REGRESSION FORECASTING OF PRE-MONSOON RAINFALL IN BANGLADESH ABSTRACT

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1 Proceedings of the SAAC Seminar on Agricultural Applications of Meteorology, held on December 2003, published by SMC in 2004 EGESSION FOECASTING OF PE-MONSOON AINFALL IN BANGLADESH SAMAENDA KAMAKA Bangladesh Meteorological Department Dhaka, Bangladesh ABSTACT In this paper the interannual variation of monthly and seasonal country averaged rainfall during the pre-monsoon season has been studied. The correlations between monthly-to-monthly rainfall, monthly to seasonal rainfall during the pre-monsoon season, and between monthly winter rainfall and the monthly and seasonal rainfall during the pre-monsoon season over Dhaka, Chittagong, Khulna, ajshahi, Sylhet and Barisal divisions and also over the country have been studied. It has been found that the monthly country averaged rainfall has interannual variation and it has increasing linear trends during the pre-monsoon season. The rates of increase of country-averaged rainfall are mm/year, mm/year and mm/year in. April and May, respectively. The variation of monthly country averaged rainfall over Bangladesh maintains a similar pattern of variation with year cycle. The country-averaged rainfall of has insignificant correlation with the country-averaged rainfall of April and May. The correlation between rainfall in April and that in May is also insignificant. But the country-averaged rainfall in has strong correlations with the country-averaged rainfall of May and -May i.e. the seasonal rainfall upto of 99% level of significance. The country averaged rainfall in has a moderate correlation with the monthly and seasonal country averaged rainfall during the pre-monsoon season in Bangladesh and the correlation coefficients are significant upto 95% level of significance. The country averaged rainfall in February and the total country averaged rainfall in and February do not have significant correlation with the monthly and seasonal country averaged rainfall. The average rainfall in over Dhaka division has moderate correlation with the rainfall in -May; the correlation coefficient is , which is statistically significant upto a level of 95%. The rainfall in has a moderate correlation coefficient of with the rainfall in -May over Dhaka division and this coefficient is also significant up to 95% level of significance. The average rainfall in over Chittagong division has moderate to good correlation with the average rainfall in April, May and -May (i.e. seasonal rainfall). The rainfall in over Chittagong division has also moderate correlation with the rainfall in -May. The correlation coefficients are , , and respectively. These coefficients are statistically significant up to 95%. Over Sylhet division, the rainfall in has only correlation with the seasonal rainfall, being significant up to 95% level of significance. Over Barisal division, the average rainfall in has moderate correlation with the average rainfall in April and the total average rainfall in -May; the correlation coefficients are and respectively. A number of regression equations for the significant correlations are developed which will be used in the forecasting of monthly and seasonal average rainfall over different divisions as well as over Bangladesh. 1. Introduction Two transition periods between southwest and northeast monsoons over the India- Bangladesh-Pakistan subcontinent are characterized by local severe storms. In Bangladesh, these transition periods are known as pre-monsoon (-May) and post-monsoon (October-November) seasons. Of these, it is the pre-monsoon season when most of the local severe storms occur over different parts of Bangladesh with frequent intervals. These storms 101

2 Proceedings of the SAAC Seminar on Agricultural Applications of Meteorology, held on December 2003, published by SMC in 2004 are popularly known as Nor westers or Kalbaishakhi in Bangladesh, West Bengal and Assam of India and Adhi (dust storms) in North India. Nor wester activities in these parts of the region are directly linked up with the apparent Movement of the sun towards north causing steady rise in temperature over Southern India and gradually extending to the north. Such a situation becomes increasingly prominent from and prolongs up to the end of May. Frequency of nor westers in Bangladesh usually reaches its maximum in May, having maximum value over the Sylhet-Mymensingh-Dhaka region (Karmakar, 1998) and is comparatively minimum over the northwestern and southwestern parts of Bangladesh. There are occasions when nor westers have been observed to occur in late February and early June. The occurrences of nor westers in late February can be attributed to the early withdrawal of winter from Bangladesh, Assam, Bihar, West Bengal and adjoining area as well as due to the penetration of the easterly lows through the southern tip of India and their subsequent movement towards north-east (Das et al., 1994), while those in early June are due to delay in the onset of south-west monsoon over this region. The delay in the occurrence of nor westers over Bangladesh is due to presence of intense subtropical high over the Bay of Bengal. Nor westers/thunderstorms bring the first rain after a long dry and hot weather during the pre-monsoon season. This rain is very important for agriculture. The knowledge of rainfall in any region is very helpful in sound cropping. In India a number scientists made efforts to correlate summer monsoon rainfall with different meteorological parameters (Venkataraman, 1955; Kulkarni and Pant, 1969; Banerjee et al., 1978; Mooley and Shukla, 1989; Upadhyay et al., 1990). But no works have been done to predict the pre-monsoon rainfall in a country like Bangladesh. In the present study, attempts have been made to correlate the monthly and seasonal rainfall during the pre-monsoon season rainfall with that of the previous months in Bangladesh and develop the regression equations accordingly. 2. Data used \Monthly rainfall data for through May for the period 1971 to 2000 for 35 stations (as shown in Fig. 1) of Bangladesh Meteorological Department (BMD) have been collected and utilized in the present study. It may be mentioned that rainfall of some stations are not available for one or two years. These data have been obtained by the interpolation method using win surfer software. 3. Methodology Linear regression equation has been used to correlate the monthly and seasonal country averaged rainfall in Bangladesh (here-in-after called rainfall in Bangladesh) with the rainfall of previous months (predictands) during the pre-monsoon season. The correlation coefficients are also computed. The significant test for the correlation coefficient has been made with the help of Student s t-distribution (Alder and oessler, 1964) given by r t ( n 2) (1 r 2 ) Where r is the correlation coefficient, (n-2) is the degree of freedom and n is the sample size. At 5% level (i.e., 95% level of significance), if t cal t 0.05, the correlation coefficients are said to be significant. 102

3 Proceedings of the SAAC Seminar on Agricultural Applications of Meteorology, held on December 2003, published by SMC in Sayedpur angpur Dinajpur BANGLADESH INDIA 25 Bogra Mymensingh Sylhet I N D I A ajshahi Ishurdi Chuadanga Jessore Satkhira Khulna Tangail Faridpur Mongla Dhaka Chandpur Madaripur Barisal Bhola Patuakhali Comilla M.Court Feni Srimangal Sitakunda Sandwip angamati Hatiya Chittagong A D I N I 22 Khepupara Kutubdia 21 B A Y O F B E N G A L Coxs_bazar Teknaf Fig. 1: Station location map of BMD 4. esults and discussion The interannual variation of monthly and seasonal country averaged rainfall during the premonsoon season has been studied. The correlations between monthly to monthly rainfall, monthly to seasonal rainfall during the pre-monsoon season, and between monthly winter rainfall and the monthly and seasonal rainfall during the pre-monsoon season over Dhaka, Chittagong, Khulna, ajshahi, Sylhet and Barisal divisions and also over the country have been studied. The scattered diagrams for each case have been constructed and the regression equations have been obtained. 4.1 Interanual variation of pre-monsoon rainfall in Bangladesh The interannual variation of monthly country averaged rainfall during the pre-monsoon season has been studied and the linear and polynomial trend curves have been fitted. The variation is shown in Fig

4 Proceedings of the SAAC Seminar on Agricultural Applications of Meteorology, held on December 2003, published by SMC in 2004 May: y = 1E-06x x x x 3 + 2E+08x 2-1E+11x + 4E+13 2 = April: y = -1E-05x x x 4 + 2E+06x 3-3E+09x 2 + 3E+12x - 9E+14 2 = : y = -2E-05x x x 4 + 2E+06x 3-4E+09x 2 + 3E+12x - 9E+14 2 = May: Yellow y = x = April: Pink y = x = : Black y = x = Country-averaged rainfall (mm) Years Fig. Trends of monthly rainfall during the pre-monsoon season of From the figure it is clear that the monthly rainfall has interannual variation and that the monthly rainfall has increasing linear trends in all the months during the pre-monsoon season. The rates of increase are mm/year, mm/year and mm/year in, April and May respectively. The polynomial trends show that the variation of monthly rainfall maintains a similar pattern of variation with year cycle, which implies that there must be correlation among the monthly rainfall during the pre-monsoon season in Bangladesh 4.2 Correlation of country averaged monthly and seasonal rainfall during the pre-monsoon season The correlation of rainfall of with rainfall of April, May and -May is given in Table 1 along with the correlation coefficients. The table shows that the rainfall of has insignificant correlation with rainfall of April. The table also shows insignificant correlation between rainfall in April and that in May. But the rainfall in has strong correlations with the rainfall of May and -May i.e. the seasonal rainfall upto of 99% level of significance. The scattered diagrams of these correlations are prepared and the regression equations are developed. One example of the scattered diagram is given in Fig

5 Proceedings of the SAAC Seminar on Agricultural Applications of Meteorology, held on December 2003, published by SMC in 2004 Table 1: Correlation coefficients of country averaged monthly rainfall and seasonal rainfall during pre-monsoon season Correlation of Correlation coefficient Level of significance ainfall of Vs rainfall of April Insignificance ainfall of Vs rainfall of May Insignificance ainfall of Vs rainfall of April-May Insignificance ainfall of Vs rainfall of -May % ainfall of April Vs rainfall of May Insignificance ainfall in -May y = x = (r= , Sig. 95%) ainfall (mm) in Fig. 3: Scatter diagram of country-averaged rainfall in Vs rainfall in -May during The regression equations corresponding to the significant correlation is: May (1) The equation (1) can be used to forecast the seasonal rainfall during the pre-monsoon season using the rainfall in. 4.3 Correlation of country averaged monthly winter rainfall and the monthly and seasonal rainfall during the pre-monsoon of season Attempts have been made to correlate the country averaged monthly rainfall of and February with that of the monthly and seasonal rainfall during the pre-monsoon season. The correlation coefficients are given in Table 2. The table shows that the rainfall in has 105

6 Proceedings of the SAAC Seminar on Agricultural Applications of Meteorology, held on December 2003, published by SMC in 2004 a moderate correlation with the with the rainfall of, April, May, April-Mayand the seasonal rainfall during the pre-monsoon season in Bangladesh and the correlation coefficients are significant up to 95% level of significance. The rainfall in February has correlation with the rainfall of and total rainfall in and February has correlation with the rainfall in and seasonal rainfall of the pre-monsoon season (-May), the correlation coefficients are significant at 99 % level of significance. The scattered diagrams of rainfall in with the monthly and seasonal rainfall during the pre-monsoon season have been prepared and the regression equations corresponding to the correlation of rainfall with monthly and seasonal rainfall are developed as given below: April May (2) (3) (4) April May May (5) (6) (7) February (8) February May (9) February Equations (2) (9) can be used to forecast corresponding country-averaged monthly and seasonal rainfall during the pre-monsoon season in Bangladesh using the rainfall of and the total rainfall of and February. Table 2: Correlation of country averaged monthly winter rainfall and the monthly and seasonal rainfall during the pre-monsoon season Correlation of Correlation coefficients Level of significance ainfall in Vs rainfall in % ainfall in Vs rainfall in April % ainfall in Vs rainfall in May % ainfall in Vs rainfall in April-May % ainfall in February Vs rainfall in -May % ainfall in February Vs rainfall in % ainfall in February Vs rainfall in April Insignificant ainfall in February Vs rainfall in May Insignificant ainfall in February Vs rainfall in -May Insignificant ainfall in February Vs rainfall in April-May Insignificant ainfall in -February Vs rainfall in % ainfall in -February Vs rainfall in April Insignificant ainfall in -February Vs rainfall in May Insignificant ainfall in -February Vs rainfall in April-May Insignificant ainfall in -February Vs rainfall in -May % 106

7 Proceedings of the SAAC Seminar on Agricultural Applications of Meteorology, held on December 2003, published by SMC in Correlation of average monthly winter rainfall, monthly and seasonal rainfall during the pre-monsoon of season over different divisions The correlation of average monthly rainfall in with the monthly and seasonal average rainfall during the pre-monsoon season has been studied for the six divisions such as Dhaka, Chittagong, Sylhet, ajshahi, Khulna and Barisal of Bangladesh. The correlation of average monthly rainfall in with the monthly and seasonal average rainfall during the pre-monsoon season has also been studied. The correlation coefficients are given in Table 3. Table 3: Correlation of average monthly winter rainfall, monthly and seasonal average rainfall during the pre-monsoon season over different divisions Divisions Correlation of Correlation coefficients Level of significance Dhaka ainfall in Vs rainfall in Insignificant ainfall in Vs rainfall in April Insignificant ainfall in Vs rainfall in May Insignificant ainfall in Vs rainfall in -May % ainfall in Vs rainfall in April Insignificant ainfall in Vs rainfall in May Insignificant ainfall in Vs rainfall in -May % Chittagong ainfall in Vs rainfall in Insignificant ainfall in Vs rainfall in April % ainfall in Vs rainfall in May % ainfall in Vs rainfall in -May % ainfall in Vs rainfall in April Insignificant ainfall in Vs rainfall in May Insignificant ainfall in Vs rainfall in -May % Sylhet ainfall in Vs rainfall in Insignificant ainfall in Vs rainfall in April Insignificant ainfall in Vs rainfall in May Insignificant ainfall in Vs rainfall in -May Insignificant ainfall in Vs rainfall in April Insignificant ainfall in Vs rainfall in May Insignificant ainfall in Vs rainfall in -May % ajshahi ainfall in Vs rainfall in Insignificant ainfall in Vs rainfall in April Insignificant ainfall in Vs rainfall in May Insignificant ainfall in Vs rainfall in -May Insignificant ainfall in Vs rainfall in April Insignificant ainfall in Vs rainfall in May Insignificant ainfall in Vs rainfall in -May % Khulna ainfall in Vs rainfall in Insignificant ainfall in Vs rainfall in April Insignificant ainfall in Vs rainfall in May Insignificant ainfall in Vs rainfall in -May Insignificant ainfall in Vs rainfall in April Insignificant ainfall in Vs rainfall in May Insignificant ainfall in Vs rainfall in -May % Barisal ainfall in Vs rainfall in Insignificant ainfall in Vs rainfall in April % ainfall in Vs rainfall in May Insignificant ainfall in Vs rainfall in -May % ainfall in Vs rainfall in April Insignificant ainfall in Vs rainfall in May Insignificant ainfall in Vs rainfall in -May % 107

8 Proceedings of the SAAC Seminar on Agricultural Applications of Meteorology, held on December 2003, published by SMC in 2004 The table shows that the average rainfall in has moderate correlation with the average rainfall in -May over Dhaka division; the correlation coefficient is , which is statistically significant upto a level of 95%. The average rainfall in has a moderate correlation coefficient of with the average rainfall in -May over Dhaka division and this coefficient is also significant up to 95% level of significance. The average rainfall in over Chittagong division has moderate to good correlation with the average rainfall in April, May and -May (i.e. seasonal rainfall). The average rainfall in over Chittagong division has also moderate correlation with the average rainfall in -May. The correlation coefficients are , , and respectively. These coefficients are statistically significant up to 95%. Over Sylhet division, the average rainfall in and does not have any correlation with the monthly and seasonal average rainfall during the pre-monsoon season. But the rainfall in has only correlation with the seasonal rainfall (up to 95%) over Sylhet division and not with the rainfall of other months. Same is the case over ajshahi and Khulna divisions except that the average rainfall in has only statistically significant correlation with the average rainfall in -May, the correlation coefficients being and respectively (both significant at 95% level of significance). Over Barisal division, the average rainfall in has moderate correlation with the average rainfall in April and the total average rainfall in -May; the correlation coefficients are and respectively. The scattered diagrams for the above significant correlations are prepared (e.g. Fig. 4) and the regressions so obtained given below. Dhaka Division May May (10) (11) 108

9 Proceedings of the SAAC Seminar on Agricultural Applications of Meteorology, held on December 2003, published by SMC in 2004 Chittagong Division April May (12) (13) May May Sylhet Division (14) (15) May (16) ajshahi Division (17) May Barisal Division April May May (18) (19) (20) The rainfall in February and total rainfall in and February have no significant correlation with the monthly and seasonal rainfall in any division. 5. Conclusions On the basis of the results presented in this paper, the following conclusions can be drawn: i) The monthly country averaged rainfall has interannual variation and it has increasing linear trends during the pre-monsoon season. The rates of increase of country-averaged rainfall are mm/year, mm/year and mm/year in. April and May respectively. The variation of average monthly rainfall over Bangladesh maintains a similar pattern of variation with year cycle. ii) The country-averaged rainfall of has insignificant correlation with the country-averaged rainfall of April and May but it has a statistically significant correlation with the seasonal rainfall during the pre-monsoon season. The correlation between rainfall in April and that in May is also insignificant. The country-averaged rainfall in has a moderate correlation with the monthly rainfall of, April, May, the total rainfall of April and May and 109

10 Proceedings of the SAAC Seminar on Agricultural Applications of Meteorology, held on December 2003, published by SMC in 2004 the seasonal country-averaged rainfall during the pre-monsoon season in Bangladesh, the correlation co-efficient are significant at 95% level of significance. iii) iv) The country-averaged rainfall in February has correlation with the rainfall in, and the total country-averaged rainfall of and February has correlation with the rainfall in and also with the rainfall march-may, the coefficients during the pre-monsoon season in Bangladesh and the correlation coefficients are significant at 95% level of significance. The country averaged rainfall in February and the total country averaged rainfall in and February do not have significant correlation with the monthly and seasonal country averaged rainfall. v) The average rainfall in over Dhaka division has moderate correlation with the rainfall in -May; the correlation coefficient is , which is statistically significant at 95% level. The rainfall in has a moderate correlation coefficient of with the rainfall in -May over Dhaka division and this coefficient is also significant up to 95% level of significance. vi) vii) viii) ix) The average rainfall in over Chittagong division has moderate to good correlation with the average rainfall in April, May and -May (i.e. seasonal rainfall). The rainfall in over Chittagong division has also moderate correlation with the rainfall in -May. The correlation coefficients are , , and respectively. These coefficients are statistically significant at 95% level. Over Sylhet division, the average rainfall in does not have any correlation with the monthly and seasonal average rainfall during the premonsoon season. But the rainfall in has only correlation with the seasonal rainfall (significant at 95%) over Sylhet division. Same is the case over ajshahi and Khulna divisions except that the average rainfall in has only statistically significant correlation with the seasonal rainfall in -May, the correlation coefficient being and respectively (both significant at 95% level of significance). Over Barisal division, the average rainfall in has moderate correlation with the average rainfall in April and the seasonal rainfall in -May; the correlation coefficients are and respectively. egression equations corresponding to the significant correlation are developed which will be useful in the forecasting of monthly and seasonal average rainfall over different divisions as well as over Bangladesh. Acknowledgement The author wishes to thank the Director of Bangladesh Meteorological Department for providing the relevant data and Mr. S.M. Quamrul Hassan, Assistant Meteorologist for his painful job in the compilation of relevant data. 110

11 Proceedings of the SAAC Seminar on Agricultural Applications of Meteorology, held on December 2003, published by SMC in 2004 eferences Alder, H. L. and oessler, E. B., "Introduction to Probability and Statistics". 3rd Ed., W. H. Freeman and Company, pp Banerjee, A.K., Sen, P.N., and aman, C..V., 1978, On fore-shadowing southwest monsoon rainfall over India with mid-tropospheric circulation anomaly of April, Indian J. Met. Hydrol. Geophys., 29, 1 and 2, pp Das,.C., Munim, A.A., Begum, Q.N. and Karmakar, S., 1994, A diagnostic study on some local severe storms over Bangladesh, Journal of Bangladesh Academy of Sciences, 18, 1, pp Samarendra Karmakar, 1998, "Climatology of thunderstorm frequency over Bangladesh during the pre-monsoon season" (Not yet published). Kulkarni, N.S. and Pant, M.B., 1969, The cumulative frequency distribution of rainfall of different intensities, Indian J. Met. Geophys., 30, 2, pp Mooley, D.A. and Shukla, J., 1989, Index of activity of the monsoon trough over India, Mausam, 40, 3, pp Upadhyay, D.S., Kaur, S., Misra, M.S. and Gupta, M.K., 1990, Space correlation structure of rainfall over India, Mausam, 41, 4, pp Venkataraman, K.S., 1955, Study of Sutcliffe s theory of development in relation to rainfall in Indian area, Indian J. Met. Geophys., 6, 1, pp

PROBABILISTIC EXTREMES OF THE FREQUENCY OF THUNDERSTORM DAYS OVER BANGLADESH DURING THE PRE- MONSOON SEASON

225 PROBABILISTIC EXTREMES OF THE FREQUENCY OF THUNDERSTORM DAYS OVER BANGLADESH DURING THE PRE- MONSOON SEASON Samarendra Karmakar Bangladesh Meteorological Department, Sher-e-Bangla Nagar, Agargaon Dhaka-1207,