The diurnal cycles of precipitation over the northern
coast of West Java have been studied using the Tropical Rainfall Measuring
Mission (TRMM) Real Time Multi-satellite Precipitation Analyses (TMPA-RT)
products with records spanning from 2000 to 2016, with emphasis on the
occurrences of early morning precipitation peaks. Diurnal precipitation over
the study area during November to March is basically characterized by
precipitation peaks that occur in the afternoon to evening time
(15:00–21:00 LT), but secondary peaks in the night to morning time (01:00–07:00 LT) are also pronounced in January and February. The harmonic analysis method
was then applied to data of January and February to objectively determine
the diurnal phase and classify the timing of precipitation for each day into
three categories, i.e. afternoon-to-evening precipitation (AEP), early
morning precipitation (EMP), and late morning precipitation (LMP) with peaks
that occur in the time windows of 13:00–24:00, 01:00–04:00, and
05:00–12:00 LT, respectively. In terms of frequency of occurrence, AEP,
EMP, and LMP constitute 55 %, 18.9 %, and 26.1 % of total samples
of precipitation events. In spite of the smallest percentage, EMP events are
characterized by seaward (as well as landward) propagation, flat phase
distribution, and large mean amplitudes. The propagating characteristics of
EMP are more prominent, with indications of stronger connectivity between
precipitation systems over land and ocean, when data are composited by
taking the 99th percentile values in each grid to represent extreme
precipitation events. The flat phase distribution of EMP events suggests
that the timing of coastal precipitation is not necessarily locked to the
phase of land–sea breezes, thus allowing precipitation to occur more
randomly. Furthermore, the role of the South China Sea cold tongue (SCS-CT) and
cross-equatorial northerly surge (CENS) as influencing factors for the
occurrences of the EMP event have also been investigated. In agreement with
previous studies, we confirmed that the SCS-CT generally prevails in January
and February, and morning precipitation events over the northern coast of
West Java mainly occurred when there was more enhanced sea surface
temperature (SST) cooling in the South China Sea. Additionally, we found
that CENS is the most differential factor with regard to the phase of
coastal precipitation. In this case, CENS is positively correlated with
SCS-CT and, when associated with EMP events, concurrent enhancement of CENS
and SCS-CT is connected to a narrow channelling of strong surface northerly
wind anomalies just offshore of Indochina and the Malay Peninsula.
Introduction
Modelling the diurnal cycle of precipitation over the Maritime Continent
(MC) is still one of the biggest challenges in tropical meteorology today
because of significant biases in simulated rainfall (Love et al., 2011; Teo
et al., 2011; Li et al., 2017). While models can capture the gross feature,
they may overestimate or underestimate the diurnal cycle amplitudes in the
simulation results, especially near coastal regions (Love et al., 2011;
Trilaksono et al., 2011; Li et al., 2017). Simulation results are also found
to have problematic phase errors compared to satellite rainfall data (Love
et al., 2011), with a bias as large as 2–4 h over both land and ocean
in the MC (Li et al., 2017). The bias in rainfall simulation is even higher,
up to 12 h over the northern coast of West Java and varying from 3 to 15 h over the western coast of Sumatra, compared to the Tropical
Rainfall Measuring Mission (TRMM) satellite dataset (as interpreted from
Fig. 5 in Im and Eltahir, 2017). Such deficiencies in the models cannot be
remedied by simply increasing spatial resolution (e.g. Im and Eltahir,
2017; Li et al., 2017), which indicates that models still lack key
capabilities to capture fundamental mechanisms involved in the diurnal
convective activities over land and ocean.
It is well known that there are contrasting temporal patterns of diurnal
convective activities in the MC (e.g. Nitta and Sekine, 1994; Yang and
Slingo, 2001), which result in predominantly afternoon and morning
precipitation peaks respective to land and ocean (Mori et al., 2004; Qian,
2008; Qian et al., 2010; Biasutti et al., 2012; Yamanaka, 2016; Yamanaka et
al., 2018). Following Houze et al. (1981), many investigators believe that
this canonical pattern of diurnal convection in the MC is driven by sea–land
breeze interaction with the prevailing background monsoon flows.
Observations of land–sea breezes over the MC have been reported in previous
studies (Hadi et al., 2000, 2002; Araki et al., 2006) and the
land–sea breeze explanation for the diurnal convection seems to be
consistent with the climatological study by, among others, Yang and Smith (2006) that shows relatively constant phases of the diurnal convection
throughout the year, although amplitudes may vary seasonally. In addition to
the sea–land breezes, mechanisms involving gravity waves have also been
proposed to explain diurnal convections over land and adjacent seas outside
and in the MC (Mapes et al., 2003; Love et al., 2011). Interestingly,
phase-locking characteristics between zonally propagating gravity waves and
mesoscale convective systems in the MC have recently been confirmed by
Ruppert and Zhang (2019).
On the other hand, Kikuchi and Wang (2008) pointed out that phase
propagation of diurnal precipitation is prominent along the land–sea
boundaries, which characterizes the coastal regime of the MC and other
regions of the world. In an earlier study, Mori et al. (2004) reported that
precipitating systems may propagate seaward or landward from coastal regions
in Sumatra. A more recent study by Trismidianto et al. (2016)
indicates that convection over coastal regions in Sumatra can also be
induced by a decaying oceanic convective system. From results of a numerical
study, Li et al. (2017) show that offshore or onshore rainfall propagation
may occur over the coastal region of the MC all year round. These studies imply
that rainfall over coastal regions may have peaks at different times in the
day depending on the initial location, the direction, and the phase speed of
the propagating convective systems.
Apart from the land–sea contrast of afternoon–morning precipitation peaks,
diurnal precipitation analysed from TRMM data by Im and Eltahir (2017) seems
to indicate that maximum precipitation over the coastal region of West Java
occurs frequently in the early morning around 01:00 LT during the wet season
in the December–January–February (DJF) period. Additionally, diurnal
variations of precipitating clouds analysed from boundary layer radar (BLR)
data observed during DJF also indicate that there is a slight increase
(decrease) in the frequency of occurrences of deep convective (stratiform)
clouds between 03:00 and 06:00 LT (as in Fig. 9 of Renggono et al., 2001)
at Serpong, which is situated in the coastal plain of West Java. These
previous studies did not seem to pay much attention to the peculiar timing
of precipitation over the coastal region but, based on results of numerical
simulation, Koseki et al. (2012) mentioned that the South China Sea cold
tongue (SCS-CT) phenomenon might affect the morning precipitation over
the northern coast of Java by shifting the offshore convection system to
move closer to the coastal area. In a case study, maximum rainfall at 04:00 LT observed during 16–25 January 2018 over northern West Java was due to a
cross-equatorial northerly surge (CENS) activity that coincided with the SCS-CT
event (Mori et al., 2018). Thus, early morning precipitation peaks seem to
have a strong relationship with propagating convective systems over the coastal
region under the influence of certain background synoptic conditions. Li et
al. (2017) suggested that background winds were the most important factor
for the offshore and onshore propagation of precipitation from the coastal
region, but there are other possible mechanisms as proposed by Mori et al. (2004). Hence, phenomena related to early morning precipitation peaks and
possible influencing factors like the occurrences of CENS and SCS-CT need to
be more thoroughly studied to improve our understanding of the diurnal
convection in the MC.
In the present study, we are particularly interested in analysing the
climatology of early morning precipitation peaks over the northern coast of
West Java, not only because documentation regarding this phenomenon is still
limited, but also because extreme precipitations occurring in the early
morning time are potentially linked to severe flooding in the Jakarta area
(Wu et al., 2007, 2013; Trilaksono et al., 2011, 2012; Sulistyowati et al.,
2014; Nuryanto et al., 2019). Our basic approach is to analyse the
prevalence of early morning precipitation peaks over the area of interest by
classifying diurnal precipitation into several dominant patterns with regards
to the phase or time of occurrences. We also analyse statistical
correlations between the early morning precipitation and predominant
background conditions, especially those related to CENS and SCS-CT, and
attempt to interpret the results in conjunction with propagating
characteristics of convective systems over the coastal region.
In the next section of this paper, we describe detailed methods to identify
the occurrences of early morning precipitation peaks and their possible
relationship with CENS and SCS-CT events, while the results are discussed in
Sect. 3. The last section presents the summary of our important results
and findings.
Data and method
In order to investigate diurnal cycles of precipitation over the northern
coast of West Java, we used the Tropical Rainfall Measuring Mission (TRMM) Real-Time Multi-satellite Precipitation Analyses (TMPA-RT) product of the 3B41RT dataset, the so-called TMPA-RT dataset
with a spatial resolution of 0.25∘× 0.25∘ at
an hourly time interval. The TMPA-RT datasets of version 6 (2000–2010) and
version 7 (2011–2016) (Huffman et al., 2007; Huffman, 2012; Yong et al., 2015)
contain precipitation estimates calibrated by the TRMM Combined Instrument
and TRMM Microwave products (Harris et al., 2007; Liu et al., 2008; Yong et
al., 2015). These data are archived, being updated in near (pseudo)
real time with a latency of 6 to 10 h, and are free for download from NASA's
website at https://pmm.nasa.gov/data-access/downloads (last access: April 2017). In this case, we have
obtained a 16-year data record for the November through March (NDJFM) periods
of November 2000 to March 2016. Other data involved in our analyses are
daily wind data with a spatial resolution of 2.5∘× 2.5∘ derived from the National Center for Environmental
Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis
(Kalnay et al., 1996) and optimal interpolated sea surface temperature
(OISST) using the advanced very high-resolution radiometer (AVHRR) dataset (Reynolds et al., 2007; Banzon et al., 2016) that has a spatial resolution of 0.25∘× 0.25∘ at a daily time interval.
It should be noted that the TMPA-RT dataset was intended to improve
applications in flood prediction, with a downside of producing more false
alarms in peak flow (Harris et al., 2007). However, as has been
demonstrated in many previous works (e.g. Mori et al., 2004; Kikuchi and
Wang, 2008), it should still be possible to extract consistent patterns of the diurnal cycle in precipitation from the satellite data by averaging over
a relatively long period of time. Moreover, Liu et al. (2008) have used the
TMPA-RT dataset to analyse propagating rainfall episodes in the Bay of
Bengal in May to September periods during 2002 to 2004.
In this study, we first analysed the composite diurnal patterns from the
time series of gridded hourly precipitation averaged over the coastal area
defined as the red-line bordered polygon in Fig. 1 for the months of November
through March. Additionally, we also analysed the diurnal composite of
longitudinally averaged precipitation over the area defined by the blue-dashed
rectangle in Fig. 1. Results of these analyses are then used to draw
the charts shown in Fig. 2 and will be discussed in Sect. 3.
A map showing the Maritime Continent and its surrounding
area. The main area of interest in this study is depicted as the inset, with
coastlines and topographic contours. The red-line bordered polygon (with land
masks over the northern coastal plain of West Java) defines the area for
spatial averaging of gridded precipitation data. The blue-dashed rectangle
defines the area for longitudinally
(105.5–108.5∘ E) averaged precipitation in Hovmöller analysis
of Fig. 2.
(a) Climatology of area-averaged (see Fig. 1) diurnal
precipitation over the northern coast of West Java, depicted as line plots
of 24 h composite time series in different colours for the months of
November (solid black), December (dashed black), January (solid red),
February (solid blue), and March (dashed-dotted black) analysed from the
TMPA-RT dataset of the 2000–2016 period. Other panels show the corresponding
time–latitude cross sections (Hovmöller diagrams) of the diurnal
composites for the months of (b) November, (c) December, (d) January, (e) February, and (f) March. Dashed vertical black lines in panels (b)–(f) denote
the latitudes of the northern coastal area of West Java. Averaged precipitation
rates in mm h-1 are shown as shaded contours.
In order to classify the phase or timing of precipitation, we picked samples
of 24 h data (area-averaged over the red-line bordered polygon in Fig. 1). We then decomposed each sample into its harmonic constituents (similar
method to that of Nitta and Sekine, 1994) by fitting the data (after
subtracting by the daily mean) using a sinusoidal function
yk(t)=Ckcos2πkTt-θk,
where yk is the precipitation anomaly contributed by the kth harmonic
component, Ck is amplitude (in mm h-1), t is time in hours, T is the
data period (in this case, 24 h), and θk (in radians) is the phase
lag. It should be noted that, for each sample, the 24 h cycle starts from
13:00 and ends at 12:00 LT on the following day by assumption that
precipitation is generally minimum during those hours. By obtaining the
phase angle of the first harmonic constituent θ1, the timing of
diurnal precipitation can be defined by an integer t=n that maximizes y1 in Eq. (1) and satisfies
θ1≈2π(t=n)24,n=0,…,23.
Note that precipitation data are given as discrete time series at an hourly
time interval, but θ1 is computed as a real
number. The classification of phase or timing of precipitation is made in
two steps. In the first step, the timing of precipitation is classified
into
afternoon-to-evening precipitation (AEP) that occurs between 13:00 and 24:00 LT, or 0≤θ1≤(1112)π, and
night time-to-morning precipitation (NMP) that occurs between 01:00 and
12:00 LT on the following day, or π<θ1<(2312)π.
Then, in the second step, samples belonging to NMP are further classified
into
early morning precipitation (EMP) that occurs between 01:00 and 04:00 LT (on
the following day), or π<θ1≤(1512)π, and
late morning precipitation (LMP) that occurs between 05:00 and 12:00 LT, or
(1612)π<θ1<(2312)π.
The first classification is performed in order to check whether early
morning precipitation events are distinguishable from other samples, whereas
the second classification is to quantify the prevalence of early morning
precipitation relative to all samples in the NMP category.
Furthermore, to investigate factors that influence early morning
(01:00–04:00 LT) precipitation on the northern coast of West Java, we
carried out analyses of meridional wind and SST fields attributive to CENS
and SCS-CT phenomena. In this case, we followed Hattori et al. (2011) and
Koseki et al. (2012) to calculate indices for CENS and SCS-CT, respectively.
Details of the calculations can be briefly described as follows.
The CENS index is calculated from daily mean surface (925 hPa) level
meridional wind data spatially averaged over the area of (0–5∘ S,
105–115∘ E) as depicted in Fig. 8. Considering that the lifetime of a
cold surge event is around 5–14 d with an average duration of
approximately 7 consecutive days (Zhang et al., 1997), the resulting daily
time series of wind data are then smoothed by a 5 d moving average. The
smoothed spatially averaged time series data of the meridional wind thus
define the daily CENS index, and a threshold of -4.5 m-1 is used to
determine the occurrence of CENS.
A similar method applies for obtaining the SCS-CT index, but with the SST data
averaged over the area of (2–10∘ N, 105–112∘ E) as
shown in Fig. 9. In this case, a day with a SCS-CT event is defined when the
index is lower than the threshold temperature of 26.4∘ (see Koseki et
al., 2012, for more detailed procedures and explanations).
The CENS and SCS-CT indices are then used to calculate the relative
frequency of occurrences of the coincidence between CENS and/or SCS-CT
events and the three phases of precipitation peaks determined from
previously mentioned harmonic analysis.
As we determined the phase of precipitation peaks based only on the first
harmonic constituent that has the largest amplitude, we have one uniquely
classified sample for each day. The samples of dates collected in these
analyses were then used to make various composite plots.
Results and discussionClimatology of early morning precipitation peaks
The diurnal composites of area-averaged precipitation over the northern
coast of West Java (red-line bordered polygon in Fig. 1), for the months of
November to March during 2000 to 2016, are depicted in Fig. 2a. It can be
seen that the dominant pattern is characterized by precipitation peaks
occurring in the afternoon to evening time (15:00–21:00 LT). However,
diurnal precipitation patterns in the months of January and February exhibit
significant secondary precipitation peaks in the early morning to morning time
(01:00–07:00 LT). The Hovmöller diagrams in Fig. 2b–f show the
time–latitude variations of longitudinally averaged rainfall over the
blue-dashed rectangle in Fig. 1. It should be clear that the coastal region
(marked by vertical black-dashed lines) is affected by precipitation systems
of both inland and oceanic origins. Figure 2d and e also clearly
indicate that the oceanic precipitation system penetrates further inland
during January and February, which appear as secondary peaks occurring
between 01:00 and 07:00 LT as shown in Fig. 2a. In addition to two
stationary patterns of precipitation over land and sea, Fig. 2c–f also show
weak signals of land-to-sea propagation of precipitation over the coastal region
during the night-to-morning transition between 23:00 and 03:00 LT on the
following day.
The timing of precipitation peaks over the coastal region has been further
analysed from the results of harmonic analysis (described in the previous
section). Herein, we focus on the data of January and February that show
strong signals of morning precipitation peaks (see Fig. 2a). Fig. 3a and Fig. 3b show the first harmonic (diurnal) constituent of each sample of
24 h rainfall time series, which were constructed from hourly data
averaged over the coastal region defined in Fig. 1. In this case, the total
number of samples that have been analysed is 373, which is not equivalent to
the total number of days in all January–February months during the 16-year
(2001 to 2016) period because time series with a diurnal cycle amplitude less
than 0.2 mm h-1 are excluded to ensure selection of significant
rainfall events (see e.g. El Kenawy et al., 2015, for thresholds of
moderate rainfall in TMPA data).
The first harmonic components analysed from diurnal
precipitation data that are grouped into (a) afternoon-to-evening
precipitation (AEP) between 13:00 and 24:00 LT and (b) night
time-to-morning precipitation (NMP) between 01:00 and 12:00 LT based on
the phase angles (see text for explanation); (c) composite sinusoidal curves
of AEP (red line) with those of NMP split into early morning precipitation
(EMP) between 01:00 and 04:00 LT and late morning precipitation (LMP) between
05:00 and 12:00 LT. Percentage indicates frequency of occurrence relative to
total samples for AEP and NMP, but those of EMP and LMP are relative to NMP
samples.
Figure 3a shows the results for data that are
categorized as AEP with maximum values between 13:00 and 24:00 LT, while
Fig. 3b shows data of the NMP category with maximum values between 01:00 and
12:00 LT. Figure 3a and b clearly demonstrate that the application of the
harmonic analysis method was successful in classifying the timing of diurnal
precipitation. It is found that AEP is a dominant feature of the diurnal
precipitation, which contributes 55 % of the diurnal rainfall patterns
over the studied area. However, NMP also has a high frequency of occurrence,
contributing 45 % of the total samples in January and February throughout
the 2001 to 2016 period.
In Fig. 3c, three composites of the first harmonic signals are depicted. The
first composite (red curve) represents the mean sinusoidal signal of AEP,
while that of NMP has been further split into EMP that occurs between 01:00
and 04:00 LT (green curve) and LMP whose time of occurrence is between 05:00
and 12:00 LT. The frequencies of occurrence of EMP and LMP are 42 % and 58 %, respectively, relative to total NMP samples, or 18.9 % and 26.1 %
relative to all samples. It is of interest to note that, in spite of the
smallest percentage, EMP has the largest mean amplitude.
Figure 4 shows more detailed analysis of the phase distribution and the mean
amplitude for all of the three 24 h cycle precipitation patterns, based
on the results of harmonic analysis. It can be seen that phases of AEP
exhibit a nearly normal distribution centred around 19:00 LT with two
maxima of mean amplitudes around 18:00 and 22:00 LT. On the other hand,
LMP shows a distribution that decreases with time, resembling a gamma-like
distribution with a maximum mean amplitude around 06:00 LT. These
characteristics of the distribution of frequency of occurrence of the two
types of precipitation are indicative of the concentration times of
convective activities, which are around 19:00 over land and 06:00 LT over
sea. These results are in agreement with many previous investigations (e.g.
Qian, 2008) and seem to signify the canonical pattern of land–sea contrast
in the phase of precipitation, which are believed to be mainly controlled by
land–sea breezes and their interactions with the monsoonal flows (e.g.
Houze et al., 1981). However, it is quite interesting that EMP events have
not only a flat phase distribution, which indicates more random events,
but also high mean amplitudes with peaks around 04:00 LT. The statistical
significance of these amplitudes is indicated by the sampling errors (±σ/n) plotted as vertical bars in Fig. 4.
The random occurrence of early morning precipitation might be closely
related to propagating systems, as indicated from Fig. 2b–f, since their
phases are not necessarily locked to the timing of sea–land breezes.
Diurnal distribution of the mean amplitude of the first
harmonic components (solid black line) and frequency of occurrence of the
peak precipitation time (coloured bar chart) differentiated into three
groups (same as in Fig. 3): AEP (red), EMP (green), and LMP (blue).
The sampling errors (±σ/n) are
plotted as vertical bars.
Relationships with propagating convective systems and extreme
precipitation events
In order to investigate the relationship between early morning precipitation
and the propagating characteristics of the precipitation system over the coastal
region, we constructed Hovmöller diagrams similar to those of Fig. 2b–f
but with data classified according to the previously defined three types of
precipitation peaks, i.e. AEP, EMP, and LMP, and show the results in Fig. 5.
Propagation of precipitation connecting two relatively stationary systems
over land and sea can be distinctly seen from Fig. 5a and b. The
propagating precipitation can be associated with rainfall events that occur
during the late night (Fig. 5a) and early morning (Fig. 5b) time over the coastal
region. It should be noted that in Fig. 5a the direction is mainly one way
from land to sea, while in Fig. 5b there is also a discernible landward
pattern of propagation. On the other hand, Fig. 5c illustrates more detached
precipitation systems over land and sea. In this case, late morning
precipitation over the coastal region can be seen as an extension of the
oceanic precipitation system.
Hovmöller diagrams of composite diurnal precipitation,
similar to Fig. 2 except that data are classified by the phases or the timing of
peak precipitation (as in Fig. 3) (a) AEP, (b) EMP, and (c) LMP. The black-dashed line indicates the boundary of the coastal area. The black solid arrows
represent the direction of propagation (following Mori et al., 2004), with
seaward propagation symbolized by α
and landward propagation indicated by β.
We have discussed earlier in Sect. 1 that EMP may be linked to extreme
precipitation events over the northern coast of Java. We performed
other analyses using Hovmöller diagrams, similar to those in Fig. 5, but
with averaged precipitations of the 99th percentile (P99) to
represent extreme values in each data grid. The results, which are depicted
in Fig. 6, strongly suggest that extreme rainfall events over the coastal
region mainly occur between 01:00 and 04:00 LT and are characterized by the
existence of propagating systems from both land and ocean (Fig. 6b). On the
other hand, extreme coastal precipitation events that occurred during
late night (Fig. 6a) and late morning (Fig. 6c) seem to originate only
from either land-based or oceanic convection. Hence, there are distinct
characteristics of propagating systems associated with extreme AEP, EMP, and
LMP events over the coastal region. Furthermore, while oceanic convection
seems to be major determinant for the occurrences of morning coastal
precipitation, linkage to land-based precipitation may also be important for
the case of EMP.
Hovmöller diagrams, same as Fig. 5, except for the
99th percentile (P99) values of the gridded hourly
precipitation.
Relationships with CENS and SCS-CT
As discussed in the previous section, we have objectively identified EMP events
over the northern coast of West Java from satellite data using harmonic
analysis. We have also found that EMP is strongly associated with
propagating precipitation systems that, in some cases, show connection
between land-based and oceanic convections. While we have insufficient data
to analyse the propagating precipitation systems in more detail, synoptic
conditions favourable for their occurrence may be investigated from global
reanalysis data. We also mentioned earlier that such favourable conditions
may develop under the influence of CENS, or SCS-CT, or the combination of
both.
We analysed the frequency of occurrence of CENS and SCS-CT events
corresponding to each classification of diurnal precipitation patterns of
AEP, EMP, and LMP. Figure 7a shows the frequencies of occurrence of CENS, SCS-CT,
and combined events, which are calculated relative to the number of samples
in each diurnal precipitation, namely, 205 for AEP, 50 for EMP, and 118 for
LMP. It should be noted that the frequencies do not sum up to 100 %
because some samples are considered unclassified if they are not associated
with either CENS or SCS-CT, and there are also an overlapping number of samples
between CENS and SCS-CT events. Bearing in mind that we focus this analysis
on data of January and February, it can be seen from Fig. 7 that SCS-CT
generally prevails, with a frequency of occurrence above 50 % in all
classified events. However, SCS-CT exhibits stronger association with
morning precipitation events, with a frequency of occurrence above 70 % in
both the EMP and LMP categories. This seems to indicate the possible influence of
SCS-CT that may induce landward shifting of the precipitation system over the
Java Sea (Koseki et al., 2012).
(a) Frequency of occurrence of SCS-CT (black), CENS
(orange), and CENS-SCS-CT (purple) corresponding to the
three diurnal phases of peak precipitation AEP, EMP, and LMP; (b) scatter
plot between SCS-CT and CENS indices for AEP, LMP, and EMP as indicated by
red, blue, and green circles, respectively. Threshold values for SCS-CT
(26.4 ∘C) and
CENS (-45 m s-1) indices are
indicated by black dotted lines.
On the other hand, the frequency of occurrence of CENS is the highest for
samples belonging to EMP and almost negligible for the AEP category. It should
be noted that the number of CENS events are almost the same for both EMP and
LMP categories and, although we uniquely classify the timing of diurnal
precipitation using harmonic analysis, some of the events may be
interrelated due to subsequent processes from the previous day or on the
following day. As is evidenced in Fig. 7a and depicted more clearly in
Fig. 7b, most CENS events are in coincidence with SCS-CT, and their
magnitudes are positively correlated.
Spatial distribution of mean (composite) 850 hPa horizontal wind
vectors and magnitude of meridional wind (shaded) corresponding to the three
diurnal phases of peak precipitation (a) AEP, (b) EMP, and (c) LMP. The
dark-blue solid rectangle represents the CENS index area (following Hattori
et al., 2011).
Same as Fig. 8 but for SST (solid line contour) and SST anomaly
relative to the SCS-CT threshold of
26.4 ∘C (shaded). The
dark-blue solid rectangle represents the SCS-CT index area (following Koseki
et al., 2012).
The composite maps of 850 hPa meridional wind fields (also wind vectors) and
SST, corresponding to SCS-CT and CENS that are classified according to the
phase of precipitation, are shown in Figs. 8 and 9, respectively. In Fig. 9b
and c, contours of the negative SST anomalies (after being subtracted by the
threshold value of 26.4 ∘C) protrude relatively more
closely towards the Equator, which indicates the strengthening of the SCS-CT
when associated with morning precipitations over the northern coast of West
Java. Although differences in the SST fields for EMP and LMP events are not
quite discernible, the composite maps of wind fields in Fig. 8 show
significantly different patterns. The enhancement of CENS associated with
EMP events (Fig. 8b) is connected to an elongated channelling of strong
northerly wind anomalies offshore from the peninsula of Indochina, Malaysia, and down
to the Karimata Strait; the northerly winds are even observed to cross Java. On the other hand, LMP events (Fig. 8c) are characterized by an
enhancement of the northerly winds at a larger spatial scale and stronger
connection with extratropical regions but weaker magnitudes over the Java
Sea compared to that of the EMP case. The enhancement of SST anomalies that are
associated with combined EMP and CENS plus SCS-CT, in comparison to EMP and
SCS-CT only, events can be more clearly seen in Fig. 10.
Same as Fig. 9 but with a smaller domain and for a composite of EMP
samples: (a) SCS-CT without CENS, (b) SCS-CT with CENS.
So far, our results indicate and confirm previous works (Wu et al.,
2007; Hattori et al., 2011; Mori et al., 2018) that CENS is an important factor for
the occurrence of EMP events over the northern coast of West Java, and cooler
SST in the South China Sea seems to provide a favourable background
condition. In addition, we found that the enhancement of northerly winds
during the EMP events may have more localized origin over the South China
Sea. The coincidence of SCS-CT and CENS events brings forward the aspect of
mesoscale sea–air interaction with all its possible feedbacks, which is
quite interesting because the cold surge that reaches the tropics is
traditionally viewed as the effects of midlatitude weather during boreal
winter (e.g. Chang et al., 2005), but more works are needed to make further
analysis.
Summary and conclusion
In the present work, we have studied the diurnal cycles of precipitation
over the northern coast of West Java, with emphasis on the occurrences of
early morning precipitation peaks, mainly using the TMPA-RT dataset
spanning from 2000 to 2016. The diurnal composites of precipitation averaged
over the study area for the months of November to March are dominantly
characterized by precipitation peaks that occur in the afternoon to evening
time (15:00–21:00 LT). However, diurnal precipitation patterns in January
and February show secondary precipitation peaks in the night to morning time
(01:00–07:00 LT). Results of Hovmöller analysis revealed that, beside
the canonical pattern of afternoon (morning) convections over land (sea),
there are weak signals of land-to-ocean propagation during the
night-to-morning transition time. This study was further motivated by this
propagating characteristic of precipitation over coastal regions in the MC
that have been suggested by previous studies (Mori et al., 2004; Kikuchi and
Wang, 2008; Li et al., 2017).
We then applied the harmonic analysis method to objectively determine the phase
of precipitation peaks over the northern coast of West Java, focusing on data
of January and February, whereby the timing of precipitation for each day
has been classified into three categories: (1) afternoon-to-evening
precipitation (AEP) that occurs between 13:00 and 24:00 LT, (2) early
morning precipitation (EMP) that occurs between 01:00 and 04:00 LT, and (3) late morning precipitation (LMP) that occurs between 05:00 and 12:00 LT. The
EMP contributes 18.9 % of the total samples analysed, which is a minority
compared to that of the LMP (26.1 %) and AEP (55 %). However, we
found that the EMP events are characterized by (1) seaward as well as
landward propagation with indication of stronger connectivity between
land-based and oceanic precipitation systems, (2) flat phase distribution,
and (3) high mean amplitudes peaking around 04:00 LT. These propagating
characteristics of EMP are more prominent when data are composited by taking
the 99th percentile (P99), which signifies the importance of EMP in
association with extreme precipitation events. The flat phase distribution
of EMP events suggests that the timing of coastal precipitation is not
necessarily locked to the phase of land–sea breezes, thus allowing
precipitation to occur more randomly. This implies that, even for as short
as 24 h lead time, probabilistic forecast may be necessary to assess the
hazard of heavy precipitation in this region.
Following the lead of previous studies, we investigated the role of SCS-CT
and CENS as influencing factors for the occurrences of the EMP event. By
calculating the frequencies of occurrence of SCS-CT and CENS that correspond
to the three timings of the coastal precipitation, we find that SCS-CT
generally prevails in January and February, but morning precipitation events
over the northern coast of West Java mainly occurred in coincidence with
enhanced SST cooling in the South China Sea, which is in agreement with the
numerical modelling results of Koseki et al. (2012). We also found that CENS
is the most important factor that influences the occurrence of EMP and its
association with extreme precipitation events over the northern coast of
West Java, which is consistent with results from previous studies (Wu et
al., 2007; Hattori et al., 2011; Mori et al., 2018). Most of the CENS events
identified in this study occurred in coincidence with SCS-CT and both
indices exhibit positive correlation when associated with EMP. In this case,
the enhancement of CENS and SCS-CT is connected to an elongated channelling
of strong northerly wind anomalies just offshore of Indochina, Malay
Peninsula, and even observed to cross Java.
In the present study, we are aware of the issues concerning the TMPA-RT
dataset in representing diurnal precipitation in the MC, but we have presented
results that are consistent with previous studies and produced some new
documentations about the propagating precipitation systems over the coastal
region. Similar studies or an extension to this study will draw more benefit from
better satellite rainfall products. We have not yet investigated the
mechanisms responsible for the propagation of the coastal precipitation, but
direct sea–land breeze effects do not seem to give satisfactory explanation.
Moreover, it can be roughly estimated from Figs. 5a and 6a that the speed of
the offshore-propagating precipitation systems is around 7 m s-1.
Ruppert and Zhang (2019) pointed out that coupling between a slow gravity wave
mode, with a phase speed of about 7 m s-1, and offshore-propagating
convections in the MC has not been well understood. Alternatively, the role
of a cold pool (e.g. Teo et al., 2011; Trismidianto et al., 2016) may serve
as a concrete and testable hypothesis to be the manifestation of the
so-called self-replicating mechanism proposed by Mori et al. (2004). We
additionally found that the CENS associated with EMP may have a more localized
origin and seems to involve mesoscale air–sea interactions over the South
China Sea, which is also of interest to address in our future study.
Data availability
The datasets of NCEP/NCAR, OISST AVHRR, and TMPA-RT generated during and/or analysed in the current study are publicly available in the direct links
https://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.pressure.html (Kalnay et al., 1996),
https://www.ncei.noaa.gov/thredds/catalog/OisstBase/NetCDF/AVHRR/catalog.html (Reynolds et al., 2007; Banzon et al., 2016),
and https://disc.gsfc.nasa.gov/datasets/TRMM_3B41RT_7/summary (10.5067/TRMM/TMPA/3H-E-IR/7, Huffman et al., 2007; Huffman, 2012), respectively.
Author contributions
Author EY drafted the initial manuscript and conducted the main data processing, including the composite and statistical analyses. Author TWH revised the initial manuscript with substantial corrections in the story line and improved discussions and the overall content of the manuscript. Author NSN performed further revision to the manuscript with detailed compositions and provided additional analysis related to oceanographic aspects. Author MRS conducted harmonic analysis and contributed in the production of related figures. All the authors were involved in discussions during the review process and read and approved the final manuscript.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The co-authors were partially supported by ITB-P3MI Research Grants 2017–2019. The authors would also like to express their appreciation to Shigeo Yoden of Kyoto University and other research team members under the JSPS and DG-RSTHE Joint Research Program for FY 2018–20 for their useful discussions and suggestions. The authors are very grateful to Marc Salzman, topical editor of Annales Geophysicae, and two anonymous reviewers for their constructive comments and suggestions, which substantially improved the clarity of this work.
Financial support
This research has been supported by the Indonesia Endowment Fund for Education (LPDP) (grant no. 20151021024633).
Review statement
This paper was edited by Marc Salzmann and reviewed by two anonymous referees.
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