CLIMATOLOGY OF THUNDERSTORMS AND THUNDERSTORM DAYS
2.1 Introduction
Thunderstorms are the mcso-sca!e phenomena, which develop from cumuloninibus clouds and are characterized by lightning discharges. They are often accompanied with strong gusts, hail, heavy rains and sometimes tornadoes. Because of the associated strong gusts and hails, these storms create hazards to aviation and river navigation in a country like Bangladesh, and cause enormous damage to standing crops, lives and properties every year.
In Bangladesh and the adjoining states of India, thunderstorms of destructive nature, locally called nofwester or Kalbaishakhi", occur during the pre-monsoon season (March-May) when the temperature begins to rise over these areas and attains maximum in April or May.
During this period, drought like weather sometimes prevails over some parts of Bangladesh and this has a significant negative impact on the agricultural products and so to the economy of the country. As a result, a thunderstorm with rain or shower has a beneficial effect on agricultural products though sometimes the destruction is also enormous. The distribution of
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nor' westers/thunderstorms along with thunderstorms days and their probabilistic frequency, both in time and space, over Bangladesh is important for aviation and agricultural purposes.
Besides these, the climatology of thunderstorms, the inter-annual variability and trends of thunderstorm frequency are important for the operational meteorologists and the agriculturists of a country like Bangladesh.
In India and Bangladesh, 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 (Bhan 1955; Rao and Raman, 1961; Williams 1961; Gupta and Chorghade, 1962; Guha, 1986; Chowdhury and Karmakar, 1986;
Chowdhury et al., 1991). Kamblc and Kanoujia (1990) made a critical analysis of the multiple squalls over central India (Vidarbha and Madhya Pradesh) for the 10-year period from 1969 to 1978. They also examined various characteristics of the multiple squalls and
their association with weather parameters. According to them, the frequency of occurrence of multiple squalls is maximum in the months of May and June at Nagpur and Bhopal whereas it is maimum in April and May at Jagdalpur. Three-hourly period between 0930-1230 UTC is the most favourable period for the development of multiple squalls over central India. But no study has so far been made on the frequency of thunderstorms and thunderstorm days in Bangladesh.
In the present chapter, an attempt has been made to study the temporal and spatial variation and the variability of monthly and seasonal thunderstorm frequency and the frequency of thunderstorm days over Bangladesh during the pre-monsoon season. The inter- annual variability and the trends of thunderstorm frequency have also been studied. The optimum time of occurrence of nor'westers/lhunderstorms has also been identified.
2.2 Data used
Source: Climate Division of Bangladesh Meteorological Department (I3MD) Period: (a) March through May, i.e. pre-monsoon season.
(b) 1972-1993 (i.e. 22 years).
Stations: 22 stations all over Bangladesh (as shown in Fig. 2.1)
Variable: Monthly nor'westers/thunderstorms and thunderstorm days, which include surface codes of 17, 29 and 95-99 [17(i.e. Thunderstorm but no
4 precipitation at the time of observation), 29 {i.e. Thunderstorm (with or without precipitation) + hail, small hail, snow during the preceding hour but not 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: Thunderstorm, slight or moderate, with hail at the time of observation; 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)1 and the dates and time of occurrence of nor' westers/thunderstorms.
It is important to note that some data are not available in some stations for a few years. These data have been considered as missing and have not been used in the statistical computations.
S
• Bogra • Syihet
Mymensingh
Rajshalli I
.5 Srirnangal lshurdi
'-
1aridptr
• Cornilla
• Jessore
• i thh1Th •Majdi Court
Satkl
'J ,and.yip Ranarnati
•f, '•)
•
Chittagong1
' Khêpiipara
a
1.
'Cox's Bazar Bay of Bengal
•Tek af
89 90 91 92
Fig. 2.1: Stations location map 2.3 Methodology
The dates and time of occurrence of nor'westers/thunderstorms have been used to identify the optimum time of occurrence of maximum thunderstorms/nor' westers in Bangladesh. Monthly frequency of nor'westers/thunderstorm and thunderstorm days is used to obtain the seasonal frequency of thunderstorms and thunderstorm days. These monthly and seasonal frequencies of nor'westers/thunderstorm and thunderstorms days are used to compute their mean, standard deviation and co-efficient of variation (CV). The CV of monthly and seasonal thunderstorms are computed by the relation:
lOOxa
19
1.
26 I)
25
24
23
22
21
35 30 25
4
20 15 10 5 0
31
/420
Where x is the mean frequency of nor'westers/thunderstorms or thunderstorm days and cv is the respective standard deviation. The spatial and temporal distributions of monthly mean frequency of thunderstorm and thunderstorm days with their CV and spatial distribution of seasonal thunderstorm frequency and its CV over Bangladesh have been analyzed.
The country-averaged frequency of thunderstorms has been obtained by averaging the frequency of21 stations in each year.
4'
2.4 Results and discussion