RESEARCH PUBLICATION NO. 6/2016
An Observation of the Synoptic Features during a
Borneo Squall Line Formation
By Fadila Jasmin Fakaruddin, Mat Kamaruzaman Mat
Adam, Yip Weng Sang, Nursalleh K Chang, Fariza Yunus
and Muhammad Helmi Abdullah
All rights reserved. No part of this publication may be reproduced in
any form, stored in a retrieval system, or transmitted in any form or
by any means electronic, mechanical, photocopying, recording or
otherwise without the prior written permission of the publisher.
Perpustakaan Negara Malaysia
Cataloguing-in-Publication Data
Published and printed by
Jabatan Meteorologi Malaysia
Jalan Sultan
46667 PETALING JAYA
Selangor Darul Ehsan
Malaysia
Contents
No. Subject
Page
Abstract
1.
Introduction
1
2.
Objective
3
3.
Methodology
3.1 Research Area
3.2 Data Distribution and Events Selection
3.3 Data Analysis
4.
5.
Results and Discussions
4.1 Observation of Borneo Squall Line (BSLA), NonBorneo Squall Line (NoBSL) and Borneo Squall
Line with the Presence of Tropical Cyclone
(BSLTC) Events
4.2 Analysis of BSLA, NoBSL and BSLC
4.2.1 Analysis of BSL
4.2.2 Analysis of NoBSL
4.2.3 Analysis of BSLTC
4.3 Analysis of Outgoing Long Wave Radiation
(OLR)
4
4
6
7
9
13
14
17
Conclusions
22
References
23
An Observation of the Synoptic Features during a Borneo Squall Line
Formation
Fadila Jasmin Fakaruddin, Mat Kamaruzaman Mat Adam, Yip Weng Sang, Nursalleh K
Chang, Fariza Yunus and Muhammad Helmi Abdullah
Abstract
Squall line is a significant weather phenomenon generally observed during the Southwest
Monsoon (SWM) season in Malaysia, particularly along the waters off the west coast of
Peninsular Malaysia and coastal areas of Borneo Island facing the South China Sea (SCS). A
squall line is dangerous to aviation, coastal activities and residents along the coastal areas, as
it produce high-intensity rainfall, frequent lightning, strong gusting winds and shear as it
passes. Understanding the synoptic features that trigger the formation of BSL will aid
forecasters in their prognostic reasoning in issuing forecast and early warning of this extreme
weather event. This study focuses on the formation of this synoptic system over the waters
off Borneo, which referred as Borneo Squall Line (BSL) from May to September from 2012
to 2014. The BSL occurrence are observed using MetMalaysia weather radars, while the
synoptic features that lead to the formation of BSL are investigated using the European
Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim and the Japan
Reanalysis-55 (JRA-55) datasets. This study revealed that MJO is the main feature that
manifests the formation of BSL, meanwhile the westward propagation of tropical easterly
waves is found as the feature that manifests these events when there is no major MJO event
during the period. This perturbation interfered the strong southwesterly flow over the SCS to
a weaker flow, which then enhances the topographic effects over the waters off Borneo. At
the same time, the temperature difference between the terrains over Borneo Island is existed
and then promotes interaction between the mountain-valley circulation and land breeze,
which enhance the convection. This study also found that September is the maximum
occurrence of BSL due to high frequency of major MJO events, and the numbers of tropical
cyclones influence the westward propagation of the easterly waves.
1.
Introduction
The Northern Hemisphere summer monsoon or better known as the Southwest
Monsoon (SWM) season in Malaysia is associated with relatively dry period during the
active phase of the monsoon. During this season, the prevailing southwesterly winds
encourage a more stable atmosphere and less intense convective development over the
Malaysian region. However, the significant weather phenomenon during this season known
as the 'squall lines' are often observed from May to September every year along the waters off
the west coast of Peninsular Malaysia and Borneo Island mainly over Sarawak and Sabah
coastlines facing the South China Sea (SCS).
A squall line is a type of mesoscale convective feature characterised by several active
thunderstorms arranged along a line. A squall line is typically formed in an unstable
atmospheric environment where low-level air can rise unaided after being initially lifted to
the point where condensation of water vapour occurs. Heat is released during condensation,
resulting in the rising air becoming lighter than nearby air at the same height and leads to an
increase in the speed of the rising air. The illustration of a squall line formation is depicted in
Figure 1 (Uiuc, 2016).
Figure 1: Formation of a Squall Line
Borneo Island is located in the western part of the Maritime Continent. Its climate is
strongly dominated by the Southeast Asia - Maritime Continent monsoon, where much of the
active convection occurs in response to the interactions of several time-space-scale
circulation systems over and around Borneo (Chang, 2005). The Maritime Continent contains
a complex distribution of several large islands with elevated orography from 100 S to 200 N
and 900 to 1500 E (Ramage, 1968). This region is thought to exert a strong influence on global
1
climate due to the very large input of moisture into the upper troposphere and the large scale
effects of the Indonesian throughflow (Houze, 1977; Neale et al., 2003).
Frank (1978) identified that tropical squall lines occur under conditions of relatively
strong low-level shear. Yang et al. (2001) indicated that the modulation of low-level
convergence by land and sea breezes, topography effects as well as mesoscale features play a
crucial role in the heat and moisture budget over the maritime continent, meanwhile Ichikawa
et al. (2006) and Qian et al. (2012) agreed that the diurnal cycle of tropical regions are
dominated by mountain-valley circulations as well as land and sea breeze interactions, with
the structure and sources of these disturbances depending on wind speed. Murakami (1983)
identified that there exists a distinct contrast in the phase of diurnal cycle between the land
and the adjacent ocean due to the temperature contrast during day and night, where maximum
convective activity over land appears in the evening, while enhancement convective activity
over adjacent ocean appears in the early morning. Agreeing with these previous studies,
Chang (2005) also found that the annual cycle of Borneo Island is dominated by the
interaction between the terrains and reversal of surface monsoonal wind.
Apart from the diurnal circulations, Chang (2005) and Ichikawa et. al. (2006) revealed
that the lower tropospheric westerly or easterly winds over the Southeast Asian region
correspond to the period of active or inactive convection associated with the passage of
intraseasonal atmospheric disturbances related to MJO or gravity waves. The frequency of
deep convection correlating with changes in upper tropospheric circulation has been verified
in many studies. Sherwood (1999) identified that buoyant instability, water vapour above the
trade wind inversion, upper-level divergence and upper-level vorticity are all favourable for
organised convection.
In Borneo Island, a squall line initially forms over the western part of the Borneo
coastline at midnight and then develops into a well-organised convective system as it moves
along the coastline, resulting in heavy precipitation from midnight through the morning
(Ichikawa et al., 2006). A few events of squall line over Borneo that had been reported in the
year 2012 to 2014 are shown in Figure 2 (The Borneo Post, 2016; MySabah Press, 2016 and
The Star Online, 2016).
2
Figure 2: Squall line over Sembulan, Kota Kinabalu, Sabah on 12 June 2012 (left), squall line over Sembulan
that was reported by the Sabah Times newspaper (middle) and squall line which produced a waterspout over
Bintulu, Sarawak on 18 October 2014 (right)
Previous research investigated the diurnal variation and convective activities over the
Maritime Continent, however, limited studies of these convective activities and their
propagation have been performed over Borneo (Ichikawa et al., 2006), especially concerning
specific weather phenomenon like those of squall lines because these convective activities
interact with/ modulate diurnal variation over the Borneo Island. This study focuses on the
formation of this synoptic system over the waters off Borneo during the SWM season,
referred as BSL. Similar process was also observed during the Northeast Monsoon (NEM)
season, especially during the presence of a Borneo vortex over the SCS (Chang, 2005).
However, this synoptic system will be discussed later in further studies. Understanding the
synoptic features that trigger the formation of BSLs will aid forecasters in their prognostic
reasoning in issuing forecast and early warning of this extreme weather event.
2.
Objective
The goals of this study are:
i.
To identify the synoptic features of atmospheric conditions that lead to the formation
of a BSL during the SWM season, and
ii.
To investigate the relationship between the existence of tropical cyclones over the
Western North Pacific (WNP) Ocean and the formation of a BSL.
3
3.
Methodology
3.1
Research Area
This study was carried out within the waters off Sabah and Sarawak in the Borneo
Island, located in the Southeast (SE) Asian region, bounded between 5⁰S to 8⁰N, 105⁰E to
120⁰E (as depicted in the green shaded area in Figure 3 (left panel) (Wikispaces.com, 2016).
The topographic map of Borneo Island is shown in Figure 3 (right panel) (Mapsof.net, 2016).
Figure 3: The study area (left panel) and topographic map of Borneo (right panel)
3.2
Data Distribution and Events Selection
This study used radar observations in Sabah and Sarawak (Kuching, Bintulu, Miri and
Kota Kinabalu) provided by Malaysia Meteorological Department (MetMalaysia) for nine
events, selected during May to September from 2012 to 2014, which include three cases of
squall lines (BSLA), three cases of non-squall line – clear weather (NoBSL), and three other
squall line cases with the presence of tropical cyclone (BSLTC) to describe the synoptic
features of a BSL formation and its interaction with the existence of tropical cyclones over
the WNP Ocean. BSLA, NoBSL, and BSLTC will be used in this study to refer to these
categories.
4
BSLA cases were defined based on the following criteria:
i.
ii.
Multi-cells thunderstorm are organised in a line;
Thunderstorm cells are formed over the Borneo coastline and propagate along the
coastline as inferred by radar observation;
iii.
There should be clear weather over inland areas of Borneo. This is to eliminate any
influence of another synoptic weather system prior to the formation of BSLA; and
iv.
No tropical cyclone formed over the WNP Ocean as observed by JMA Tropical
Cyclone Tracks.
For NoBSL, the following must be considered:
i.
ii.
No sign for development of thunderstorm over the study area; and
No tropical cyclone formed over the WNP Ocean as observed by JMA Tropical
Cyclone Tracks.
For BSLTC, the following criteria were considered:
i.
ii.
Thunderstorms formed as described in (i), (ii) and (iii) of BSLA cases; and
Tropical cyclone formed over the WNP Ocean and moved into the MetMalaysia
monitoring region (Latitude: 00 – 200 N, Longitude: 950 E – 1300 E) as observed by
JMA Tropical Cyclone Tracks.
Based on these conditions, three cases of each category have been selected as follow:
i.
BSLA – 29 May 2012, 12 May 2013 and 5 June 2014;
ii.
NoBSL (clear weather) – 30 June 2012, 23 July 2013 and 25 June 2014; and
iii.
BSLTC – 3 June 2012, 11 August 2013 and 15 September 2014.
5
3.3
Data Analysis
ECMWF ERA-Interim dataset was used in this study as it has the lowest overall bias in
6-hourly data (Decker et al., 2012), meanwhile the JRA-55 dataset was used to describe the
precipitation variations as the performance of this dataset was described as the best in
monitoring precipitation variations in East Asia (S.-J. Sohn et al., 2012). ECMWF ERAInterim dataset covers the period from 1989 to present (D.P. Dee et al. 2011) with resolution
0.125⁰ x 0.125⁰ (14 km by 14 km) grid points and available at 3 and 6-hour intervals,
whereas the JRA-55 dataset covers the period from 1958 to present (Harada et al. 2015) with
resolution 1.25⁰ x 1.25⁰ (138 km by 138 km) grid point and available at 3 and 6-hour
intervals.
This study utilised the 2m air temperature, wind at 10m height from sea level, as well
as Liquid Water Content (LWC) at the 850-hPa pressure level from the ECMWF ERAInterim dataset to identify the low-level atmospheric circulation that indicates the formation
of a BSL (Frank, 1978). LWC was analysed to describe the variation of the rainfall as it was
proven to show a positive relationship with water vapour, whereby convective clouds have a
higher LWC than stratiform clouds (Chakraborty, 2012).
The relative humidity (RH), divergence (D) and vorticity (Vo) at the 500-hPa pressure
level from the same dataset were analysed to observe the upper-level circulation that
encouraged low-level convergence (Sherwood, 1999). High RHs are associated with rising
air or moisture advection (convergence) from upstream. The convergence of horizontal winds
causes air to rise, whereas the divergence of horizontal winds causes subsidence of the air. A
negative value of D at the upper level is often referred as convergence at the lower level. Vo
is caused by troughs and ridges and other embedded waves or height centres (speed and
directional wind changes in relation to a vertical axis). A wind flow through a Vo gradient
will produce regions of PVA (Positive Vorticity Advection) and NVA (Negative Vorticity
Advection). PVA contributes to rising air or upper-level divergence.
Besides the annual cycle, the lower tropospheric disturbances during summer monsoon
also correspond to the period of active or inactive convection over Borneo, which is
associated with the passage of intraseasonal disturbances or large scale circulation systems
called the Madden-Julian Oscillation (MJO) (Chang, 2005; Ichikawa et. al., 2006). The
6
Outgoing Long Wave Radiation was analysed using the JRA-55 to observe the influence of
MJO to a BSL formation, meanwhile, the MJO index used in this study was obtained from
the Australian Bureau of Meteorology (BOM). In addition, the tropical cyclone information
from the JMA Tropical Cyclone Best Track was used to describe the relationship between
occurrences of a tropical cyclone over the WNP Ocean with the BSL formation, while the
tropical cyclone
intensity
was
taken
from the Regional Specialized
Meteorological
Center (RSMC) Tokyo, JMA.
4.
Results and Discussions
4.1
Observation of Borneo Squall Line (BSLA), Non-Borneo Squall Line (NoBSL) and
Borneo Squall Line with the Presence of Tropical Cyclone (BSLTC) Events
Based on radar observations located in Kuching, Bintulu, Miri and Kota Kinabalu, 60
cases of squall lines were observed over the waters off Borneo during May to September
from the year 2012 to 2014, with maximum occurrence during September (18 cases). Statistic
of BSLs occurrence during May to September from the year 2012 to 2014 as observed by the
radars observation is depicted in Table 1.
Table 1: Statistic of BSLs occurrence during May to September from the year 2012 to 2014 as observed
by the radars
Number of BSL Occurrence
Year/
Month
May
Jun
July
August
September
2012
2013
2014
2
8
9
2
13
3
3
7
3
4
1
2
2
1
1
Total BSL
(by month)
6
13
17
6
18
Total Cases:
60
Maximum
occurrence of
BSL during
September
As observed by the radars, average initial times of squall line formation were at
0200LT or 0300LT. Generally, these convective activities initially dominated the west part of
the Borneo (Sarawak) coastline, and then developed into a well-organised convective system
as it moved along the coastline. These convective activities seem to decrease in the morning
(at average time 0800LT to 0900LT) and gradually dissipated by noon. The selected BSLA,
7
NoBSL and BSLTC cases over the waters off Borneo as observed by the radars are depicted
in Figure 4, Figure 5 and Figure 6.
29 May 2012 (0500LT)
12 May 2013 (0200LT)
5 June 2014 (0830LT)
Figure 4: BSLAs over the waters off Borneo as inferred by the radar observations
30 June 2012 (0800LT)
23 July 2013 (0800LT)
25 June 2014 (0800LT)
Figure 5: NoBSL over the waters off Borneo as inferred by the radar observations
3 June 2012 (0200 LT)
11 August 2013 (0800 LT)
15 Sep 2014 (0400 LT)
Figure 6: BSLTCs over the waters off Borneo as inferred by the radar observations
8
4.2
Analysis of BSLA, NoBSL and BSLC
4.2.1
Analysis of BSL
The 10-m wind analyses as depicted in Figure 7(i) (top) revealed the major peak of
winds speed convergence and winds flow confluence over the SCS and Borneo waters,
especially over the central region. These analyses showed that the southwesterly winds were
strong over the SCS (around 10 to 15 knots) and weakening when approaching the coastline
(less than 10 knots). This condition contributed to wind shear mainly over the waters of
central Borneo.
At the same time, the southwesterly winds converge with the southeasterly winds
from the inland areas of Borneo along the coastal areas and enhanced the shear zones. These
shears were very strong in some cases as we can observe the cyclonic vortex over the waters
of Borneo, mainly over Sarawak (Figure 7(i) (bottom)).
(knots)
(knots)
(knots)
(knots)
Figure 7(i): The 10-m wind streamline analyses on 5 June 2014 at 0200LT (top – left) and 0800LT (top – right)
over Borneo represented the synoptic features of BSLA, meanwhile the 10m wind streamline analyses on 29
May 2012 at 0200LT (bottom – left) and 0800LT (bottom – right) showed the strong shears forming a cyclonic
vortex over the waters off the western part of Borneo.
9
This analysis also revealed that a dominant outflow wind which was centred over the
inland areas of Borneo reversed the southwesterly winds to southeasterly winds and moved
further to the coastal areas (Chang, 2005). We assume that the southeasterly winds over the
inland areas of Borneo originated from the mountain valley circulation at the mountain range.
This mountain valley circulation interacted with the land breezes from the coastal areas and
triggered convection along the coastline (Ichikawa et. al., 2006; Qian et. al., 2012; and Yang
et. al., 2001).
This study then analysed the 2-m air temperature as depicted in Figure 7(ii) to
investigate the temperature differences between the mountain range, the valley and the ocean
to describe the topografic effects which contributed to the formation of convective activities
over Borneo waters (Murakami, 1983).
From the analysis in the early morning (0200LT), higher temperatures (29 0 C) were
observed over the waters off Sarawak compared to the mountain range (22 0 C) and the valley
(250 C). The analysis showed that significant temperature differences over Borneo Island
existed. This condition caused an interaction to happen between the mountain-valley
circulation and the land breeze which contributed to the formation of convective activities or
squall lines. Analysis at 0800LT showed that the temperature over the waters off Sarawak
dropped by 10 C, from 290 C to 280 C, while the temperature over the valley remained at 250 C
and the temperature over the mountain range increased by 1 0 C from 220 C to 230 C. As the sun
rises in the morning, the temperature differences between the land and ocean were getting
smaller and insignificant. The disturbances in the synoptic wind flow along the coastline also
decreased and the squall line gradually dissipates (Ichikawa et. al., 2006).
( 0C)
( 0C)
Figure 7(ii): The 2-m air temperature analyses on 5 June 2014 at 0200LT (left) and 0800LT (right) over Borneo
represented the synoptic features of BSLA
10
The Liquid Water Content (LWC) at 850-hPa level over the Borneo (depicted in
Figure 7(iii)) were analysed to investigate the existence of water source as the supporting
features for these convections. The analysis showed the wind shear and convergence gathered
a large amount of moisture over the central region of Borneo, with Stratus or Cumulus clouds
observed over that particular region (based on the LWC values of cloud types as described by
Thompson (2007)). This analysis coincided with the 10-m winds analysis whereby intense
convergence was observed. Based on this analysis, we can conclude that the moisture or
LWC played a crucial role in supporting and sustaining a BSL formation.
(g/m3)
(g/m3)
Figure 7(iii): The 850-hPa level of wind and liquid water content (LWC) analyses on 5 June 2014 at 0200LT
(left) and 0800LT (right) over Borneo represented the synoptic features of BSLA. The LWC are plotted in units
of g/m3 .
Thompson (2007) defined that the clouds with low densities such as cirrus clouds,
contain very little water, thus resulting in relatively low liquid water content, whereas clouds
with high densities, such as cumulonimbus clouds, have much higher liquid water content.
The LWC values of cloud types as describe by Thompson (2007) are given in Table 2.
Table 2: LWC values of cloud types as describe by Thompson (2007)
Cloud Type
Cirrus (Ci)
Fog
Stratus (St)
Cumulus (Cu)
Stratocumulus (Sc)
Cumulonimbus (Cb)
LWC (g/m3 )
0.03
0.05
0.25 – 0.30
0.25 – 0.30
0.45
1.0 – 3.0
11
Cressman (1953) revealed that the 500-mb pressure level is at the mean equivalent
barotropic surface or nearly geostrophic, therefore the synoptic scale vorticity is best
analysed and plotted at this level; and the maxima and minima of absolute vorticity can be
associated with individual troughs, ridges and regions of large lateral shear. Since the
existence of wind convergence and shear were significant at low-level, relative humidity
(RH), divergence (D) and vorticity (Vo) at the 500-hPa level were analysed to further
examine the existence of these convergences and shears.
From the analysis, RH and D did not discriminate much between BSLA and NoBSL
over the Borneo coastline. High RH values and negative values of D were dominant over
SCS and Borneo in all cases, which showed that the air was rising over those particular
regions.
However, a significant feature was identified in the Vo analysis. The 500-hPa level
analysis of Vo as depicted in Figure 7(iv) showed positive values over Borneo, especially
over Sarawak. These conditions contributed to rising air motions at this level (500-hPa) and
coincided with the existence of wind convergence and wind shear at the lower level (10m
wind analysis) over this particular region.
( 0C)
( 0C)
Figure 7(iv): The 500-hPa level wind and vorticity (Vo) analyses on 5 June 2014 at 0200LT (left) and 0800LT
(right) over Borneo represented the synoptic features of BSLA
12
4.2.2
Analysis of NoBSL
NoBSL cases show a significant contrast as compared to BSL cases. The low-level
10m wind analysis of NoBSL as depicted in Figure 8(i) showed that there was no wind speed
convergence over the SCS. The strong southwesterly winds (10 - 20 knots) over the SCS
became dominant and prevented clouds development over the coastal waters off Borneo such
that the topographic effects were neglected in this case, as shown in Figure 8(i). This
analysis coincides with the 850-hPa level wind circulation pattern which showed that there
was a buffer zone near the equator (between 1 0 N and 10 S) over the western part of Borneo
(southern Sarawak) as depicted in Figure 8(ii). This anti-cyclonic buffer strengthened the
southeasterly wind from the waters off Bunguran to strong southwesterly over the SCS. The
LWC analysis at the 850-hPa level also showed no indication of moisture for cloud
development over the waters off Borneo. Meanwhile, the 500-hPa levels of Vo analysis
showed negative values prominent over Borneo, mainly over central Sarawak (Figure 8(iii)).
These conditions contributed to sinking air at this level which indicated suppressed
convection leading to relatively dry conditions over the coastal waters off Borneo.
(knots)
(knots)
Figure 8(i): The 10m wind streamline analyses on 23 July 2013 at 0200LT (left) and 0800LT (right) over
Borneo represented the synoptic features of NoBSL
13
(g/m3)
(g/m3)
Figure 8(ii): The 850-hPa level of wind and LWC analyses on 23 July 2013 at 0200LT (left) and 0800LT
(right) over Borneo represented the synoptic features of NoBSL
((00C)
C)
( 0C)
( 0C)
Figure 8(iii): The 500-hPa level of vorticity (Vo) analyses on 23 July 2013 at 0200LT (left) and 0800LT (right)
over Borneo represented the synoptic features of NoBSL
4.2.3
Analysis of BSLTC
Previous studies by Richie et al. (1999) found that one of the characteristics of
tropical cyclone genesis in the WNP, which originate from the easterly waves, is the
monsoon shear line. The monsoon shear line provides an environment for sustained moist
convection, which becomes an organiser in MCS that normally develop mid-level vortices.
In this study, the radar observations and the tropical cyclone information obtained
from the JMA Tropical Cyclone Best Tracks were compared in order to investigate the
influence of tropical cyclone over the WNP Ocean and the formation of a BSL. The tropical
cyclones, which were considered in the current analysis, formed over the WNP Ocean and
moved into the MetMalaysia monitoring region (Latitude: 0 0 to 200 N and Longitude: 950 E to
1300 E). Table 3 shows the list of tropical cyclones formed over the WNP Ocean that moved
into the MetMalaysia monitoring region and the BSL dates from May to September of 2012
to 2014.
14
From the observations and analyses, it was observed that 28 cases out of 60 BSLs
(approximately 47%) occurred in the presence of tropical cyclone over the MetMalaysia
monitoring region.
Table 3: List of tropical cyclones which formed over the WNP Ocean and moved into the MetMalaysia
monitoring region (Latitude: 00 – 200 N, Longitude: 950 E – 1300 E) and BSL occurrences from May - Sept (2012
– 2014)
No.
Date of BSL Occurrence
Name of
Tropical
Cyclone
(TC)
1.
1, 2, 3, 5 June 2012,
6 June 2012 (morning),
6 June 2012 (evening)
Mawar
1 - 6 June 2012
75
Typhoon
Guchol
Talim
Doksuri
Vicente
Saola
Kai-Tak
Tembin
Sanba
13 - 20 June 2012
17 - 20 June 2012
25 - 30 June 2012
21 - 24 July 2012
28 July - 3 August 2012
13 - 18 August 2012
19 - 30 August 2012
11 - 18 September 2012
100
50
40
80
70
65
80
110
Super Typhoon
Severe Tropical Storm
Tropical Storm
Typhoon
Typhoon
Typhoon
Typhoon
Super Typhoon
Jelawat
20 September –
1 October 2012
110
Super Typhoon
30 August 2013
16 September 2013
Yagi
Leepi
Bebinca
Rumbia
Cimaron
Jebi
Mangkhut
Utor
Trami
Pewa
Kong-Rey
Usagi
8 - 12 June 2013
18 - 21 June 2013
20 - 24 June 2013
27 June - 2 July 2013
16 - 18 July 2013
31 July - 3 August 2013
6 - 8 August 2013
9 - 15 August 2013
18 - 22 August 2013
18 - 25 August 2013
26 - 30 August 2013
16 - 23 September 2013
45
40
40
50
40
50
40
105
60
45
55
110
Tropical Storm
Tropical Storm
Tropical Storm
Severe Tropical Storm
Tropical Storm
Severe Tropical Storm
Tropical Storm
Super Typhoon
Severe Tropical Storm
Tropical Storm
Severe Tropical Storm
Super Typhoon
27, 30 September 2013
Wutip
65
Typhoon
75
Typhoon
40
100
90
70
105
75
45
Tropical Storm
Super Typhoon
Super Typhoon
Typhoon
Super Typhoon
Typhoon
Tropical Storm
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
29 July 2012
30 August 2012
14,15 September 2012
20, 23, 24, 26
September 2012
12 June 2013
16, 17, 18 July 2013
11 August 2013
18 August 2013
24.
Fitow
25.
26.
27.
28.
29.
30.
31.
Hagibis
Neoguri
Rammasun
Matmo
Halong
Kalmaegi
FungWong
10 July 2014
12 July 2014
15 September 2014
Date of TC
Maximum
wind near
the centre
(knots)
Peak Intensity
27 September –
1 October 2013
30 September –
7 October 2013
14 - 17 June 2014
3 - 11 July 2014
12 - 19 July 2014
17 - 25 July 2014
29 July - 11 August 2014
12 - 17 September 2014
17 - 24 September 2014
15
Analyses of 10-m wind for BSLTC (Figure 9(i)) found that during Typhoon Utor
(location at 0800LT: Latitude – 14.50 N, Longitude – 125.90 E, with maximum wind near the
centre – 85 knots), the synoptic features of BSLTC were quite similar to NoBSL. However,
the southwesterly winds over these areas were even stronger (10 to more than 20 knots)
compared to NoBSL (10 – 20 knots) due to the influence of the tropical cyclone. These
strong winds create the winds confluence over the northern part of Borneo waters as the
tropical cyclone
propagates
and
strengthens.
The wind
analysis also
showed
the
southwesterly winds were parallel to the coastline, which drive the wind shear even stronger
along the coastline. The synoptic feature of Typhoon Utor became dominant such that the
topographic effects were negligible in this case.
(knots)
(knots)
Figure 9(i): The 10-m wind streamline analyses on 11 August 2013 at 0200LT (left) and 0800LT (right) over
Borneo represented the synoptic features of BSLTC. Present synoptic system, Typhoon Utor (location at
0800LT: Latitude: 14.50 N, Longitude: 125.90 E, with maximu m wind near the centre: 85 knots)
The LWC at 850-hPa level analyses in Figure 9(ii) shows that the moisture flux
occurred according to the wind confluence. The 500-hPa level analyses of Vo as depicted in
Figure 9(iii) found that positive values were very prominent over SCS and the coastal waters
of Borneo. These conditions contributed to the rising air at this level and coincide with the
existence of winds convergence and shear over those regions at the lower level (10-m wind
and 850-hPa level wind analyses).
This analysis concluded that the formations of BSLTC were influenced by the wind
confluence and moisture flux that derived from the formation of tropical cyclone over the
WNP Ocean. The formation of tropical cyclone over the WNP Ocean will bring more
moisture to the SCS and Borneo region.
16
(g/m3)
(g/m3)
(g/m3)
(g/m3)
Figure 9(ii): The 850-hPa level of wind and LWC analys es on 11 August 2013 at 0200LT (left) and 0800LT
(right) over Borneo represented the synoptic features of BSLTC
( 0 C)
( 0C)
Figure 9(iii): The 500-hPa level of vorticity (Vo) analyses on 11 August 2013 at 0200LT (left) and 0800LT
(right) over Borneo represented the synoptic features of BSLTC
4.3
Analysis of Outgoing Long Wave Radiation (OLR)
Based on the wind analysis in Section 4.2.1, BSLA occurred in terms of weaker
southwesterly winds over SCS and Borneo region. Agree with the assumption based on
Chang (2005), which revealed that the lower tropospheric westerly or easterly winds over the
Southeast Asian region correspond to the period of active or inactive convection associated
with the passage of intraseasonal atmospheric disturbances related to MJO or gravity waves,
this study analysed the OLR data obtained from the JRA-55 and MJO index obtained from
the Bureau of Meteorology (BOM), Australia to describe the correlation between MJO and
the BSL formation. Liebmann et al. (1994) revealed that even though tropical cyclones
preferentially occur during the active phase of MJO, the formation of tropical cyclogeneses
over the WNP Ocean is not influenced by MJO.
17
The time series of OLR anomalies (Figure 10) indicated the presence of MJO, where
the eastward propagating waves (indicated by purple arrows) were very prominent over the
Borneo region (1050 E to 1200 E) during these periods. Positive OLR anomalies (red shaded)
indicated suppressed convection and negative OLR anomalies (green shaded) indicated
enhanced convection.
This analysis also showed that during these periods, mainly during July to September,
MJO was accompanied by the influx of westward propagating convective systems associated
with the tropical easterly waves (indicated by blue arrows) into the Borneo region in some
BSL cases. This condition contributed to the enhancement of a BSL. This analysis also
showed that the tropical cyclone occurrences over the WNP Ocean during the selected period
were correlated with the westward propagation of easterly waves from the WNP Ocean
(Richie et. al., 1999).
Based on Figure 10, there was no indication of the eastward propagating waves
associated with the presence of MJO in most of the NoBSL cases. This analysis showed that
Positive OLR anomalies (red shaded) very prominent in NoBSL cases, which indicated
suppressed convection during those particular events.
18
-2
0
0
Figure 10: Time series of OLR anomalies (Wm ) averaged between 8 N - 8 S during May to September of the
0
0
year 2012, 2013 and 2014 (Borneo Island is located between 105 E to 120 E)
19
The OLR analysis was consistent with the MJO indexes obtained from the BOM, which
is shown in Figure 11.
Major MJO events dominated during
Figure 11: Comparison between the MJO Index obtained from ……....................................
September - Coincidence with radar
Bureau of Meteorology, Australia (BOM) and the BSL dates as …………………………..
observations of maximu m BSLs
observed by MetMalaysia radars observation. Major MJO events………………………………
occurrence
are indicated by amplitude greater than one (exceeding the red dashed line).
The phases and amplitudes of MJO were defined according to the MJO index based on
Wheeler et al. (2004), (WH04). Based on WH04, enhanced convection phase over the
Malaysian region is define happens in Phase 4 and 5 (maritime continent region). Meanwhile,
major MJO events were defined whereby the amplitude of the WH04 index was greater than
one (Hidayat et al., 2010). According to Tangang (2012), Malaysia is geographically located
between two large oceans, which are the Pacific Ocean and the Indian Ocean, hence its
climate is also strongly influenced by the natural climate variability associated with these
oceans. Based on WH04 and Tangang (2012), MJO Phase 3, 4, 5, and 6 was considered in
this analysis.
20
Based on Figure 11, the minimum days of MJO occurrences were observed in
August, with two-day occurrences, while maximum days were observed in September with
up to 30-day occurrences. This analysis also revealed that major MJO events dominated in
September, which corresponds with the maximum occurrence of BSLs, as observed by the
radars observation (the BSL dates as observed by the radars observation is indicated by the
light blue dashed lines).
By comparing the radar observations and the MJO indexes, this study discovered the
high possibility of BSL occurrences during MJO. Approximately 70% (40 cases) from the
total of 60 squall cases were observed during MJO. From these 40, 14 cases were occurred
during MJO events alone, while balance 28 cases occurred during MJO together with the
influx of the westward propagating of easterly waves or during tropical cyclone. Four cases
occurred (approximately 7%) during tropical cyclone or influence by the westward
propagating of easterly waves (no MJO event), 14 cases (approximately 23%) occurred due
to other factors. Based on these observations and analyses, we can conclude that the
atmospheric conditions over the Borneo region during BSL were strongly affected by the
eastward propagation of the MJO. Meanwhile, the influxes of the westward propagating
convective system associated with the tropical easterly waves into the Borneo region during
the same period brings more moisture to SCS and Borneo regions and inclined the formation
of a BSL (Richie et al., 1999). The westward propagation of tropical easterly waves was
considered as the feature that manifests the formation of the BSL when no major MJO event
occurred during that period. These observations and analyses also agree with Liebmann et al.
(1994), which revealed that MJO does not influence the occurrences of tropical cyclones over
the WNP Ocean, however, the existence of tropical cyclones over WNP Ocean influence the
formation of a BSL.
21
5.
Conclusions
The SWM season in Malaysia is associated with the relatively dry period during the
active phase of the monsoon. However, the BSL formation along the waters off Borneo
Island mainly over the Sarawak and Sabah coastlines interacts with or modulated by the
diurnal variation over the Borneo Island. The effects of the low-level wind and the large-scale
circulation field upon the formation of a BSL were observed and discussed in this study. This
study reveals that:
(i)
MJO is the main feature that manifests the formation of a BSL;
(ii)
Influxes of the westward propagation of the easterly waves at the same period bring
more moisture to SCS and Borneo regions, encouraging the formation of a BSL.
The westward propagation of tropical easterly waves is considered as the feature
that manifests the formation of a BSL when there is no major MJO event during the
period;
(iii)
This
perturbation
(MJO
or tropical easterly waves) interfered
the strong
southwesterly flow over the SCS to a weaker flow which further enhances the
topographic effects over the waters off Borneo to become predominant;
(iv)
The temperature difference between the terrains over Borneo Island, as evidenced in
BSL cases, leads to the conclusion that there exist interactions between mountain
valley circulations and land breezes. These interactions encourage the convergences
over the central Borneo and contribute to the formation of the BSL. On the other
hand, we assume that this might also be the reason of the BSL propagation
southwestward along the coastlines which seldom penetrate into the inland areas of
Borneo;
(v)
The maximum occurrence of BSL is in September due to high frequency of major
MJO events in our region which was explained in the previous section; and
(vi)
The number of tropical cyclones over the WNP Ocean influences the westward
propagation of the easterly waves.
22
Some significant features observed in this study are important as indicators of
favourable synoptic conditions for the formation of BSL. Further study with more cases can
be conducted in the future to develop a forecasting guide tool to aid forecasters in monitoring
and forecasting the formation and evolution of BSL during the SWM season. With proper
usage of the understanding of the main features discussed during this study, a timely and
accurate early warning can be issued to reduce the risk of deaths, property damage and
economic loss caused by BSLs.
REFERENCES
Chang, C.-P., Z. Wang, J. McBride, and C.-H. Liu, 2005: Annual Cycle of Southeast Asia –
Maritime Continent Rainfall and the Asymmetric Monsoon Transition, J. Climate,18, 287–
301.
Cressman, G.P., 1953: An Application of Absolute Vorticity Charts, J. Meteo., 10, 17 – 24.
Decker, M. et al., 2012: Evaluation of the Reanalysis Products from GSFC, NCEP, and ECMWF
Using Flux Tower Observations, J. Climate, 25, 1916 – 1944.
D. P. Dee, S. M. Uppala, A. J. Simmons, P. Berrisford, P. Poli, S. Kobayashi, U. Andrae, M. A.
Balmaseda, G. Balsamo, P. Bauer, P. Bechtold, A. C. M. Beljaars, L. van de Berg, J. Bidlot,
N. Bormann, C. Delsol, R. Dragani, M. Fuentes, A. J. Geer, L. Haimberger, S. B. Healy, H.
Hersbach, E. V. Hólm, L. Isaksen, P. Kållberg, M. Köhler, M. Matricardi, A. P. McNally, B.
M. Monge-Sanz, J.-J. Morcrette, B.-K. Park, C. Peubey, P. de Rosnay, C. Tavolato, J.-N.
Thépaut, F. Vitart, 2011: The ERA-Interim reanalysis: configuration and performance of the
data assimilation system, Q. J. R. Meteorol. Soc., 137, 553 – 597.
Frank, W. M., 1978: The Life Cycles of GATE Convective Systems, J. Atmos. Sci., 35, 1256 – 1264.
Harada, Y., Kamahor, H., Kobayashi, C., Endo, H., Ota, Y., Onoda, H., Onogi, K., Miyaoka,
K., and Takashi, K., 2015: The JRA-55 Reanalysis: Representation of Atmospheric
Circulation and Climate Variability, Jou. of the Met. Soc. of Jap., Ser.II, 94,3, 269–
302.
Houze, R. A. Jr., 1977: Structure and Dynamics of a Tropical Squall-Line System, Mon. Wea. Rev.,
105, 1540 – 1567.
Ichikawa, H., and T. Yasunari, 2006: Time Space Characteristics of Diurnal Rainfall over Borneo and
Surrounding Oceans as Observed by TRMM-PR, J. Climate, 19, 1238 – 1260.
Liebmann, B., Hendon, H. H. and Glick, J. D., 1994: The Relationship between Tropical Cyclones of
the Western Pacific and Indian Oceans and the Madden-Julian Oscillation, Jou. of the Met.
Soc. of Jap., 72, 3, 401 – 412.
23
Murakami, M., 1983: Analysis of the Deep Convective Activity over the Western Pacific and
Southeast Asia, Part I: Diurnal Variation, J. Meteor. Soc. Japan, 61, 60 – 75.
Neale, R., and J. Slingo, 2003: The Maritime Continent and Its Role in the Global Climate: A GCM
Study, J. Climate, 16, 834 – 848.
Qian, T., C. E. Epifanio, and F. Zhang, 2012: Topographic Effects on the Tropical Land and Sea
Breeze, J. Atmos. Sci., 69, 130 – 149.
Ramage, C.S., 1968: Role of a Tropical "Maritime Continent" in the Atmospheric Circulation, Mon.
Wea. Rev., 132, 1201 – 1219.
R. Hidayat, and S. Kizu, 2010: Influence of the Madden–Julian Oscillation on Indonesian Rainfall
Variability in Austral Summer, Int. J. Climatol, 30, 1816 – 1825.
Richie, E. A. and Holand, G. J., 1999: Large–Scale Patterns Associated with Tropical Cyclogenesis in
the Western Pacific, Mon. Wea. Rev., 127, 2027 – 2043.
S. Chakraborty, et al., 2012: A Comparative Study of Cloud Liquid Water Content from
Radiosonde Data at a Tropical Location, Int. J. Geosci, 3, 44 – 49.
Sohn, S.J., Tam, C.Y., Ashok, K., and Ahn, J.B., 2012: Quantifying the Reliability of
Precipiation Datasets for monitoring large-scale East Asian Precipitation Variations,
Int. J. Climatol, 32, 1520 – 1526.
Sherwood, S. C., 1999: On Moistening of the Tropical Troposphere by Cirrus Clouds, J. Geophys.
Res., 104, 11,949 – 11,960.
Tangang F. T., Juneng, L., Salimun, E., Sei, K. M., Le, J. and Muhamad, H., 2012: Climate Change
and Variability over Malaysia: Gaps in Science and Research Information, Sains Malaysiana,
41, 11, 1355 – 1366.
Thompson, Anne, 2007: Simulating the Adiabatic Ascent of Atmospheric Air Parcels using the Cloud
Chamber, Department of Meteorology, Penn State, accessed September 15, 2016,
https://en.wikipedia.org/wiki/Liquid_water_content
Wheeler, and Hendon, 2004: An All-Season Real-Time Multivariate MJO Index: Development of an
Index for Monitoring and Prediction, Mon. Wea. Rev., 132, 1917 – 1932.
Yang, G. Y, and J. Slingo, 2001: The Diurnal Cycle in the Tropics, Mon. Wea. Rev., 129, 784 – 801.
Uiuc,
University
of
Illinois,
“Squall
Line”,
accessed
September
15,
2016,
http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/svr/modl/line/squall.rxml
Borneo Post, “Calm weather after heavy rains likely”, accessed September 1, 2016,
http://www.theborneopost.com/2012/06/15/calm-weather-after-heavy-rains-likely/
MySabah
Press,
“Dragon
in
the
Cloud”,
accessed
September
1,
2016,
http://www.mysabah.com/wordpress/dragon-in-the-cloud/
The
Star
Online,
“Waterspout
sighted
off
Bintulu”,
accessed
September
1,
2016,
http://www.thestar.com.my/news/nation/2014/10/18/waterspout-in-bintulu/
24
Mapof.net,
“Borneo
Topography
-
Malaysia
maps”,
accessed
September
1,
2016,
http://mapsof.net/malaysia/borneo-topography
Wikispaces.com, “LOCATION, GEOGRAPHY AND POPULATION”, accessed September 1, 2016,
https://dayakinfo.wikispaces.com/Geography+and+Population
25