ND R'WE STE RB
5.4 Results and discussion
5.4.1 Precipitable water content of the troposphere and its relation with instability indices and rainfall due to nor'westers
5.4.1.2 Correlation of precipitable water content of the troposphere with different instability indices
Since water vapour goes up into the troposphere due to the presence of convergence and instability of the troposphere, it may have some relation with the instability indices.
Keeping this fact in mind, attempts have been taken to correlate the precipitable water content of the troposphere with different instability indices. The results are given in Table 5.1.
Table 5.1: CC of precipitable water with different instability indices at 0000 UTC over Dhaka
Correlation of CC
precipitable _water _with
SEE Regression Equations
TT 0.35574** ±6.63 W = 0.3692*TT+19.299
SWI 0.58237** ± 5.76 W = 0.033*SWI+27.3 14
SWI/TT 0.561696** ± 5.86 W= 1.6937*SWIITT+26.431
MSWI 0.36005** ± 6.61 W= 0.02*MSWI+26.808
El 0.67304** ± 7.09 W = 0.6864*EI+31.709
MEl 0.41686** ± 6.44 W = 0.4449*MEI+3 1.432
** Significant at 99% level of significance
The table shows that the precipitable water content of the troposphere at 0000 UTC over Dhaka has significant correlations with different instability indices namely TT, SWI, SWI/TT, MSWI, El and MEl of the troposphere at 0000 UTC over Dhaka. The CC is significant at 99% level. SEE is significantly smaller ranging between 5.76 and 7.09. The regression equations corresponding to the significant correlations are developed with the help of scatter diagrams. The equations will be useful in the computation of precipitable water content of the troposphere.
5.4.1.3 Specific humidity and weighted average specific humidity (WAq) of the troposphere at 0000 UTC in relation to nor'westers
The specific humidity is maximum near the surface and generally decreases with height for obvious reason as can be seen from Fig. 5.4. It has been found that the specific humidity may be of the order of 15-20 g kg near the surface on the dates of occurrence of nor'westers. The specific humidity has been found to increase on the dates of occurrence of nor'westers in Bangladesh as can be seen from Fig 5.4.
I
121
100 200 300 400 500 600 700 800 900 1000
4 0 5 10 15 20
Difference in specific humidity in gfkg
Fig. 5.4:Vertical cross-section of the difference in specific humidity on 8 May (date of occurrence of nor'westers) and 5 May (date of non-occurrence of nor'westers) 1994 over
Dhaka at 0000 UTC
Frequency of WAq of the troposphere over Dhaka on the dates of occurrence of nor'westers in Bangladesh has been computed and the diagrams are prepared accordingly.
A
One example is given in Fig. 5.5.
50 45 40
- 35 30
25 20 15 10 0
9
17
'0 00
o 0 ID -
— '0 - 0 0 C
Weighted average specific humidity (g/kg)
Fig. 5.5: Frequency of WAq of the troposphere between 1000-500 hPa over Dhaka at 0000 UTC on the dates of occurrence of nor'westers in Bangladesh during 1990-1995 The frequency of WAq also indicates the frequency of nor'westers. It has been found that the maximum number of nor'westers occur when the WAq between the surface and 500 hPa is 8-12 g kg', the maximum frequency being 43 out of 108 (i.e. 39.18%) in the range of 8-10 g kg*
The spatial distribution of WAq between the surface and 500 hPa on the dates of nor'westers has also been studied. The distribution pattern is given in Fig. 5.6 as an example.
It has been found that nor'westers occur near or at the eastern/northeastern end of maximum 122
WAq. the maximum WAq being 14 -16 g kg in most of the cases. Nor'westers have been found to occur near the point of inter-section of the axes of moist and dry zones.
35
::
20 -
9.7
-' '5--
)
•-'to-
U
-1 70 75 80 85
-
90 95 100
Fig. 5.6: Spatial distribution of WAq at 00 UTC on 14 May 1992 5.4.1.4 Statistical prediction of 24-hr rainfall due to nor'westers in Bangladesh
With a view to predicting 24-hour rainfall due to nor'westers, attempts have been made to correlate a number of parameters of the troposphere over Dhaka at 0000 UTC on the dates of occurrence of nor'westers such as precipitable water (mmlhr), MSWI, SWI, SWI/TT, difference in specific humidity at 1000 and 850 hPa (ql000-q850), WAq with next 24-hour rainfall at Dhaka, next 24-hour maximum rainfall in Bangladesh and country averaged 24- hour rainfall.
The correlation co-efficient, regression equations corresponding to the significant CC and the SEE has been computed. The results are given in Table 5.2.
The table shows that the CC are relatively small but are statistically significant and SEE are higher. The small CC are significant because of the large number of data (usually 106 or 108). Since the SEE are large, the regression equations cannot be used to predict the 24-hour rainfall due to nor'westers. Therefore, it is very difficult to predict rainfall due to nor'westers statistically.
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Table 5.2: CC of 24-hr rainfall with different parameters on the dates of occurrence of nor' westers
Correlation of CC SEE Regression Equations
W (mrn/hr) at 0000 UTC vs 0.24915** ± 8.43064 y = 0.306x - 1.3696 next 24-hr CArain
x=W. y = CArain MSWI at 0000 UTC vs next 0.30425** ± 8.52495 y0.0207x-0.1335 24-hr CArain in Bangladesh
x=MSWl, y = CArain W (mm/hr) at 0000 UTC vs 0.24660* ± 20.22573 y = 0.7261 x - 11.136 next 24-hr rainfall at Dhaka
xW, y=next 24-hr at Dhaka
W (mm/hr) at 0000 UTC vs 0.23215* ± 35.41168 y = I.I923x + 13.296, x = W, y =
next 24-hr maximum rainfall next 24-hr maximum rainfall in
in Bangladesh Bangladesh
LnW at 000 UTC vs 24-hr 0.24941** ±20.23195 y=20.977x-59.876 rain at Dhaka
x=lnW. y=next 24-hr rain at Dhaka LnW at 0000 UTC vs next 0.227099* ± 35.46158 y = 37.199x - 76.594, x = In W, 24-hr maximum rainfall in y=next 24-hr maximum rain in
Bangladesh Bangladesh
LnW at 0000 UTC vs 24-hr 0.245557** ± 8.43861 y = 10.484x - 27.796, x = In W,
Carain in Bangladesh y=next 24-hr CArain in Bangladesh
SWI at 0000 UTC vs next 24- 0.22718* ± 20.32454 y = 0.0379x + 4.8865, x = SWI,
hr rainfall at Dhaka y=next 24-hr rain at Dhaka
SWI at 0000 UTC vs next 24- 0.29504** ±34.78567 y = 0.0859x + 33.154, x = SWI, hr maximum rainfall in y=next 24-hr rain in Bangladesh Bangladesh
MSWI at 0000 UTC vs next 0.19853* ± 35.68164 y0.0566x+29.651 24-hr maximum rainfall in
xMSWI, y=next 24-hr maximum Bangladesh
rain in Bangladesh SWI/TT at 0000 UTC vs next 0.25588** ± 35.19424 y= 3.9628x+ 33.453 24-hr maximum rainfall in
xSWI/TT, y=next 24-hr maximum Bangladesh
rain in Bangladesh
q1000-q850 at 0000 UTC vs next 0.212758* ± 20.39245 y = -1,0167x + 22.956, x=q1000-q350,
24-hr rainfall at Dhaka y=next 24-hr rain at Dhaka
q1000-q850 at 0000 UTC vs next 0.32268** ± 8.239486 y = -0.6432x + 14.627, x=q1000-q850.
24-hr CArain in Bangladesh y= next 24-hr Carain in Bangladesh WAq at 0000 UTC vs next 0.24844** ± 20.2159 y = 3.1863x - 17.265, x=WAq,
24-hr rainfall at Dhaka y=next 24-hr rain at Dhaka
WAq at 0000 UTC vs next 0.22882* ± 35.44041 y5.1191x+4.3875 24-hr maximum rainfall in
x=WAq, y=next 24-hr maximum Bangladesh
rain in Bangladesh WAq at 0000 UTC vs next 0.20912* ±8.51268 y1.1187x-L6665 24-hr CArain in Bangladesh
x=WAq, y=CArain in Bangladesh
CArain = Country Averaged rain, W = Precipitable water, * Significant at 95% level of significance, ** Significant at 99% level of significance.
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