some extreme rainfall records in the weak UHI
cases are located near the coastline.
The strong-UHI subgroup of the local/shear line
type mainly (13 out of 17 events) initiates and deve-
lops during the daytime particularly in the afternoon-
to-evening hours (Fig. 14a). This subgroup has the
hourly extremes and rainfall accumulation mostly
over the inland portion of the PRD urban cluster
(Fig. 14b),wheretheUHIismostintense(Fig. 4a).
In contrast the weak-UHI subgroup has 3 out of 8
events initiating in the nocturnal, 1 in the morning,
and 4 in the afternoon (Fig. 14c). The weak-UHI
subgroup produces rainfall mostly near the coastline
(Fig. 14d), attributable to more (less) favorable
FIG. 13. (a) Vertical velocity (210
Pa s
) at 700 hPa, (b) horizontal wind speed (m s
) at 850 hPa, (c) wind
convergence (210
) at 925 hPa, and (d) precipitable water (mm) averaged over the key region using the
ERA-Interim reanalysis data. The red (blue) symbols represent the 18 (20) local/SW wind type EXREs with a
strong (weak) UHI, and the red (blue) lines denote the corresponding average values.
AUGUST 2019 W U E T A L . 1813
thermodynamic conditions [i.e., more (less) moisture
and larger (smaller) convective available potential
energy] to the south/north of the shear line (Fig. 10b).
The different features in the temporal and spatial
distributions of the two subgroups support possible
convective intensification and rainfall enhancement
by the strong UHI over the inland PRD urban region
for the local/shear line type EXREs.
In addition to the UHI effect, sea breezes could
also play an important role in convection development
and rainfall production over the PRD region; for ex-
ample, a close association between sea breezes and in-
land propagation of warm-season rainfall over the PRD
coastal region during the daytime was noticed (Chen
et al. 2016;Z. N. Jiang et al. 2017). Moreover, extreme
precipitation events over coastal South China can be
influenced by the trumpet-shaped topography of the
PRD (Huang et al. 2019) and the coastal mountains
(Wang et al. 2014;Wu and Luo 2016). Interactions be-
tween sea breezes, UHI-induced circulation, larger-
scale southwesterly air flows (Fig. 10a), and topography,
as well as their influence on the timing, location, and
evolution of rain storms in the local/SW wind type of
EXREs deserve further study.
c. Comparison between migratory EXREs with a
strong and weak UHI
is expected to impact the rainfall intensity and distri-
bution over the key region rather than the timing
of these EXRE types. Visual examination of radar
reflectivity animation indeed suggests notable inten-
sification of radar reflectivity when the rain storms
approach the strong-UHI region especially over the
inland PRD, which is not observed in the weak-UHI
cases. This expectation is also confirmed by examin-
ing the rainfall distribution. Therefore, this subsection
will discuss mostly the spatial distributions of ex-
treme hourly rainfall records and accumulated rain-
fall amounts, although the timing of the EXREs will
still be shown.
For the migratory-NW type, which accounts for
60% of the total number of the migratory EXREs,
both the hourly precipitation extremes and rainfall
FIG. 14. As in Fig. 12, but for the local/shear line type of the 2011–16 EXREs.
accumulation tend to concentrate over the inland portion
of the PRD urban cluster in the strong-UHI subgroup
(Fig. 15b), while those of the weak-UHI subgroup are
contrastingly situated near the coastline with smaller
rainfall amounts in the inland urban region (Fig. 15d).
The larger rainfall amounts near the coastline could
be attributed to higher equivalent potential tempera-
ture (u
) of air masses near the coastline in the weak-
UHI cases (not shown).
The migratory-SW type consists of only 3 EXREs
with strong UHI. Despite of the small number of events,
it is still noted that both the hourly precipitation ex-
tremes and rainfall accumulation are mainly concen-
trated over the urban cluster (Fig. 16c). In contrast
the 10 migratory-SW EXREs with weak UHI have the
hourly extremes mainly distributed over the southwest
coastal area (Fig. 16d).
All the 6 migratory-NE EXREs are accompanied
by a strong UHI and their extreme hourly precipita-
tion records are mostly located over the urban cluster
(Fig. 17b). The rain storms in these cases intensify im-
mediately when enter the urban cluster and weaken
quite rapidly when move out from the southwest bound-
ary of the urban region. Moreover, three extra rainfall
events that similarly have the rain storm moving out
from a TC or its remnant centered around the Taiwan
Strait to influence the PRD region are found. These
three events have weak UHI and do not produce
hourly precipitation .60 mm h
. The radar animation
(not shown) suggests that the rain storms weaken
or dissipate when they pass the PRD urban region,
opposite to the evolution of rain storms in the 6 strong-
UHI cases.
The 4 migratory-S EXREs are also accompanied by
a strong UHI. Examination of the radar anima-
tion (not shown) suggests convective intensification
when the storms move over the UHI. However, the
storms produce accumulative rainfall mainly over
the coastal region, with a secondary rainfall center
over the northwest PRD urban region (Fig. 17d).
FIG. 15. As in Fig. 12, but for the migratory-NW type of the 2011–16 EXREs. The arrow in (c) and (d) denotes the
movement direction of the extreme-rain-producing storm.
AUGUST 2019 W U E T A L . 1815
Stronger evidence of rainfall intensification by the UHI
is limited by the small number of this EXRE type.
5. Summary and conclusions
This study investigates the changes in extreme hourly
precipitation in coastal areas of South China, focusing
on its relationship with the PRD urbanization through:
1) exploring long-term changes of hourly extremes
(defined by the 95th percentile) using gauge-based
hourly precipitation observations at 61 national-level
surface stations over 46 years (1971–2016); and 2) ana-
lyzing the 120 extreme rainfall events (with at least one
record of .60 mm h
hourly rainfall in the key region)
in 2011–16 using observations collected by densely dis-
tributed AWSs and radar network in coastal South
China. The major findings are summarized as follows:
1) Statistically significant increase of hourly precipitation
intensity leads to higher annual amounts of both total
and extreme precipitation over the PRD urban cluster
in the rapid urbanization period (about 1994–2016)
than during the preurbanization era (1971–about
1993), suggesting a possible link of rainfall enhance-
ment to the rapid urbanization. This tends to change
the pattern of climatological-mean precipitation dis-
tribution as the urban cluster is situated among the
three centers of climatological-mean precipitation
amount over coastal South China and tends to connect
the centers.
2) Those urbanization-induced positive trends of the
extreme hourly rainfall amount and frequency
over PRD are found in the majority of stations
over the PRD when the TC-produced extreme
rainfall records are excluded. The trends are more
closely related to more frequent occurrence of the
short-duration (#6 h), abrupt rainfall events, than
the longer-duration, continuous, or growing rain-
fall events.
3) The 120 EXREs in 2011–16 consist of 67 (53) cases
with a strong (weak) UHI. They are categorized into
six types according to the originating location and
movement of the extreme-rain-producing storms,
namely, the local/SW wind (31.7%), local/shear
FIG. 16. As in Fig. 12, but for the migratory-SW type of the 2011–16 EXREs. The arrow in (c) and (d) denotes the
movement direction of the extreme-rain-producing storm.
line (20.8%), and migratory-NW, migratory-SW,
migratory-NE, or migratory-S types (28.3%, 10.8%,
5.0%, 3.3%). Irrespective of synoptic conditions and
seasons (April–September) in which the EXREs
take place, rainfall intensification by the strong UHI
is noticeable in all the six types, especially over
the inland urban region where the UHI intensity is
strongest, because of the strong UHI-induced larger
CAPE to feed the rain storms. The UHI thermal
perturbation probably plays an important role in the
convective initiation and intensification of the locally
developed rain storms in the afternoon.
This study provides observed evidence of hourly
rainfall intensification over the PRD urban cluster,
especially its inland portion, in a large ensemble of
EXREs under various synoptic conditions, consistent
with the findings from the long-term trend analysis.
The results build a scientific basis for future in-depth
analysis and modeling studies to better understand the
individual and combined impacts of local urban envi-
ronment and global warming on the change of hourly
precipitation extremes over the coastal South China
during the past decades. Physical mechanisms about
the impacts of urban cluster on evolution of extreme-
rain-producing storms (e.g., the UHI-induced destabi-
lization, anthropogenic aerosol emissions) can also
be better understood using high-resolution modeling
Acknowledgments. This research is supported by
National (Key) Basic Research and Development
Program of China (2018YFC1507400), The National
Natural Science Foundation of China (41775050), and
the Basic Research and Operation Funding of Chi-
nese Academy of Meteorological Sciences (CAMS)
(2017Z006). We also acknowledge the support from
the National Center for Atmospheric Research
(NCAR) Water System and the USDA-NIFA Agri-
culture and Food Research Initiative (Awards 2015-
67003-23508 and 2015-67003-23460). We thank Dr.
identify the TC-induced precipitation and Prof. Da-
Lin Zhang (University of Maryland) for helpful dis-
cussions. The land-use data were downloaded from
the Resources and Environment Scientific Data Cen-
ter, Chinese Academy of Sciences (
FIG. 17. As in Fig. 12, but for the (a),(b) migratory-NE type and (c),(d) migratory-S type of the 2011–16 EXREs.
Both types only have strong-UHI events. The arrow in (c) and (d) denotes the movement direction of the extreme-
rain-producing storm.
AUGUST 2019 W U E T A L . 1817
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... The specific procedures for calculating the extreme precipitation indices are as follows. For each of the 5,384 meteorological grid points in the Sichuan-Chongqing area, precipitation events with a daily precipitation amount of ≥1 mm during the 1979-2015 period were sorted in the ascending order (Zhai et al., 2005;Zhang et al., 2013;Wu et al., 2019), and the 95th percentile was estimated as the extreme precipitation threshold. When the precipitation amount of a given day exceeded this threshold, an extreme precipitation event was considered to have occurred. ...
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China has undergone rapid urbanization over the past few decades, and accordingly, changes have occurred in the extreme precipitation events. However, few studies have focused on the relationships between rapid urbanization and extreme precipitation events in southwest China, particularly in the Sichuan–Chongqing area, which has a complex topography and has experienced rapid urbanization over the past few decades. This is the first study to analyze the impact of urbanization on the amount, frequency, and intensity of extreme summer (June–August) precipitation events over the past 30 years. Our results indicate that extreme precipitation events primarily occurred in the urban-dominated Sichuan basin, particularly during the fast urbanization development stage (FUDS) of 1994–2015. Extreme precipitation amounts and intensities increased during the FUDS, implying the greater probability of individual precipitation events developing into heavy or extreme events in a particular area. In addition, the probability distribution functions of the occurrence and volume of strong convective events significantly increased during the FUDS. Finally, the annual increase in urban-scale land surface air temperature, increase in wet convection, and changes in wind speed are identified as essential factors leading to extreme precipitation events in this region.
... e study found that both the summer precipitation and frequency of HEP were affected by terrain, and the values were higher in the windward slope area. Wu et al. [27] used hourly precipitation data of 61 rain gauges from 1971 to 2016 to investigate the variation of extreme hourly precipitation (EXHP) in the Pearl River Delta (PRD) region of South China. Sen Roy and Rouault [28] analyzed hourly precipitation data at 102 stations in South Africa from 1998 to 2007 to understand trends in extreme hourly precipitation events. ...
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Based on the hourly precipitation data of 34 meteorological stations in Chongqing in the summers (June to August) from 1996 to 2015, the spatial distribution and daily variation of precipitation amount (PA), precipitation intensity (PI), precipitation frequency (PF), and precipitation extremes in Chongqing are analyzed. The results show that, from the perspective of spatial distribution, the precipitation amount (PA) in Chongqing presents a distribution pattern of more around and less in the middle; the area with high precipitation intensity (PI) is mainly located in the northeast of Chongqing; the large value centers of precipitation frequency (PF) are located in the south and west of Chongqing and near Chengkou. On the spatial distribution of hourly precipitation, the precipitation in most areas of Chongqing is mainly concentrated at night [0200–0900 BT (1800–0100 UTC)], and the rain belt spreads from west to east with the passage of time. On the whole, interannual evolution characteristics of summer precipitation amount, precipitation intensity, and precipitation frequency in Chongqing are basically the same, showing a fluctuation characteristic without obvious trend, but there are some peaks and valleys. From the perspective of diurnal cycle, a larger peak of PA in Chongqing appears near 0300 BT (1900 UTC), another lower peak around 1200 BT (0400 UTC), a larger peak of PI around 0300 BT (1900 UTC), another smaller peak around 1500 BT (0700 UTC), and only one peak of PF around 0700 BT (2300 UTC). The extreme precipitation of different duration in summer in Chongqing is closely related to the topographic characteristics and weather system, the extreme centers of each diachronic precipitation are mainly located near Shapingba, Kaizhou, Youyang, and Shizhu, and the time evolution characteristics of the extreme precipitation are not obvious, but the trend of the extreme precipitation accumulated in 1 h, 3 h, 6 h, or 12 h is basically the same.
... In addition, an interesting phenomenon is observed, showing the capital city Xi'an and its surrounding cities have relative low mean values of EPIs (indicated by PRCPTOT, R10mm, R20mm, R95P, RX1day, and RX1day). Most previous studies have shown that rapid urbanization will increase the intensity and frequency of extreme precipitation (Fu et al. 2019;Liang and Ding 2017;Wu et al. 2019), which seems to contradict our results. However, as found by Kaufmann et al. (2007), urbanization reduces precipitation in the Pearl River Delta of China. ...
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Extreme precipitation poses a severe threat to the natural ecosystem, socioeconomic development, and human life. Investigating the spatiotemporal variations in extreme precipitation and exploring the potential drivers have implications for disaster risk reduction and water resource management. In this study, we analyzed the changes in nine extreme precipitation indices (EPIs) over the Wei River Basin (WRB) during 1957–2019. Furthermore, we assessed the effect of geographic factors (latitude, longitude, and altitude) on the spatial distribution of EPIs and the potential impact of ocean–atmosphere circulation on the temporal variability of EPIs. The results indicate that six EPIs present a downward trend and three EPIs show an upward trend, but all the trends are not significant. In the seasonal scale, max 1-day precipitation amount (RX1day) increases significantly in summer (P < 0.05), while the trends in max 5-day precipitation amount (RX5day) are not significant in all seasons. The period of about 8 years and less than 3 years were observed in most EPIs. The mean values of EPIs except consecutive dry days (CDD) gradually increase from northwest to southeast of the WRB. Latitude, longitude, and altitude are important factors affecting the spatial distribution of the extreme precipitation. Southern Oscillation Index (SOI) and Atlantic Multidecadal Oscillation (AMO) contribute the most to EPIs variation. Interdecadal and interannual oscillations occur between most EPIs and ocean-atmospheric circulation factors, but their phase relationships are different. Our findings highlight the importance of examining global and local driving factors of trend in regional extreme precipitation by a systematic approach, and help to further understand the precipitation changes in the WRB.
... According to the Clausius-Clapeyron (CC) relation, a saturated atmosphere can hold approximately 7% more moisture per 1 • C increase in temperature (Lenderink and van Meijgaard, 2010;Molnar et al., 2015). Hence, assuming that the atmosphere is saturated during extreme rainfall events (Ali and Mishra, 2017;Mishra et al., 2012;Pan et al., 2019), UHIs are expected to contribute to an intensification of short-duration extremes (Wu et al., 2019). When aerosol concentrations are sufficiently high, they can act as a source of cloud condensation nuclei on which water can condense, enhancing cloud formation and precipitation; however, too elevated aerosols concentrations can also have the effect of slowing down the condensation of cloud water because smaller drops are generated, thus lowering the chances of drop precipitation (Han et al., 2014). ...
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Short-duration extreme rainfall events are the main trigger of flash and pluvial floods in cities. Depending on the local climate zone and urban fabric that affect meteorological variables such as air temperature, humidity, and aerosol concentration, the built environment can either intensify or reduce extreme rainfall intensity. This study examined how urbanization in a large metropolitan area characterized by open low-rise buildings, affected sub-daily extreme rainfall intensities over the period between 2000 and 2018. The research was conducted in the metropolitan region of Phoenix, Arizona, which is supported by a large and dense rain-gauge network (168 stations). The built area increased by 6% between 2001 and 2016 and the number of residences by 300,000. Over the study period, sub-daily extreme rainfall intensities intensified both in the urbanized area and in its rural surroundings but the intensification trend within the built area was considerably larger (3 times larger). We calculated a negative trend in aerosol concentration (−0.005 AOD y⁻¹) but a positive trend in near-surface air temperature that was considerably larger in the urban areas (0.15 °C y⁻¹) as compared to the rural counterpart (0.09 °C y⁻¹) for the period between 2005 and 2018. Although built surfaces and open low-rise buildings contributed to an increase in air temperature, they did not affect air humidity. Changes in rainfall extremes approximately follow the Clausius–Clapeyron relation within the urban area with an increase at a rate of 7% °C⁻¹. These results demonstrate that the warming effect associated with a low-rise urban area can cause an intensification of sub-daily rainfall extremes that is significantly larger than in nearby rural areas.
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The northern mountainous area of the Pearl River Delta (PRD) is a center of convection and heavy rainfall of coastal southern China during the summer monsoon season. Although there are frequent nighttime convective activities and rainfall over the region, the key dynamical and physical processes responsible for the nocturnal convection initiation (CI) are complex and not yet well documented. In this study, high‐resolution reanalyses from the Variational Doppler Radar Assimilation System (VDRAS) were used to explore the triggering mechanisms of a representative nocturnal convection event that occurred over the northern PRD on August 14, 2014. Results showed that the enhanced prevailing low‐level southerly winds, deflected easterly winds caused by the blocking of multi‐scale orography, and northern downslope winds associated with an orographic cooling effect were crucial for generating low‐level convergence and strong updrafts before CI. The low‐level convergence and moisture flux in the boundary layer started to increase at 00:20 local solar time (LST), and by the CI time (01:00 LST), the convergence intensity more than doubled. The intensification of low‐level convergence and moisture transport was strongest before 00:40 LST and was dominated by a zonal component that was closely related to the deflected easterly winds. Using Doppler radar data assimilation, this is the first study to reveal how the deflected flows and the thermally induced downslope winds combine with prevailing monsoonal winds to affect a nocturnal CI over the complex mountainous region of the South China coast.
Coastal South China is frequently exposed to substantial rainfall within fronts-free environments during the presummer rainy season, which recurrently leads to severe disasters due particularly to the heavy short-term rainfall (HSR). The present study is motivated to identify the HSR hotspots over coastal South China through 10 notorious episodes of warm-sector heavy rainfall, with emphasis on the mesoscale features linking with the HSR hotspots. These episodes suggest complex rainfall features in the occurrence frequency and intensity. HSR mainly occurs rather over the coastal areas than the windward piedmont farther inland, and contributes over 80% to the rainfall accumulation in the regions of Yangjiang and Shanwei. Mesoscale topographies in both regions play an important role in the presence of HSR hotspots, but in different ways. Convergence perturbation line related to the inland mountains migrates southward, and enhances the preexisting convection along the coast resulted from the differential surface friction between land and ocean as well as small hills, which is the primary cause for the high frequency of HSR in coastal Yangjiang. While concave terrain drives cyclone-like perturbation, modulating the convection activity prior to the presence of HSR over Shanwei region. A cold dome induced by the previous convection dominates the mesovalley, furthering the subsequently convective enhancement at coastal zone. Collectively, both inland mountains and ocean-to-land friction regulate the mesoscale surface processes by governing the convergent wind perturbations and upscale convective growth, responsible for the HSR hotspots over the low-altitude coastal areas.
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Plain Language Summary When extreme precipitation occurs, one will easily connect it to intense storms such as those producing numerous lightning flashes. However, some rainstorms display little severe signatures, therefore, they are even more challenging from the monitoring or nowcasting point of view. This study investigates the warm‐season extreme precipitation events (EPEs thereafter) with weak convection (weak upward motion indicated by satellite proxy) in the tropics and subtropics using 15 years of satellite observations. Surprisingly, EPEs with weak convection (WeEPEs) over land are as frequent (∼30%) as EPEs with intense convection, highlighting the significance of WeEPEs. Over land, WeEPEs preferentially develop over deep monsoon regions and their maxima are located over or near coastal mountains, suggesting the important role of orographic lifting/convergence in producing WeEPEs. Spaceborne radar, microwave, and lightning observations all indicate that WeEPEs have a weak convective intensity and limited ice contents above the melting level. WeEPEs are featured by vertical radar profiles persistently increase downward below the melting level (by ∼15 dBZ). Our findings collectively suggest that heavy rainfall in WeEPEs is largely caused by precipitation process in the warm‐cloud layer, instead of the melting of large amount of ice particles.
Over the recent years, characteristics of precipitation in Shanghai have received extensive attention. Scholars used multiple methods to study the precipitation time series in Shanghai. However, most of the existing studies only focus on the statistical characteristics of the precipitation series rather than the internal dynamic characteristic. The visibility graph algorithm has been proved to be an efficient algorithm for transforming the information stored in time series into the structure of complex network. By studying the topological properties of the network, we can master the internal dynamics characteristics of time series. Therefore, this paper exploits the visibility graph algorithm to analyze the daily precipitation time series in Shanghai from 2000 to 2020. Some characteristics of precipitation during this period are obtained: Firstly, Typhoon Mesa and Typhoon Fitow had a profound impact on Shanghai’s precipitation; Secondly, there is a certain similarity of the precipitation structure in different periods; Finally, only a few nodes in the precipitation network have a profound impact on the precipitation in Shanghai. In general, our research provides a new perspective and method for studying the characteristics of precipitation in Shanghai.
Conference Paper
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Studies concerning the effects of urbanization on heavy precipitation events mostly focused on the summertime convective precipitation events. In these studies, the Urban Heat Island (UHI) effect was prominent over the urbanized region before the event, changing the spatial and temporal distribution of the precipitation. We aim to reveal the impact of urbanization over Ankara on the springtime frontal precipitation event of 5 May 2014, when the ground heating and UHI effects are not as strong as those in the summertime. We performed two different simulations based on the land-use scenarios with urban (URBAN) and without urban areas (NOURBAN) over Ankara, integrating the CORINE Land Use dataset into the Weather Research and Forecasting Model (WRF v3.8) and replacing the urban areas with the dominant land use category over the region. Four sub-regions with the identical area coverages corresponding to the upwind, central, and downwind parts of the city center are defined to have a lucid spatial and temporal representation of the event. The two simulation results agreed reasonably with the observations. In the simulation (URBAN) with the urban land use included, the spatial average of the daily rainfall amounts over the predefined sub-regions slightly decreased, especially the sub-regions to the upwind and downwind of the highly urbanized area. However, the difference in precipitation amount in the vicinity of the urbanized area between the two different simulations is not of significance in comparison to what was observed in other summertime precipitation studies. On the other hand, the UHI effect might be crucial in determining the impact of urban land use on the distribution and magnitude of the heavy springtime rainfall. To support this idea, we performed a similar analysis for a summertime convective precipitation event over Ankara and compared the results.
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Record‐breaking rainfall of 524.1 mm in 24 h occurred in the coastal metropolitan city of Guangzhou, China during 06–07 May, 2017, and caused devastating flooding. Observation analysis and a nested very‐large‐eddy simulation (VLES) with Weather Research and Forecasting (WRF) model were conducted to investigate various factors that contributed to the heavy rainfall, including synoptic weather pattern, topographic effects, cold pool, and urban effects. Firstly, the warm and moist southerly flow in the lower troposphere over the trumpet‐shaped topography of the Pearl River Delta continuously provided fuel for the development of the severe rainfall. Consequently, the southerly flow from the sea in the south strengthened with the development of the convection. Meanwhile, the precipitation‐produced weak cold pool supported a stationary outflow boundary, where new convective cells were continuously initiated and drifted downstream. The interaction between the cold outflows and the warm moist southerly flows in the lower troposphere formed a back‐building convective system, which produced local persistent heavy rainfall that lasted for more than 5 hours and reached record levels. Sensitivity experiments in which the urban area was removed from the model indicate that the urban forcing affected the timing and location of convective initiation and helped concentrate the maximum rain core. The nested WRF‐LES successfully simulated this heavy rainfall, and the model's advantages are noted for forecasting such local severe weather.
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In this study, an extreme rainfall event of 451 mm in 20 hr that occurred in coastal South China on 11 May 2014 during the Southern China Monsoon Rainfall Experiment is investigated using integrated observations from the dual-Doppler radar pair, polarimetric radar, extensive mesonetwork, and enhanced upper-air soundings. Results show the generation of the extreme rainfall by two consecutive mesoscale convective systems (MCSs) consisting of multiple meso-β-scale rainbands. The two MCSs are maintained by lifting southerly oceanic flows over a quasi-stationary mesoscale outflow boundary (MOB) along the coastline that are enhanced by convectively generated weak cold pool. Northeastward “echo training” of convective cells, under the influence of environmental southwesterly flows, leads to the formation of the multiple rainbands in each MCS. Their subsequent propagations in a “rainband training” form, together with the echo training, along the coastline account for the production of extreme rainfall. The second MCS is characterized with a leading bowing rainband showing a process of rapid splitting and reestablishment (RSRE), which contributes to the formation of the rainband training. The occurrence of the RSRE process requires ample supply of unstable upstream oceanic air mass, the quasi-stationary MOB, and a bowing rainband intersecting with the MOB. The second MCS produces more precipitation than the first one as a result of more rainbands, stronger convective intensity, and more moderate-sized raindrops with larger maximal sizes. The above findings, especially the RSRE process and its associated storm internal circulation, appear to add new Insights into the formation and maintenance of training rainbands and their roles in heavy rainfall production.
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A spectral analysis of daily rainfall data has been performed to investigate extreme rainfall events in south China during the presummer rainy seasons between 1998 and 2015 (excluding 1999, 2006, 2011, and 2014). The results reveal a dominant frequency mode at the synoptic scale with pronounced positive rainfall anomalies. By analyzing the synoptic-scale bandpass-filtered anomalous circulations, 24 extreme rainfall episodes (defined as those with a daily rainfall amount in the top 5%) are categorized into "cyclone" (15) and "trough" (8) types, with the remaining events as an "anticyclone" type, according to the primary anomalous weather system contributing to each extreme rainfall episode. The 15 cyclone-type episodes are further separated into (11) lower- and (4) upper-tropospheric migratory anomalies. An analysis of their anomalous fields shows that both types could be traced back to the generation of cyclonic anomalies downstream of the Tibetan Plateau, except for two episodes of lower-tropospheric migratory anomalies originating over the South China Sea. However, a lower-tropospheric cyclonic anomaly appears during all phases in the former type, but only in the wettest phase in the latter type, with its peak disturbance occurring immediately beneath an upper-level warm anomaly. The production of extreme rainfall in the trough-type episodes is closely related to a deep trough anomaly extending from an intense cyclonic anomaly over north China, which in turn could be traced back to a midlatitude Rossby wave train passing by the Tibetan Plateau. The results have important implications for understanding the origin, structure, and evolution of synoptic disturbances associated with the presummer extreme rainfall in south China.
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Changes in extreme precipitation are among the most impact-relevant consequences of climate warming, yet regional projections remain uncertain due to natural variability and model deficiencies in relevant physical processes. To better understand changes in extreme precipitation, they may be decomposed into contributions from atmospheric thermodynamics and dynamics, but these are typically diagnosed with spatially aggregated data or using a statistical approach that is not valid at all locations. Here we decompose the forced response of daily regional scale extreme precipitation in climate-model simulations into thermodynamic and dynamic contributions using a robust physical diagnostic. We show that thermodynamics alone would lead to a spatially homogeneous fractional increase, which is consistent across models and dominates the sign of the change in most regions. However, the dynamic contribution modifies regional responses, amplifying increases, for instance, in the Asian monsoon region, but weakening them across the Mediterranean, South Africa and Australia. Over subtropical oceans, the dynamic contribution is strong enough to cause robust regional decreases in extreme precipitation, which may partly result from a poleward circulation shift. The dynamic contribution is key to reducing uncertainties in future projections of regional extreme precipitation. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
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Correlations of the Urban Heat Island Intensity (UHII) and key surface variables with the Short-Duration Intense Rainfall (SDIR) events are examined for the Beijing urban areas by applying hourly data of a high-density Automatic Weather Station (AWS) network. Higher frequencies (amounts) of the SDIR events are found in or near the central urban area, and most of the SDIR events begin to appear in late evening and nighttime, but tend to end in late night and early morning. Correlations of the UHII with the SDIR frequency (amount) are all highly significant for more than 3 hours ahead of the beginning of the SDIR events. Although the UHII at immediate hours (<3h) before the SDIR occurrence is more indicative to the SDIR events, their occurrence more depends on the magnitude of the UHII at earlier hours. The UHII before the beginning of the SDIR events also shows the high-value centers in the central urban area, which is generally consistent with the distribution of the SDIR events. The spatial and t...
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Warming of the climate is now unequivocal. The water holding capacity of the atmosphere increases by about 7% per °C of warming, which in turn raises the expectation of more intense extreme rainfall events. Meeting the demand for robust projections for extreme short-duration rainfall is challenging, however, because of our poor understanding of its past and future behaviour. The characterization of past changes is severely limited by the availability of observational data. Climate models, including typical regional climate models, do not directly simulate all extreme rainfall producing processes, such as convection. Recently developed convection-permitting models better simulate extreme precipitation, but simulations are not yet widely available due to their computational cost, and they have their own uncertainties. Attention has thus been focused on precipitation–temperature relationships in the hope of obtaining more robust extreme precipitation projections that exploit higher confidence temperature projections. However, the observed precipitation–temperature scaling relationships have been established almost exclusively by linking precipitation extremes with day-to-day temperature variations. These scaling relationships do not appear to provide a reliable basis for projecting future precipitation extremes. Until better methods are available, the relationship of the atmosphere's water holding capacity with temperature provides better guidance for planners in the mid-latitudes, albeit with large uncertainties.
Five-day means show that over south China there are certain persistent variations in the march of rainfall during the wet season (March–November). These are explained in terms of seasonal meteorological trends in the surrounding regions. The view is advanced, supported by an ancient Chinese farmers' calendar, that the rainfall pattern of south China has changed little in the past 2000 years.
Summer precipitation plays critical roles in the energy balance and the availability of fresh water over eastern China. However, little is known regarding the trend in local-scale precipitation (LSP). Here, we developed a novel method to determine LSP events in the summer afternoon throughout eastern China from 1970 to 2010 based on hourly gauge measurements. The LSP occurrence hours decrease at an annual rate of 0.25%, which varies considerably by region, ranging from 0.14% over the Yangtze River Delta to 0.56% over the Pearl River Delta. This declining frequency of LSP is generally accompanied by an increase in rain rate of LSP but a decrease in visibility, whose linkage to LSP events was investigated. In particular, more LSP events tended to form when the atmosphere was slightly polluted. Afterwards, LSP was suppressed. These findings have important implications for improving our understanding of the climatology of daytime precipitation at local scales.
Using the hourly precipitation records of meteorological stations in Shanghai, covering a period of almost a century (1916–2014), the long-term variation of extreme heavy precipitation in Shanghai on multiple spatial and temporal scales is analyzed, and the effects of urbanization on hourly rainstorms studied. Results show that: (1) Over the last century, extreme hourly precipitation events enhanced significantly. During the recent urbanization period from 1981 to 2014, the frequency of heavy precipitation increased significantly, with a distinct localized and abrupt characteristic. (2) The spatial distribution of long-term trends for the occurrence frequency and total precipitation intensity of hourly heavy precipitation in Shanghai shows a distinct urban rain-island feature; namely, heavy precipitation was increasingly focused in urban and suburban areas. Attribution analysis shows that urbanization in Shanghai contributed greatly to the increase in both frequency and intensity of heavy rainfall events in the city, thus leading to an increasing total precipitation amount of heavy rainfall events. In addition, the diurnal variation of rainfall intensity also shows distinctive urban-rural differences, especially during late afternoon and early nighttime in the city area. (3) Regional warming, with subsequent enhancement of water vapor content, convergence of moisture flux and atmospheric instability, provided favorable physical backgrounds for the formation of extreme precipitation. This accounts for the consistent increase in hourly heavy precipitation over the whole Shanghai area during recent times.
Impacts of urbanization and anthropogenic aerosols in China on the East Asian summer monsoon (EASM) are investigated using version 5.1 of the Community Atmosphere Model (CAM5.1) by comparing simulations with and without incorporating urban land cover and/or anthropogenic aerosol emissions. Results show that the increase of urban land cover causes large surface warming and an urban frictional drag, both leading to a northeasterly wind anomaly in the lower troposphere over eastern China (EC). This weakens the southerly winds associated with the EASM and causes a convergence anomaly in southern China (SC) with increased ascent, latent heating, and cloudiness. The enhanced latent heating reinforces surface convergence and upper-level divergence over SC, leading to more northward advection in the upper level into northern China (NC) and descending between 30° and 50°N over East Asia. Cloudiness reduction, adiabatic heating, and warm advection over NC all enhance the urban heating there, together causing anomalous tropospheric warming at those latitudes over East Asia. Anthropogenic aerosols cause widespread cooling at the surface and in the troposphere over EC, which decreases the summer land-ocean thermal contrast, leading to a weakened EASM circulation with reduced moisture transport to NC. This results in wetter and drier conditions over SC and NC, respectively. When both the urbanization and anthropogenic aerosols are included in the model, aerosols' cooling is partially offset by the urban heating, and their joint effect on the circulation is dominated by the aerosols' effect with a reduced magnitude. In the combined experiment, surface and tropospheric temperatures are also altered by the decrease (increase) in cloudiness over NC (SC) with most of the cooling confined to SC, which further weakens the EASM circulation.