4.5 Results and discussion
4.5.2 Spatial distribution of different instability indices
The different stability indices of the troposphere on the dates of occurrence of a number of nor'westers have been computed by using the rawinsonde data of Dhaka and Chittagong and a number of Indian stations at 0000 UTC and their spatial distributions have been studied critically.
The spatial distributions of SI and LI on the dates of occurrence of 26 nor'westers have been studied and examples of distribution are given in Figs. 4.3-4.4.
It has been seen from the analyses of these Stability Indices (SI and LI) that maximum negative values lay over the eastern Madhya Pradesh, Bihar, West Bengal and adjoining Bangladesh indicating the highly unstable area. The negative areas of the Indices in combination with the low-pressure area over Bihar, West Bengal and adjoining Orissa and Bangladesh as well as the cyclonic circulation in the upper air up to 3-4 km or above are favourable for the occurrence of nor'westers in Bangladesh. Normally, nor'westers have been found to occur at the northeastern or eastern part of the area of maximum instability, where the gradients of instability indices are steep.
__2.. o•
M, F
-1.
:
Fig. 4.3: Spatial distribution ofSl on 14 April 1990, the Fig. 4.4: Spatial distribution of LI on 14 April 1990, date of occurrence of nor'wester in Bangladesh the date of occurrence of nor'wcstcr in Bangladesh
The spatial distributions of CT, VT and TT for 26 cases of nor'westers have been examined. Examples of spatial distribution are given in Figs. 4.5-4.7. From the analysis of CT, it has been found that the area of maximum values exists over eastern Uttar Pradesh and adjoining eastern Madhya Pradesh, Bihar, Sub-Himalayan West Bengal extending towards north
91
10
Bangladesh in 9 out of 26 cases, over Bangladesh and adjoining area in 16 out of 26 cases and in I case over north Orissa and adjoining Gangetic West Bengal and Bangladesh. These areas indicate the highly unstable area. Nor'westers have been found to occur over the area of maximum CT or at the northeastern or eastern part of the area of maximum CT/unstable area.
In case of VT, the area of maximum value i.e. unstable area lies over Uttar Pradesh and adjoining Madhya Pradesh, Bihar, Sub-Himalayan West Bengal with its trough extended to Bangladesh in 18 out of 26, over north Orissa, Gangetic West Bengal and adjoining Bangladesh with its trough extended northeastwards in 6 out of 26 and in only 2 cases the area of maximum VT lies over Bangladesh. Considering the Ti', it has been found that the area of maximum TI lies over Eastern Uttar Pradesh and adjoining Madhya Pradesh, Bihar, Sub-Himalayan West Bengal with its trough extended to Bangladesh in 13 out of 26 nor'westers, over Bangladesh in 6
Ac cases and over north Orissa and adjoining Gangetic West Bengal with its trough extended northeastwards up to Bangladesh. The gradient of instability indices is maximum over Bangladesh. All these results substantiate that nor'westers occur at the eastern/northeastern part of the maximum unstable area or over the area of maximum instability in Bangladesh.
35
30,
25
201
10
' 22 \
I I---- - -
35
30
/ •i -' _./_
- M
251
aE
k.1 5 1 '34 7 20
I
15
,207 207
10 0
0
70 75 80 85 90 95 ISO
0 1
70 75 00 85 50 95 100
Fig. 4.5: Spatial distribution ofCTon 26 March 1993 Fig. 4.6: Spatial distribution ofVTon 17 May 1992
The spatial distribution of El is shown in Fig. 4.8 as an example. It has been found from the analysis of El that in 9 out of 26 nor'westers the area of minimum El lies over Uttar Pradesh and adjoining eastern Madhya Pradesh or Bihar, West Bengal and adjoining Bangladesh, where the surface low and low-level circulation exist as has been observed from the analyses of surface
92
- j---- -s'-- - 35
00 '•
00
20
15-
10
5
and pilot charts (not shown here). In these cases, the trough of minimum El is found to extend east/southeastwards up to Bangladesh.
- The area of minimum El has been found to exist over Bangladesh in 12 out of 26 nor'westers and over Gangetic West Bengal and adjoining area in 5 out of 26 nor'westers. In this case, the trough of minimum El has been found to extend northeastwards across Bangladesh. The nor'westers have been found to occur over the area of minimum El or at the northeastern or eastern part of the area of minimum El, where the injection of moisture takes place from the Bay of Bengal. This area of minimum El indicates the area of unstable atmosphere, which is favourable for the occurrence of nor'westers in Bangladesh.
The spatial distributions of SW! and K! are shown in Fig. 4.9 and Fig. 4.10 respectively as examples. Form the analysis of SW!, maximum SW! has been found to exist over Uttar Pradesh and adjoining Madhya Pradesh or Bihar and sub-Himalayas in 6 out of 26 nor'westers with extended trough up to Bangladesh. In 12 out of 26 nor'westers, maximum SW! lies over Bangladesh, and over Orissa, Gangetic West Bengal and adjoining region in 7 out of 26 nor'westers with elongated area northeastwards over Bangladesh. In only one case, it is found over the area northeast of Bangladesh. In all the cases, Bangladesh has the area of instability on the dates of occurrence of nor'westers.
I
35
7-
3
468
20
L
rs 10
I
0•
70 75 00 85 50 95 100 IN 70 75 80 85 10 95 100
Fig. 4.7: Spatial distribution of iT on 10 April 1993 Fig. 4.8: Spatial distribution of El on 14 May 1992, the date of occurrence of nor'wester
93
ER
30
75 ,S
20
'5
IS 35
25 30
70
¶5 \\ (
S S
0
70 75 80 85 90 95 QD
0
70 75 60 85 30 95 100
Fig. 4.9: Spatial distribution of SWI on 14 May 1992, Fig. 4.10: Spatial distribution olKl on 21 April 1993,
-I the date of occurrence of nor'wester the date of occurrence of nor'wester
In case of KI, the maximum values have been found to exist over eastern Uttar Pradesh and adjoining Madhya Pradesh or Bihar, Sub-Himalayan West Bengal and adjoining area in 9 out of 26 nor'westers, and over Orissa and adjoining Gangetic West Bengal in 5 out of 26 nor'westers. Maximum values of K! have been found to lie over Bangladesh in 9 out of 26