THEIR FLUXES IN RELATION TO NOR'WESTERS
6.4 Results and discussion
6.4.5 Comparison of the DSE and MSE components of the troposphere over Dhaka between the dates of occurrence and non-occurrence of
nor'westers
The DSE and MSE at each level of the troposphere over Dhaka on 25 dates of occurrence of nor'westers at Dhaka as well as over Bangladesh have been compared with those on the dates of non-occurrence. The dates of non-occurrence are the dates one day earlier than the dates of occurrence except in one case where it is two days earlier when no nor'wester occurred at Dhaka or the dates on which nor'wester occurred at one/two places over Bangladesh far away from Dhaka. The frequency table for the increase/decrease in DSE and MSE at each level of the troposphere on the dates of occurrence of nor'westers with respect to those on the dates of non-occurrence has been constructed and is given in Table 6.9.
155
Table 6.9:Comparison of DSE over Dhaka between the dates of occurrence and non- occurrence of nor'westers
Level Increase/decrease in DSE Increase/decrease in MSE (hPa)
Class f Increase in DSE Class (i/gm) f Increase in MSE
(i/gm) (% cases) (% cases)
-6to-4 1 -18to-12
Surface -4 to -2 0 80.00 -12 to -6 1 84.00
-2to0 4 -6to0 2
0to2 16 0to6 13
2to4 3 6to12 7
4to6 1 12to18
-6 to -4 1 -24 to -18 1
1000 -4to-2 1 84.00 -18to-12 0 80.00
-2to0 2 -12to-6 1
0to2 16 -6to0 3
2to4 4 0to6 13
4to6 I 6to12 4
l2 to* 18 3
-6to-4 0 -12to-6
925 -4 to -2 3 68.00 -6 to 0 3 84.00
-2to0 5 0to6 13
0to2 13 6to12 6
2to4 4 12to18 2
4to6 0
850 -6to-4 0 -12to-6 0
-4 to —2 3 56.00 -6 to 0 6 76.00
-2to0 8 0to6 7
0to2 10 4 to 12 10
2to4 3 12to18 2
4to6
-8to-6 1 -l8to-12
-6to-4 0 -12to---6 1
700 -4 to —2 I 72.00 -6 to 0 8 60.00
-2to0 5 0to6 11
0to2 15 5 to 12 4
2to4 3 12to18 0
4to6 0
-6to-4 I -12to-6 0
500 -4 to —2 4 56.00 -6 to 0 11 56.00
-2to0 6 0to6 12
0to2 8 6to12 2
2to4 4 12to18 0
4to6 2
-6to-4 I -12to---6 0
400 -4 to —2 2 56.00 -6 to 0 10 60.00
-2to0 8 Otoó 14
0to2 11 6to12
2to4 2 12to18 0
4to6
156
A T able 6.9 (Cont'd)
-6to-4
300 -4 to —2
-2to0 0to2 2to4 4to6
1
2 64.00
6 11
2 3
-l2to-6 -6 to 0
0to6 6to12 12to18
0
9 64.00
16 0 0
-8to-6 I -12to-6
200 -6 to -4 1 48.00 -6 to 0 12
-4to-2 7 0to6 12
-2to0 4 6to12 4800
0
0to2 5 12to18 0
2to4 6
4to6
-10to-8 I -12to-6 1
-8to-6 0 -6to0 13
100 -6 to -4 1 44.00 0 to 6 11
-4to-2 4 6to12 0
-2to0 8 12to18 0 44.00
0to2 8
2to4 3
f = frequency
6.4.5.1 Dry static energy (DSE) - Table 6.9 shows the frequency distribution of the changes in DSE at each level of the troposphere over Dhaka on the dates of occurrence of nor'westers from that on the dates of non-occurrence of nor'westers. The table shows that the increase in DSE occurs in most of the cases from the surface to 300 hPa, the maximum being in the lower troposphere on the dates of occurrence as compared to that on the dates of non- occurrence of nor'westers. Sometimes, the increase in DSE occurs up to the top of the troposphere. The DSE increases in 80, 84, 68, 56 and 72% cases at the surface, 1000, 925, 850 and 700 hPa levels respectively. The table also shows that the increase in DSE occurs in the range of 0-2 Joule/gm in most of the cases at each level except at 200 hPa level where the range is 2-4 Joules/gm in most of the cases. The vertical profiles of DSE for some cases are shown in Figs. 6.21 (a-c). The increase in DSE is not so significant. In some cases, the increase in DSE is also found to occur in the range of 2-4 Joules/gm and 4-6 Joules/gm.
4
6.4.5.2 Moist static energy (MSE) - The Table 6.9 shows that the increase in MSE occurs in most cases from the surface to 300 hPa level on the dates of occurrence of nor'westers as compared to that on the previous days - the dates of non-occurrence. The increase occurs in maximum cases starting from the surface to 300 hPa level having 84, 80, and 84. 76, 60, 56, 60 and 64% cases at the surface, 1000, 925, 850, 700, 500, 400 and 300 hPa levels respectively. The increase in MSE also occurs in 48 and 44% cases at 200 and 100-hPa levels
157
*
respectively. The primary and secondary maximum increases in MSE are in the ranges of 0-6 Joule/gm and 6-12 Joules/gm respectively at each level except 850-hPa level where the primary maximum increase is in the range of 6-12 Joules/gm and the secondary maximum in the range of 0-6 Joule/gm. This implies that a significant increase in MSE occurs in the lower atmosphere on the dates of occurrence of nor'westers as compared to that on the dates of non- occurrence. This significant increase in MSE is also evident from the Figs. 6.22 (a-c) for three cases. The increase in MSE on the dates of occurrence of nor'westers is mainly due to the contribution of LH released prior to the occurrence of nor'westers. In one case there is a significant decrease in MSE of magnitudes -16.25 Joules/gm and -19.76 Joules/gm at the surface and 1000-hPa level on 29 April 1990 as compared to that on 28 April 1990. This decrease in MSE may be due to the accumulation of MSE at a distant place, Magura, from Dhaka where a tornado occurred on 29 April 1990.
p'P 100 200 300 '.00 s00 600 700
•00 900
100
200
arlh
300 / 1
'.00 L 900 600
300 // .__..._._.21350 Il (nonoccurrcnc.)
600 .---23.3.90
:: 290 300 310 320 330 340 350 .360 310 JeJ(e/grn
- hP
b. April C.M.y
100 200 300 400
500 900 accunnc.) 700 coecurro n.) oo
goo
...19.5 52 n o—occurrence .--.--.16.5 92
(cccurrtnce]
300 310 320 320 240 350 360 370 290 300 310 320 330 3.0 750 360 370 JouL. / gm .Jout. /
Figs. 6.21 (a-c): Vertical profiles of DSE over Dhaka at 0000 UTC on the dates of occurrence and non-occurrence of nor'westers
158
-.155.92 (non _eecUrrtnce)
-.16.5 92 (OcCurrence) 0
0 10
ID 10 50 ID 100
200 300 600 500 600 700
$00 900
inn.,
290 300 310 320 330 340 350 30 370 .Joot, /gm
100 b. Apri' 10
200
300 30
400 4c
500 Sc
C, 2
600
20 6(
700
800 91
900 91
10 290 300 310 320 30 360 350 360 370
Joule igm
C. May
290 300 310 320 330 340 350 360 370 jouls /gm
Figs. 6.22 (a-c): Vertical profiles of MSE over Dhaka at 00 UTC on the dates of occurrence and non-occurrence of nor'westers
6.4.6 Comparison of the DSE and MSE components of the troposphere