Comparison of different stability indices with modified stability indices on the dates of occurrence of nor'westers in Bangladesh

4.5 Results and discussion

4.5.4 Comparison of different stability indices with modified stability indices on the dates of occurrence of nor'westers in Bangladesh

Different stability indices on the dates of occurrence of nor'westers have been compared with the corresponding modified stability indices and are discussed in the following.

The CT and the MCT have been computed using the relevant data on the dates of occurrence of nor'westers (108 cases) in Bangladesh during the period March-May of 1990-1995 with the data at 0000 UTC over Dhaka and their comparison is shown in Fig. 4.11.

Fig. 4.11 shows that the MCT is always greater than CT. It can be seen that CT values are less than 150C in many cases and even less than 10°C. But MCT is almost greater than 150C except one or two cases. This indicates that CT shows relatively stable atmosphere in the morning, which leads the forecaster to be confused with the morning data for forecasting of nor'westers in the afternoon/evening. In that respect, MCT gives much greater values indicating unstable atmosphere, which is favourable for the occurrence of nor'westers in Bangladesh.

MVT and MTT as shown in Fig. 4.12 and Fig. 4.13 respectively show the similar results as in the case of MCT. The reason for the increased values of the MCT, MVT and MTT is due to the use of 925 hPa level data, where the temperature and dew-point temperature are greater than those of 850 hPa level. Therefore, it may be concluded that the modified indices are better to be used to determine the instability of the troposphere for the occurrence of nor'westers in Bangladesh.

101

-

Index values

tJ 0 Ut 0 •a- 0

10/3/1990 ¹

17-4-90 _S

9/5/1990

8/5/1991

.4

_S

4 18-5-91

22-4-92

16-5-92 ⁴

30-5-92 12/4/3993 24-4-93

5/5/1993 ^{- t'J}

l +

12/5/1993 ⁰⁰

20-3-94 _

29-3-94

20-4-94

41 16-5-94

4C 9/4/1995 ^.-•

26-4-95

at (D CD

a a) a

a- (D a

CD

.-

• -

M

CD

Q 7

— a

a a ^-

C)

a) 'i - •

a- a

o a a

a la-

0

a-H

GO o -a a-

CD CD

0

CD 0

CL

H a- .-

a a-a' a-

-a

CD

•-

a- a a- a

a) a' a' o —

-t

o ^a a'

0 a H

C)

CD

a) a-

a

a' a

0-a a) CD 9,

-1•

C-) 0 a, -t

0 a

0 -4)

C) H a) a 0

•I• C)

a a

(I) a- a a'

a) a) r

a ^_

GO

a' 0

a

a-C H

C)

0 a a-a a- a)

a GO

0 o a a

-C -C

a a a a 0 -4)

Index values

0 ^Ut 0 Ut 0 Ut 0 Ut 0 Ut

14-4-90 L ^•_.

:;-

4--- -

I

3/5/1991 _-

1/5/1991

22-4-92 _•

14-5-92

— - .

20-5-92 ^-

r

8/4/1993 ^:z •-i+

16-4-93 . ^{•_ -.}

e

26-4-93 U 5/5/1993 _,

II5/199 +

20-5-93

GO

00 26-3-94

_____ •

-

_, 'f ,_,

I

21-4-94

K

36-5-94

-

8/4/1995 ^-

20-4-95 — S

23-5-95

cases. The variation of ME! and El are also the same as shown in Fig. 4.15. The figure reveals that the MEl s much lower than the El, showing relatively greater instability of the troposphere.

The reason is that the moist static energy at 925 hPa generally remains greater than that of 850 hPa. Therefore, MKI and MEl at 0000 UTC are better to be used for the forecasting of nor'westers in Bangladesh.

It)

60

0) 50

40 30 20 10

o 0 0 - - - t.l (1 C.t ., In - . 1. i - 'r

0' 0' 0' 0' 0' 0'

4 ' 1

0 - ci ct (N 4C> - (N ^{00 o} (N - .L 00 6_

Dates ^{(N - (N (N (N — (N (N}

Fig. 4.13: Comparison between TT and MTT at 0000 UTC at Dhaka on the dates of occurrence of nor'westers in Bangladesh

In the same way, the MSWI is higher than the SW! at 0000 UTC on the dates of occurrence of nor'westers (Fig. 4.16).

70 60

C) 50

> 40 30

- 20 10 0

y ⁼ 00555x + 4

- R2 =00199

y=-0.0356x+40.I32 R2 ⁼ 0.0076

K!

t 1 ' P -1 - -r -r ^{.'- v In '} rt

0' 0' 0' 0' C' 0' 0' C' 0' C' 0' C' C' 0' C'

44 4

t 'r 00

- - rN (N - (N - (N (N (N

Dates

Fig. 4.14: Comparison of K! and M Kl at Dhaka at 0000 UTC on the dates of occurrence of

* nor'westers in Bangladesh

103

15.00

10.00 y=0.6163x- 12.346

5.00

El 0.00

In

• -5.00 -10.00

- -15.00 -20.00 -25.00

MEL -30.00

Dates

Fig.

4.15:

Comparison between E land ME! at 0000 UTC at Dhaka on the dates of occurrence of nor'westers in Bangladesh during 1990-1995

I UULLU

900.0 800.0 700.0

(1) 600.0

06 500.0 400.0

(1) 300.0

200.0 _{4* I}

100.0 on

IC§I 4 c' ICP ICP 41 ICE5 ,)tx41 4, 4'CO

..V b' ,t'' (

Date of occurrence

Fig. 4.16: Comparison between SWI and MSWI at 0000 UTC at Dhaka on the dates of occurrence of nor'westers in Bangladesh during 1990-1995

4.5.5

Spatial distribution of different modified stability indices in relation to the occurrence of nor'westers in Bangladesh

The different modified stability indices of the troposphere on the dates of occurrence of a number of nor'westers have been computed by using the rawinsonde data at different stations under study at 0000 UTC and their spatial distributions have been studied critically.

104

35 ..

30

-25

25

:

10

0.

70 75 80 85 90

35.

'..

30

/

25

'427.

11z;r

^r1>'

20

- ..

15 \

..

309

\, ( ^-

10

5

0

ç.

70 75 80 85 90 95 100

1

95 100

21.9

The spatial distributions of MCT, MVT, MTT, MKI, MSWI and ME! on the dates of occurrence of nor'westers are given in Figs. 4.17 ^- 4.22 respectively as examples.

The distribution patterns of MCT, MVT and MTT are shown in Figs. 4.17-4.19 for examples. It has been found that MCT is maximum over Bangladesh in 13 out of 26 (50%

cases), over eastern Madhya Pradesh, Bihar, Sub-Himalayan West Bengal with its elongated area up to Bangladesh in 5 out of 26 (19.23% cases), and over Orissa and Gangetic West Bengal with northeastward extension up to Bangladesh in 8 out of 26 (30.77% cases).' These maximum areas of MCT indicate the highly unstable area. Nor'westers have been found to occur over the area of maximum MCT or at the northeastern or eastern part of the area of unstable area.

Fig. 4.17: Spatial distribution of MCT (°C) on Fig. 4.18: Spatial distribution of MVT (°C) on 20 May 28 April 1993, the date of occurrence of 1992, the date of occurrence of nor'wester in

nor'wester in Bangladesh Bangladesh

In case of MVT, maximum values lie over eastern Uttar Pradesh and adjoining eastern Madhya Pradesh, Bihar and Sub-Himalayan West Bengal in 19 out of 26 (73.08%) nor'westers with extension up to Bangladesh in most of the cases. In 5 out of 26 (19.23%) nor'westers, the maximum MVT lies over Orissa and Gangetic West Bengal with an extension up to southwestern Bangladesh, and in 2 cases (7.69%), the maximum MVT lies over Bangladesh and adjoining area. Orissa and Gangetic West Bengal have the maximum MTT with an elongated

105

portion northeastwards across Bangladesh in 30.86% cases of nor'westers (10 out of 26), Bangladesh has maximum MTT in 30.77% cases (8 out of 26) and eastern Uttar Pradesh, eastern Madhya Pradesh, Bihar and Sub-Himalayan West Bengal of India has also maximum MTT in 30.77% cases (8 out of 26) with an elongated area extended eastwards up to Bangladesh. The maximum areas of MCT, MVT and MTT in combination with the low pressure area over Bihar, West Bengal and adjoining Orissa and Bangladesh as well as the cyclonic circulation extending to 3-4 km or above (not discussed here) are favourable for the occurrence of nor'westers in Bangladesh.

35 t.

SS

47

30

;

S4

³⁰

is-

\

^'L

\

46

10 1:

0

ç _ç -

70 75 80 85 90 95 tOO 70 75 80 85 90 95 100

Fig. 4.19: Spatial distribution of MTT (°C) on 25 May 1992, the date of occurrence of nor'wester in Bangladesh

Fig. 4.20: Spatial distribution of MKI (°C) on 30 May 1992, the date of occurrence of nor'wester in Bangladesh

The spatial distribution patterns of MKI and MSWI are shown in Fig. 4.20 and Fig. 4.21, respectively as examples. It has been found that the highly unstable area with maximum MKI exists over Bangladesh (in 9 out of 26 i.e. 34.62 %) and adjoining area, and also over eastern Uttar Pradesh, Bihar and sub-Himalayan West Bengal of India (34.62 %) with an elongated area eastwards/southeastwards up to Bangladesh. Another area of maximum MKI exists over Orissa and Gangetic West Bengal (30.77%) with an elongated portion extended northeastwards over Bangladesh. MSWI has also maximum values over Bangladesh and adjoining area in 12 out of 26(46.15%). In 34.62% cases, MSWI is maximum over Orissa and Gangetic West Bengal with

106

-- ---

35-

30 /

25

(487V

c*'

^'

20 _-400.

\\

Is

to-

U

0• - -

70 75 80 85 90 95 tOO

30

25-i '

20

IS-

10-

'1 -\

1

1 US = Unstable, S tabIe 743

I -'7

'I 7\

ST

0-

70 75 80 85 90 95 100

an elongated portion extended towards Bangladesh, and only in 19.23% cases the MSWI has maximum value over eastern Uttar Pradesh, and adjoining Madhya Pradesh, Bihar and Sub- Himalayan West Bengal with its extended area up to Bangladesh. The maximum areas of MKI and MSWI, indicating the maximum instability, are almost over the surface low-pressure area over Bihar, West Bengal and adjoining Bangladesh and nor'westers have been found to occur at the eastern end of the area of maximum instability.

The spatial distribution of the MEL for different nor'westers has also been studied: An example of the distribution is shown in Fig. 4.22. It has been found from the analysis of the spatial distribution of MEl that the maximum negative area (indicating greater instability) exists over Bangladesh and adjoining West Bengal in 50% cases of nor'westers, in 34.62% cases maximum negative MEL exists over Orissa and Gangetic West Bengal having a trough extended up to Bangladesh, and the area over eastern Uttar Pradesh and adjoining Madhya Pradesh, Bihar and Sub-West Bengal has maximum negative MEl in 15.38% cases with a trough extended eastwards up to Bangladesh. In general, the maximum negative area has been found over the surface low-pressure area. It may be noted that sometimes more than one unstable area are found to exist over India.

Fig. 4.21: Spatial distribution of MSWI on 16 Fig. 4.22: Spatial distribution of MEl (Joule gm') on April 1993, the date of occurrence of nor'wester in 14 May 1992, the date of occurrence of nor'wester in

Bangladesh Bangladesh

107

4.5.6 Correlation among different modified and unmodified instability indices