京都大学防災研究所年報 第 56 号 B 平成 25 年 6 月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 56 B, 2013

Influence of Dry-line on the Formation of Severe Local Convective Storms of
Bangladesh: A Case Study

Fatima AKTER and Hirohiko ISHIKAWA
Synopsis
This study analyzes the atmospheric instability and trigger mechanisms of a
Severe Local Convective Storm (SLCS) initiation over Bangladesh and adjoining Indian
Territory of 4 April 2011. Two types of data, JRA-25 1.125 degree resolution reanalysis
data and Global Spectral Model (GSM) 20 km resolution analyses data, both by Japan
Meteorological Agency (JMA), have been used and comparisons between two products
are illustrated for this case. JRA-25 reanalysis data analyses specific humidity gradientgreater than 3 g/kg over 100 km, whereas the GSM data analyses specific humidity
gradient greater than 10 g/kg over 100 km since sharper gradient of specific humidity is
resolved in this analysis. GSM data significantly identify dry-line and can calculate
instability precisely which suggests that the dry-line influences the initiation of
convection in this case.
Keywords: Severe Local Storms, Tornado, Bangladesh disaster, Dry-line

1.

Introduction

Severe Local Convective Storms (hereinafterreferred to as SLCS) are very devastating
phenomena in Bangladesh and North-eastern part ofIndia in the pre-monsoon seasons (March, April andMay). They cause huge damages to our lives andproperties in a very short time. These storms are
locally termed as Nor’wester (Kalaishakhi- in
Bengali) since the systems migrate from north-west
to south-east. Those are often associated withtornadoes. Every year the SLCS of Bangladesh
cause the highest death toll in the World. Annual
death toll accounts for 179 deaths per year causedby only from tornadoes in Bangladesh from theperiod of 1967-96 (Ono 2001).
In the pre-monsoon season, sufficient low
level southerly warm moist wind flows from Bay of
Bengal and moderate to strong cold dry westerly
wind flows from mid-upper level towards
Bangladesh that influences the convection. The
cold advection often associated with westerly jet.

These settings of wind pattern in this season are
identified the favorable environmental condition for
SLCS development (Weston, 1972; Prasad, 2006;
Yamane and Hayashi, 2006). Previous studies
identified the preferred location of SLCS formationin Bangladesh (Peterson and Meheta, 1981; Yamane
et al 2010a). Some other studies also focused on the
dynamic and thermodynamic aspects of theinitiation of SLCS (Lohar and Pal, 1995), a study
also identified the propagation and modes oforganization of Mesoscale Convective Systems(Dalal et al 2012), and another study indicates thelower and mid-upper tropospheric features for
initiating of Nor’westers (Ghosh et al 2008).
Contrarily, the physical process and trigger
mechanisms of SLCS organization are rarely
studied due to the scarcity of observation data in the
study area.
SLCS are very frequent and well-studied
over the Great Plains of the United States.
Thunderstorms over the Great Plains are frequently
formed along dry-lines. Number of climatological― 299 ―

studies proves that the initiations of severe storm
outbreaks in the United States are triggered by
dry-line (Reha 1966; Bluestein and Parker 1993;
Ziegler et al. 1997 and Hane 1997). Dryline is amesoscale narrow boundary separating moist
maritime tropical air masses of Gulf of Mexicofrom continental dry air of the arid areas in thewestern Great Plains during the warm season(Fujita 1958; Miller 1959; Rhea 1966).

Fig 1. Schematic vertical cross section of the
dry-line and its relationship with topography(Adapted from Bluestein 1993)
The gently sloping terrain of the Great Plains
contributes to dry-line formation, because dry-lines
essentially represent the intersection of the top of
the maritime tropical boundary layer with theground (Fig.1, Bluestein 1993). Climatologicalstudy of the Great Plains found the dry-line
positions in the region where the topography of theplains rapidly increases from east to west (Hochand Markwowski 2005).
Bangladesh belongs to the Gangetic Deltaand its topography is very flat nearly sea level. The
altitude gradually increases to the West andelongated to Chotanagpur platue (23˚N 85˚E;
average elevation of 539 m above sea level) in the
Indian Territory. Although topography plays
significant role on dry-line formation, no evidences
has been demonstrated in the study region. Dry-linefeatures in North-east India were identified byWeston (1972). He mentioned that pre-monsoon
deep convection is favored near the region of
low-level moisture gradients. Strong localized
convergence and abundant moisture often make thedry-line a preferred location for SLCS initiation.
Carlson (1998) and Bluestein (1993) alsomentioned about presence of dry-line in Northern
India. However, the origin, structure andmechanism of dry-line in this region are rarely
investigated.
The major objective of this study is to
identify the dry-line and investigate whether it

influences the development of SLCS in Bangladesh
in a way analogous to the features in the UnitedStates. Therefore, a detailed climatologicalinvestigation is carried out to determine
pre-monsoon dry-line characteristics along withsynoptic environment. A case of tornadic outbreak,which swept through seven districts of northern
Bangladesh on 4 April, 2011 in the afternoon,
causing twelve deaths and more than one hundredand fifty injuries, was closely investigated as an
example. Section two define dry-lines for the study
region, section three includes description of thedata sources and dry-line identification methods,
Section four contains results and discussion, andSection five provides a summary in this paper.
2.

Definition of dry-line

Dry-line is a confluent boundary separating dry
air masses from more moist air masses. The mostsignificant signature of dry-line is a sharp moisture
discontinuity (Schaefer 1974a). In the present
study horizontal specific humidity was used todetermine the moisture discontinuities. A set ofvariables were considered to identify dry-lines in
the Great Plains. Specific humidity was used incoordination with dew point temperature (Schaefer
1974a; Hoch and Markowski 2005) since the natureof the elevation differs across the study region.
Unlike Great Plain region the elevation does notvary in the target area of the present study.
Therefore we only consider specific humidity
gradient that was equal or exceeding 3g/kg/100km(Schaefer 1974a) as the dry-line features forclimatological studies. We also consider surface
temperature, pressure and wind profiles to avoid
any localized disturbances. To exclude local
variations within an air mass, the discontinuity had
to remain identifiable for at least 6 hours. Moreover,
we used terrain following data for our calculation.
3.

Data and Methodology

Initially, we used JRA-25 1.125 degree
resolution reanalysis model level data by Japan
Meteorological Agency (JMA) to analyze synoptic
features for 35 years climatology (between 1979
and 2013) in the pre-monsoon seasons. The― 300 ―

Japanese 25-year reanalysis (JRA-25) was createdwith an up-to-date numerical assimilation andforecast model. It covers the period 1979 to 2004,
which are 25-years. The data set is continued to thisday as the JMA Climate Data Assimilation System(JCDAS) in every 6 hours. Temperature, wind,
relative humidity, air pressure (on model-level),brightness temperature (satellite), and precipitablewater have been assimilated. The model calculation
was performed with a spectral resolution about
1.125 degree and 40 height levels (top at 0.4 hPa).
Alternatively we used JMA’s GSM analysis
data to see the more localized features. The data
are one of very high horizontal resolution of 0.1875
degrees, approximately 20 km among theoperational global model in the world. In thevertical, it has 60 layers from surface to 0.1 hPa.
The vertical resolution is finer near the surface forbetter representation of the planetary boundary
layer processes. The data are provided 6 hourly
from 2007. The available variables are wind (zonal
and meridional), temperature, specific humidity,
surface pressure and cloud water content.
In the present study to understandenvironmental stability condition several stability
indices are computed such as Lifted Index (LI),
Convective Available Potential Energy (CAPE),
Showalter Stability Index (SSI) etc. and the detailsof the indices are already explained to our previous
work Yamane et al. (2010b). To calculate LI, SSI
and vertical profile we converted original GSM
model level data to pressure level data.
Dry-lines are identified from a horizontal
specific humidity gradient. Specific Humidity
gradient is denoted as ∇ Q. Here q is specifichumidity (g/kg).

𝜕𝑞 2
𝜕𝑞 2
√
|∇. 𝑄| = ( ) + ( )𝜕𝑥𝜕𝑥
The critical magnitude of defining dry-line
should depend on the horizontal resolution of thedata. A criterion of 3x10-8m-1, equivalent to 3 g/kg
difference of specific humidity over 100 km, wasused by Schaefer (1974a) Hoch and Markowski(2005) and Coffer et al. (2013). The same criterion
is applied in our analysis with 1.125˚ resolution of
JRA-25 data. For GSM analysis of 0.1825 degree
resolution, however, the more strict criterion of

1x10-9m-1 , equivalent to 10 g/kg difference ofspecific humidity over 100 km, was used sincemore sharp gradient of specific humidity is resolvedin GSM analysis. For dry-line analysis time, wemainly refer to the field at 12 UTC (18 BDT,
Bangladesh Local Time) and 18 UTC (00 BDT),
since the surface dry-line feature is the mostevident in these synoptic times.
Though the data are global, we restrict ouranalysis area to south Asia in bigger domain
between 65˚-100˚E longitude and 05˚-29˚N latitude.
For detail analysis we take smaller domain between
84˚-94˚E longitude and 18˚-26˚ N latitude which
focus on Bangladesh and the West Bengal of India.
To see the large scale convective phenomena the IR
imageries of Multi-functional Transport Satellite(MTSAT) of JMA are used. MTSAT provides
imagery for the target area every 30 minutes, which
are used to monitor clouds, sea surface
temperatures and distribution of water vapor.
4.
4.1

Result and Discussion

Synoptic overview
Synoptic scale processes play verysignificant role in regulating dry-line formation andintensity. Climatology of pre-monsoon seasonsshows typical diurnal variations from morning toevening. In the present study southerly or
south-westerly tongue of moist warm wind flowfrom the Bay of Bengal to the land and dry warm
westerly wind flowing from the Indian high lands to
Bangladesh are identified. Low level temperature
exhibits comparatively higher values in the central
India than in Bangladesh. At upper troposphere,
Westerly to North-westerly wind is very strong overBangladesh. Cold and low moisture air flows
toward Bangladesh at 500 hPa. Most of the days in
April upper air westerly is much stronger than in
March and May over Bangladesh. Surface moist
tongue of southerly to southwesterly is very
prominent in April and May.
The study also identifies very high surface
moisture content everywhere over Bangladesh andsurface specific humidity values approximately~16 g/kg (max. in May18g/kg, min. in March
12g/kg). In contrast moisture content is very low
over Indian Territory.

― 301 ―

ab

Fig. 2 The two panels show surface analysis, shaded temperature (K) and vector wind (m/s) by (a)JRA-25 and by (b) GSM for the SLCS case on 12UTC 4 April, 2011

Fig. 2 shows strong southerly to south
westerly wind flows northward from the Bay of
Bengal.
Temperature
distribution
shows
comparatively higher value over Indian high land.
Severe convection usually requires intense heatingnear the surface and it influences upward motion
for ascending layer parcels.

Fig. 3 (a-d) shows strong south westerlies
over 500 hPa. From surface to 800 southerly tosouth westerly is strong over Bangladesh. Dry hot
air blowiwng toward Bangladesh from Indian
Territory is analyzed. Above 700 hPa dry cold air
blows over Bangladesh. Surface temperature
increases over Indian Territory as day advances.

a

b

c

d

Fig 3. Vertical distribution contour of specific humidity g/kg and wind vector m/s a) JRA -25, b)
GSM; wind vector m/s and temperature (K) c) JRA-25, d) GSM on 12UTC 4 April, 2011

― 302 ―

In the upper atmosphere above 300 hPa, analysis
shows stronger south westerly wind and the windspeed becomes stronger over the Indian
subcontinent at the mid of the day.
From the vertical cross section we can see
east to west decrease of specific humidity content
near the surface at 87°E that corresponds to theposition of dry-line (referred to Fig 3 a-b again).
Surface specific humidity reaches approximately~16 g/kg all over Bangladesh. The distribution ofvertical temperature shows no gradient between dry
and moist air masses (Fig 3 c-d). Intense heating ofmoist air near the surface produces significant
upward motion for ascending layer parcel.
A Specific humidity gradient is observed
between moist and warm air masses that is the con-fluent zone for separating moist air masses from
Bay of Bengal in the south to dry continental air
masses in the west (Fig. 4a-b). JRA-25 reanalysis
data analyses specific humidity gradient greaterthan 3 g/kg but GSM data analyses specifichumidity gradient greater than 10 g/kg over 100
km (Fig. 4c-d).
Southerly wind blows at the east side of
dry-line which carries sufficient moisture to theland. This is a potential source for the initial growthof clouds.
Sufficient moisture and localized
upward motion near dry-line enables successive
convection occurrence and maintains long lastingstorms. This feature is consistent with Great Plaincase reported by Hane et al. (1997).

a

b

c

d

Fig 4. Shaded gradient of specific humidity (g/kg/100km) (a) JRA -25 (b) GSM: Shaded temperature andcontoured specific humidity gradient; (c) (≥3g/kg/100km) using JRA-25 data; d) (≥10g/kg/100km) by
GSM on 12 UTC 4th April, 2011

― 303 ―

ba

Fig 5. Shaded black body temperature TBB and contoured specific humidity gradient (a) (≥3g/kg/100km)
using JRA-25 data; (b) (≥10g/kg/100km) by GSM on 12 UTC 4th April, 2011
The satellite IR imageries of Multi-functional
Transport Satellite (MTSAT) of JMA are coupledwith specific humidity gradient at 12UTC showssome eastward moving cloud initiated along the
dry-line (Fig 5 a-b). From this figure we can depict
that dry-line exists near the cloud initiation position
which suggests that dry-line contributes to theinitiation of SLCS in this case.
4.2 Atmospheric Instability
Atmospheric instability is a major determinant
to estimate the possibility of the development of
SLCS. Many examples of localized convective
development also along the dry-line in Great Plains
have been noted Bluestein and Parker (1993) andHane (1997).

aYamane and Hayashi (2006) showed that themagnitude of potential instability and vertical wind
shear increase in the pre-monsoon seasons which isan important factor to form severe thunderstorms.
Atmospheric environment has high potential
instability on severe local convective storms daysof Bangladesh (Yamane et al, 2010b; 2012).
According to Yamane et al, (2010b; 2012)
Lifted Index (LI), Precipitable Water (PW) andConvective Available Potential Energy (CAPE)
have relatively higher forecast skill for SLCS
(Yamane 2010b). They also mentioned that spatial
distribution of CAPE and PW value are higher and
LI is lower over sea than the land (Yamane et al.
2012).

b

Fig 6. Shaded CAPE J/kg a) JRA-25, b) GSM at 1200 UTC on 4th April, 2011

― 304 ―

ab

Fig 7. At the cross section of 22° N latitude and 87° E longitude a) 2667 J/kg by JRA-25,b) 2471 J/kg by GSM data
shaded CAPE J/ a) JRA-25, b) GSM at 1200 UTC on 4th April, 2011

Fig. 6 a-b shows CAPE distribution over the
study area using JRA-25 data and GSM data
respectively. CAPE values are high in the east side(moist part) of the gradient and it is significantly
increased as the day advances. It reaches more than2400 (J/kg) over the 22° N latitude and 87° E
longitude at 1200 UTC. The Lifted Index (LI) is ameasure of the thunderstorm potential which takes
into account the low level moisture availability andcan be used as a predictor of latent instability.
Karmakar and Alam (2006) studied a number ofthunderstorm cases over Dhaka and concluded thatthunderstorms are likely to occur over Bangladesh
when LI ≤ 0 and, severe Nor’westers with tornadic
intensity are possible when LI < -3 overBangladesh. In this study LI values -6 in bothanalyses (Fig 7a; 7b) at the same area. Negative
Showalter Stability Index (SSI) is an indicator for
possible severe convective activity. SSI is analyzed-1 by JRA-25 and -5 by GSM at the same location(figure didn’t include). Two different analysis dataof different horizontal resolution indicates that the
instability around the moist part of dry-line which
triggers the severe convection over the region.
Though the JRA-25 analysis only gives vague
information of unstable region, the use of fine grid
data specifically locates the very unstable region
where convection be initiated.

5.

Summary

In the present study, we examined theenvironmental condition for the severe storm caseon April 4, 2011. The interest is focused on the
dry-line feature which is formed between the moist
air originates in Bay of Bengal and the dry air from
Indian sub-continent. This case study suggests thatthe dry-line has certain influence to trigger severe
storm similar to the dry-line in the Great Plain of
the United States. The use of fine grid analysis
proved to be very powerful to locate the specificlocation of convection initiation.
The low resolution JRA-25 data is available
since 1979 and the fine resolution data from 2008.
Combining both data, climatological study on therelation of dry-line and SLCS is to be continued
following the present study.

Acknowledgements
The JMA data used in this study wasprovided by way of “Meteorological ResearchConsortium”, a framework for research cooperationof JMA and MSJ.

― 305 ―

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(Received June 11, 2013)