THEIR FLUXES IN RELATION TO NOR'WESTERS
6.4 Results and discussion
6.4.4 Dry static energy and moist static energy contents of the troposphere
6.4.4.2 Comparison of DSE and MSE components on the dates of occurrence of nor'westers with the normals
6.4.4.2.1 Dry static energy (DSE) - The DSE of each layer from the surface to 100 hPa over Dhaka on the dates of occurrence of 79 nor'westers which occurred at Dhaka has been compared with the monthly, weekly and daily normals. The number and percentage number of cases in respect of the increase and decrease in DSE are given in Table 6.6
Table 6.6 shows that the DSE on the dates of occurrence of nor'westers over Dhaka increases in maximum cases at the surface, 850, 700 and 100 hPa level as compared to the 6-yearly monthly normal values. These increases occur in 53.16, 53.16, 63.16 and 67.09% cases of nor'westers at those levels respectively. The cases of increase in DSE at other levels range from 36.71 to 48.10% having the minimum number at 300-hPa level. The increase in DSE is
150
mainly due to the increase in SH of the troposphere. As compared to the monthly normal, the DSE decreases in 55.13, 54.43, 54.43, 56.96, 63.29 and 50.63% cases of nor'westers at 1000, 925, 500, 400, 300 and 200 hPa levels respectively. The decrease in DSE at 1000 and 925 hPa levels may be due to the presence of relatively colder air in the morning in absence of the sunlight. In studying 79 cases, it has been found that nor'westers have occurred on many consecutive days in many occasions. In each occasion, there has been increase in DSE in almost all levels on the starting day followed by decrease at almost each level on the subsequent days. This is for obvious reason that the nor'westers generally occur in the afternoonlevening/night and after that occurrence, there exists no source of energy or insolation up to 0000 UTC of the next day. As a result, the DSE remains less at 0000 UTC on the subsequent days of occurrence as compared to the previous weekly normal DSE. As the earth's surface receives sun's insolation with the progress of daytime, energy is assumed to be accumulated in the troposphere thereby creating a favourable condition i.e. instability for the occurrence of nor'westers provided that there must be favourable synoptic conditions and upper air circulation up to a reasonable height. The decrease in DSE occurs in more than 50%
cases (i.e. 54.43-63.29%) in the layer 500-300 hPa, which is mainly due to the cold air advection in the middle and adjoining upper troposphere. This cold air advection in the middle and adjoining upper troposphere is necessary in producing the required instability for the occurrence of nor'westers when warm and moist air advection takes place in the lower troposphere as the sun's heating increases with time. Since the increase in DSE in the lower troposphere does not occur in 100% of the cases, the another reason for the corresponding decrease in DSE may be due to the presence of low pressure area where the geopotential height is relatively low and as a result the PE becomes lower which can not be compensated by the increase in SH of the troposphere.
From the comparison of DSE on the dates of occurrence of nor'westers at Dhaka with 6- yearly weekly normal, it can be seen that the DSE increases in 50.63, 50.00, 57.50 and 56.96% of the cases of nor'westers at the surface, 1000, 700 and 100 hPa levels respectively.
The increase in DSE at other levels ranges from 40.51 to 49.37% cases. In contrast, the decrease in DSE occurs in maximum cases of 54.43, 55.70, 54.43 and 59.49% at 925, 500, 400 and 300 hPa levels respectively. At other levels, the decrease in DSE ranges from 43.04 to 51 .90% cases.
As compared to the daily normal, the DSE increases in 50.00, 53.16 and 54.431%
cases of nor'westers at 1000. 700 and 100 hPa levels respectively (Table 6.6). The increase in 151
DSE ranges from 35.44 to 46.84% cases at other levels, the minimum being 35.44% cases at 300-hPa level where the decrease occurs in 64.56% cases i.e. the maximum cases.
Table 6.6: Comparison of DSE over Dhaka on the dates of occurrence of nor'westers at Dhaka with the monthly normal, weekly normal and daily normal
Level (hPa)
Cases of norwesters
Monthly normal Weekly normal Daily normal Increase Decrease in Increase Decrease Increase Decrease studied
in DSE DSE in DSE in DSE in DSE in DSE (% cases) (% cases) (% cases) (% cases) (% cases) (% cases)
Surface 79 53.16 46.84 50.63 49.37 45.57 54.43
1000 78 44.78 55.13 50.00 50.00 50.00 50.00
925 79 45.57 54.43 45.57 54.43 43.04 55.70
850 79 53.16 46.84 48.10 51.90 46.84 53.16
700 78 63.29 36.71 57.50 41.77 53.16 45.57
500 79 45.57 54.43 43.75 55.70 45.57 54.43
400 79 43.04 56.96 45.57 54.43 44.33 55.70
300 79 36.71 63.29 40.51 59.49 35.44 64.56
200 79 48.10 50.63 49.37 50.63 44.30 55.70
100 79 67.09 32.91 56.96 43.04 54.43 45.57
All the results substantiate that the increase in DSE occurs in most cases at 700 hPa level in the lower troposphere and at 100 hPa level in the upper troposphere whereas the decrease in DSE occurs in maximum cases at 300 hPa level as compared to the monthly, weekly and daily normals prior to the occurrence of nor'westers
6.4.4.2.2 Moist static energy (MSE) - The increase and decrease in MSE of the troposphere over Dhaka on the dates of occurrence of nor'westers at Dhaka from the monthly, weekly and daily normals are given in Table 6.7. The table shows that there are increase in MSE from the monthly normal startiig from 1000 to 700 hPa level in most of the cases having the maximum value (69.62% of cases) at 925 hPa level followed by a decrease in the layer 500-200 hPa having the decrease in 67.09, 65.82, 63.29 and 54.43 % of cases at 500, 400, 300 and 200 hPa levels respectively. The MSE increases again in 67.09 % of cases at 100-hPa level. The increase in MSE in the lower troposphere is mainly due to the contribution of LH. This means that the increase in MSE takes place within the boundary layer where condensation of moisture takes place and cloud formation starts. The decrease in MSE at 500 hPa and above can be attributed to the dry and cold air advection from the north.
At 100 hPa level, the increase in MSF. is mainly due to the contribution of SI-I.
152
f
-k
The MSE of the troposphere over Dhaka on the dates of occurrence of nor'westers at Dhaka increases in most of the cases in the lower troposphere as compared to the weekly normal MSE having the maximum of 75.64% cases at 925-hPa level. The increase in MSE occurs in 51.28, 53.25, 75.64, 66.67 and 6 1.54% of cases at the surface, 1000, 925, 850 and 700 hPa levels respectively. Then the decrease in MSE occurs in most of the cases in the layer 500-200 hPa having 60.26, 62.82, 58.97 and 50.00% of cases at 500, 400, 300 and 200 hPa levels respectively. This decrease in MSE is followed by an increase in 56.41% of cases at 100-hPa level.
V
The comparison of MSE of the troposphere over Dhaka on the dates of occurrence of nor'westers at Dhaka with the daily normal MSE (Table 6.7) also shows that the increase in MSE occurs in most of the cases starting from the surface to 700 hPa level having the increase in maximum cases of 76.92% at 925 hPa level - the top of the boundary layer. The increase in MSE occurs in 50.00, 57.14, 76.92, 66.67 and 61.54% cases at the surface, 1000, 925, 850 and 700 hPa levels respectively. Increase in MSE also occurs in 55.13% of cases at 100 hPa level as compared to the daily normal. In the layer 500-200 hPa, the MSE decreases in most of the cases having 57.69, 57.69, 62.82 and 53.85% of cases at 500, 400, 300 and 200 hPa levels respectively.
Table 6.7: Comparison of MSE over Dhaka on the dates of occurrence of nor'westers at Dhaka with the monthly normal, weekly normal and daily normal
Level (hPa)
Cases of nor' westers
S U d d ie
Monthly normal *Weekly normal *Da ily normal Increase Decrease in Increase Decrease Increase Decrease
in MSE MSE in MSE in MSE in MSE in MSE (% cases) (% cases) (% cases) (% cases) (% cases) (% cases)
Surface 79 48.10 51.90 51.28 48.72 50.00 48.52
1000 78 55.85 46.15 53.25 45.45 57.14 42.86
925 79 69.62 30.38 75.64 24.36 76.92 23.08
850 79 67.09 32.91 66.67 33.33 66.67 33.33
700 78 62.03 37.97 61.54 31.18 61.54 38.46
500 79 32.91 67.09 39.74 60.26 41.03 57.69
400 79 34.18 65.82 37.18 62.82 42.31 57.69
300 79 3 6.7 1 63.29 41.03 58.97 35.90 62.82
200 79 45.57 54.43 50.00 50.00 46.15 53.85
100 79 67.09 32.91 56.41 43.59 55.13 44.87
7 *No. of cases studied is I less than the 2nd column
15 J.)
Table 6.7 substantiates that the MSE of the troposphere over Dhaka on the dates of IL occurrence of nor'westers at Dhaka increases in most of the cases in the lower troposphere having the increase in maximum cases of 69.62, 75.64 and 76.92% at 925 hPa level as compared to the monthly, weekly and daily normals respectively. This increase in MSE is followed by a decrease in the layer 500-200 hPa that is again followed by an increase at 100 hPa level.
6.4.4.3 Comparison of DSE and MSE components on the dates of occurrence of