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Earth and Space Science
RESEARCH ARTICLE
10.1002/2016EA000181
Key Points:• The climatological relationshipbetween the diurnal migration of
rainfall and ambient wind is clariﬁed• The diurnal characteristics ofrainfall-system migration depend
systematically on the backgroundwind• Advection by background wind issuggested to be an importantcomponent of the diurnal rainfall
propagation mechanism

Correspondence to:
A. Yanase,yanase@satellite.isee.nagoya-u.ac.jp

Citation:
Yanase, A., K. Yasunaga, and H. Masunaga(2016), Relationship between the
direction of diurnal rainfall migrationand the ambient wind over theSouthern Sumatra Island, Earth andSpace Science, 4, doi:10.1002/
2016EA000181.
Received 16 MAY 2016
Accepted 23 DEC 2016
Accepted article online 29 DEC 2016©2016. The Authors.
This is an open access article under theterms of the Creative Commons
Attribution‐NonCommercial‐NoDerivs
License, which permits use and distribution in any medium, provided theoriginal work is properly cited, the use isnon‐commercial and no modiﬁcations
or adaptations are made.

YANASE ET AL.

Relationship between the direction of diurnal
rainfall migration and the ambient windover the Southern Sumatra Island
A. Yanase1, K. Yasunaga2,3, and H. Masunaga4

1

Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan, 2Department of Earth Science, University of
Toyama, Toyama, Japan, 3Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan, 4Institute for Space-EarthEnvironmental Research, Nagoya University, Nagoya, Japan

Abstract This study investigates the climatological relationship between diurnal rainfall migration andthe ambient wind over the southern Sumatra by using observation and reanalysis of 15 years of datafrom the Tropical Rainfall Measuring Mission satellite precipitation radar. When winds in the lower
troposphere (1000–500 hPa) are westerly, convective rainfall systems migrate eastward, while rainfall
systems dominated by stratiform precipitation propagate to the west in association with mid-upper level
easterly winds. When the winds are easterly throughout the troposphere, both convective and stratiform
precipitation systems propagate only to the west. Rainfall system migration appears to be generally consistent
with ambient zonal wind. It is suggested that the advection by background wind is, in general, a dominantcomponent of the propagation mechanism of diurnal rainfall.

1. Introduction
The Maritime Continent (MC) is a unique region between the Indian and Paciﬁc Oceans (warm pool), consisting of many islands surrounded by warm water. The MC is known as a region of especially high convective
activity, and latent heat released from active convection there plays an important role in driving atmosphericgeneral circulation [Ramage, 1968; Neale and Slingo, 2003]. Over the MC, the diurnal precipitation cycle is oneof the most dominant variations among a broad range of time scales of convective activity, ranging from several hours to a year. Accordingly, diurnal variations of rainfall over the MC have drawn great climatologicalinterest for several decades [Hendon and Woodberry, 1993; Nitta and Sekine, 1994; Yang and Slingo, 2001;
Kikuchi and Wang, 2008]. Many features of these variations have been well documented to date, e.g., theinitiation of convection in a mountainous area in the afternoon, the peak of rainfall early in the night, andthe movement of rain areas from the mountains to coast or offshore areas around midnight [Nitta andSekine, 1994; Nesbitt and Zipser, 2003; Mori et al., 2004; Sakurai et al., 2005; Ichikawa and Yasunari, 2006;
Love et al., 2011]. However, several fundamental issues still remain unexplained, such as the offshore shiftmechanism, interactions with large-scale (longer-period) variations, and local orography effects, although
they have been discussed by some previous studies [Ohsawa et al., 2001; Mapes et al., 2003; Ichikawa andYasunari, 2008; Wu et al., 2009]. Owing to our poor understanding of diurnal rainfall variation, current
state-of-the-art general circulation models, which are used for weather forecasting and climate projection,
still have difﬁculty reproducing the diurnal precipitation cycle (e.g., amplitude and phase of rainfall).
This study focuses on the migration of precipitation systems, which is among the unresolved issues in thediurnal rainfall cycle over the MC. The relevant processes reported in previous studies include land breeze
convergence [Houze et al., 1981], advection by background wind [Mori et al., 2004; Sakurai et al., 2009,2011], self-replication of convective cells [Sakurai et al., 2009, 2011], convergence of cold outﬂows producedby rainfall systems [Fujita et al., 2010], and gravity waves triggered by daytime heating of the mixed layer onland [Mapes et al., 2003] or convection on land [Love et al., 2011].
Southwest Sumatra is a suitable location for investigating precipitation system migration. Above all, clear seasonal variations of diurnal cloud and precipitation systems are present in this region. Diurnal cloud system
migration and its seasonality, associated with the ambient wind, over Sumatra Island have been investigatedby Sakurai et al. [2005], as described below. Moreover, diurnal rainfall cycles show clear seasonal contrasts
over southern Sumatra Island in Figure 1: an inland peak in the boreal winter (Figure 1a) and an offshore peakin the boreal summer (Figure 1b). Hence, this region offers an ideal test bed to investigate the potential

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Figure 1. The geographical distribution of the square root of power spectrum density (i.e., amplitude) corresponding to the diurnal cycle calculated by applying thefast Fourier transform (FFT) method to TRMM 3B42 rainfall data in (a) boreal winter and (b) boreal summer. The rectangular area in the panels indicates the analysis
domain of this study.

effects of seasonality on the diurnal propagation of precipitation systems. Furthermore, there are no small
islands off the coast that could potentially modulate rainfall system propagation, and the region’s topography
is relatively simple. Thus, we focus on Sumatra Island and its vicinity to explore the migration mechanisms.
Sakurai et al. [2005] revealed seasonal variations in diurnal cloud system migration over Sumatra Island, making use of blackbody temperature (TBB) data from the Geostationary Meteorological Satellite. A westward
cloud migration occurs in almost all months over southern Sumatra Island because easterly wind dominates
throughout the year in the upper troposphere. On the other hand, eastward cloud migration exists only when
the low-level tropospheric wind is westerly. Although they discussed seasonality with focus on the monthly
averaged zonal wind, lower troposphere wind direction varies each month. Note that TBB data as a proxy ofcloud top temperature are not always a reliable measure of precipitation system migration as an extended
area of upper level cirrus veils lower level convection.
Mori et al. [2004] looked into the diurnal cycle of rainfall over Sumatra Island by using Tropical Rainfall
Measuring Mission (TRMM) satellite precipitation radar (PR) data and intensive radiosonde soundings.
Rainfall systems develop along the southwestern foot of the mountains and propagate to the east or west.
The propagation mechanisms, although discussed by Mori et al. [2004], remain not entirely clear. It is partly
because Mori et al. [2004] analyzed a transient period of seasonality (November), when zonal wind direction
ﬂuctuates in the lower troposphere.
From a case study using Geostationary Operational Environmental Satellite (GOES-9) and observational datain the ﬁrst campaign of the Coupling Processes in the Equatorial Atmosphere (CPEA-I) project, the migration
direction and speed of convective systems with precipitation were found to be related to the direction andspeed of horizontal winds in the lower troposphere [Sakurai et al., 2009]. They suggested that the rainfall
migration is explained by a self-replication of convective cells and advection caused by the background windin the lower troposphere. The analysis period of the case study is limited to April and May 2004. Hence, therobustness is not clear because the lower tropospheric zonal wind seasonally varies in direction.
Therefore, further investigation with an extended period of time is needed to clarify the climatological relationship between the ambient wind and rainfall system migration and to test and further elaborate existing
hypotheses in order to explain the propagation mechanism. In this study, we examined seasonal changes ofthe propagation features of the diurnal rainfall by making use of long-term (15 years) observations from
TRMM-PR and reanalysis data. In particular, the purpose of our study is to shed light on the potential rolesof the direction and magnitude of the ambient wind in the migration of diurnal rainfall.

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The data and composite analysis method are described in section 2, and the results from the composite
analysis are presented in section 3. Section 4 is devoted to discussion on the results, and a summary is given
in section 5.

2. Data and Methodology
2.1. Data
We utilized two TRMM-PR products in this study’s composite analysis (TRMM 2A25 version 7 and 3G68).
TRMM-PR was the ﬁrst space borne rain radar, and it could directly observe the three-dimensional structureof precipitation systems. The TRMM 2A25 product contains the surface rainfall rate and vertical distributionsof attenuation-corrected reﬂectivity factor and rainfall rate retrieved from its reﬂectivity. It also includes theclassiﬁcation of rain types (stratiform, convective, and others) based on the existence of bright band or horizontal homogeneity of radar echoes at the surface. In our analysis, the data were compiled into an hourly
gridded product (0.25° × 0.25° spatial resolution) from 1 January 1998 to 31 December 2012.
It takes several days for the TRMM-PR data to recur to the same location. This is because the TRMM satellite
completed about 16 orbits per day and the PR instrument had a swath width of only 215 km (247 km after 23
August 2001). To conﬁrm the inﬂuence of the infrequent sampling, we used the TRMM 3G68 data product asa supplement in our analysis. TRMM 3G68 is an hourly gridded product (0.25° × 0.25° spatial resolution) that
contains rain rate estimates at the surface from the TRMM 2A12 data product, making use of a retrieval
algorithm for the TRMM Microwave Imager (TMI) and TRMM-PR (2A25). Although we used both the PR and
TMI data, the results using TMI data were consistent with the outcomes using PR data and did not alter
the conclusions; thus, we do not show them in this paper.
TRMM 3B42 version 7 is also used in the fast Fourier transform (FFT) analysis in section 1. TRMM 3B42 provides
gridded rain rate by using microwave and IR estimation with gauge adjustment, with a 3 h temporal resolution
and a 0.25° × 0.25° spatial resolution. The power spectrum density corresponding to the diurnal cycle of rain rateis computed by applying an FFT method using the TRMM 3B42 product from January 1998 to December 2012.
For ambient wind information, we use reanalysis data, known as ERA Interim products, offered by the European
Centre for Medium-Range Weather Forecasts (ECMWF). The time resolution of the ERA Interim product is 6 h,and its horizontal-grid interval is 0.75°. We acquired the data spanning from 1 January 1998 to 31 December
2012 to match the temporal coverage of TRMM products. The 6 h wind data are averaged to daily wind, anda 5 day running mean is applied to the daily wind time series to smooth out short-term ﬂuctuations.
2.2. Methodology
We focused on the southern Sumatra Island area described in section 1. The domain for the composite analysis is indicated by the rectangular area (96°E–110°E, 5°S–3°S) in Figure 1, where two separate peaks arefound in the onshore (boreal winter, Figure 1a) and offshore (boreal summer, Figure 1b) regions.
Figure 2a shows that the time series of zonal wind averaged for lower troposphere (1000–500 hPa) over therectangular domain of Figure 1. The temporal mean of the time series is Ū = 0.4 m s 1, and its standard deviation is σ = 3.8 m s 1. Using these values, we identify four regimes and make a composite plot of diurnal rainfall
variations in each regime: (1) strong westerly (SW) regime where the zonal wind exceeds mean plus standard
deviation (U ≥ Ū + σ), (2) weak westerly (W) regime where the zonal wind is between 0 and mean plus standard deviation (0 < U < Ū + σ), (3) weak easterly (E) regime where the zonal wind is between 0 and mean
minus standard deviation (Ū σ < U < 0), and (4) strong easterly (SE) regime where the zonal wind is smaller
than mean minus standard deviation (U ≤ Ū σ).
The seasonal variations of the number of days falling into the four regimes (SW, W, E, and SE) deﬁned above
are shown in Figure 2b. During the boreal winter (December to March), the SW and W regimes are more
dominant than the E and SE regimes. On the other hand, during the boreal summer (May to October), theE and SE regimes are more dominant than the SW and W regimes. We identiﬁed April and November astransient periods between the regimes, when the days of westerly wind (SW and W) are comparable to thatof easterly wind (E and SE). The composite plots in the SW and W regimes may be considered to primarily
represent the feature during the boreal winter, while the E and SE regimes are representative of theboreal summer.

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Figure 2. (a) Time series of the zonal wind averaged for the lower troposphere (1000–500 hPa) over the rectangular domain
in Figure 1. The gray line shows the time mean of the time series Ū, and the red and blue lines indicate the mean plus andminus standard deviation, respectively (Ū + σ and Ū σ), which are used as an index to identify four wind regimes (SW,strong westerly; W, weak westerly; E, weak easterly; SE, strong easterly). (b) Seasonal variations of the four wind regimes.
The pink, red, green, and blue lines indicate the SW, W, E, and SE regimes, respectively.

3. Results
Figures 3a–3d show the time-longitude cross sections of composite surface precipitation from TRMM 2A25
for the rectangular area indicated by Figure 1 in each regime. In the SW regime (Figure 3a), the rainfall
develops to the east of the island (104°E–106°E) from the evening to midnight local time (1800–0000 LT).
The rainfall peak then migrates to the eastern offshore area (108°E–109°E) at about 8 m s 1. It should be

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Figure 3. (a–d) Time-longitude cross sections of the composite surface rain rate and (e–h) the composite ratio to the total
precipitation from TRMM 2A25 in SW (Figures 3a and 3e), W (Figures 3b and 3f), E (Figures 3c and 3g), and SE regimes(Figures 3d and 3h). The area between the two dashed lines from 102°E to 106°E corresponds to the land region. In
1
Figures 3e–3h, the solid line indicates the subjectively estimated migration speed of 8 m s . The area with surface rain
1
rates below 0.2 mm h is blanked out in Figures 3a–3d.

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noted that the precipitation system redevelops when it arrives over the ocean east of the island (107°E) frommidnight to early morning (0000–0300 LT). On the other hand, the westward propagation signal of a diurnal precipitation system is much weaker than the eastward propagation. The rainfall system over the oceanwest of Sumatra Island (96°E–102°E) exhibits a notable development from early morning to midmorning(0300–0900 LT), although not completely vanishing during the rest of the day.
In the W regime (Figure 3b), rainfall systems are generated over the inland area (102°E–106°E) in theafternoon (1200–1500 LT) and intensify over the central part of the island (103°E–105°E) until evening(1800–2100 LT). Although the eastward migration signal is weaker in the W regime than in the SW regime,
zonally symmetric propagation from the western coast to the Indian Ocean is found from the evening(1800–2100 LT) to the morning (0600–0900 LT) in the W regime. Both eastern and western migration systems
redevelop off the coast (107°E and 101°E) from the evening to the morning (1800–0900 LT). The eastward andwestward migrations have speeds of approximately 5 m s 1 and 4 m s 1, respectively.
In the E regime (Figure 3c), rainfall is initiated over the western coast of the island (102°E–104°E) and develops
from the evening to the early morning (1800–0300 LT), slightly earlier than in the W regime. After initiation,rain systems in this regime propagate solely to the west without any eastward counterparts. Similarly, a westward propagation is found in the SE regime (Figure 3d). However, rainfall in the SE regime grows in intensity
continuously as it propagates farther off the western coast of the island (96°E–98°E). Propagation speeds inthe E and SE regimes are estimated to be 8 m s 1 and 8–12 m s 1, respectively. These results are qualitatively
consistent with the outcomes using TRMM 2A12 TMI data (not shown).
Figures 3e to 3h show the time-longitude cross sections of the composite ratio of convective rain to total
precipitation (sum of convective and stratiform rainfall). The eastward migrating systems in the SW and W
regimes are characterized by convective rainfall. In contrast, the westward migrating systems in the SW
and W regimes are dominated by stratiform rainfall. In the SW regime, stratiform rain almost entirely extends
off the west coast of Sumatra Island throughout the day, except in the vicinity of 98°E from 1500 to 0000 LT. In
the W regime, the migration signal of stratiform rainfall from the west coast of Sumatra Island (102°E) to thesea (100°E) is clearer than in the SW regime. In the westward propagating systems of the SE and E regimes,
convective precipitation is more dominant than stratiform rainfall. However, convective rain gradually weakens as the precipitation system travels over the oceanic regions.
The sequences of the vertical proﬁles of the convective and stratiform reﬂectivity from TRMM 2A25 data areshown in Figure 4. In the SW regime (Figure 4a), intense convective rain systems with a deep structuredevelop in the evening (1800 LT) over the eastern portion of the island (104°E–106°E). Convective systems,with their structure and intensity being maintained, propagate from land to the eastern sea (107°E) in the latenight (0000 LT). Vertical structure and reﬂectivity intensity get lower and weaker in the morning (0600 LT).
The sharp bright bands in stratiform systems, which form as an organized convective system matures, areshown at midnight (0000 LT) over 104°E–107°E. The stratiform rain systems also exist through the day off
the western coast of Sumatra Island (96°E–102°E). It especially enhances the intensity and yields the brightband structure in the morning (0600 LT) over 96°E–102°E and at noon (1200 LT) over 98°E–100°E.
In the W regime (Figure 4b), penetrating convective rainfall develops over 102°E–106°E in the evening(1800 LT). It propagates eastward and gradually weakens from midnight (0000 LT) to morning (0600 LT).
Although moderate stratiform rain persists throughout the day off the western coast of Sumatra Island(96°E–102°E), deep stratiform rain with bright bands develops in the evening (1800 LT) over 103°E–
105°E. It migrates westward, and the intensity of stratiform rain increases from the evening (1800 LT) over
103°E–105°E to the morning (0600 LT) over 99°E–102°E.
In the E regime (Figure 4c), deep convective rain takes place in the evening (1800 LT) over 102°E–105°E andthe rain system gradually weakens from midnight (0000 LT) over 100°E–103°E to the morning (0600 LT) over
99°E–101°E. Stratiform rainfall with bright band structures occurs over the western coast of Sumatra Island(101°E–103°E) in the evening (1800 LT) and then matures during the night (00 LT) over 100°E–102°E. It
propagates westward, maintaining its structure from the evening (1800 LT) over 101°E–103°E to noon
(1200 LT) over 97°E–99°E.
In the SE regime (Figure 4d), the penetrating convective rainfall system develops in the evening (1800 LT)
over 101°E–104°E and propagates westward with upkeep of strong reﬂectivity intensity and its tall structure.

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Figure 4. The sequence of the vertical distributions of the (left) convective and (right) stratiform reﬂectivity from TRMM 2A25 in the (a) SW, (b) W, (c) E, and (d) SE
regimes. The area between the two dashed lines at 102°E and 106°E corresponds to the land region.

The deep stratiform rainfall, which occurs in the evening (1800 LT) over 101°E–104°E with the bright band,
gradually becomes stronger from the evening (1800 LT) over 101°E–104°E to noon (1200 LT) over 96°E–97°E
and maintains its bright band structure.

4. Discussion
4.1. Seasonality of Diurnal Rainfall Migration
As mentioned in the previous section, it was shown that the direction of diurnal rainfall propagation is related
to the direction of ambient zonal wind. In this section, the seasonal variations of diurnal precipitation migration are discussed in terms of propagation direction. To this end, we equally divide the year into four periods
into a seasonal-mean composite plot in a similar manner to Figures 3a–3d (Figure 5). The four periods are
December, January, and February (DJF; Figure 5a); March, April, and May (MAM; Figure 5b); June, July, andAugust (JJA; Figure 5c); and September, October, and November (SON; Figure 5d).
In DJF (Figure 5a), rainfall systems occur in the noon (1200 LT) over land (102°E–106°E) and propagate both
eastward and westward. The eastward propagation of diurnal rainfall is similar to the result in the SW andW regimes (Figures 3a and 3b), while the westward migration is associated with the W and E regimes

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Figure 5. Time-longitude cross sections of the composite surface rain rate from TRMM 2A25 for (a) December, January, and February; (b) March, April, and May;(c) June, July, and August; and (d) September, October, and November.

(Figures 3b and 3c). Figure 2b shows that this period (DJF) is dominated by the SW and W regimes with asecondary contribution of the E regime.
In MAM (Figure 5b), the rainfall extends over land (102°E–106°E) from noon to midnight (1200–0000 LT) andsimultaneously migrates to the east and west with an amplitude overall weaker than in DJF (Figure 5a). This propagation direction and its characteristics might be interpreted as a combination of the W and E regimes(Figures 3b and 3c). This is supported by Figure 2b, showing that MAM is dominated by the W and E regimes
with the exception of March, for which the SW regime makes a comparable contribution. Mori et al. [2004] also
show that diurnal precipitation migrates eastward and westward as seen in Figures 5a and 5b of this study.
In JJA (Figure 5c), rainfall systems over the western portion of the island (102°E–104°E) in the afternoon andevening (1500–2100 LT) migrate westward only. The propagation signal is as observed in the E regime(Figure 3c), although weaker in strength. These characteristics of the direction of diurnal precipitation migration
are as expected, given that the E regime dominates in this period (Figure 2b).
In SON (Figure 5d), precipitation systems generated from the west coast to the land (102°E–106°E) propagate
to the east and west. The westward propagation signal, extending over a distance from the starting location(96°E–98°E), is especially stronger than the eastward one. It should be noted that its westward signal appears

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to be composed of two propagation speeds (slower and faster), presumably explained by a mixture of the E
and SE regimes (Figures 3c and 3d). The weak eastward propagation may be ascribed to the W regime, which
gradually becomes more frequent as time proceeds in the second half of the SON period (Figure 2b).
Based on these results, the seasonality of the diurnal propagation is interpretable in terms of the seasonallychanging proportion of the different wind regimes as implied in Figure 2. These results are consistent withthe seasonality of diurnal cloud migration reported by Sakurai et al. [2005].
4.2. Comparison Between Diurnal Rainfall Propagation and Zonal Wind Speed
The relationship between diurnal rainfall propagation and ambient zonal wind was qualitatively shown insections 3 and 4.1. In this section, we quantitatively compare the propagation speeds with the zonal windspeed (Figure 6) and discuss possible migration mechanisms.
In the SW regime, the wind is westerly from 1000 hPa to 400 hPa (Figure 6a) and the wind speed from 800 hPa
to 600 hPa matches the migration speed (approximately 8 m s 1) of the tall convective precipitation systems(Figures 3a and 4a). It is suggested that the eastward propagation of diurnal rainfall dominated by convective
rain may be primarily owing to the lower level (800–600 hPa) background wind in the SW regime.
In the W regime, the eastward migration speed (approximately 5 m s 1) of convective rainfall systems(Figure 3f) is slightly faster than the wind speed (approximately 0–4 m s 1) in the lower troposphere(1000–500 hPa) (Figure 6b). Thus, simple advection by lower level wind cannot completely account for
convection migration speed. The difference in speeds (1–2 m s 1) might be insigniﬁcant taking into accountthe horizontal resolutions of the reanalysis data, local circulation, and uncertainty about estimated propagation
speed. Westward migration of stratiform rainfall (about 4 m s 1), on the other hand, is completely opposite ofwind direction in the lower troposphere (1000–500 hPa) in the W regime. Therefore, it is unlikely that advection
by the lower level wind (1000–500 hPa) is critical for the movement of stratiform rain systems. In this regime,
however, wind is easterly above 500 hPa (Figure 6b) and the composite wind speed for 400–250 hPa (about
3–6 m s 1) agrees with the migration speed (about 4 m s 1) of deep stratiform rain (Figure 4b). It is suggested
that the maximum peak, which rainfall systems redevelop over the sea (101.5°E) in the morning (0600 LT) in
Figure 3b, might be caused by local effects (e.g., land-sea breeze). On the other hand, the weak signal of
westward diurnal rainfall migration that dominates stratiform rainfall is shown from 1800 LT to 0600 LT. It might
be suggested that the precipitation system, produced by local circulation over the west coast, is strengthenedby merging with the stratiform rainfall system driven by the ambient wind from land to sea in the mid-upper
troposphere. Hence, we speculate that the wind in the mid-upper troposphere may play a role in the westward
migration of rainfall systems.
In the E regime, the westward propagation speed (about 8 m s 1) of convective precipitation is comparable tothat reported by Mori et al. [2004]. In this regime, the speed of the composite zonal wind is less than 4 m s 1 in thelower (1000–500 hPa) level (Figure 6c), which is much slower than migration speed. It is unlikely that the westward propagation is mostly controlled by the lower tropospheric wind, although we cannot completely exclude
the possibility that a zonally nonhomogeneous local circulation accelerates the westward advection associatedwith ambient ﬂow in the lower level. On the other hand, the speed of about 8 m s 1 corresponds to the zonal
wind in the 400–250 hPa levels (Figure 6c). The rainfall system is composed of not only convective-type rainfall
but also stratiform-type rainfall (Figure 4c). Thus, the advection of stratiform precipitation by the mid-upper
level wind might be partly responsible for the westward movement of the precipitation system.
In the SE regime, the westward propagation speed (approximately 8–12 m s 1) of convective rainfall is thefastest among all regimes. In this case, we found the relationship between the movement of the system
and ambient zonal wind to be similar to those in the E regime. The propagation speed exceeds that of thecomposite zonal wind in the lower (1000–500 hPa) troposphere (less than 6 m s 1) and better agrees withthat in the mid-upper (500–200 hPa) troposphere (Figure 6d). Therefore, we also speculate that mid-upper
level wind may play an essential role in the westward movement of rainfall systems.
Sakurai et al. [2009] investigated the propagation of diurnal cloud systems over Sumatra Island based on acase study by using intensive observation (CPEA-I) data. They discovered diurnal cloud systems with ahorizontal scale of several hundred kilometers propagating eastward and westward and that cloud system
directions are parallel to lower troposphere zonal wind. The relationship between the convective-type diurnal

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Figure 6. Vertical proﬁles of the zonal wind averaged over the rectangular domain of Figure 1 for (a) SW, (b) W, (c) E, and (d) SE regimes. Error bars indicate one
standard deviation at each level.

rainfall propagation directions and ambient wind in the lower troposphere (1000–500 hPa) is consistent with
our ﬁndings.
Note that westward-propagating systems redevelop off the western coast in the W, E, and SE regimes(Figures 3b–3d). Some previous studies suggest that cold pool dynamics are important to the redevelopmentof rainfall systems over the ocean [Sakurai et al., 2009, 2011; Fujita et al., 2010]. Cold pools, however, are associated with intense downdrafts produced in the convective core [Feng et al., 2015]. Although the stratiform rainfall areas dominate the convective areas over the ocean west of Sumatra Island, convective rainfall coexists(Figures 3e–3h), which might be critical to maintain systems over the ocean [Moteki et al., 2008]. On the other
hand, the importance of local effects also has been discussed in some previous studies [Wu et al., 2009; Sakurai
et al., 2011; Fujita et al., 2013]. They suggest that mountainous topography and thermally induced local circulation can play an important role in the organization of heavy rainfall during the night over the west coast of
Sumatra Island. Wu et al. [2009] uses numerical models to show that heavy nighttime rainfall is caused by offshore ﬂow from mountainous topography and convection. These local effects may explain aspects of the redevelopment of rainfall systems off the west coast (e.g., 101°E at 0600 LT in the W regime) as found in this study.
The “feeder-seeder” mechanism [Locatelli et al., 1983] also seems like a plausible explanation for the westward propagation of precipitation systems and their redevelopment over the ocean, which is schematicallyshown in Figure 12 of Mori et al. [2004]. The feeder related to the low-level clouds off the coast of western
mountainous region is enhanced by receiving the seeder associated with the stratiform clouds (e.g., anvil
cloud) in the mid-upper (500–200 hPa) troposphere advected by the ambient wind. As we focus on the different aspects, wave dynamics have been discussed by previous studies, e.g., gravity waves associated withupper level heating and lower level cooling of the stratiform rainfall [Mapes, 2000] or induced by diurnal heating of the daytime mixed layer on land [Mapes et al., 2003]. The propagation speeds of their results are
20 m s 1 and 15 m s 1, respectively. In this study, we have found no evidence implying that the propagationof rainfall systems agrees with the gravity wave speed and do not explore this possibility further.
These discussions suggest that a probable mechanism of the eastward migrating systems characterized byconvective-type rain is the advection by background wind in the lower troposphere and/or self-replicationof convective systems associated with the diurnal cycle. The same mechanism may be at work with respect
to the westward propagation of stratiform-type precipitation. However, the mechanism of westward propagating systems characterized by convective rainfall remains unclear in our study. Although the backgroundwind seems important in the migration direction of diurnal cloud systems with precipitation, an assessmentof other aspects of dynamical and thermodynamic elements needs to be investigated by making use of anumerical model of intense observations with a long enough time span in future work.

YANASE ET AL.

DIURNAL RAINFALL MIGRATION OVER SUMATRA

10

Earth and Space Science

10.1002/2016EA000181

5. Conclusion
This study investigates the climatological relationship between the direction of diurnal rainfall migration andambient wind over southern Sumatra Island by using long-term data (15 years) from TRMM-PR satellite
observation and reanalysis.
In the strong westerly (SW) regime, diurnal convective-type rainfall propagates to the east and its speed isconsistent with lower level tropospheric wind (800–600 hPa). In the weak westerly (W) regimes, convective
diurnal rainfall migrates eastward, although its signal is weaker than in the SW regime. In contrast, diurnal
stratiform-dominated rainfall propagates westward in association with easterly background wind in the
mid-upper troposphere. On the other hand, in the weak easterly (E) and strong easterly (SE) regimes, the convective and stratiform rainfall associated with the diurnal cycle migrate only to the west. The propagation
speed of the diurnal rainfall matches the wind speed in the mid-upper level in each regime. The advectionof stratiform precipitation by the mid-upper level wind might be partly responsible for the westward
movement of precipitation systems.
Based on these results, the direction of diurnal rainfall migration appears to be generally consistent with
ambient zonal wind. Although some features have been reported by previous studies [Mori et al., 2004;
Sakurai et al., 2005; Sakurai et al., 2009], this is the ﬁrst study to report the propagation direction of diurnal
rainfall systems through a whole year over Sumatra Island.
Acknowledgments
The TRMM 2A25 product was provided
by the Japan Aerospace Exploration
Agency. TRMM 3G68 product information can be found at ftp://trmmopen.
gsfc.nasa.gov/pub. TRMM 3B42 productwas distributed by NASA Goddard Earth
Sciences Data and Information Services
Center. ERA Interim data were provided
by ECMWF (https://climatedataguide.
ucar.edu/climate-data/era-interim). This
work is partly supported by the Japan
Society for the Promotion of Science
Grant-in-Aid (KAKENHI) for Grant-in-Aid
for Scientiﬁc Research (B) (26287113)
and for Challenging Exploratory
Research (26610150). The authors thankanonymous reviewers for providing
positive comments for improvement of
this paper.

YANASE ET AL.

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