THEIR FLUXES IN RELATION TO NOR'WESTERS
6.4 Results and discussion
6.4.1 Sensible heat content (SH) of the troposphere
6.4.1.1 SH of the troposphere over Dhaka at 0000 UTC during the pre-monsoon season
The vertical cross-section of the SH of the troposphere over Dhaka at 0000 UTC has been studied for the pre-monsoon season. The vertical distribution pattern is shown in Fig.
6.2. The figure shows that the SH of the troposphere decreases with height for obvious reason as the temperature decreases with height.
IOU
200 300 400 500 600 700 800 900 1000
180 200 220 240 260 280 300 320 CpT (Joule/gm)
JH
Bancjade ) -
Fig. 6.2: Vertical cross-section of SH on 23 March 1990 (date of occurrence) over Dhaka at 0000 UTC
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4-
Table 6.1: Monthly variation of SH (Joule/gm) of the troposphere over Dhaka at 0000 UTC during the pre-monsoon season
hPa March April May
1000 296.52 298.98 300.05
925 294.18 297.46 297.73
850 290.62 294.75 295.28
700 279.78 282.16 284.40
500 264.18 265.19 268.44
400 252.86 254.02 258.48
300 237.24 239.89 244.42
200 221.06 221.64 223.68
100 199.62 199.31 196.80
During the pre-monsoon season, mean monthly SH of the troposphere has been computed by using the data for the period 1990-1995 and the results are given in Table 6.1.
The table shows that the SH of the troposphere increases as the season progresses for obvious reason.
6.4.1.2 Comparison of the SH of the troposphere over Dhaka at 0000 UTC on the dates of occurrence and non-occurrence of nor'westers
The SH of the troposphere over Dhaka at 0000 UTC on the dates of occurrence and non-occurrence of nor'westers has been compared with each other. The vertical cross-section of the difference in SH on the dates of occurrence and non-occurrence has been studied, and some results are shown in Figs. 6.3-6.5.
'pp 20 300 400
500 600 700 Soo 900
-5 -4 -3 -2 -1 0 1 2 3 4 5 Difference in CpT (Joule/gm)
Fig. 6.3: Vertical cross-section of the difference in SH on 23 March (date of occurrence) and 22 March (date of non- occurrence) 1990 over Dhaka at 0000 UTC
-1 0 1 2 3
Difference in CpT (Joule/gm)
Fig. 6.4: Vertical cross-section of the difference in sensible heat on 7 April (date of occurrence) and 6 April (date of non- occurrence) 1995 over Dhaka at 0000 UTC 139
-C
-4 -2 0 2 4
Difference in CpT (Joute/gm)
Fig. 6.5: Vertical cross-section of the difference in SH on 10 March (date of occurrence) and 9 March (date of non-occurrence) 1990 over
Dhaka at 0000 UTC
It has been found that the SI-I of the troposphere on the dates of occurrence of nor'westers increases significantly from near the surface to about 3 00-200 hPa in most of the cases, although it has inter-layer variations. Of course, there are some exceptions especially in the lower troposphere near the surface as can be seen from Fig. 6.5, where there exists decrease in SH of the troposphere at 0000 UTC. These exceptions may be due to the decrease in temperature on some occasions near the surface in the early morning because of cold air advection from the northlnorthwest.
6.4.1.3 Comparison of the SH of the troposphere over Dhaka at 0000 UTC on the dates of occurrence of nor'westers with the weekly normal
The SH of the troposphere over Dhaka at 0000 UTC on the dates of occurrence of nor'westers has been compared with the weekly normal. The vertical cross-section of the deviation of SH on the dates of occurrence from the weekly normal has been studied and some results are shown in Figs. 6.6-6.7.
It has been found that the SH of the troposphere on the dates of occurrence of nor'westers increases significantly as compared to the weekly normal from near the surface to about 3 00-200 hPa or more in most of the cases, although it has inter-layer variations. The deviation of SH decreases and becomes normal or negative in the layer between 700 and 500 hPa levels. This may be due to the decrease in temperature in the early morning because of cold air advection from the north/northwest. This cold air advection in this layer from the
140
x
north/northwest is very essential to create the required instability for the formation of thunderstorms in conjunction with the warm and moist air advection in the lower troposphere.
200 300 400 500 600 -
900
-2 0 2 4 6 8 10
Deviation of sensible heat (Joule/gm)
100 200 300 400 500
I 600
700 800 900 1000
0 4
Deviation of sensible heat (Joule/gm)
Fig. 6.6: Vertical profile of the deviation of Fig. 6.7: Vertical profile of the deviation of SH of the troposphere over Dhaka at 0000 SH of the troposphere over Dhaka at 0000 UTC on 10 April 1990 (date of occurrence) UTC on 16 May 1992 (date of occurrence) from the weekly normal from the weekly normal
6. 4.1.4 Comparison of the SH of the troposphere over Dhaka at 0000 UTC on the dates of occurrence of nor'westers with the monthly normal
The SH of the troposphere over Dhaka at 0000 UTC on the dates of occurrence of nor'westers has been compared with the monthly normal. The vertical cross-section of the deviation of SH on the dates of occurrence from the monthly normal has been studied and the vertical variation pattern is shown in Fig. 6.8 as an example.
100 200 300 400 500 600 800 900 1000
-15 -5 5 15
Deviation of sensible heat (Joule/gm)
Fig. 6.8: Vertical profile of the deviation of SH of the troposphere over Dhaka on 10 April (date of occurrence) from the monthly normal
141
It has been seen that the deviation of SH from the monthly normal is positive in the jr lower troposphere and then it becomes negative in the layer between 850 and 650 hPa levels.
Then the deviation increases tremendously in the upper troposphere, having maximum in the layer 400-300 hPa. The negative value in the layer between 850 and 650 hPa levels may be attributed to the cold air advection.