Bangladeshi picking up hail
What are the causes of the Bangladesh severe weather alley?
An illustration of the Bangladesh/East Indian severe storm environment can be found here.
Land-Sea distribution and topography
Elevated terrain of north India is EML source region
Ranchi, India
Albuquerque(June 7 1990)
Albuquerque(June 21 1960)
The dryline separates the hot and dry EML source region and the most air over Bangladesh
April 14, 1969
April 14, 1986
Tibetan Plateau enhances the mid and high level flow over south Asia
Nocturnal storms over the Khasi Hills near Cherrupunji leave outflow boundaries
over northern Bangladesh. These nocturnal storms are probably caused by
the low
level jet impinging on the Khasi Hills of Meghalaya, India.
Average Rainfall for April
May 13 1996
Apr 14 2004
May 04 2003
A front of
sorts often stretches across central Bangladesh and into north India.
Apr 08 1995
Apr 14 2004
High level jet
The sounding
The result is classic
"loaded gun" type sounding that we see here in the plains and in South
Africa.
May 22 1981 Oklahoma City
June 02 1995 Friona, TX
May 17 2000 North Platte
December 19 1992 Durban, South Africa
April 14 2004 Dhaka, Bangladesh
April 15 2004 Dhaka, Bangladesh
Places in the world that have a pronounced severe weather season
typically have elevated mixed layers.
Sounding Comparison
The Timeline
Several factors lead to a very
short but active severe
weather season across Bengal.
North and central India heats
up and dries out in late March or early April. A deep, dry mixed
layer develops. Low level
flow from the Bay of Bengal increases markedly during this time.
Westerly mid-level flow
around the Tibetan Plateau advects the Indian mixed layer over
the Bengal
moist tongue. This leads to the elevated
mixed layer. Note that parts of the Indian
desert are
"elevated"(1-3000ft) compared to Bangladesh which is near sea level.
The mid level flow is still
fairly strong in April with 30-50kt 700mb flow and 35 to 50 kt 500mb
flow fairly common.
The high level jet is usually over or just north of the Bengal in April.
The southern branch of
the polar jet often retreats north of the Tibetan Plateau by May, leaving
light mid to high level flow across the Bengal region.
By June the high level flow is light.
All these factors result in a tornado maximum in early to mid April. In short, vertical wind shear
and instability are maximized and the jet is in a favorable position during this time.
Bangladesh and east Indian Tornado Documentation
Previous tornado documentation
Peterson and
Mehta(1981 and 1995), Ono(1997), Qayyum and others
Individual tornado studies in
journals
Peterson and Mehta (1981 and 1995) documented 36 tornadoes. (Most comprehensive)
Ono (1997) documented 28 spring events from 1990 to 1994 using a fairly liberal
set of criteria
Goldar(2001) documented 36 "possible tornadoes" which partially filled the gap
in the 1890s
and early 1900s. Again a liberal definition was used.
Myself and Ashraf M. Dewan searched through old newspaper microfilm for March-May from
the 1950s through the 1980s.
We came up with a comprehensive tornado climatology for Bangladesh
which builds on the previous work by Peterson, Mehta and Ono.
Thirty-three tornadoes were previously undocumented.
All tornadoes
30 or more deaths
tornado paths
tornado details
Tornado Cases
Surface and upper air charts for Bangladesh and east Indian tornadoes can be found here.
April 14 2004
March 20 2005
May 04 2003
Mar 23 2005 High winds and hail(possibly tornadoes)
May 13 1996
Surface
500mb
200mb
200mb reanalysis
April 26 1989
Surface
500mb
200mb
200mb reanalysis
Reanalysis Composites for all tornadoes
62 tornado cases from 1951 to 2005
200mb wind m/s
climatology
anomaly
250mb wind m/s
climatology
anomaly
500mb wind m/s
climatology
anomaly
700mb wind m/s
climatology
anomaly
500mb height
anomaly
700mb height
300mb temps
anomaly
400mb temps
anomaly
500mb temps
anomaly
700mb temps
850mb temps
Sea Level Pressure
anomaly
Reanalysis Composites for May tornado cases
14 cases
200mb wind m/s
climatology
anomaly
250mb wind m/s
climatology
anomaly
500mb wind m/s
climatology
anomaly
500mb height
climatology
anomaly
300mb temp
climatology
anomaly
500mb temp
climatology
anomaly
Implications for Tornado Forecasting
Look for nose of high level jet over Bangladesh
Watch out for outflow boundaries and trough/frontal positions
The dryline/front intersection is a favored area for storm initiation.
Optimal wind speeds at different levels for tornadoes:
adequate low level flow from Bay of Bengal
700mb 260 to 290 at 40kts
500mb 260 to 300 at 50 kts
200mb 260 to 300 at 60 to 80 kts
Optimal morning sounding
April 14 2004
Surface dewpoints usually range from 74F to 81F when tornadoes occur.
