Numerical experiments on the late night-early morning maximum ofrainfall in the northeastern Bangladesh
Aya Kataoka, Takehiko Satomura( Division of Earth and Planetary Science, Graduate School of Science, Kyoto University, Kyoto, Japan )Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan.

email: kataoka@kugi.kyoto-u.ac.jp

ABSTRACT
Numerical experiments over the Bengal region were conducted by using MM5. The experiments were performed
during 14th - 21st June 1995 included the active period when the rainfall amount was above normal over Bangladesh.
We focused on the northeastern Bangladesh where the late night-early morning maximum of rainfall was observed byusing the data from rain gauges and radar in Dhaka.
At first, the diurnal variation of Ic (convective index) and 3 hourly precipitation of the model results wereinvestigated to check the reliability of the model results. Ic had a peak around 06 LT - 15 LT which was about 3 hours
later than that of precipitation. According to previous paper, this time lag between Ic and precipitation indicated that themodel results agreed with Ic. The characteristics of the simulated precipitation and equivalent potential temperature (θe)
from late night to early morning well agreed with those of squall line. Compared with the results without northernmountains, it’s considered that the low θe area in the region framed in by northern and eastern mountains affects thelifecycle and movement of the precipitation system.
Keywords: Bangladesh, late night-early morning rainfall, diurnal variation, topography, squall line

1.

Introduction

2.

Model

Bangladesh is one of the heaviest rainfall regions inFor numerical experiments, we used the PUS/NCARSouth Asia. Climatic rainfall over Bangladesh is about

mesoscale model (MM5). NCEP-Reanalysis2 data and6000 mm during summer monsoon in some regions.

NCEP weekly SST data were used for the input data. TheEspecially northeastern Bangladesh has much rainfall,model domains are indicated Fig. 1. Domain 1 was thewhere the late night-early morning maximum rainfall

mother domain, domain 2 was two-way nested with

was observed by rain gauges and the radar in Dhaka.

domain 1 and domain 3 was one-way nested with domain(Ohsawa et al., 2000, 2001; Terao et al., 2002; Islam et

2. The analyzed period was 16th - 21st June 1995 whichal., 2004). They suggested that the late night-early

included the active period when the rainfall amount was

morning maximum is associated with the Shillong

above normal over Bangladesh.

Plateau, east-west elongated mountainous range whose
highest peak is about 2000 m located right in the north ofthe late night-early morning maximum rainfall region.
Though the Shillong Plateau was suggested to work as abarrier to the southwesterly monsoon wind, themechanism of the late night-early morning maximumrainfall is still not fully understood. The purpose of ourstudy is to understand the mechanism of the latenight-early morning maximum by using a numericalmodel.
Fig. 1: Model domains

3.

Results
Results of domain 3 are shown because we focusedon the northeastern Bangladesh.

3.1 Diurnal variations of convective index and 3
hourly rainfall
In order to check the model result, the activities ofdeep convective clouds derived from hourly GMS IR1
Tbb data was used. We used the following index Ic

Fig. 3: Same as Fig. 2, except mean diurnal variation of 3

defined byhourly rainfall of the model results averaged above 3Ic=230 - Tbb= 0: Tbb ≤ 230 K

mm/3 h.

: Tbb > 230 K.

Mean diurnal variation of 3 hour averaged Ic wascompared with 3 hourly rainfall of model results

3.2 Hourly rainfall and equivalent potential
temperature of the model results
Fig. 4 shows hourly rainfall from 00 LT to 07 LT

Fig. 2 shows the mean diurnal variation of 3 hour

17th June 1995. Around midnight, precipitation appeared

averaged Ic. Looking at the southern foot of theright south foot of Shillong Plateau. The intensity of themountain, Ic had a peak around 06 LT – 15 LT. Fig. 3

precipitation became stronger and the precipitation areashows the mean diurnal variation of 3 hourly rainfall of

became to a line form until around 03 LT. The

the model results averaged above 3 mm/3 h. The peak ofprecipitation area broadened southward and the south tip

the rainfall in the southern foot of the mountain wasof the area formed an arc over early morning. Thearound 03 LT – 12 LT. The time lag between Ic and 3

intensity of the precipitation weakened after 06 LT.

hourly rainfall was about 3 hours. Because Ic has a peaka few hours later than that of rain (Nitta and Sekine1994)., the model result described above agreed with thatof Ic in the southern foot of the mountain. The model
well simulated the precipitation from late night to early

00 LT 17th

04 LT

01 LT

05 LT

02 LT

06 LT

morning found in the previous studies.

Fig. 2: Mean diurnal variation of 3 hour averaged Ic(time-latitude section). The values were averaged 16th 21st June 1995 and from 90.5 E to 93.5 E. The left figure

03 LTA

07 LT

is the terrain averaged from 90.5 E to 93.5 E.
Fig. 4: Hourly rainfall of the model results from 00 LT to
07 LT 17th June 1995 (domain 3).

Fig. 5 shows the vertical cross section equivalentpotential temperature (θe) cutting along line A in Fig. 4.

00 LT 17th

03 LT

06 LT

09LT

In the vicinity of surface, a low θe area was found in thesouthern side of mountain. Low level high θe air ran intoand advected upward on the low θe air. Strong upward
flow existed in the boundary of these two air masses. In
the north of the strong upward flow, downward flow withlow θe air was simulated. It seems that the low θe area
descended from middle level (around 7 km) to thesurface. This downward flow divided into northward andsouthward at the surface. The boundary of the low andhigh θe moved southward about the speed of 10 m s-1
from 03 LT to 04 LT (not shown). These characteristics

Fig. 6: Distribution of θe at the altitude of 100 m fromwere consistent well with those of squall line.

00 LT to 09 LT 17th June 1995

3.3 Sensitivity to the Shillong Plateau
To compare with the results of realistic terrain(control run), we ran the model using terrain taken off
the Sillong Plateau. Fig. 7 shows hourly rainfall of themodel result without Shillong Pleateau every 3rd hour.
Precipitation

existedaroundthenortheastern

Bangladesh-Indian border and moved southward similar
to model results in the control run. The precipitation area
was almost the same as that of control run at 06LT. The
areas were, however, located in the north of the results ofFig. 5: Vertical cross section of θe averaged 20 km eachcontrol run at 00 LT and 03 LT. This result indicated thatside of line A in Fig. 4 at 03 LT 17th June 1995 (colorSillong Plateau plays an important role in the lifecyclecontour). Vector shows wind in the direction of paralleland movement of the precipitation. The line shaped

to the cross section. The solid and dashed line shows 1g

precipitation area was not clear in this no plateau case.

-1

Fig. 8 shows the horizontal cross section of the θe at-1

the altitude of 100 m. At 03 LT, the low θe was located

kg of precipitation hydrometeor mixing ratio and 0.1 g
kg of cloud mixing ratio, respectively.

almost the same place as the precipitation area, that wasFig. 6 shows horizontal cross section of θe at thenorth of the control run. From 06 LT to 09 LT the low θealtitude of 100 m. At 00 LT 17th June, before thearea stayed concave space as in Fig. 6 and also low

precipitation area formed in a line, a low θe area existed

virtual potential temperature area existed there (notin the south of the mountain. From 03 LT to 06 LT whenshown). These characteristics indicate that the concave

the intensity of precipitation became strong, lower θespace has somewhat important role on the lifecycle andarea appeared and broadened in the region framed in by

movement of the precipitation.

northern and eastern mountains. Since a low virtual
potential temperature area was well consistent with thelow θe area, the cold air existed in this concave spacewhen precipitation became heavier (not shown). At 09
LT when precipitation weakened, the contrast of high andlow θe area became weak.

of the precipitation hardly recognized.
It is necessary to make a more frequent observation than
one hour interval.
The model results successfully simulated the late
00 LT 17night-early morning maximum rainfall. The simulated

thresults agreed with Ic.
It is suggested from this study that1) Squall line system produced the precipitation fromlate night to early morning in the northeastern
Bangladesh (Fig. 4, 5).
03 LT2) Shillong Plateau and the concave space framed in
by northern and eastern mountain possibly affect thelifecycle and movement of the precipitation (Fig. 6-8).

Reference
Islam, M. N., T. Terao, H. Ueda, T. Hayashi and K. Kikuchi,

06 LT

2004: Spatial and temporal variation of precipitation inand around Bangladesh. submitted to J. Meteor. Soc.

Fig. 7:

Hourly rainfall of the model results without

Shillong Plateau 00 LT, 03 LT and 06 LT 17th June 1995
(domain 3)

Japan.
Nitta, T. and S. Sekine, 1994: Diurnal variation of convective
activity over the tropical western pacific. J. Meteor. Soc.
Japan, 72, 627-641.
Ohsawa, T., T. Hayashi and Y. Mitsuta, 2000: Intraseasonal

00 LT 17th03 LTvariation of monsoon activities associated with therainfall over Bangladesh during the 1995 summer
monsoonseason.

J.

Geop.,

Res.,

105,

D24:29445-29459.
Ohsawa, T., H. Ueda, T. Hayashi, A. Watanabe and J.
06 LT09LTMatsumoto, 2001: Diurnal variations of convective
activity and rainfall in tropical Asia. J. Meteor. Soc.
Japan, 79, 333-352.
Terao, T., T. Hayashi, M. N. Islam, J. Matsumoto and T. Oka,
2002: Observational study on the diurnal variations ofconvective activity and circulations over Bangladesh inFig. 8: Same as Fig. 6, except the results using the terrain

the Summer Monsoon Season. Annuals of the Disas.

without Shillong Plateau

Prev. Res. Inst. Kyoto Univ., 45 B, 233-244.

4.

Conclusion and discussion
Though the study using the radar in Dhaka (Islam,2004) didn’t suggested precipitation in the northeastern
Bangladesh came from squall line, the simulated results
indicated squall line system. The results of the radar wereaveraged over one month or one summer monsoon
period, additionally time resolution was 3 hour (notobserved 03 LT). Therefore the lifecycle and movement