Bangladesh is one of the countries at particular risk of devastating lightning mortality and morbidity. A lightning fatality database in Bangladesh developed by Dewan et al. (2017a) from 1990 to mid-2016 reported a total of 5468 casualties, composed of 3086 fatalities and 2382 injuries. They also reported more fatalities during the pre-monsoon season of March through May than during the monsoon of June through September through the year. Kumar and Kamra (2012a) and Siingh et al. (2014) mentioned Bangladesh as one of the most lightning-prone regions in Indian subcontinent mostly due to orography and local meteorological factors. The annual lightning death toll for Bangladesh was estimated 500 to 1000 by Ono and Schmidlin (2011). As the lightning contributed more than 25% of the total electricity-related casualties in Bangladesh (Mashreky et al., 2012), a precise synoptic anomaly based lightning climatology is of utmost important to reliable weather forecasts (Kandalgaonkar et al., 2010) and safety education (Roeder et al., 2015).
Lightning casualties are however, the highest during the months from April to June with its peak on May (Farukh et al., 2017; Karmakar, 2001). Thunderstorm formation and frequency distribution with their physical characteristics were documented for India and Bangladesh by Rao and Raman (1961), Gupta and Chorghade (1962), Guha (1986), Chowdhury and Karmakar (1986) and Chowdhury et al. (1991). In Bangladesh, the lightning activity increases considerably since the past few years with the annual fatality rates of 0.9 (Gomes et al., 2006), but they appear to be lower than expected in terms of the reported deaths (Holle and Lopez, 2003). The neighboring countries like India got a total of 1,755 deaths per year (Illiyas et al., 2014) and 49 annual fatalities were reported for SriLanka (Gomes et al., 2006). The topic of multiple lightning fatalities within a short period drew a great deal of attention during the last several years in Bangladesh. The month of May has both the most lightning deaths and the most lightning strokes with frequent alarmist news reports every year when multiple lightning fatalities occur (Holle and Islam, 2017). Eight of the top ten lightning fatality days since 1990 were in May and the other two were in early June (Dewan et al., 2017a). They also concluded that, the very frequent strokes in April, May, and June indicate the most vulnerable time of year for lightning in Bangladesh. The country’s maximum fatality during the pre-monsoon season occurs due to frequent lightning coincident with labor-intensive agricultural practices. In Bangladesh, lightning remained an underestimated natural hazard (Dlamini, 2009) whereas, the majority of the population continues to be engaged in subsistence agriculture (Holle, 2016b), live and work in lightning-unsafe environments (Holle, 2009). For April and May, the lightning fatalities are frequent during both morning and afternoon. An average of 1.73 deaths per day in the pre-monsoon, 0.71 in the monsoon, and very small averages in other seasons is reported from 2013 to 2017 in Bangladesh (Holle et al., 2019). This study also found that all of the top ten fatality days in recent years, comprised of 19 to 51 deaths on each day, occurred during May and early June. Dewan et al. (2017b) pointed out that the lightning injury locations in subtropical and equatorial regions have sharply defined maxima related to the local timing of the arrival of the monsoon. The Global Lightning Dataset (GLD360) network detected 23.9 million strokes in the pre-monsoon, 11.4 million in the monsoon, 1.6 million in the post-monsoon, and 0.4 million in the winter season over Bangladesh with an average of 434,043; 2,049,779; 2,294,359; and 1,085,439 for March, April, May, and June, respectively (Holle et al., 2019). The pre-monsoon season daily lightning strokes detected by is around 300,000 (Holle and Cooper, 2019).
The tropical regions are estimated to account for 78% of global lightning (Christian et al., 2003) where there are marked elevation changes and land–water boundaries (Albrecht et al., 2016; Holle and Murphy, 2016). Murugavel et al. (2014) identified convective available potential energy (CAPE), orography and prevailing local meteorological conditions are the causes of thunderstorm formation and lightning is northeast India. The highest lightning frequencies are related to the regions of greatest instability (Kandalgaonkar et al., 2003; Williams, 2005) over a country’s atmosphere. Therefore, the lightning discharges in thunderstorms are due to atmospheric convection (Petersen et al., 1996) occurs as a result of heating of the boundary layer by solar radiation during the day or by the mixing of air masses of different densities. The resultant lightning activity is thus an indication of convective rainfall (Petersen and Rutledge, 1998) and distribution of thunderstorms (Chaudhuri and Middey, 2013) for a particular area. The increased frequency and intensity of lightning could rise more lightning fatalities (Zhang et al., 2011). Several studies reported that the northeast parts of Indian subcontinent are a lightning hotspot with unique spatial and temporal attributes (Lal and Pawar, 2009; Ranalkar and Chaudhari, 2009; Kandalgaonkar et al., 2003, 2005, 2010; Dewan et al., 2017a; Tinmaker et al., 2010, 2014, 2015; Murugavel et al., 2014; Siingh et al., 2014; Chaudhuri and Middey, 2013; Tinmaker and Chate, 2013; Tinmaker and Ali, 2012; Kumar and Kamra, 2012a, b; Nath et al., 2009). Denoting Bangladesh as a lightning hotspot only a few studies have been reported like Karmakar (2001), Chowdhury and De (1995), Mashreky et al. (2012), Siingh et al. (2014), Saha and Quadir (2016), SMRC (2010), Karmakar and Alam (2005), Tinmaker and Chate (2013), Nath et al. (2009), Holle et al. (2019), Dewan et al. (2017a,b).
Since past decade a growing number of studies around the world has started to quantify lightning hazard with a goal of developing initiatives to mitigate the impacts of lightning. As the lightning casualty has been identified as one the main causes of weather-related deaths in Bangladesh, research on lightning activity in regard to synoptic climatology is worth to go through. A couple of studies have been conducted on synoptic climatology context but particularly on Indian territory to characterize spatial and temporal variations, model relationships, satellite based lightning activity etc. (Holle et al., 2019; Albrecht et al., 2016; Yuan et al., 2016; Cecil et al., 2015; Dai et al., 2009; Ranalkar and Chaudhari, 2009; Kandalgaonkar et al., 2005; Kodama et al., 2005; Qie et al., 2003; Bond et al., 2002; Boccippio et al., 2001; Cardoso et al., 2014). Lightning characteristics along with worldwide lightning activity over land have also been documented by numerous studies (Turman and Edgar, 1982; Orville and Henderson, 1986; Christian et al., 1999).
Farukh et al. (2011a, b) characterized lightning occurrence in Alaska using various instability indices. The CAPE, a conditional instability parameter of the tropical atmosphere (Williams and Renno, 1993), appears to play a vital role in the occurrence of lightning in Bangladesh. The Lifted Index (LI) is a measure of upper level instability considering elevated convection which tends to occur when upper level disturbances move across unstable equilibrium environment aloft (Tinmaker et al., 2017) whilst, negative LI indicates the possibility of convection (Basu and Mondal, 2002; Chaudhari et al., 2010; Litta et al., 2012). The K Index (KI) has been proved useful in indicating the probability of air mass thunderstorms (Anderson, 1991, Holle et al., 1992, Reap, 1994, and Reap and Foster, 1979). KI values higher than 35ºC indicate high probability for the development of numerous and/ or severe thunderstorms (Tinmaker et al., 2017). Nag et al. (2017) indicated that the annual, seasonal, and diurnal cycles of lightning occurrence are unique to Bangladesh and this combination does not occur elsewhere in the Indian subcontinent. Apart from factors noted above, enhanced aerosol in the atmosphere caused by increased anthropogenic activities over the country during the last few decades may also influence enhanced lightning activity (Siingh et al., 2014; Pathak and Bhuyan, 2014; Kar et al., 2009; Bell et al., 2008). The typical pre-monsoon seasonal maximum in lightning fatalities in Bangladesh can now be placed into the context of knowing its synoptic climatology thoroughly. Therefore, the present piece of work explores the synoptic climatology behind severe lightning occurrence during the pre-monsoon season over Bangladesh that neither of the study includes. Three connected research questions are addressed: (i) what is the scenario of pre-monsoon severe lightning events over Bangladesh in terms of death, (ii) what are the synoptic climatological features behind this severe weather phenomena, and (iii) what are conditions of atmospheric stability indices related to pre-monsoon severe lightning events? The issues are essentially relevant for Bangladesh where near real-time weather forecasting for lightning activities is unavailable due to limitations in resources and technical expertise. The order of presentation on these issues are: section 2 will be a description of data and methodology used to conduct the study; section 3 will be focusing on results and discussions on pre-monsoon death pattern, pre-monsoon surface and upper temperature conditions, anomaly in pre-monsoon synoptic features based on 1981–2010 climatology, results of instability indices using radiosonde parameters; and finally the study summarizes by linking the above results in section 4.