SURFACE AND UPPER AIR SYNOPTIC CONDITIONS IN RELATION TO
3.4 Analyses
3.5.1 Surface synoptic and upper air conditions associated with nor'westers .1 Mean sea level pressure distribution
3.5.1.4 Upper air synoptic conditions
A number of upper air synoptic charts (both constant pressure and pilot charts) on the dates of occurrence of nor'westers during 1990-2003 have been examined to find out the favourable synoptic as well as circulation patterns associated with nor'westers in Bangladesh. It has been found that cyclonic circulation or a strong trough must be present over the surface low at lower levels extending to about 1-1.5 km or more (Figs. 3.17-3.20, as for examples).
In many cases the cyclonic circulation has been found to strengthen and extend to greater heights in the afternoon. In the pilot chart, low-level circulation is more prominent as compared to that in the constant pressure chart. The cyclonic circulation is responsible for the influx of moisture from the Bay of Bengal. In some cases, it has been found the significant cyclonic circulation needs to be extended to greater heights at the beginning of the season or after some days without nor'westers. After the occurrence of one or two nor'westers, the moisture is present in the troposphere when low-level circulation up to 1 km is enough to cause thundcrstorms/nor'westers. When trough in the upper air westerly jet stream is present over Bihar/West Bengal of India or over Bangladesh, severe nor'westers are found to occur in Bangladesh. The speed of the westerly jet stream has also been found to contribute to the severity of the nor'westers.
It has been difficult to find out the place of occurrence of nor'westers in Bangladesh. So, a several cases of nor'westers during the pre-monsoon season of 1990- 2003 have been examined by studying the tropospheric synoptic charts. In these cases, the axis of the upper air maximum jet stream at 300 hPa or 200 hPa has been superimposed on the axis of the low-level maximum wind at 850 hPa at 0000 UTC. Severe nor'westers have been found to occur near the point of intersection of the axis of the maximum jet stream at 300/200 hPa and the axis of maximum wind at 850-hPa level [Fig. 3.21 (a-b)].
This method has been found to be effective in operational forecasting of nor'westers.
M.
-
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- i
.---- • -'--•- '-:
.-
-
-
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700 75 •O 00'
.. --- 75
Fig. 3.18: Circulation pattern at 0.3 km (1000 ft) at 0000 UTC on 29 April 2002 (Tornado at Syihet in the afternoon)
Fig. 3.17: Circulation pattern at 850 hPa at 00 UTC on 19 April 1990
R7_____ --..--.*-• .__:.._ :----
r----
( 7:
2
7.
Fig. 3.19: Circulation pattern at 1.5 km (5000 11) at Fig. 3.20: Circulation pattern at 3 km (10,000 fi) at 0000 UTC on 29 April 2002 (Tornado at Sylhet in 0000 UTC on 29 April 2002 (Tornado at Sylhet in
the afternoon) the afternoon)
i . _ • _..$ _9 L. YJL (i" I' _ . _4'
(a) 28 Max± 1990 14 April 1990
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Kis
i 7
Fig. 3.21 (a-b): Superimposition of the axes of the upper air jet stream and the lower level maximum winds on the dates of occurrence of severe nor'wcsters
(Solid lines =' 200 hPa and dashed lines =' 850 hPa, o=> place of occurrence)
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3.5.2 Synoptic and upper air conditions associated with tornadoes 3.5.2.1 Synoptic conditions on 26 April 1989
The surface synoptic condition at 0000 UTC of 26 April 1989 was characterized by the presence of a westerly low over western part of Bangladesh and adjoining West Bengal of India with a prominent trough extending to northeast.
Moderately strong pressure gradients were found to exist over Bangladesh. The same synoptic conditions prevailed up to 1200 UTC with a little exception when there appeared a low over Madhya Pradesh and adjoining area and the pressure gradient became strong (Fig. 3.22 and 3.23).
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-
Fig. 3.22: Distribution of MSL pressure at 1200 UTC on 25 April 1989
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t t)
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•' • .... ...----. -
II •- ,. •. - .-'/I
/
Fig. 3.23: Distribution of MSL pressure at 1200 UTC on 26 April 1989
At 0000 UTC on 25 April, strong cyclonic circulation was found to exist up to about 1.5 km (5000 ft). Westerly jet stream of speed 80 knots was found over northern India and Kolkata at about 12 km (40,000 ft). The same eyelonic circulation in the lower troposphere intensified further, extended up to 3.0 km (10,000 ft) and prevailed throughout the subsequent hours up to 0600 UTC on 26 April 1989. Westerly jet stream of speed
11 about 130 knots was found over Dhaka at about 10.6 km (35,000 ft) at 0000 UTC on 26 April. A strong trough in the westerly jet appeared at 200 hPa (12 km) level over Bangladesh at 1200 UTC on 26 April (Fig. 3.24). The jet core of 100 knots was found to extend up to the place of occurrence as could be seen in the Fig. 3.24.
The above-mentioned synoptic conditions and the lower tropospheric circulation patterns together with upper tropospheric strong jet stream were favourable for the occurrence the severe tornado over Bangladesh.
50
Is
Fig. 3.24: 200 hPa circulation pattern at 1200 UTC on 26 April 1989 (Finch, J.D., 2004) 3.5.2.2 Synoptic conditions on 13 May 1996
The tornado originated under the influence of prominent westerly low at the surface over Bihar and adjoining West Bengal of India with its associated trough extended up to Assam across central Bangladesh having low level wind converging over the district of Tangail. Fig. 3.25 gives the spatial distribution of MSL pressure at 0000 UTC on 13 May 1996 showing the presence of prominent westerly low over Bihar and adjoining West Bengal of India with its trough extending towards northeast across northern Bangladesh.
:
\ \ /
- _/• '/
_\ \ --
--.'
\ \ -.
• ___ _____
Fig. 3.25: Spatial distribution of MSL Fig. 3.26: Circulation pattern at 0.6 km pressure at 0000 UTC on 13 May 1996 (2000 ft) at 0000 UTC on 13 May 1996
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51
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q 13 RX, .>Wc-
Fig. 3.27: Circulation pattern at 3.0 km Fig. 3.28: 200 hPa circulation pattern at 0000 (10,000 ft) at 0000 UTC on 13 May 1996 UTC on 13 May 1996 ((Finch, J.D., 2004)
The circulation patterns at 0.6 km (2000 ft) and 3.0 km (10,000 ft) show the presence of strong westerly trough over northwestern Bangladesh and adjoining area (Figs. 3.26- 3.27). A strong trough in the westerly jet appeared at 200 hPa (12 km) level over northern Bangladesh at 0000 UTC on 13 May 1996 (Fig. 3.28). The jet core of 100 knots was found to extend up to the place of occurrence as could be seen in the Fig. 3.28.
The surface synoptic conditions and the low-level circulation together with strong trough in the westerly jet were favourable for the occurrence of the tornado over Bangladesh.