JOURNAL OF GEOPHYSICAL RESEARCH: ATMOSPHERES, VOL. 118, 3163–3177, doi:10.1002/jgrd.50337, 2013
Influence of summer monsoon diurnal cycle on moisture transport
and precipitation over eastern China
Guixing Chen,1 Weiming Sha,1 Masahiro Sawada,2 and Toshiki Iwasaki1
Received 8 October 2012; revised 20 February 2013; accepted 14 March 2013; published 25 April 2013.
[1] Diurnal variability of the summer monsoon over China, a key element affecting regional
climate, is examined using the latest reanalysis dataset and satellite rain estimates. Diurnal
variation of low-level wind is found to be pronounced over South China during active
monsoon days. Mean wind speed attains a maximum (minimum) in early morning (afternoon),
with a diurnal range of 2–3 m/s, double that of inactive monsoon days. The largest amplitude
typically appears at 850 hPa, consistent with radiosonde observations. Such a monsoon diurnal
cycle can strengthen low-level moisture transport at night by about 20% more than during the
day. Nocturnal moisture fluxes converge toward Central China and lead to a meso-synopticscale moisture sink during the late night and morning, which plays a role in regulating the
regional water budget on a diurnal time scale. Monsoon flow also helps provide substantial
moisture over low-lying areas in the morning hours. Correspondingly, morning rainfall
undergoes a remarkable increase and greatly contributes to the diurnal cycle of summer rainfall.
The strongest response comes from meso-a-scale rain events that not only become prominent
during active monsoon days but also possess a dominant morning peak. These morning events
occur mainly on the basins and plains of Central China, where the monsoon diurnal cycle
promotes nighttime mesoscale convection. These tend to shift northward from June to August,
with the progress of the monsoon diurnal cycle, thereby producing the morning-peak summer
rainband. The findings point to an efficiency of nocturnal monsoon flow influencing the warmseason weather and climate over eastern China.
Citation: Chen, G., W. Sha, M. Sawada, and T. Iwasaki (2013), Influence of summer monsoon diurnal cycle on moisture
transport and precipitation over eastern China, J. Geophys. Res. Atmos., 118, 3163–3177, doi:10.1002/jgrd.50337.
1.
Introduction
[2] The summer monsoon has a profound influence on the
weather and climate over East Asian countries. The monsoon
variability, ranging from diurnal to interdecadal time scales,
is a much researched subject [e.g., Ding, 1992; Chen et al.,
2004]. In recent decades, the diurnal variation in winds,
convective activities, and precipitation has been recognized
as being a key aspect of the warm-season climate in tropical
and monsoon regions [Ohsawa et al., 2001; Wang et al.,
2004; Hirose and Nakamura, 2005; Yang and Smith, 2006;
Yu et al., 2007; Chen et al., 2009a; Mao and Wu, 2011].
Gaining further knowledge of the monsoon climate on a
diurnal time scale has important hydrological implications
for improved heavy rain prediction and water resource
management and remains a topic for current and future works.
1
Department of Geophysics, Graduate School of Science, Tohoku
University, Sendai, Japan.
2
Atmosphere and Ocean Research Institute, The University of Tokyo,
Tokyo, Japan.
Corresponding author: G. Chen, Department of Geophysics, Graduate
School of Science, Tohoku University, A519, Physics A, 6-3, Aoba, Aramaki,
Aoba, Sendai, Miyagi 980-8578, Japan. (chen@wind.gp.tohoku.ac.jp)
©2013. American Geophysical Union. All Rights Reserved.
2169-897X/13/10.1002/jgrd.50337
[3] The monsoon flow, on a short time scale, is characterized by remarkable diurnal variation [Ramage, 1952]. Over
the Asian continent, the summer monsoon exhibits a
pronounced diurnal pulsation of its large-scale divergent circulation [Krishnamurti and Kishtawal, 2000]. Over eastern
China (Figure 1), the diurnal variation is clearly seen in the
low-level winds on a regional scale. For example, the southwesterly of tropical monsoon is most evident at night, as illustrated by the observations made in Hong Kong [Johnson,
2006]. The low-level wind speed, recorded at mountain sites
in South China, tends to reach a peak in the predawn [Yu et
al., 2009]. In particular, the diurnal wind variation over eastern
China attains large amplitude in the warm season after the onset of the summer monsoon [Chen et al., 2009b]. The
increased velocity of the southwesterly wind during the night
helps to establish the low-level jet (LLJ) over South China
[Rife et al., 2010]. These observational studies reveal that
nocturnal strong southwesterlies, with a meso-synoptic span
of several hundred to a thousand kilometers, are fairly common occurrences over eastern China. Although such events
seem to be rooted in the monsoon activities, a direct survey
of the diurnal cycle of monsoon flow over eastern China has
not been fully researched.
[4] Diurnal variation in the low-level wind has been linked
to the moisture supply and moist convection over eastern
China. The low-level southerly flow is shown to enhance
moisture transport from South China to Central China during
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Figure 1. Daily mean of TRMM rainfall and 850 hPa horizontal winds on (a) active and (b) inactive monsoon days. In
Figure 1a, the low-level convergence over eastern China stronger than –1 106 s1 is hatched. Eastern China implies
the area of mainland China east of 105 E, including three
subdomains as marked by the dashed rectangles: South China
(105 –120 E, 21 –27 N), Central China (105 –120 E,
27 –35 N), and North China (105 –120 E, 35 –38 N). The
long dashes mark the elevations of 1500 m and 3000 m.
the night [Li et al., 2007]. The intensity of this moist
inflow to the Meiyu frontal zone is a deciding factor in the
duration of the nocturnal rainband over Central China
[Yamada et al., 2007]. The nocturnal southwesterlies, in addition to transporting moisture, also strengthen the low-level
moisture convergence, frontogenesis, and convective instability that favor nighttime convection growth [Chen et al., 2009b;
Chen et al., 2010; Yuan et al., 2010; Ueno et al., 2011]. Climatologically, the mesoscale convective systems prefer to form
and grow in the environment of an enhanced, moist southerly
at night [Miller and Fritsch, 1991]. Observations have been
made of the significant relationship between the nocturnal
LLJ over South China and the extreme heavy rainfall over
Central China [Monaghan et al., 2010].
[5] Precipitation diurnal variability has also been the subject
of research to illustrate the possible influence of the summer
monsoon over eastern China on a short time scale. The
rainband along the Meiyu front is found to attain a morning
peak, while the rainfall in adjacent areas exhibits an afternoon
peak [Geng and Yamada, 2007]. The morning rainfall
dominates eastern China in the Meiyu season, while the afternoon rainfall is dominant in midsummer [Xu and Zipser,
2011]. The morning-peak rainfall tends to shift northward,
along with the summer progress of the monsoon rainband
[Yin et al., 2009; Chen et al., 2009a]. It has recently been
shown that morning rainfall is pronounced over Central China
during active monsoon period, while afternoon rainfall
becomes more pronounced during monsoon break period
[Yuan et al., 2010]. In particular, research has focused on
the morning rain systems as they explain the majority of the
variance in the amount of seasonal rainfall, leading to the
anomalous wet and dry seasons over Central China [Chen
et al., 2012a]. As indicated above, the diurnal cycle of hydrological processes in relation to the summer monsoon should
be addressed as a key issue in extending our understanding
of the regional climate.
[6] Previous studies have shed insight into diurnal variability, but most have been based on a summer mean or cases of
short periods. Diurnal variability over eastern China, in relation to monsoon activities, requires further studies. In particular, the impact of the monsoon diurnal cycle on moisture
transport and summer precipitation remains to be quantified.
The objective of this study is to address such processes using
the latest archived data. We provide a unique addition to previous works in that the relative importance of monsoon diurnal
cycle in summer climate is estimated. This article is organized
as follows: Section 2 describes the dataset used in this study
and categorizes the monsoon days. Section 3 offers a brief
explanation on the general features of monsoon activities and
related rainfall diurnal cycle. Section 4 examines the spatial
structures of the monsoon diurnal cycle. Section 5 presents
the induced diurnal cycles of moisture transport and regional
water budget. Section 6 illustrates the role of mesoscale rain
events in producing the rainfall budget as a response to the
monsoon diurnal cycle. The final section presents a discussion
and summary.
2.
Dataset and Monsoon Day Categorization
2.1. Reanalysis Data, Sounding Profile and
Satellite-Derived Rainfall
[7] As in situ observations are limited by time-space
sampling, reanalysis data are generally employed for climatological studies on wind diurnal oscillation and other related
processes [Higgins et al., 1997]. In the current study, we use
the latest reanalysis product from the European Centre for
Medium-Range Weather Forecasts (ECMWF). The
ECMWF Re-Analysis (ERA-interim) assimilates surface
observations, soundings, and a variety of satellite products
[Dee et al., 2011]. The variables include the pressure-level
analysis of winds, humidity, temperature, and so on. The
spatial resolution is 1.5 1.5 , and the temporal resolution
is six hourly. With an interval of 25 hPa below 750 hPa, the
ERA-interim is suitable for resolving the diurnal variation
of wind and moisture in the lower troposphere. It also offers
a forecast for the surface variables such as precipitation and
evaporation at three hourly intervals. As shown by Dee
et al. [2011], the ERA-interim has a good representation
of the hydrological cycle because of its improved physics
scheme and data assimilation. In this study, the ERA-
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interim data from 1989 to 2010 are used to examine the longterm mean of the monsoon diurnal cycle and its related moist
processes (sections 4 and 5).
[8] The ERA-interim dataset has been shown to represent
the horizontal pattern of wind diurnal variation over many
regions around the world, including China [Rife et al., 2010;
Monaghan et al., 2010]. However, the question remains as
to whether the data capture the nature of the monsoon diurnal
cycle, particularly the vertical structure. For validation, we
used the intensive observations from the South China Sea
Monsoon Experiment [Lau et al., 2000]. Six hourly sounding
profiles are available at a dozen sites in South China during
5–21 June 1998 when the summer monsoon is active over
eastern China. These quality-control data of horizontal winds,
temperature, and moisture have a good vertical resolution in
the lower troposphere, and thus, they are able to depict the
vertical structure of monsoon diurnal cycle. A spatial average
of sounding profiles over South China gives a reference of six
hourly vertical profiles, which is compared with those in
reanalysis data in section 4.2.
[9] In this study, the Tropical Rainfall Measuring Mission
(TRMM) 3B42 product is used for rainfall observation. The
3B42 dataset offers three hourly rain estimates with a spatial
resolution of 0.25 0.25 [Huffman et al., 2007]. The rain
rate has been calibrated by rain-gauge observations on land
and thus is applicable for studying the rainfall budget. This
short-interval dataset is widely used to monitor precipitation
diurnal variability [Kikuchi and Wang, 2008; Mao and Wu,
2011]. Over eastern China, 3B42 resolves a large amount
of morning rainfall, despite an underestimate of a few
fractions [Shen et al., 2010]. In particular, it captures the difference in the rainfall diurnal cycle between the anomalous
wet and dry seasons associated with the summer monsoon
[Chen et al., 2012]. The 3B42, with its 0.25 mesh resolution,
is also useful for studying the rainfall volume by mesoscale
convection [Huffman et al., 2007; Demaria et al., 2011].
In this study, 3B42 data from 1998 to 2010 are used to record
the huge number of rain events shown in section 6. The
plentiful samples derived from 13 years of archives have
helped us quantify the role of precipitation systems in
producing the rainfall diurnal cycle as a response to
monsoon variability.
[10] Local time (LT = UTC + 8 h) is applied in this study.
The four synoptic times (14:00, 20:00, 02:00, and 08:00 LT)
in the reanalysis data correspond to the afternoon, early
evening, late night, and morning, respectively. In studying the
rainfall budget, we group three hourly 3B42 rain estimates into
P.M. (14:00, 17:00, 20:00, and 23:00 LT) and A.M. (02:00,
05:00, 08:00, and 11:00 LT) hours, i.e., the afternoon-evening
rainfall and the late night-morning rainfall.
2.2. Categorization of Active/Inactive Monsoon Days
[11] The monsoon flow passes over South China before
releasing precipitation over Central China and other East
Asian regions. The flow features a surge of low-level southwesterlies in the Meiyu season or a channel of southerlies at
the western flank of the subtropical high [e.g., Ding, 1992].
To classify monsoon days, certain variables such as wind,
rainfall, moisture, and the pressure gradient are referred to
according to different focus of studies. In this study, we specify the monsoon flow as a continuous strong southerly, similar
to studies using the meridional wind as a monsoon intensity
index [Zeng et al., 2012]. First, based on the daily mean wind
at 850 hPa, the strong southerly is identified as either an obvious southerly (v ≥ 4 m s 1) over more than 50% of all grid
points in South China or a longitudinal band of intense southerly (v ≥ 8 m s 1). Strong southerlies that last for at least 2 days
are then registered as active monsoon days, and the remaining
summer days are registered as inactive monsoon days. A few
cases of southerlies due to landfall tropical cyclones are treated
as inactive monsoon days. There are 951 (1073) summer days
in 1989–2010, which are categorized as active (inactive) monsoon days, accounting for 47% (53%) of a total 2024 summer
days. The averaged number of active monsoon days per month
for June, July, and August is 16, 20, and 7, respectively. Those
in June and July mainly come from the Meiyu period that lasts
from mid-June to mid-July. This is consistent with the rainfallbased criterion that defines a Meiyu period of approximately
1 month over Central China [e.g., Chen et al., 2004]. Active
monsoon days are less frequent in August, corresponding
to weak southerlies and a short-period monsoon revival [Chen
et al., 2004; Zeng et al., 2012]. Most of the active monsoon
days come from continuous periods of 5 days or longer, which
correspond to the persistent features of monsoon activities.
Through comparing the groups of active and inactive monsoon days, rather than presenting a summer mean, we locate
the monsoon diurnal cycle and explore its relative contribution
to the summer climate.
3. General Characteristics of the Summer
Monsoon
3.1. A Mean of Low-Level Wind and Rainfall Amount
[12] During active monsoon days, strong low-level
southwesterlies prevail over South China (Figure 1a). As
the wind speed declines northward, it exhibits a convergence feature over Central China, including the Sichuan
Basin (104 –109 E, 28 –32 N) and the East China Plain
(112 –120 E, 27 –35 N). Accordingly, a rainband stretches
zonally from the Tibetan Plateau foothills, through the
Sichuan Basin, and East China Plain to the western coasts
of Korea and Japan. The mean rainrate over Central China
reaches 8.5 mm/day. The accumulated monsoon rainfall
(365 mm) accounts for two thirds of the summer amount
(576 mm), highlighting the importance of the monsoon in
delivering the seasonal rainfall budget. Rainfall is also evident near the mountain ranges and coasts of South China,
which may be induced by the southwesterly monsoon flow
blowing against these terrains [Xie et al., 2006].
[13] During inactive monsoon days, the monsoon flow is
absent over eastern China and resides in the low latitudes
(Figure 1b). Rainfall is suppressed over most of eastern
China. In particular, the rainband in Central China vanishes
as the rainrate declines to 4.3 mm/day. Strong rainfall is seen
instead along tropical island shores or over South China
coasts, arising from the activities of the tropical monsoon
trough and tropical cyclones [e.g., Ding, 1992] as well as
the land-sea breeze [Nitta and Sekine, 1994].
3.2. Diurnal Cycle of Monsoon Rainfall
[14] To illustrate the diurnal cycle, we express the TRMM
rainfall during A.M. hours as a percentage of the daily mean
(Figure 2). In general, late night-morning rainfall occurs
mainly over the oceans particularly offshores and in valleys
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Plateaus and over the East China Plain (Figure 2a). The
A.M. rainfall can account for up to 70% of the daily mean
amount over the Sichuan Basin and ~50% over the East China
Plain. This regional morning-hour rainfall is distinct from the
afternoon-hour rainfall, which usually dominates inland during the warm season. During inactive monsoon days, the A.
M. rainfall is confined to the plateau peripheries, such as the
Sichuan Basin (Figure 2b). The percentage of A.M. rainfall
also declines to ~40% over the East China Plain, while the
amount of P.M. rainfall rises to 60% and 1.5 times the amount
of AM rainfall. The spatial pattern of the diurnal cycle shows a
clear localized feature in Figure 2b, in contrast to a smooth
regionality in Figure 2a.
[15] Figure 2c shows the difference in rainfall proportion
between Figures 2a and 2b. The proportion of AM rainfall
increases between inactive and active monsoon days, along
the summer rainband over Central China. The increase ratio
is 10–15% on the east lees of the Tibetan and Yun-gui
Plateaus and reaches up to 10%–20% over the East China
Plain. This indicates a large change in the diurnal cycle, as
the increase in the proportion of AM rainfall means a
corresponding decrease in the proportion of PM rainfall. For
example, an increase of 10% may change the ratio of A.M.
to P.M. rainfall from 50 : 50 to 60 : 40; i.e., the AM rainfall
becomes 1.5 times the amount of the PM rainfall. Therefore,
the morning signature of summer rainfall over Central China
is attributed to the active monsoon, which brings a large
amount of rainfall (Figure 1a) with an increasing proportion
in the morning hours (Figure 2c). In contrast, offshore of South
China, the A.M. rainfall proportion declines by 5%–10% on
active monsoon days, displaying the different effect of the
regional monsoon in an upstream region.
[16] Morning rainfall in the monsoon season over eastern
China has been reported in recent studies of monthly analysis
[Yin et al., 2009; Chen et al., 2009a; Yuan et al., 2010]. We
further characterized such morning rainfall to specific active
monsoon days in the above statistics, by which the importance
of the active monsoon days on the summer rainfall budget has
been evaluated in a quantitative manner. In the following
sections, we investigate the moist processes that result to this
phenomenon, with an emphasis on the monsoon diurnal cycle.
4. Summer Monsoon Diurnal Cycle Over
Eastern China
Figure 2. The percentage of TRMM rainfall in AM hours
to the daily mean on (a) active and (b) inactive monsoon
days. Figure (c) denotes the percentage difference between
Figures 2a and 2b. Long dashes mark the elevations of
1500 m and 3000 m.
and basins. In contrast, the afternoon-evening rainfall dominates over elevated plateaus, mountains, and tropical islands.
Such a terrain dependency may result from convective
activities that are intensely modulated by local land-sea or
mountain-valley breezes [Johnson et al., 1993; Ohsawa
et al., 2001]. During active monsoon days, the A.M. rainfall
is observed on the east lees of the Tibetan and Yun-gui
4.1. Horizontal Structure of the Diurnally Oscillating
Low-Level Wind
[17] We first examine the diurnal variation of the horizontal wind at low levels from the ERA-interim dataset. Figure 3
shows the diurnal veering of 850 hPa winds over eastern
China. The daily mean has been removed to highlight the
diurnal oscillation. Diurnal variations are found to be evident
on land. The wind vectors undergo a clockwise rotation and
behave like an inertial oscillation [Blackadar, 1957]. Over
South China, the wind vector exhibits an intensified southerly
at 02:00 LT and a southwesterly at 08:00 LT. In the mean flow
of southwesterly, this represents an acceleration of wind speed
during the night. The phase rotation is relatively fast to the
north of 30 N, and this is probably due to a shorter inertial
period of 24 h than that of ~28 h in South China. The veering
leads to a deviated northerly, or northwesterly at 08:00 LT,
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4.2. Vertical Structure of the Wind Diurnal Oscillation
[19] After illustrating the horizontal feature, we now focus
in this section on the vertical structure of the monsoon diurnal cycle. First, we take a detailed view of the profiles of the
meridional wind over South China (105 –120 E, 21 –27 N)
during 5–21 June 1998 (Figures 4a and 4b). It is evident that
the monsoon flow is active on 13 out of the 16 days. The
southerly maxima of 6–10 m/s occur regularly in the lower
troposphere during the nights of active monsoon days, while
the maxima are less visible on 5–6 June and on 9 June when
the monsoon is absent. Figures 4a and 4b also show that
both the daily and diurnal variations of the low-level wind
in reanalysis data are highly similar to those in observations.
The correlation coefficient between the two dataset reaches
0.93, which is above the level of statistical significance at
99.9% confidence. Such a similarity is also seen in the temperature and moisture fields (figure omitted). Although the
ERA-interim dataset assimilates the soundings twice daily,
Figure 3. Diurnal component of 850 hPa horizontal winds
at four synoptic times on (a) active and (b) inactive monsoon
days from the ERA-interim data. The daily mean at each
grid point has been removed. The long dashes mark the elevations of 1500 m and 3000 m.
which possibly combines with the strengthened southwesterly
coming from the south to enhance the low-level convergence
over Central China.
[18] Figure 3a shows that the diurnal range of low-level
winds attains 2–3 m/s over South and Central China during
active monsoon days, which accounts for at least one fifth
of the average daily speed. The peak of wind speed may
enhance the monsoon flow to form a LLJ at night. A wind
streak of up to 12 m/s can be observed at 02:00/08:00 LT on
71% of active monsoon days, compared to only 47% at
14:00/20:00 LT. This appears to explain the frequent events
of nocturnal LLJ over South China [Rife et al., 2010]. Figure 3b shows that the diurnal range, however, declines to 1–
1.5 m/s during inactive monsoon days. It seems clear that the
large diurnal wind variation is primarily integrated with the
monsoon flow. As suggested by Chen et al. [2009b], such a
meso-synoptic-scale phenomenon can be designated as the
monsoon diurnal cycle over eastern China. It manifests a
diurnal pulsation of regional-scale monsoon flow, compared
to a continental-scale concept of the Asian monsoon circulation [Krishnamurti and Kishtawal, 2000].
Figure 4. Vertical profiles of the meridional wind over South
China. (a) The SCSMEX six hourly radiosonde observations
during 5–21 June 1998; (b) is the same as Figure 4a but for
the ERA-interim dataset; (c) the composite six hourly profiles
during 5–21 June 1998 from the SCSMEX observations, with
the daily mean removed; (d) is the same as Figure 4c but for
all of the active monsoon days in the summers of 1989–2010
by the ERA-interim dataset.
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it appears to be reliable in capturing the nature of the monsoon diurnal cycle over South China.
[20] The diurnal cycle is better illustrated using the composite six hourly profiles with the daily mean removed.
The meridional wind profiles in Figure 4c show that the
southerly is suppressed at 14:00 LT in the planetary boundary layer (PBL) at below 800 hPa and remains weak at 20:00
LT in the upper PBL near 850 hPa. The enhanced southerly
appears at 02:00 LT in the PBL and lasts until 08:00 LT in
the upper PBL. The wind oscillation thus exhibits a phase
lag with height, and the diurnal range at 850 hPa attains
2.5 m/s. Figure 4d reveals that the largest amplitude at
850 hPa is also seen in the reanalysis profiles of active
monsoon days. This maxima layer is higher than that which
usually occurs at ~950 hPa in other weather regimes [Higgins
et al., 1997; Parker et al., 2005]. The southerly wind tends to
maximize at 02:00 LT, and the zonal wind maximum follows
at 08:00 LT (figure omitted). Such a feature of the monsoon
flow seems to be reflected in the predawn wind peak recorded
over South China [e.g., Yu et al., 2009]. We note that the peak
time in reanalysis is somewhat earlier than that of
observations, as shown by the decaying profiles of 20:00 LT
and 08:00 LT in Figure 4d. Other discrepancies between the
two datasets are also seen in the oscillation of near-ground
winds, probably arising from the different resolution of the
data and the average period used. Although the wind oscillation is strongest in the PBL, it seems to extend upward
into the middle–upper levels (Figures 4c and 4d); the possible
mechanisms are discussed later. During inactive monsoon
days, the wind profiles exhibit a similar phase as those in
Figure 4d but have much smaller amplitude in the PBL
(figure omitted).
[21] The vertical structure of the diurnal cycle is further
illustrated in the pressure-longitude section of wind oscillation
(Figure 5). At 20:00 LT, easterly anomalies dominate the midlower troposphere east of 100 E, while westerly anomalies
appear at the upper level as return flow (Figure 5a). These
indicate an establishment of mountain-plain and land-sea
solenoids between the Tibetan Plateau, the lowlands of eastern
China, and the ocean. Such solenoidal circulations extending
deep in troposphere are commonly seen on the east lees of high
terrains [Carbone and Tuttle, 2008; Bao et al., 2011; Jin et al.,
2012]. With a span of thousands of kilometers, they are also
recognized as continental-scale “land-sea” breezes [Dai and
Deser, 1999]. They are driven by a pressure gradient force,
as negative (positive) pressure anomalies occupy the Asian
continent (Pacific ocean) in the mid-lower troposphere [Huang
et al., 2010]. As the pressure pattern is present during both
active and inactive monsoon days (figure omitted), solenoidal
circulations seem to bring a summer background of wind
diurnal oscillation over East Asia.
[22] At 02:00 LT, the low-level wind then shifts to become
a southerly, while the upper wind becomes a northerly as
large-scale solenoids evolve (Figure 5b). The strongest
southerly occurs in the PBL at a zone of 105 E–118 E and
relates to a meso-synoptic-scale monsoon diurnal cycle. A
regional mechanism creating this extra amplitude may involve
the variation of PBL turbulent mixing that relaxes frictional
drag at night and regulates wind inertial oscillation
[Blackadar, 1957; Parker et al., 2005; Jiang et al., 2007].
The diurnal cycle of the PBL mixing effect is noted to have
Figure 5. Longitude-pressure section of the wind diurnal
oscillations during active monsoon days in terms of (a) zonal
wind at 20:00 LT and (b) meridional wind at 02:00 LT. The
variables are averaged over a zone of 25.5 –27 N where
monsoon flow enters Central China, with the daily mean of
active monsoon days removed. The average topography of
25.5 –27 N is shaded in black.
a particularly large amplitude during active monsoon days,
as shown in the ERA-interim data (figure omitted), and appears to combine with the large-scale solenoids, to give a
strong diurnal variation of monsoon flow. Future estimates
using observations or modeling at a higher spatiotemporal resolution may offer more insights into the formation physics of
monsoon diurnal cycle.
5. Impacts on Moisture Transport and
Water Budget
5.1. Diurnal Cycle of the Moisture Transport
[23] The summer monsoon plays an important role in
transporting moisture to eastern China [e.g., Ding, 1992;
Zhou and Yu, 2005]. On a short time scale, moisture transport may intensify at night due to the diurnally varying
low-level winds [Li et al., 2007; Yamada et al., 2007; Chen
et al., 2009b; Chen et al., 2010; Yuan et al., 2010]. That may
cause a moisture sink over eastern China and regulate the
water budget on a diurnal time scale in summer [e.g., Chen,
2006]. From the angle of monsoon diurnal cycle, we propose
two questions: First, “How much does the monsoon diurnal
cycle explain the diurnal amplitude of moisture transport?”
and second, “To what extent does it contribute to the
regional water budget?”
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[24] To estimate the moisture transport and water budget,
we employ the governing equation of vertically integrated
precipitable water [Trenberth and Guillemot, 1995]:
@w
þ rQ ¼ E P
@t
(1)
in which the precipitable water w and water vapor flux Q are
given by
w¼
1
g
Z
0
psfc
qdp; Q ¼
1
g
Z
psfc
Vqdp
(2)
0
and E, P, V, q, g, p, and psfc are evaporation, precipitation,
horizontal velocity vector, specific humidity, gravity, pressure, and surface pressure, respectively. For consistency,
all these variables are given by the ERA-interim data. It is
noted that the diurnal cycle pattern of precipitation forecast
from the ERA-interim (figure omitted) is similar to the
observed one shown in Figure 2c. Water vapor resides
primarily in the lower troposphere and is largely transported
by low-level winds [Chen, 2006]. As moisture fluxes converge (r Q < 0) toward the region of moisture sink (E P
< 0), a link between the monsoon flow and the regional
hydrological cycle is measured.
[25] First, we examined the daily mean of the monsoon
moisture fluxes and then clarified the relative magnitude of
the diurnal cycle. Figure 6a shows that on active monsoon
days, the column-integrated moisture fluxes onto eastern
China originate mainly from South China and the South
China Sea. These fluxes have a strength ~350 kg m 1 s 1
over South China and account for up to 80% of the
northward transport of summer moisture. More than three
quarters of the fluxes come from the lower-tropospheric
transport below 700 hPa. The largest transport occurs at
around 110 E where the monsoon flow is the strongest.
[26] Figure 6b shows the large diurnal variations of lowlevel moisture fluxes seen over eastern China. The deviated
fluxes are directed northward at 02:00 LT and eastward at
08:00 LT, and present as enhanced moisture transport through
the night. The diurnal range is estimated at ~70 kg m 1 s 1,
about one fifth of the daily mean. This indicates that during
active monsoon days, the moisture transport is strengthened
at night by about 20% of that occurring during the day. Note
that a diurnal range of specific humidity is ~5% of the daily
mean, which is smaller than that of wind oscillation (~20%).
Diurnal variation of the moisture fluxes thus results mostly
from the oscillation of the wind, as also indicated by the similarity between Figures 3a and 6b. During inactive monsoon
days, the diurnal amplitude of the moisture fluxes becomes
much smaller (figure omitted), due to a weak wind variation
and moisture deficit.
5.2. Diurnal Variation of the Hydrological Processes
Contributing to the Water Budget
[27] To illustrate the regional effect of the monsoon
diurnal cycle, we estimated the hydrological processes contributing to the water budget over Central China (Figure 7).
We divided the water amount from the low-tropospheric moisture fluxes below 700 hPa by an area size of Central China;
such that it was normalized to a unit of 1 m2. The regional
mean of water vapor storage, evaporation, and precipitation
was also estimated using equation (1), while the contribution
Figure 6. Water vapor fluxes during active monsoon days
in terms of (a) the daily mean and (b) the diurnal component
in the lower troposphere. The contour in Figure 6a denotes
the percentage of the lower-tropospheric fluxes to the total
fluxes. Long dashes mark the elevations of 1500 m and
3000 m.
of cloud water was neglected. It should be noted that a balance
among these terms may not be reached at a short time interval.
Although the six hourly snapshots in Figure 7 may not contain
the conservation of water mass, they offer us an instantaneous
view of the diurnal variation of hydrological processes in the
context of the monsoon diurnal cycle.
[28] Figure 7 shows a strong moisture inflow into the
southern boundary of Central China during active monsoon
days. This southerly inflow is in its suppressed phase at
14:00 LT, while both evaporation and precipitation are
active over Central China (Figure 7a). The 6 h sum of evaporation and moisture flux convergence reaches 3.23 kg/m2 and
exceeds the precipitation of 2.65 kg/m2. Accordingly, the
storage of atmospheric moisture increases by ~0.5 kg/m2 from
14:00 LT to 20:00 LT (Figures 7a and 7b). At 20:00 LT, the
fluxes at the western boundary switch from being inflow to
outflow (Figure 7b), as the upslope wind blows toward the
plateaus. An enhanced outflow is also observed at the northern
boundary, while a reduced outflow is seen at the eastern
boundary. The resulting net moisture flux convergence over
Central China declines from 1.00 kg/m2 at 14:00 LT to
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CHEN ET AL.: MONSOON DIURNAL CYCLE OVER CHINA
Figure 7. Diurnal variations of the water budget variables
over Central China during active monsoon days. Moisture
fluxes are normalized by the area size of Central China and
plotted as the vectors at four boundaries. Their net value is labeled at the bottom right of each box. The atmospheric storage
of water vapor is enclosed by a diamond. Evaporation (E) and
precipitation (P) are marked in brackets. All variables are
given a unit of kg/m2 (equivalently, mm/m2) per 6 h.
0.48 kg/m2 at 20:00 LT; precipitation decreases from 2.65 to
1.22 kg/m2, respectively.
[29] At 02:00 LT, the inflow at the southern boundary
increases to 3.94 kg/m2 (Figure 7c), due to the speed-up of
the monsoon flow. An inflow also appears at the western
boundary, arising from a reversal of the mountain-plain solenoid. The moisture flux convergence increases to 1.01 kg/m2,
suggesting an enhancement of moisture convergence over
Central China. This leads to a growth of precipitation from
1.22 to 1.94 kg/m2 in past 6 h. The precipitation fallout and
decayed evaporation decrease the moisture storage at night.
At 08:00 LT, there is a slight decline of the inflows at
the southern and western boundaries, while the outflow at
the northern boundary declines from 0.98 to 0.59 kg/m2
(Figure 7d). As a result, the moisture flux convergence remains as strong as 1.16 kg/m2. It combines with a resumed
evaporation to support the precipitation, which increases to
2.57 kg/m2 during the morning.
[30] An overview of Figure 7 shows that the moisture
supply from advection is primarily a result of the southerly
monsoon inflow and partly a result of the westerly inflow.
The accumulated daily supply is 15.23 kg/m2 per day, 4 times
that of evaporation which is 3.6 kg/m2 per day. The variation
in the southerly inflow is induced by the monsoon diurnal
cycle, while that of westerly one is mainly due to the
mountain-plain solenoid. Both two inflows are suppressed at
20:00 LT and result in a weak convergence of moisture fluxes.
They become active at 02:00/08:00 LT to generate an enhancing convergence. A transition then occurs at 14:00 LT, with
a small decrease in the convergence from 08:00 LT related
to the decayed southerly monsoon inflow but strong westerly
inflow. On inactive monsoon days, the moisture supplied by
the southerly inflow declines to 2.79 kg/m2 per day (figure
omitted). In particular, the moisture flux convergence at
02:00/08:00 LT is weak at ~0.3 kg/m2, which is much smaller
than an amount of ~1.0 kg/m2 during the morning hours of active monsoon days. It seems clear that monsoon diurnal cycle
combines with large-scale solenoids to play a crucial role in
regulating the summer water budget over Central China.
5.3. Spatial Pattern of the Moisture Flux Convergence
and Moisture Content
[31] Section 5.2 emphasized the importance of the moisture flux convergence affecting regional water budget. To
further clarify the spatial pattern of convergence, we split the
lower-tropospheric moisture fluxes into two parts: stream
function and velocity potential. We focused on the velocity
potential component, because it relates closely to the moisture
flux divergence and convergence. The summer daily mean at
each grid point was removed to show the diurnal oscillation
on active monsoon days.
[32] Figure 8a shows that at 14:00 LT, moisture fluxes
converge toward the coasts of South China and tropical
islands, where convection develops in the early afternoon
[Ohsawa et al., 2001]. At 20:00 LT, moisture fluxes diverge
over the western Pacific and China coast, while they converge toward the Tibetan and Yun-gui Plateaus (Figure 8b).
This suggests that the large-scale sea breezes not only transport moisture to terrain as reported by previous studies
[Huang et al., 2010; Yuan et al., 2012] but also play a key
role in regulating the moisture divergence and convergence.
Such patterns appear to collocate with the convective activities dominating the plateau slope in early evening but which
are suppressed over the ocean or even dissipating over the
eastern China coast [Asai et al., 1998; Hirose and Nakamura,
2005; Chen et al., 2009a].
[33] At 02:00 LT, the moisture flux divergence is located
over the tropical regions (Figure 8c). The convergence
occurs on the plateau foothills, the Central China, and the
Korean coast, which corresponds with the nocturnal rainfall
that forms over low-lying areas (Figure 2a). Figures 8b and
8c also show that the moisture divergence and convergence
exhibits as an east-west dipole pattern at 20 LT and rotates
clockwise to a south-north pattern at 02:00 LT. This is
thought to result from the diurnal veering of the low-level
wind, as shown in Figures 3a and 6b. Similar to studies on
a continental-scale mode [Krishnamurti and Kishtawal,
2000; Chakraborty and Krishnamurti, 2008], this depicts a
poleward moisture discharge at night driven by the diurnal
pulsation of the monsoon flow on a regional scale.
[34] At 08:00 LT, the moisture fluxes turn to converge
toward Central China, particularly over the East China
Plain (Figure 8d). This convergence feature is associated
with the deviated southwesterly from South China and
the northwesterly from North China. The intensity of the
fluxes attains ~20 kg m 1 s 1 at 02:00/08:00 LT,
amounting to about 50% of the anomalous moisture fluxes
(cf. Figures 8c, 8d, and 6b). Such a meso-synoptic-scale
convergence is shown to persist through the late night and
morning. This corresponds to the largest increase of morning
rainfall over the East China Plain during active monsoon
days (Figure 2c). During inactive monsoon days, however,
the morning flux convergence is suppressed over Central
China and retreats to the South China coast (figure omitted).
We can therefore clearly see that the primary contribution to
the strong moisture sink on summer mornings over Central
China comes from the monsoon diurnal cycle.
[35] The importance of the monsoon diurnal cycle
compared to local breezes is further estimated in terms of
the horizontal pattern of moisture content. We assume that
the local breezes are shallow as indicated in Figure 4 and
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CHEN ET AL.: MONSOON DIURNAL CYCLE OVER CHINA
Figure 8. Six hourly anomalies of the velocity potential of low-level moisture fluxes during active
monsoon days. The daily mean of summer days has been removed at each grid point. The contour interval
is 0.4 107 g∙kg1m2s1.
can be plotted by the near-ground winds at 975 hPa. We focus on mesoscale pattern, as the ERA-interim data are
less able to resolve the very localized pattern than the in situ
records (cf. Figures 4c and 4d) or the high-resolution data
[Rife et al., 2010]. Figure 9a shows that the moisture content
is concentrated at the foothills and offshores, where the local
breezes are convergent, at 08:00 LT on inactive monsoon
days. Such a terrain dependency implies a dominant effect
of the local mountain-valley or land-sea breezes [Johnson
et al., 1993]. Figure 9b illustrates that local breezes are still
visible on active monsoon days; morning moisture however
has changed notably from Figure 9a. Figure 9c shows the
moisture difference between Figures 9a and 9b, highlighting
the effect of the monsoon diurnal cycle. The moisture content is clearly enhanced over Central China but suppressed
on the South China coast, due to nocturnal monsoon transport. Such a spatial pattern corresponds well with the change
in morning rainfall (Figure 2c). Moreover, Figure 9c shows
that the moisture anomaly induced by the monsoon diurnal
cycle has a magnitude of O(1) kg/m2, which is comparable
to that of the local breezes in Figure 9a. This anomaly looks
strong enough to modify the regional diurnal cycle, and it
even reverses the phase over the East China Plain and off
the shores of South China.
6.
Impacts on Precipitation Diurnal Variability
6.1. Rainfall Budget by the Various-Size Rain Events
[36] In this section, we examine the role of the population
of rain events in the context of the rainfall diurnal cycle as a
response to the monsoon diurnal cycle. In this respect, we
use the 3B42 data to categorize the rain events according
to the size of their area. We employ a concept of the contiguous rain area enclosed within a specified isohyet [Ebert and
McBride, 2000]. An isohyet threshold of 1 mm/h is applied
to mask out drizzle and so that isolated (organized) systems
can be recorded as small (large) events. The rainfall volume
of mesoscale events derived from this method is found to
correlate well with the ground observations [Demaria
et al., 2011]. The location of an event is indicated by the
rain-weighted center of a rainy area. Using a 13 year archive
of TRMM rain estimates enabled us to document the size,
location, and rainfall volume of a huge population of
summer rain events.
[37] Figures 10a shows the diurnal cycle of the rainfall
budget over Central China on inactive monsoon days.
Meso-a-scale rain events which have an area size of at least
2 104 km2 are capable of producing 143 mm of rainfall in
one summer. These rain events explain 77% of the total rainfall amount of the moderate to intense rainrate (≥1 mm/h) on
inactive monsoon days. The largest contribution is from rain
events with an area size of ~105 km2, in relation to mesoscale organized convection. The resulting rainfall diurnal cycle exhibits a major peak at 17:00 LT and a secondary peak
at 08:00 LT. On the other hand, the small–medium events
with an area less than 2 104 km2 yield a smaller rainfall
amount of 43 mm and display a single peak in the afternoon.
[38] Figure 10b shows that the rainfall produced by small–
medium events on active monsoon days remains as weak as
on inactive monsoon days. The rainfall from meso-a-scale
events, however, increases remarkably on active monsoon
days. The accumulated rainfall amount of meso-a-scale
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CHEN ET AL.: MONSOON DIURNAL CYCLE OVER CHINA
events in one summer attains 254 mm, with 134 mm
recorded in A.M. hours and 120 mm in P.M. hours. It is
particularly interesting to note that the rainfall peak at
05:00–08:00 LT becomes dominant and exceeds the peak
of 17:00 LT. Such a plenty of morning-peak rainrate
delivers a major source of the warm-season morning rainfall.
The largest amount comes from the rain events with an area
of ~2 105 km2 (i.e., the size is twice of those of inactive
monsoon days).
[39] Nighttime rain events over eastern China have been
linked to the propagating organized convections that often
reach maturity in the late night and early morning [e.g., Asai
et al., 1998; Wang et al., 2004; Chen et al., 2012a]. In many
previous studies, these rain events are observed as active
alongside the large diurnal variation of low-level winds. Their
distinct activities during active monsoon days, as revealed in
Figure 9. Diurnal components of lower-tropospheric water
vapor content (shaded) and 975 hPa wind (vector) at 08:00
LT during (a) inactive and (b) active monsoon days. (c) The
difference of water vapor content at 08 LT between active
and inactive monsoon days. Long dashes mark the elevations
of 1500 m and 3000 m.
Figure 10. Three hourly 3B42 rainfall amount averaged
over Central China, accumulated from the rain events with
respect to area-size categories during (a) inactive and (b) active
monsoon days for a summer mean between 1998 and 2010.
The triangle marks a size threshold of 2 104 km2 that
separates small–medium and meso-a-scale rain events. This
threshold approximates a horizontal scale of ~150 km, which
is slightly smaller than the widely accepted 200 km. The time
duration of rain events is not considered.
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CHEN ET AL.: MONSOON DIURNAL CYCLE OVER CHINA
Figure 11. Meso-a-scale rain events in (a) P.M. and (b) A.M. hours of inactive monsoon days, and
(c) P.M. and (d) A.M. hours of active monsoon days. The dots denote the location of rain events. The density
(shaded) is estimated as an occurrence within 150 km to any grid point and adjusted to one summer. Long
dashes mark elevations of 1500 m and 3000 m.
this statistics, highlight a strong connection between the
mesoscale convection and the monsoon diurnal cycle.
6.2. Spatial Distribution of Meso-a-Scale Rain Events
[40] As meso-a-scale rain events respond to the monsoon,
their spatial distribution is examined to clarify the
regionality of the rainfall diurnal cycle. Figures 11a and
11b show that on inactive monsoon days, there are much
fewer rain events over Central China than those over the
coasts of South China and tropical islands, and there is a
greater occurrence in the P.M. hours than in the A.M. hours
over most of Central China, except over the plateau foothills.
This explains the weak rainfall over Central China, which
has a major afternoon peak on inactive monsoon days. The
suppressed rain events in the A.M. hours may result from a
moisture deficit, and a small diurnal variability that is less
supportive of nocturnal mesoscale convection.
[41] Figures 11c and 11d show that rain events over
eastern China become more frequent on active monsoon
days. In the P.M. hours, rain events mainly occur over the
southeastern part of the Tibetan Plateau, South China, and
the mountain ranges at 110 E (Figure 11c). In the A.M.
hours, rain events usually occur to the east lees of the
plateaus, the East China Plain, and the East China Sea
(Figure 11d). Such a shifting between P.M. and A.M. hours
suggests a strong dependency of the diurnal occurrences on
major terrains, when they yield the double peaks of rainfall
seen in Figure 10b. In particular, the active A.M. occurrence
gives the maxima of morning rainfall along the summer
rainband over the low-lying areas (Figures 11d and 2c).
Meso-a-scale rain events are thus responsible for the
regionality of the rainfall diurnal cycle on active monsoon
days. On the other hand, these rain events possess a
relatively large area and are likely to be of long duration.
As rain events migrate, they may therefore cast a rain
footprint over a wide region and result in a relatively smooth
signature of the diurnal cycle (Figure 2a), which is distinct
from the localized one during inactive monsoon days
(Figure 2b).
[42] At the east lees of the Tibetan Plateau, the nocturnal
rain events usually originate from the convective systems
that form on the plateau slope near midnight and then move
eastward [Asai et al., 1998; Wang et al., 2004; Sugimoto and
Ueno, 2012], which is a similar phenomenon to those seen in
North America [Carbone et al., 2002; Carbone and Tuttle,
2008]. These rain events are greatly supported by such
features as the low-level convergence of the southwesterly
monsoon [Chen et al., 2010; Ueno et al., 2011; Chen
et al., 2012a], the rising motion of mountain-plain solenoid
reversal [Bao et al., 2011; Jin et al., 2012], and the convective instability by moisture transport [Chen et al., 2009b;
Yuan et al., 2012]. The propagation of the system is also
facilitated by an enhancement of the vertical wind shear
due to the strong low-level southwesterly [Wang et al.,
2012]. Because of its large amplitude, the monsoon diurnal
cycle appears to play an active role in the above mechanisms
supporting convective initiation, growth, and migration at
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CHEN ET AL.: MONSOON DIURNAL CYCLE OVER CHINA
night. It not only increases moisture transport at night
(Figures 6b and 9c) but also strengthens low-level convergence (Figures 8c and 8d) and vertical wind shear (Figure 4).
In dosing so, the monsoon flow acts to promote the frequent
nocturnal occurrence of self-sustaining convective systems
which inherently reach maturity in the late night-early morning and result in an increasing amount of morning rainfall on
active monsoon days.
[43] Over the East China Plain, the southwesterly monsoon flow may impinge on the Meiyu front to help produce
a zone of moisture convergence, convective instability, and
mesoscale lifting, in which nocturnal convection is promoted [Yamada et al., 2007; Chen et al., 2012a]. The local
mountain-plain solenoid also helps to initiate convection,
which can be enhanced by the nocturnal LLJ over the plain
[Bao et al., 2011; Sun and Zhang, 2012]. As the East China
Plain is located downstream of the wind diurnal veering, the
strong moisture supply and the low-level convergence may
persist through the late night and morning (Figures , 6b,
8c, and 8d). In section 5, it was stressed that these persistent
conditions are most evident in the presence of the monsoon
diurnal cycle. As such conditions favor the nighttime growth
of long-lived rainfall systems [Chen et al., 2009b; Chen
et al., 2010; Yuan et al., 2010], they are expected to yield
the largest increase of morning rainfall over the East China
Plain during active monsoon days.
[44] It is well known that the summer monsoon undergoes
a northward march from June to August. The monsoonrainfall link on a diurnal time scale needs to be further evaluated with consideration of the summer progress. Figure 12
shows the monthly distribution of wind oscillation and
morning rain events on the active monsoon days from June
to August. The nocturnal enhanced southwesterly tends to
extend northward, suggesting the progress of the monsoon
diurnal cycle. The northward progress is most evident over
the East China Plain. By August, the monsoon diurnal cycle
can reach the northern part of Central China or the southern
boundary of North China (~35 N). Correspondingly, the
location of morning rain events shift northward over Central
China. The morning events occur mainly over the low-lying
areas within the monthly rainband, toward which the nocturnal monsoon flow is converging. An increasing occurrence
is also observed over North China in July and August, which
can be supported by the nocturnal southwesterly [He and
Zhang, 2010; Chen et al., 2012b]. It seems clear that the response of morning rain events to the monsoon diurnal cycle
is established during the summer progress. This is thought to
explain the morning-peak rainband that moves northward
during the summer season [Yin et al., 2009; Chen et al.,
2009a].
7.
Summary and Discussion
[45] Using the latest reanalysis data and satellite rain estimates, we estimated the hydrological impacts of the monsoon diurnal cycle over eastern China. We highlighted that
the summer monsoon flow becomes more efficient at night
in transporting moisture and promoting morning rainfall
and that this greatly affects the warm-season weather and
climate. The findings are summarized as follows:
Figure 12. Meso-a-scale rain events in the A.M. hours and
850 hPa deviated winds at 02:00 LT on active monsoon days
of (a) June, (b) July, and (c) August. The dots denote the
location of rain events with monthly density shaded. Wind
vectors greater than 1 m/s are plotted. Long dashes mark
the elevations of 1500 m and 3000 m.
[46] 1. The diurnal variation of low-level wind is found to
be evident over South China during active monsoon days.
The largest diurnal amplitude typically occurs at 850 hPa
where the monsoon flow prevails, as consistent with radiosonde observations. The wind speed attains a maximum in
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CHEN ET AL.: MONSOON DIURNAL CYCLE OVER CHINA
the early morning and a minimum in the afternoon. The diurnal range attains a magnitude of 2–3 m/s, which accounts
for one fifth of the daily mean wind speed and which may
strengthen the southwesterly monsoon to form the frequent
events of the nocturnal LLJ over South China. This can be
viewed as the meso-synoptic-scale diurnal pulsation of the
summer monsoon that develops in the lower troposphere
over eastern China. In contrast, the diurnal range of lowlevel wind declines to 1–1.5 m/s during inactive monsoon
days.
[47] 2. Moist air is advected poleward by the summer
monsoon over eastern China. Due to the monsoon diurnal
cycle, the moisture transport over South China is
strengthened by about 20% at night. Nocturnal moisture
fluxes exhibit a strong convergence at their northern
terminus, which leads to a meso-synoptic-scale moisture
sink over Central China through the late night and morning. Such features express the diurnal pulsation of the
summer monsoon, which drives the moisture discharge
from South China to Central China and thereby regulates
the regional water budget on a diurnal time scale. The
nocturnal monsoon flow also combines with the local
breezes to strengthen moisture over the basins and plains
of Central China in the morning, while reducing moisture
over the South China coast. Such impacts seem to explain the evident morning rainfall over Central China
and the suppressed rainfall over the South China coast
during active monsoon days.
[48] 3. The mean rainrate over Central China reaches
8.5 mm/day on active monsoon days, twice that on inactive
monsoon days. What is of great interest is that the proportion of morning rainfall increases by 10% or more on active
monsoon days. The abundant morning rainfall greatly
contributes to the summer diurnal cycle. Statistical analysis
reveals that the majority of summer rainfall comes from the
meso-a-scale rain events relating to mesoscale organized
convection. These rain events not only become more
pronounced on active monsoon days but also exhibit a
dominant morning peak that exceeds the afternoon peak,
in a contrast to those on inactive monsoon days. Such
morning rain events generally occur on the low-lying areas
of Central China along the summer rainband, where the
monsoon diurnal cycle and related moist processes promote
an increased nocturnal occurrence of mesoscale organized
convections that generally reach maturity during the late
night-early morning. The increased occurrences of morning
events gradually shift northward from June to August, in
response to the progress of the summer monsoon and its corresponding diurnal cycle.
[49] More effort is needed to explore the physical link
between the summer monsoon and the precipitation diurnal
variability. What is the importance of the formation dynamics underlying the monsoon diurnal cycle? An oscillation of
pressure has been noted to induce the diurnal variation of
low-level wind over the summer continent [Holton, 1967;
Dai and Deser, 1999; Huang et al., 2010]. Other PBL processes such as diurnally varying friction may also regulate
the wind inertial oscillation [Blackadar, 1957]. Jiang et al.
[2007] suggested that diurnal oscillations of the pressure gradient force and that of vertical diffusion should be combined,
in order to gain a realistic modeling of nocturnal LLJ over the
Great Plains. Parker et al. [2005] found that the wind diurnal
cycle is well established in the West African monsoon layer,
where both the pressure gradient and the convective boundary layer are evident. It is still unclear how these physical
processes are incorporated in driving the diurnal variation
of low-level winds in the strong monsoon environment over
eastern China.
[50] In our statistics, nocturnal mesoscale convection has
been recognized as the most responsive system to monsoon
diurnal cycle. The underlying mesoscale processes embedded in regional-scale monsoon flow should be regarded as
a key issue. Because in situ observations and reanalysis data
are coarse, it may be necessary to apply high-resolution
modeling to resolve the detailed timing, intensity, and duration of mesoscale systems. To achieve this, numerical experiments need therefore to capture not only the LLJ activities
but also regional forcings such as the Meiyu front, the daytime heating/moistening [Yamada et al., 2007], and the
mountain-valley solenoid [Sun and Zhang, 2012]. As indicated by the current study, a reliable reproduction of the
wind diurnal oscillation on both regional and local scales
will be particularly meaningful for further investigation.
Our ongoing works involves estimating the relative importance of the monsoon diurnal cycle in the genesis and development of nocturnal mesoscale convection, compared with
other known forcings.
[51] The feedback of diurnal variability to variations on
longer timescales also remains to be estimated over eastern
China. Morning rainfall explains a key part of the seasonal
budget [Yu et al., 2007; Yin et al., 2009] and a large proportion of the rainfall variance [Yuan et al., 2010; Chen et al.,
2012a]. Morning rainfall has undergone a decreasing trend
over the last four decades of weakening monsoon, contributing to a long-term decrease of annual rainfall in the northern
region of Central China [Yin et al., 2011]. The long-term
rainfall variations can be closely connected with those of
the low-level wind gradient [Zeng et al., 2012]. Here we
see that the wind gradient is most evident during the night
of active monsoon days. This implies that a long-term trend
of the monsoon diurnal cycle could function as an important
climate forcing over eastern China. More studies will be able
to shed further light on this interesting relationship that has
valuable implications for the changes in the regional hydrological cycle, the energetics of summer monsoon, and the
evaluation of climate model performance [Chakraborty
and Krishnamurti, 2008].
[52] Acknowledgments. The authors are grateful to three anonymous
reviewers for their constructive comments. Thanks also go to ECMWF for
providing the reanalysis dataset and to GSFC and TSDIS of NASA for
providing the satellite rainfall dataset.
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