Occurrence of Meso-Scale Disturbances and Seasonal and Spatial Variation ofCAPE in the South Asia
*Yusuke Yamane and Taiichi Hayashi
Disaster Prevention Research Institute, Kyoto University(Gokasho, Uji, Kyoto 611-001, Japan. E-mail yamane@rcde.dpri.kyoto-u.ac.jp)
Abstract
Meso-scale disturbances occur frequently in the northeastern Indian subcontinent, and cause devastative
damages in this region every year. These meso-scale disturbances mainly occur in the pre-monsoon season (fromMarch to May). However, differences in an atmospheric environment between the pre-monsoon season and theother season (monsoon and dry season) are not clarified. In this study, seasonal and spatial variation of CAPE
(Convective Available Potential Energy) is examined in the South Asia as the index of the thermal instability. The
thermal instability is very important for the initiation of meso-scale disturbances. In the center of Bangladesh,
CAPE is high in the pre-monsoon season and highest on May (>2400J/kg), and lower in the monsoon and dryseason.

Keyword: CAPE, Meso-scale disturbance, Northeastern Indian subcontinent

1. Introduction

causing meso-scale disturbances. Through the analysis

Meso-scale disturbances, for example, tornadoes andof CAPE, it is attempted to reveal quantitative

hail storm, frequently occur in the northeastern Indian

estimation that meso-scale disturbances frequently

subcontinent (the northeastern part of India andoccur in the pre-monsoon season and in theBangladesh) in the pre-monsoon season (from Marchnortheastern Indian subcontinent.

to May), and cause much loss of life, house and crop
ever year. In 2004, the tornado occurred in

2. The general features of meso-scale disturbances

Maymensingh division in Bangladesh on 14 April andin the northeastern Indian subcontinent

caused the deaths of about 70 people. Two launches

Meso-scale disturbances including tornado is one ofwere capsized by high gust during a thunderstorm in

the most remarkable meteorological phenomena in thethe Meghna River in Bangladesh on 23 May 2004.

northeastern Indian subcontinent. Peterson and Dewan

These meso-scale disturbances are locally called as(2002) show that about 60% of “Nor’westers” occur“Nor’wester” in the northeastern Indian subcontinent,inwhich is derived from thunderstorm coming from theAccording to tornadoes, their climatology has beennorth-west direction.

studied by many researchers. Fig.1.2 shows the spatial

In this study, the seasonal and spatial distribution of

distribution of tornadoes in the world (Fujita (1973)).

CAPE (Convective Available Potential Energy) is

In the South Asia, the place of tornado occurrence

examined in the South Asia. CAPE is the index forconcentrates only over the northern and northeasternevaluation of the thermal instability of the atmosphere.

area of the Indian subcontinent. Fig.1.3 shows

The thermal instability of the atmosphere is very

monthly frequency of tornado in India from 1839 toimportant for the initiation of convective clouds

1999 (Goldar et al. (2001)). About 70% of tornadoes

AprilandMayin

Bangladesh(Fig.1.1).

occur in the pre-monsoon season (from March toBangladesh and the northeastern India is one of theMay). Fig.1.4 shows monthly frequency of tornado

highest regions of the thermal instability in the South

from 1961 to 2000, and about 80% of tornadoes

Asia. This indicates that meso-scale disturbances

occurs from March to May and the largest figure is infrequently tend to occur in Bangladesh and theApril. Fig.1.5 indicates the distribution of the places

Northeastern Indian in the pre-monsoon season.

of tornado occurrence in Bangladesh for same

The cause that CAPE increases in pre-monsoon

duration as Fig.1.4. Tornadoes seem to occur in theseason in the northeastern India and Bangladesh could

center of Bangladesh.

be considered as follows: In the pre-monsoon season,
the southwesterly wind becomes predominant in the3. Data and CAPE analysis

lower troposphere over northeastern India andThe ECMWF 40 years reanalysis data (ERA-40)

Bangladesh. However, the westerly cold flow

were used for the calculation of CAPE. These data(subtropical jet) is still prominent in the upper

sets could be downloaded freely from the following

troposphere (Hirosawa (1996)). So, the lapse rate of

web site:http://data.ecmwf.i-nt/data/d/era40_daily/.

temperature between the lower and the upper

Fig.2.1 shows seasonal variation of daily CAPE attroposphere is large. On the other hand, the heating oflongitude 90°E and latitude 22.5°N (around thethe Tibetan plateau causes relative higher temperature

center of Bangladesh) in 2001. The ECMWF data sets

region in the upper troposphere in the monsoon season

are archived for 00UTC, 06UTC, 12UTC and 18UTC

and the lapse rate of temperature is small. Moreoverper day. CAPE was calculated four times in a day andthe strong southwesterly inflow transports moisture in

the daily CAPE was made of the average of these four

the lower troposphere from the pre-monsoon season tovalues. Daily CAPE is increasing from late February,monsoon season (The Bay of Bengal is on the southhighest on early May in the year, decreasing graduallyside of this region). The thermal instability is higher inin the monsoon season. In the dry season (fromthe pre-monsoon season over the northeastern India

December to February), daily CAPE is very lowand Bangladesh.

(around 0 (J/kg)).
Fig.2.2 shows spatial distribution of monthly CAPE

4. Summaryin the South Asia in (a) January (dry season), (b) May

Seasonal and spatial distribution of CAPE is(pre-monsoon season) and (c) July (monsoon season)

examined in the South Asia using ECMWF 40 years

respectively. Monthly CAPE is the monthly average ofdata. The relatively higher CAPE region is just overdaily CAPE’s. In January, CAPE is very low over thethe northeastern India and Bangladesh. CAPE isSouth Asia. In May, relatively higher CAPE region

highest on May around the center of Bangladesh.

(>2400 (J/kg)) is recognized around Bangladesh andThese quantitative facts indicate that meso-scalein the northern and southern area of India. In July,

disturbances frequently occur in the pre-monsoonmonthly CAPE is lower than on May in theseason and especially in the northeastern India andnortheastern India and Bangladesh, and continues

Bangladesh.

relatively high in the northern India with a similar
pattern on May.
These results show that the thermal instability isquantitatively highest on May (pre-monsoon season)
in a year in Bangladesh and the northeastern India.

ReferenceMonthly freque ncy of tornado in India(1839-1999)
30

Peterson, R.E. and Dewan, A.M., 2002: Damaging
25
25Nor’westers in Bangladesh, Proc. 21th Conf. on Sever

389-392

Frequency20Local Storm, San Antonio, TX, Amer. Meteror. Soc.,

1514108

Hirosawa, J., 1996: Tornado in Bangladesh (in5Japanese), Kishou, Vol.40, No.8, 4-8444AUG

SEP

OCT22100

NOV

DEC00JAN

FEB

MAR

APR

MAY

JUN

Fujita, T., 1973: Tatsumaki (in Japanese), Kyoritu

JUL

MONTH

Publication, pp.228.

Fig.1.3 Monthly frequency of tornado in India fromGoldar, R.N., Banerjee, S.K. and Debnath, G.C., 2001:

1839 to 1999)

TornadoinIndia

andneighborhood

predictability,India.

Met.

Dept.,

anditspre-publishedscientific report, No.2Average Monthly Occurrence of Damaging Thunderstorm Days in Parcent
35
31.4

31.2

30
25
Percent Occurrence
2015
11.2

10.3

104.3

4.1

5
0.4

1.6

2.1

78

1.8

1.1

0.5

1112

Nor’westersin0
1

2

3

4

5

6

9

10

month

Fig.1.1

MonthlyfrequencyofBangladesh (Peterson et al. (2002))

Fig.1.4 The distribution of tornado in Bangladesh
from 1961 to 2000. (The dot indicates the number oftornado for each division).

numer of occurrenceMonthly number of tornadoes in Bangladesh(1961〜1992)

Fig.1.2 The spatial distribution of tornado in the world

10090807060
504030201008634
21011243(Fujita (1973))

4523267month89710211112Fig.1.5 Monthly number of tornadoes in Bangladesh
from 1961 to 2000

Fig.2.1 Daily CAPE at longitude 90°E and latitude 22.5°(around(a) January

the center of Bangladesh) in 2001.

(b) May(c) July
Fig.2.2 The distribution of Monthly CAPE in 2001 in the South Asia.