The observation of pre-monsoon meso-scale cloudsystems
developed over Bangladesh∗

Toru Terao(1) , Md. Nazrul Islam(2) , Fumie Murata(3) , Taiichi Hayashi(4)(1: Osaka Gakuin University, 2: Bangladesh University of Engineering and Technology, 3:
Research Institute for Humanity and Nature and 4: Kyoto University)∗ Toru Terao, Faculty of Informatics, Osaka Gakuin University, Japan, 564-8511, Kishibe-Minami,
Suita, Osaka, Japan. e-mail: terao@s.osaka-gu.ac.jp

Abstract
Using observations with relatively high temporal and spatial data resolutions such as AWS installed by ourresearch group and radar of Bangladesh Meteorological Department (BMD), the meso-scale structure of premonsoon and monsoon cloud systems were investigated. In 2004, severe tornado attacked Mymensingh districton 14 April. On 23 May, strong meso-scale system developed over the country, causing two launches capsized
by strong wind on the Meghna river somewhat downstream of Dhaka near Chandpur. The meso-scale systemdeveloped on 14 April had elliptical shape with size of several tens of kilometers. That developed on 23 May
had rather larger size. It was composed of two parts; one was a strong convective rainband with size of several
hundred kilometers and the other was trailing stratiform clouds. The data of AWS in Dhaka elucidated somecharacteristics of this convective system. The pressure increased around the convective rainband. Suddenly
the temperature falled and the wind direction changed quickly to northwesterly.
Keyword: Bangladesh, pre-monsoon, tornado, meso-system, AWS.

1. Introduction
Many natural disasters due to various types of meteorological disturbances such as heavy rainfall in the monsoon
season and the tropical cyclones in the pre- and postmonsoon seasons attack Bangladesh. Still more, in thepre-monsoon season (April to early June), damages due
to developed tornadoes and strong gust winds frequently
emerge over this country. In 2004 also, in the evening on
14 April, tornados attacked villages of Mymensingh andNetrokona districts, causing more than 70 deaths. In theearly morning on 23 May, the strong gust wind due to a developed meso-scale cloud system passed the Meghna river
basin, making two vessels sink. Local people are afraidof this type of pre-monsoon disturbances and call them
“Kalbaishakhi” or “Nor’wester”. Thus, the features ofsuch meso-scale cloud systems should be addressed more.
Needless to say, the cloud system developing in monsoon
season brings severe ﬂood disasters to this country. The
mechanisms that are responsible for such cloud systems
should be clariﬁed more.
However, in this region, there was no observation
with high spatial and temporal resolutions enough toresolve the meso-scale cloud systems. Although BMD(Bangladesh Meteorological Department) is maintaining
more than 30 synoptic surface observation stations (Fig.
1, ×), the mean distance of these stations are several tensof kilometers. While the observations are conducted 24
hours for almost all stations, its frequency is only 3 hourly,
because of the lack of the automatic instruments. Therefore, the meso-scale structures of such systems have notbeen investigated.
To enable the analysis of the meso-scale systems developing over this country, in spring 2004, we installed anautomatic weather statioon (AWS) in Dhaka city, the capital of Bangladesh in the central part of this country (Fig.
1, rectangle). The data of this AWS system are accessible through the internet in quasi-real time by courtesy ofFig. 1: BMD (Bangladesh Meteorological Department) observation sites for rainfall are indicated by×. Three points, Dhaka, Mymensingh and Chandpur,are highlighted by big rectangles. The coast lines andborder lines are shown. Horizontal and vertical axes
indicate the longitude and latitude, respectively.
ICDDR,B (Centre for Health and Population Research,Bangladesh), at which we settled the AWS. Furthermore,in 2000, the Dhaka radar of BMD was replaced and nowwe can get digital radar echo data.
Using these observations, in the present study, we willshow the meso-scale structures of pre-monsoon and monsoon cloud systems that evolved over this country in thisyear. We have conducted some statistical analysis using
Dhaka radar data (Terao et al, 2001, 2002, Islam et al,2001, 2004). However, the case study has been conducted
only for a few limited cases. This study is the ﬁrst researchof the meso-scale structure of disturbances developed overBangladesh using our AWS observations.

(a)

(b)

Fig. 2: The Dhaka radar images on 14 April 2004. The ovservation time (local time) is shown on the topof ﬁgures. The coast lines and border lines are shown. Horizontal and vertical axes indicate the longitude
and latitude, respectively.

2. Meso-scale Structures of Cloud Systems
2.1. Tornado on 14 April
In the evening on 14 April 2004, strong meso-scale cloudsystems with tornadoes attacked villages of Mymensingh(Fig. 1, rectangle) and Netrokona districts, which are in
the northern part of the country. Following newspapers,
these tornadoes occured in this area for 20 minutes from
19:45 LST. In Bangladesh, tornadoes commonly develop
in the pre-monsoon season (Peterson and Mehta, 1981;Ono, 1997). The case on 14 April is one of the archetypal
examples of such type of disturbances. The tornado occured far to the north of the AWS installed in Dhaka city.
So, we could not ﬁnd any signature of the system thatcaused the tornado in the AWS records. However, theDhaka radar system caught the echoes associated withthe system (Fig. 2).
In the radar image, many cloud systems moving eastsoutheastward had elliptical shapes with their major axes
being parallel to the direction of the movement of systems.
The strongest part of the disturbance seems to be in thecenter of the system. The size of echoes is 10 to 30 km.
Many meso-scale disturbances developed over the northern slope of the Meghalaya mountains lying about 30 km
to the north of 25◦ N from 90 to 93◦ E in the Indian teritory. They crossed the ridge and ﬂew into northeastern
Bangladesh.
The system at somewhat to the southwest of 91◦ E 25◦ N
at 19:40 LST (Fig. 2a) induced the tornadoes. Althoughthe “hook echo” is not clearly seen, the elliptic shape does
not controvert the possibility that the system has the supercell structure. This system is the southwesternmost
echo of the disturbances. This may imply that the system develops near the interface of two air masses withsomewhat diﬀerent characteristics.

2.2. Disturbance on 23 May
In the early morning (about 3:30 LST) on 23 May 2004,
some launches (vessels) capsized during a storm on theMeghna river downstream Dhaka near Chandpur (Fig.
1, rectangle). The meso-scale system that causes this disaster was captured by both the AWS system installed at
Dhaka city and the Dhaka radar system.
The radar image of this disturbance (Fig. 3) indicatesthat the system consists of strong linear shaped convective cloud band and trailing stratiform cloud area withrelatively weak echo. The rainband oriented northeastsouthwest with rather large size, and moved southeastward. It passed Dhaka at about 2:30 LST.
Figure 4 shows the result of AWS observations.
This elucidates the meso-scale structure of this rainband.
Strong rainfall with the rain rate of 3.5 mm/min is observed at 2:33 LST. After that, rain continues until 6 LSTwith relatively low rain rate. This result is consistent withthe radar image. The record of pressure (Fig. 4d) indicates that this system accompanied a meso-high pressure
system. Temperature falled and humidity rose rapidly,
and wind direction changed to northwesterly with strong
gust. These features are similar to the meso-scale rainbands described in Houze (1993).

2.3. Rainfall on 13 September
During monsoon season of 2004, severe ﬂood occured
over Bangladesh with two peaks in July and September.
The former one is mainly due to the rainfall in the upper
catchment of the Ganges, Brahmaputra (Padma, Jamnain Bangladesh) and Meghna rivers. However, the latter

(a)(b)Fig. 3: Same as Fig. 2 except for 23 May 2004.

(a)rain(d)pressure

40
30
20
100

1007
1006
1005
1004
1003
1002
1001
1000
999
998
997

precipitation

03

6

9

1215(b)temperature

36
34
32
30
28
26
24
22
20
18

18

21

24

pressure

0369121520
15

temperature

18(e)wind speed

2124

wind speed

10500369121518(c)relative humidity

10090807060502124369121518213691224LOCAL TIME15(f)wind direction

36031527022518013590450

relative humidity

00182124

wind direction

03691215182124

LOCAL TIMEFig. 4: The data of AWS installed at Dhaka city for 23 May 2004. (a)Rainfall (0.1mm/min),(b)temperature (◦ C), (c)relative humidity (%), (d)pressure (hPa), (e)wind speed (ms−1 ), and (f)winddirection (degree). The horizontal axes are local time.
one occured at the same time as a heavy rainfall overBangladesh.
The AWS shows the signature of the rainfall at Dhaka
clearly (Fig. 5). The rainfall was so strong that the daily
rainfall went up to 321.5 mm. Furthermore, the mostsigniﬁcant feature of the rainfall is the nearly constantrainfall intensity during the day. The rain rate is moderately strong at about 15 mm/hour. However, the rain
continues this intensity for more than one day.

3. Summary
In Bangladesh, remarkable meso-scale cloud systems
develop in the pre-monsoon season as well as in the monsoon season. They cause severe disasters due to tornado,
strong gust and rain. However, there was no observa-tions that resolved meso-scale structures of disturbances.
Therefore, we installed an automatic weather station at
Dhaka city. Using this, in the present study, we analyzed meso-scale cloud systems developed over this country in 2004. Besides, we utilized the Dhaka radar system
of Bangladesh Meteorological Department (BMD), which
was replaced to digital system in 2000.
In 2004, severe tornado attacked the Mymensingh andNetrokona districts on 14 April. This system was one ofcloud systems developing with elliptical shapes with size ofseveral tens of kilometers, and moving southeastward. We
could not analyze the meso-scale structure of this system
extensively because this system did not pass the Dhaka
AWS system. On 23 May, another strong meso-scale system developed over the country causing two launches capsized by strong wind on the Meghna river somewhat down-

30

RAIN(hourly)

25RAIN(mm/hr)

2015
1050-51111.51212.51313.51414.515

DAY(Sep/04)

Fig. 5: Hourly precipitations calculated from theDhaka AWS data during four days from 11 to 14
September 2004. The vertical axis shows rainfall intensity (mm/hour). The horizontal axis is the day ofSeptember 2004.

stream of Dhaka near Chandpur. This system had ratherlarger size. It is composed of two parts; one was a strong
convective rainband with size of several hundred kilometers and the other was stratiform system. The data ofAWS in Dhaka elucidated some characteristics of this convective system. The pressure increased around the convective rainband. Suddenly the temperature falled andthe wind direction changed quickly to northwesterly.
While these cloud systems are pre-monsoon disturbances, the structure is diﬀerent from each other. The
meso-scale system on 14 April seemed to develop along
the interface of two diﬀerent air masses. Although thehook echo structure could not so clearly conﬁrmed, theshape of the echo was similar to that of supercell. On
the other hand, that developed on 23 May had a typicalmeso-scale structure with convective rainband and trailing stratiform clouds. While both cloud systems in thepre-monsoon season accompany instantaneous very strong
rainfall, the rainfall in the monsoon season seems to havecontinuous moderately strong rainfall.

Aknowledgments
The authors would like oﬀer special thanks to Dr. Wagatsuma at ICDDR,B for the cooperation with our research and the AWS operations. They express their
gratitude to Mr. Md. Akram Hossain, the director ofBangladesh Meteorological Department, for his continuous support for our research. The GFD-DENNOU library
was used to draw ﬁgures. This rsearch is supported byJSPS Grant-in-Aid for Scientiﬁc Research and Disaster
Prevention Research Insitute, Kyoto University.

References
Houze, R. A. Jr., Cloud Dynamics. Academic Press,
573pp, 1993.
Islam, M. N., T. Hayashi, H. Uyeda, T. Terao and K.
Kikuchi, Diurnal variations of cloud activity in
Bangladesh and north of the Bay of Bengal in
2000. Remote Sensing of Environment, 93, 378-388, 2004.
Islam, M. N., T. Terao, T. Hayashi, J. Matsumoto andT. Oka, Estimation of monsoon rain and the useof BMD radar data. Proceedings of the Third International Symposium on Asian Monsoon System,
10-12 Dec. 2001, Okinawa, Japan, 298-303, 2001.
Ono, Y., Climatology of tornadoes in Bangladesh. J.
Met., 22, 325-340, 1997.
Peterson, R. E. and Mehta, K. C., Climatology of tornadoes of India and Bangladesh. Arch. Meteor. Geophysik, Bioklim., 29B, 345-356, 1981.
Terao, T., T. Hayashi, M. N. Islam, and J. Matsumoto,
Diurnal variation of the atmospheric vertical structure over Bangladesh region ovserved by rawinsonde intensive observation. Proceedings of theThird International Symposium on Asian Monsoon
System, 10-12 Dec. 2001, Okinawa, Japan, 174179, 2001.
Terao, T., T. Hayashi, M. N. Islam, J. Matsumoto andT. Oka, Observational study on the diurnal variations of convective activity and circulations overBangladesh in the summer monsoon season. Annual Rept. DPRI Kyoto Univ., 45-B, 223-244(inJapanese), 2002.