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, Bangladesh Abstract An attempt has been made to study the variability of the probabilistic frequency of thunderstorm days over Bangladesh during the pre-monsoon season. The monthly and seasonal probabilistic frequency of thunderstorm days have been computed for three time scales - (a) in 1 year out of 4 years, (b) in 1 year out of 10 years and (c) in 1 year out of 20 years - representing relatively frequent events, moderately extreme events and extreme events respectively. The study reveals that the distribution patterns of the monthly probabilistic thunderstorm days during March through May are similar to each other and the seasonal pattern is also the same as the monthly pattern. The probabilistic minimum and maximum frequency of thunderstorm days are highest over Sylhet region and lowest over southern and extreme southeastern parts of Bangladesh, Dinajpur-northern Rajshahi region and Rangmati-Comilla-Maijdi Court region. In 1 year out of 4 years, no thunderstorm activity is likely over southern and extreme southeastern Bangladesh, northern Rangamati region and southern Dinajpur-northern Rajshahi region. In 1 year out of 10 years, Bangladesh likely to have no thunderstorm activity except Sylhet, Dhaka and Rajshahi and in 1 year out of 20 years, no thunderstorm activity is likely over Bangladesh except Sylhet. Introduction The pre-monsoon season (March-May) is the season when most of the severe thunderstorms occur in Bangladesh It is well known that severe thunderstorms are favoured by strong convective instability, abundant moisture at low levels, strong wind shear and a dynamical lifting mechanism that can release instability (Newton, 1963). The intensity of the thunderstorms is intimately connected with the heights of cumulonimbus (Cb) clouds. Over the areas around Delhi in north India, about 73% of the cumulonimbus clouds mature and dissipate below 12 km, half of these dissipating below 9 km itself (Seshadri, 1986). Only about 10% of all cumulonimbus clouds extend beyond 15 km. The space and time distribution of the frequency of thunderstorm days and their variability together

226 with their probabilistic frequency are most essential for aviation and non-aviation purposes. Agriculture is one of the non-aviation needswhich requires the distribution of thunderstorm days, because the thunderstorm days are related to the frequency of thunderstorms as well as the rainfall due to thunderstorms. Some works have been done on the space and time variations of thunderstorms, the physical characteristics of the atmosphere for their formation and frequency distribution of days of thunder in India and Bangladesh (Rao and Raman, 1961; Gupta and Chorghade, 1962; Guha, 1986; Chowdhury and Karmakar, 1986; Chowdhury et al., 1991) but no study has so far been made on the frequency of thunderstorm days and their probabilistic frequency specially in Bangladesh. The present work is an attempt to study the spatial variation of the probabilistic extreme frequency of thunderstorm days over Bangladesh during the pre-monsoon season. The monthly and seasonal probabilistic frequency of thunderstorm days have been computed for three time scales - (a) in 1 year out of 4 years, (b) in 1 year out of 10 years and (c) in 1 year out of 20 years-representing relatively frequent events, moderately extreme events and extreme events respectively. Data used The monthly thunderstorm days over 21 stations of Bangladesh (Fig. 1) during the pre-monsoon season for 22 years (1972-1993) have been taken the Climate Division of Bangladesh Meteorological Department (BMD). It is mentioned that there are few occasions in some stations when data are not available. These data have been considered as missing and have not been used in the statistical computations. It is also important to note that the thunderstorm days are the days of thunderstorms which include surface codes of 17 (i.e. Thunderstorm but no precipitation at the time of observation), 29{i.e. Thunderstorm with or without precipitation, hail, small hail, snow pellets (not used when precipitation is observed at the time of observation)}, and 95-99 [95: Thunderstorm, slight or moderate, without hail but with rain and/or snow at the time of observation; 96: Same as 95 with hail; 97: Thunderstorm heavy, without hail but with rain and/or snow at the time of observation; 98: Thunderstorm combined with dust storm or sandstorm at the time of observation; 99: Thunderstorm, heavy, with hail at the time of observation] (BMD, 1982). Methodology The mean value x and the standard deviation σ of the monthly and seasonal thunderstorm days have been computed for all the stations under consideration. The values x and σ are then used for the computation of the probability of an event by using the Z-score. For the computation of the probability of an event, the z-score equation can be rearranged as (Alder and Roessler, 1964): x' = x Zσ (1)

227 26 Dinajpur Rangpur INDIA 25 Bogra Mymensingh Sylhet Rajshahi Ishurdi Tangail Srimangal 24 Chuadanga Faridpur Dhaka Comilla INDIA Jessore Madaripur 23 22 INDIA Khulna Satkhira Barisal Bhola Patuakhali Khepupara M.Court Sandwip Rangamati Hatiya Chittagong BAY OF BENGAL Coxs_Bazar 21 Teknaf 89 90 91 92 Fig. 1: Station location map

228 Where x' is the probabilistic estimate of monthly and seasonal thunderstorm days during the pre-monsoon season. In the present study, the probabilistic extreme frequency of monthly and seasonal thunderstorm days have been computed for three time sales: (i) (ii) (iii) in 1 year out of 4 years, in 1 year out of 10 years, in 1 year out of 20 years. These time scales represent a relatively more frequent event, less frequent event and the least frequent event respectively. The formulae for the three time scales are: (i) In 1year out of 4 years: x' = x 0.675σ (2) (ii) In 1 year out of 10 years: x' = x 1.280σ (3) (iii) In 1 year out of 20 years: x' = x 1.645σ (4) The Z-values have been calculated using the time scales and statistical tables (Alder and Roessler, 1964). Equation (2) indicates that 75% of the occurrence will be above ( x 0.675 ) and 75% of the occurrence will be below ( x + 0.675 σ) in 1 year out of 4 years. These are the frequent events that will occur in 1 year out of 4 years. Equation (3) indicates that 90% of the occurrence will be above ( x - 1.280σ) and 90% of the occurrence will be below ( x 1.280 ) in 1 year out of 10 years. These are the moderately frequent events that will occur in 1 year out of 10 years. Similarly, equation (4) indicates that 95% of the occurrence will be above ( x 1.645 ) and 95% of the occurrence will be below ( x 1.645 ) in 1 year out of 20 years. These are extreme events that will occur in 1 year out of 20 years. It may be noted that the minimum probabilistic frequencies of thunderstorm days have been found to be negative in some stations where the standard deviations have large values. These negative probabilistic frequencies have been assumed to be zero. The spatial distributions of the monthly and seasonal probabilistic thunderstorm days over Bangladesh have been analyzed and studied. Significance test for the normal distribution of the frequency of thunderstorm days For the computation of the probabilistic extreme of an event, it is necessary to check the normal distribution of the data. The normality of the monthly and seasonal data has been tested for all the stations with the help of the Cornu test

229 (Geary, 1936). It has been found that the monthly and seasonal frequency of thunderstorm days over all the stations of Bangladesh during the pre-monsoon season are normally distributed at 95% level of significance except at Dhaka, Comilla and Satkhira in March, Rangamati in April and normally distributed at 99% level of significance (Table 1). Table 1. Significance of normal distribution of monthly and seasonal thunderstorm days over Bangladesh during the pre-monsoon season Stations March April May Seasonal MD/ Significance MD/ Significance MD/ Significance MD/ Significa nce Dhaka 0.92 99% 0.71 95% 0.85 95% 0.76 95% Faridpur 0.86 95% 0.87 95% 0.80 95% 0.84 95% Mymensingh 0.86 95% 0.73 95% 0.81 95% 0.86 95% Chittagong 0.83 95% 0.78 95% 0.87 95% 0.79 95% Cox s Bazar 0.79 95% 0.77 95% 0.86 95% 0.79 95% Rangamati o.84 95% 0.90 95% 0.80 95% 0.82 95% Comilla 0.92 99% 0.82 99% 0.85 95% 0.79 95% Maijdi Court 0.77 95% 0.85 95% 0.70 99%* 0.82 Sylhet 0.87 95% 0.86 95% 0.86 95% 0.81 95% 95% Sandwip 0.80 95% 0.80 95% 0.82 95% 0.74 95% Teknaf 0.39 Nosig. 0.86 95% 0.75 95% 0.81 95% Khulna 0.81 95% 0.80 95% 0.84 95% 0.86 95% Satkhira 0.89 99% 0.76 95% 0.86 95% 0.80 95% Barisal 0.87 95% 0.79 95% 0.86 95% 0.87 95% Khepupara 0.79 95% 0.76 95% 0.84 95% 0.80 95% Jessore 0.81 95% 0.79 95% 0.83 95% 0.84 95% Rajshahi 0.84 95% 0.77 95% 0.81 95% 0.82 95% Bogra 0.86 95% 0.76 95% 0.86 95% 0.82 95% Ishurdi 0.87 95% 0.78 95% 0.88 99% 0.83 95% Rangpur 0.79 95% 0.86 95% 0.89 99% 0.87 95% Dinajpur 0.83 95% 0.82 95% 0.84 95% 0.79 95% *(For n=16). For n=30, MD/, 0.715 and 0.88 means significance at 99%, MD/ >0.74 and,0.88 means significance at 95%; MD = Mean Deviation

230 Results and discussion Probabilistic frequency of thunderstorm days The probabilistic frequency of monthly and seasonal thunderstorm days over Bangladesh during the pre-monsoon season have been computed for each station and are given in Tables 2 and 3 respectively. The spatial distribution of these probabilistic frequencies of thunderstorm days have been analyzed and studied. Probabilistic frequency of monthly thunderstorm days The spatial distributions of the probabilistic minimum and maximum frequencies of monthly thunderstorm days over Bangladesh for the time scales under study show almost similar patterns in all the months of March through May as given in Figs. 2(a-b). The probabilistic frequency of monthly thunderstorm days has high values over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions, and low values over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region. The area of highest probabilistic frequency of thunderstorm days extends southeastward from Sylhet region to Dhaka region with an elongated part extended southward up to Barisal-Khepupara region. In March, the probabilistic minimum frequency of thunderstorm days over Bangladesh ranges from 0 to 7, 0 to 4 and 0 to 2 in 1 year out of 4 years, 10 years and 20 years respectively (Table 2). In 1 year out of 4 years, the southern and extreme southeastern parts of Bangladesh, northern Rangamati region and southern Dinajpur-northern Rajshahi region have no thunderstorm day (Fig. 2). The high values of the probabilistic minimum frequency of thunderstorm days are 7, 2 and 1 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In 1 year out of 10 years, Bangladesh is likely to have no thunderstorm days except Sylhet, Dhaka and Rajshahi where the probabilistic minimum frequencies of thunderstorm days are 4, 1 and 1 respectively. Similarly, in 1 year out of 20 years, Bangladesh is likely to have no thunderstorm days except Sylhet where the frequency is 2. In April, the ranges of the probabilistic minimum frequency of thunderstorm days over Bangladesh are 1-18, 0-16 and 0-14 in 1 year out of 4 years, 10 years and 20 years respectively. In 1 year out 4 years, the low values of the probabilistic minimum frequency of thunderstorm days are 1-2, 1-3 and 1-2 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region respectively whereas the high values are 18, 4-5 and 2-4 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In 1 year out of 10 years, no thunderstorm day is found over southern and extreme southeastern Bangladesh, northern Rangamati-Maijdi Court region and southern Dinajpur-northern Rajshahi region. The high values of the probabilistic minimum frequency of thunderstorm days are 16, 2-14 and 1-2 over Sylhet,

231 Table 2. Probabilistic frequency of monthly thunderstorm days (1972-1993) over Bangladesh during the pre-monsoon season Stations March April May In 1 year out of 4 years In 1 year out 10 years In 1 year out of 20 years In 1 year out of 4 years In 1 year out 10 years In 1 year out of 20 years In 1 year out of 4 years In 1 year out 10 years In 1 year out of 20 years Min. Max Min. Max Min. Max. Min. Max. Min. Max. Min. Max. Min. Max. Min. Max. Min. Max... Dhaka 2.74 6.03 1.26 7.50 0.37 8.39 6.84 12.12 4.47 14.48 3.04 15.91 9.76 14.72 7.53 16.04 6.19 18.29 Faridpur 1.76 6.24 0 8.26 0 9.47 5.06 10.94 2.43 13.57 0.84 15.16 6.55 13.57 3.40 16.70 1.51 18.60 Mymensingh 1.10 4.26 0 5.68 0 6.54 4.49 9.72 2.14 12.07 0.73 13.48 8.36 16.38 4.77 19.97 2.60 22.14 Chittagong 0.84 4.62 0 6.31 0 7.33 4.02 9.25 1.67 11.60 0.26 13.02 6.74 11.81 4.47 14.08 3.09 15.43 Cox s Bazar 0 3.52 0 5.12 0 6.09 1.74 6.35 0 8.42 0 9.67 6.01 11.35 3.61 13.75 2.17 15.20 Rangamati 0.47 3.78 0 5.26 0 6.15 2.73 7.53 0.58 9.69 0 10.98 3.17 9.63 0.27 12.53 0 14.27 Comilla 1.11 4.89 0 6.58 0 7.60 3.24 7.52 1.33 9.44 0.17 10.59 4.36 11.07 1.36 14.07 0 15.89 Maijdi Court 0.74 4.01 0 5.47 0 6.36 1.29 5.59 0 7.51 0 8.67 2.70 7.43 0.57 9.55 0 10.83 Sylhet 6.73 13.1 3.84 16.0 2.10 17.81 18.35 23.94 15.84 26.45 14.33 27.96 19.42 25.91 16.52 28.82 14.7 30.57 8 7 6 Sandwip 0.18 3.35 0 4.77 0 5.62 1.24 6.19 0 8.41 0 9.75 2.39 6.47 0.57 8.29 0 9.39 Teknaf 0 1.78 0 2.80 0 3.41 1.06 3.88 0 5.15 0 5.91 1.78 6.22 0 8.21 0 9.41 Khulna 1.36 4.95 0 6.56 0 7.54 3.53 8.05 1.50 10.08 0.28 11.30 7.29 12.61 4.90 14.99 3.47 16.43 Satkhira 0.74 2.89 0 3.85 0 4.43 2.02 5.50 0.46 7.06 0 8.01 3.67 8.33 1.58 10.42 0.33 11.68 Barisal 1.38 4.89 0 6.46 0 7.41 3.69 8.86 1.37 11.17 0 12.57 5.16 10.48 2.78 12.86 1.34 14.30 Khepupara 0.38 4.12 0 5.80 0 6.82 1.55 5.66 0 7.49 0 8.60 3.97 8.94 1.74 11.16 0.40 12.51 Jessore 2.08 6.11 0.27 7.93 0 9.02 4.79 8.92 2.94 10.77 1.82 11.89 10.29 14.75 8.30 16.75 7.09 17.96 Rajshahi 1.64 3.91 0.63 4.92 0.01 5.53 4.08 7.74 2.44 9.38 1.45 10.37 6.76 12.15 4.34 14.57 2.88 16.03 Bogra 0.88 3.22 0 4.26 0 4.90 3.11 7.18 1.29 9.00 0.19 10.10 7.28 12.91 4.75 15.44 3.23 16.96 Ishurdi 1.38 4.25 0.10 5.54 0 6.31 4.40 8.23 2.69 9.95 1.65 10.98 6.93 12.25 4.54 14.63 3.11 16.07 Rangpur 0.37 3.25 0 4.54 0 5.31 2.30 6.84 0.27 8.87 0 10.10 6.83 14.12 3.57 17.38 1.60 19.35 Dinajpur 0.35 2.12 0 2.91 0 3.39 1.61 4.54 0.30 5.85 0 6.64 3.92 10.38 1.03 13.28 0 15.03

232 Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively In 1 year out of 20 years, the southern part of Bangladesh south of Satkhira-Khulna- Barisal-Comilla, and northwestern part of the country have no thunderstorm activity. The high values of the probabilistic minimum frequency of thunderstorm days in 1 year out of 20 years are 14 and 1-2 over Sylhet, Jessore-Faridpur- Rajshahi regions respectively. In May, the ranges of the probabilistic minimum frequency of thunderstorm days over Bangladesh are 2-19, 0-17 and 0-15 in 1 year out of 4 years, 10 years and 20 years respectively. In 1 year out of 4 years, the low values of the probabilistic minimum frequency of thunderstorm days are 1-4, 3-4 and 4-5 over southern and extreme southeastern Bangladesh, Rangamati- Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region respectively, whereas the high values are 19, 7-10 and 6-7 over Sylhet, Jessore-Faridpur- Rajshahi and Chittagong-Cox's Bazar regions respectively. In 1 year out of 10 years, the low values of the probabilistic minimum frequency of thunderstorm days are 0-2, 0-2 and 1-2 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court and Dinajpur-northern Rajshahi regions respectively and the high values are 17, 3-8 and 3-4 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In 1 year out of 20 years, the southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region have no thunderstorm day. The high values of the probabilistic minimum frequency of thunderstorm days are 15, 2-7 and 2-3 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In March, the ranges of the probabilistic maximum frequency of thunderstorm days over Bangladesh are 2-13, 3-16 and 3-18 in 1 year out of 4 years, 10 years and 20 years respectively (Table 1). In 1 year out of 4 years, the low values of the probabilistic maximum frequency of thunderstorm days are 2-4, 3-5 and 2-3 over southern and extreme southeastern Bangladesh, Rangamati- Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region respectively, and the high values are 13, 4-6 and 4-5 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In 1 year out of 10 years, the low values of the probabilistic maximum frequency of thunderstorm days are 3-6, 5-7 and 3-4 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region respectively, and the high values are 16, 5-8 and 5-6 over Sylhet, Jessore-Faridpur-Rajshahi and

233 Chittagong-Cox's Bazar regions respectively. In 1 year out of 20 years, the low values of the probabilistic maximum frequency of thunderstorm days are 3-7, 6-8 and 3-4 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region respectively, and the high values are 18, 5-9 and 6-7 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In April, the ranges of the probabilistic maximum frequency of thunderstorm days over Bangladesh are 4-24, 5-26 and 6-28 in 1 year out of 4 years, 10 years and 20 years respectively. In 1 year out of 4 years, the low values of the probabilistic maximum frequency of thunderstorm days are 4-6, 6-8 and 5-6 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region respectively, whereas the high values are 24, 8-11 and 6-9 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In 1 year out of 10 years, the low values of the probabilistic maximum frequency of thunderstorm days are 5-7, 7-10 and 6-8 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region respectively, and the high values are 26, 9-14 and 8-11 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In 1 year out of 20 years, the low values of the probabilistic maximum frequency of thunderstorm days are 6-9, 9-11 and 7-9 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region respectively, whereas the high values are 28, 10-15 and 10-13 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In May, the ranges of the probabilistic maximum frequency of thunderstorm days over Bangladesh are 6-26, 8-29 and 9-31 in 1 year out of 4 years, 10 years and 20 years respectively. In 1 year out of 4 years, the low values of the probabilistic maximum frequency of thunderstorm days are 6-9, 7-11 and 10-11 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region respectively, and the high values are 26, 12-15 and 11-12 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In 1 year out of 10 years, the low values of the probabilistic maximum frequency of thunderstorm days are 8-11, 10-14 and 13-14 over southern and extreme southeastern Bangladesh, Rangamati- Maijdi Court-Comilla and Dinajpur-northern Rajshahi regions respectively, and the high values are 29, 15-17 and 14 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In 1 year out of 20 years, the low

234 Figs. 2(a-b): Spatial distribution of the probabilistic a) minimum thunderstorm days in 1 year out of 4 years in March and b) maximum thunderstorm days in 1 year out of 20 years in May over Bangladesh

235 values of the probabilistic maximum frequency of thunderstorm days are 9-13, 11-16 and 15 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region respectively, and the high values are 31, 16-18 and 15 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. Probabilistic frequency of seasonal thunderstorm days The spatial distributions of the probabilistic minimum and maximum frequency of seasonal thunderstorm days for the three time scales show the same patterns as those of the probabilistic frequency of monthly thunderstorm days and the patterns are shown in Fig. 3(a-b). The ranges of the probabilistic minimum frequency of seasonal thunderstorm days over Bangladesh are 4-48, 1-43 and 0-40 in 1 year out of 4 years, 10 years and 20 years respectively. Table 3: Probabilistic frequency of seasonal thunderstorm days over (1972-1993) Bangladesh during the pre-monsoon season Stations Probabilistic seasonal thunderstorm days In 1 year out of 4 years In 1 year out of 10 years In 1 year out of 20 years Min. Max. Min. Max. Min. Max. Dhaka 22.23 29.96 18.77 33.42 16.68 35.51 Faridpur 15.19 28.91 9.04 35.06 5.33 38.77 Mymensingh 15.01 29.30 8.61 35.71 4.75 39.57 Chittagong 14.14 23.13 10.11 27.16 7.68 29.59 Cox s Bazar 10.44 18.47 6.84 22.07 4.66 24.25 Rangamati 7.71 19.76 2.31 25.16 0 28.42 Comilla 10.39 21.80 5.28 26.91 2.19 30.00 Maijdi Court 5.74 16.02 1.13 20.62 0 23.40 Sylhet 48.31 59.21 43.43 64.09 40.49 67.04 Sandwip 5.03 14.87 0.63 19.28 0 21.94 Teknaf 3.72 10.52 0.67 13.57 0 15.41 Khulna 13.56 24.23 8.78 19.01 5.90 31.89 Satkhira 7.59 15.55 4.02 19.12 1.86 21.28 Barisal 12.23 22.23 7.74 26.71 5.04 29.42 Khepupara 7.19 17.31 2.65 21.85 0 24.59 Jessore 19.47 27.49 15.87 31.08 13.70 33.25 Rajshahi 14.74 21.54 11.69 24.59 9.85 26.43 Bogra 12.81 21.76 8.79 25.78 6.37 28.20 Ishurdi 14.43 23.03 10.58 26.88 8.25 29.20 Rangpur 10.55 23.17 4.89 28.82 1.48 32.23 Dinajpur 6.63 16.30 2.29 20.63 0 23.25

236 In 1 year out of 4 years, the low values of the probabilistic minimum frequency of seasonal thunderstorm days are 4-7, 6-10 and 7-10 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court-Comilla region and Dinajpurnorthern Rajshahi region respectively, and the high values are 48, 14-19 and 10-14 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In 1 year out of 10 years, the low values of the probabilistic minimum frequency of seasonal thunderstorm days are 1-4, 1-5, and 2-5 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region respectively (Table 3), and the high values are 43, 9-16 and 7-11 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In 1 year out of 20 years, the low values of the probabilistic frequency of seasonal thunderstorm days are 0-2, 0-2 and 0-2 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region respectively. This shows that the southern and extreme southeastern parts of Bangladesh, Rangamati-Maijdi Court and Dinajpurextreme northern Rajshahi regions have no thunderstorm activity in 1 year out of 20 years. In this time scale, the high values of the probabilistic frequency of seasonal thunderstorm days are 40, 5-14 and 5-8 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. The ranges of the probabilistic maximum frequency of seasonal thunderstorm days over Bangladesh are 11-59, 14-64 and 15-67 in 1 year out of 4 years, 10 years and 20 years respectively (Table 3). In 1 year out of 4 years, the low values of the probabilistic maximum frequency of seasonal thunderstorm days are 11-17, 16-22 and 16-18 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court- Comilla region and Dinajpur-northern Rajshahi region respectively, whereas the high values are 59, 22-29, and 18-23 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In 1 year out of 10 years, the low values of the probabilistic frequency of seasonal thunderstorm days are 14-22, 21-27 and 21-23 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court- Comilla region and Dinajpur-northern Rajshahi region respectively, and the high values are 64, 25-35 and 22-27 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. In 1 year out of 20 years, the low values of the probabilistic maximum frequency of seasonal thunderstorm days are 15-25, 23-

237 30 and 23-25 over southern and extreme southeastern Bangladesh, Rangamati-Maijdi Court-Comilla region and Dinajpur-northern Rajshahi region respectively, and the high values are 67, 26-39 and 24-30 over Sylhet, Jessore-Faridpur-Rajshahi and Chittagong-Cox's Bazar regions respectively. Conclusions On the basis of the present study, the following conclusions can be drawn: i) The probabilistic minimum and maximum frequencies of monthly and seasonal thunderstorm days have the distribution patterns similar to each other. ii) The probabilistic minimum and maximum frequencies of monthly and seasonal thunderstorm days are highest over Sylhet region and lowest over southern and extreme southeastern parts of Bangladesh, Dinajpur-northern Rajshahi region and Rangamati-Maijdi Court-Comilla region. There exist two other regions of probabilistic high frequency of thunderstorm days. These are Jessore-Rajshahiwestern Faridpur and Chittagong-Cox s Bazar regions. The area of highest frequency of thunderstorm days extends southwestwards from Sylhet region to Dhaka region with an elongated part extended southwards up to Barisal- Khepupara region. iii) In March no thunderstorm activity is likely over southern and extreme southeastern Bangladesh, northern Rangamati region and southern Dinajpurnorthern Rajshahi region. In 1 year out of 10 years, Bangladesh is likely to have no thunderstorm activity except Sylhet, Dhaka and Rajshahi where the probabilistic values are 4, 1 and 1 respectively. In 1 year out of 20 years, no thunderstorm activity is likely over Bangladesh except Sylhet where the probabilistic frequency is 2. In April and May, no thunderstorm activity is likely at a few places in Bangladesh, especially in 1 year out of 10 years and 20 years. In 1 year out of 20 years, no seasonal thunderstorm activity is likely at Rangamati, Maijdi Court, Sandwip, Teknaf, Khepupara and Dinajpur.The probabilistic maximum frequency of thunderstorm days over Bangladesh in 1 year out of 4 years, 10 years and 20 years are significant at Sylhet, Mymensingh-Dhaka, Jessore-Rajshahi-western Faridpur and Chittagong-Cox s Bazar regions during the pre-monsoon season.

238 Acknowledgements The author wishes to express his gratitude to the Director of Bangladesh Meteorological Department for his encouragement in carrying out the present study. The author also wishes to thank Mrs. Jinnatun Nessa, Meteorologist and Mr. Enaitur Rahman Miah, Assistant Meteorologist of the Climate Division of Bangladesh Meteorological Department for their co-operation in providing the relevant data for this study. References Alder, H. L. and Roessler, E. B., 1964. "Introduction to Probability and Statistics". 3rd Ed., W. H. Freeman and Company, pp. 82-84. BMD, 1982. "Weather Observer's Hand Book". 1st Ed.: (D.1)-20 (D.1)-26. Chowdhury, M. H. K. and Karmakar, S., 1986. "Pre-monsoon nor'westers in Bangladesh with case studies". Proceedings of the SAARC Seminar on Local Severe Storms, Dhaka, Bangladesh, pp. 147-166. Chowdhury, M. H. K., Karmakar, S., and Khatun, A., 1991. "A diagnostic study on some aspects of tropospheric energy in relation to nor'westers over Bangladesh". Proceedings of the SAARC Seminar on Severe Local Storms, Colombo, Sri Lanka, pp. 19-33. Geary, R. C., 1936. "Moments of the ratio of the mean deviation to the standard deviation for normal samples". Biometrika, 28, pp. 295-307. Guha, M. K., 1986. "Statistical model on severe local storm variation". Proceedings of the SAARC Seminar on Local Storms, Dhaka, Bangladesh, pp. 73-76. Gupta, H. N. and Chorghade, S. L., 1962. "A Statistical study of thunderstorm activity over Agartala airfield". Indian J. Met. Geophys., 12, 1, pp. 109-114. Hossain, M. A. and Karmakar, S., 1998. Some meteorological aspects of the Saturia tornado, 1989 A case study, Journal of Bangladesh Academy of sciences, 22, 1, pp. 109-122. Newton, C. W., 1963. Dynamics of severe convective storms. Meteorological Monographs, 5, 27, American Meteorological Society, pp. 33-58. Seshadri, N., 1986. A radar study of heights of thunderstorm cloud over north India. Proceedings of the SAARC Seminar on Local Severe Storms, 17-21, January 1985, Dhaka, Bangladesh, Published by BMD, pp. 34-41. Rao, K. N. and Raman, P.K., 1961. "Frequency of days of thunder in India". Indian J. Met. Geophys. 12, 1, pp. 103-108.