Atmospheric Research 95 (2010) 400–406 Contents lists available at ScienceDirect Atmospheric Research journal homepage: www.elsevier.com/locate/atmos Severe local convective storms in Bangladesh: Part I. Climatology Yusuke Yamane a,⁎, Taiichi Hayashi b, Ashraf Mahmmood Dewan c, Fatima Akter d a Pioneering Research Unit for Next Generation, Kyoto University, Gokasho, Uji, Kyoto, Japan b Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, Japan c Department of Geography and Environment, University of Dhaka, Dhaka - 1000, Bangladesh d Habitat for Humanity International - Bangladesh, Dhaka - 1212, Bangladesh article info abstract Article history: This paper presents the climatology of severe local convective storms in Bangladesh. We Received 13 May 2008 collected sufficient severe local convective storm events over a long period (2,324 events from Received in revised form 30 October 2009 1990 to 2005) to compile reliable climatological statistics by surveying the literature. Severe Accepted 9 November 2009 local convective storms frequently occur during the pre-monsoon season from March to May, peaking in April. The highest frequency was found to be at 16–17 Local Standard Time (LST) Keywords: and 20–21 LST, with lower activity around midnight and early morning. It was also found that Severe local convective storm the occurrence of severe local convective storms is more concentrated in the central region of Climatology Bangladesh Bangladesh, and tends to spread northward during the pre-monsoon season. © 2009 Elsevier B.V. All rights reserved. 1. Introduction ical department reports and newspapers. They reported that “tornado” and nor'wester occurred most frequently in the Bangladesh is located in the northeastern part of the Indian Dhaka district during the pre-monsoon season. In addition, it subcontinent and faces the Bay of Bengal to the south and the was reported that the most frequent time of occurrence was in Meghalaya plateau to the northeast. Almost the entire country is the evening (in their paper, however, the exact period of less than 10 m above sea level and on a flat plane. Severe Local “evening” is not clearly specified). Convective Storms (henceforth referred to simply as SLCS) Chowdry and Karmakar (1986) investigated the climatol- frequently occur in Bangladesh during the pre-monsoon season ogy of nor'westers from 1964 to 1973 in Bangladesh based on from March to May, causing deaths and damage to property the local literature. They reported that nor'westers occurred every year. most frequently in the north central region of Bangladesh In Bangladesh, SLCS are classified depending on the during the pre-monsoon season, peaking in April. Ono (1998) magnitude of wind speed. The ones producing wind gusts collected 44 “tornado” events from 1990 to 1994 in above 42 ms−1 are defined as tornadoes, while those producing Bangladesh using newspapers published in Bangladesh and wind gusts ranging from 11 ms−1 to 42 ms−1 are defined as found that “tornado” occurred most frequently in central ‘nor'westers’.Theterm‘nor'wester’ means that SLCS come Bangladesh during the pre-monsoon season, peaking in May, mostly from the northwestern direction. Despite being highly and with most events occurring in the evening (again, the arbitrary, such criteria for classifying SLCS have been used in a exact period of evening is not clearly specified). number of climatological studies addressing SLCS in Bangla- The above climatological studies were conducted based on a desh. Afrose et al. (1981) collected data on “tornado” and survey of the local literature. In such type of approach, it is nor'wester events from 1975 to 1979 (523 events) in desirable to sample for as long a period as possible and to collect Bangladesh and examined their climatology using meteorolog- data from a variety of data sources in order to produce a reliable climatological study. Although Afrose et al. (1981) investigated many “tornado” and nor'wester events (523 cases), the period of ⁎ Corresponding author. Present affiliation: Center for Southeast Asian Studies, Kyoto University, Kyoto, Japan. study was short (from 1975 to 1979), while Ono (1998) in- E-mail address: [email protected] (Y. Yamane). vestigated 44 “tornado” events referring to a short period of 0169-8095/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2009.11.004 Y. Yamane et al. / Atmospheric Research 95 (2010) 400–406 401 Fig. 1. Annual frequency of severe local convective storms in Bangladesh from 1990 to 2005. The maximum (year), minimum (year) and annual average are shown on the upper side of the figure. 5 years (from 1990 to 1994). Chowdry and Karmakar (1986) did convections including not only the wording “tornado” and not state the number of events examined in their study. But, the “nor'wester” but also wind gusts, hail and lightning. These period of their analysis covered 10 years, as mentioned later, reports can be regarded as the evidence of occurrence of SLCS, which is shorter than that of our study (16 years). and were used to investigate their climatology in Bangladesh. Again, it is important to stress that the definition of tornadoes Wind gusts also can be induced by local topography and and nor'westers used in Bangladesh, based on arbitrary thresh- produced by tropical cyclones. Since Bangladesh is almost olds of wind speeds, may be considered flawed. It is also doubtful entirely flat, no damaging gust produced by topographically- whether the wind speed was accurately estimated. Hence, induced windstorms has been reported. We also excluded Chowdry and Karmakar (1986), that showed the climatology of wind damage reports with tropical cyclones by utilizing best nor'westers, may have included tornadic events in their study. track data and meteorological data. Based on these points, Ono (1998) may have included non-tornadic events in his we can state that the events analyzed in this study rea- climatology of “tornado”. sonably can be considered as related to local deep convec- In this study, in order to exclude such confusion, we did tion. Our objective is to produce a reliable database upon not discriminate between tornadoes and nor'westers. We which we can study the climatology of convective storms in collected reports of severe weather accompanied with deep Bangladesh. Fig. 2. Monthly frequency of severe local convective storms in Bangladesh from 1990 and 2005. 402 Y. Yamane et al. / Atmospheric Research 95 (2010) 400–406 Fig. 3. Hourly frequency of severe local convective storms in Bangladesh from 1990 to 2005. BST means Bangladesh Standard Time (GMT+6 h). Fig. 4. Geographical distribution of severe local convective storms in Bangladesh from 1990 and 2005. Dots are located at the headquarters of districts and the shade and size of a dot indicates the number of events for each district. D: Dhaka (90.3°E 23.7°N), T: Tangail (89.9°E 24.2°N), N: Netrakona (90.7°E 24.8°N), S: Sirajganj (89.6°E 24.0°N), C: Comilla (91.1°E 23.4°N) and P: Pabna (89.2°E 23.9°N), respectively. Y. Yamane et al. / Atmospheric Research 95 (2010) 400–406 403 Fig. 5. Same as Fig. 4, except for in March. B: Barisal (90.3°E 22.8°N), L: Lakshmipur (90.8°E 22.9°N), respectively. 2. Data and method of mean sea level pressure (contoured at 1 hPa intervals) and wind vector maps were prepared by utilizing ECMWF 40 years In the present study, we produced a database of SLCS in re-analysis data (ERA-40) and NCEP FNL data. ERA-40 and NCEP Bangladesh based on the survey of storm reports published in the FNL data are available on the website of ECMWF and NCEP, local literature (mostly media-based literature). This same respectively. The area of influence of the tropical cyclone was approach has been employed by many researchers. Niino et al. delineated by the outmost closed isobar associated with a tropical (1997) developed the climatology of tornadoes in Japan based on system. Any report placed within this area of influence was the survey of newspapers and reports compiled by the Japan excluded from the database in the present study. Meteorological Agency. The newspapers surveyed here included The database of SLCS constructed in the present study “The Bangladesh Observer” in English and “Daily Ittefaq” in contains date, location (district and upazila1), duration, type Bengali. In addition, the “Bangladesh Disaster Report” complied of phenomena (e.g., hail and lightning) and the damage to by the Disaster Forum (NGO in Bangladesh) and the annual property. The district of occurrence is identified for most reports of Bangladesh Space Research and Remote Sensing reports, and the upazila is also identified in some events. The Organization (SPARRSO) were also used in this study. We occurrence time is identified for some events. Details of searched and collected articles and reports of severe weather damage (e.g., the number of deaths and injuries, the damage utilizing these data sources. to buildings and crops) are also included in the database. In our database the exclusion of wind damage reports asso- In this study, we collected 2,324 SLCS events from 1990 to ciated with tropical cyclones was accomplished as follows. First, 2005 in Bangladesh. There were more events and the period we searched for tropical cyclone activity affecting Bangladesh by of analysis is longer than those of previous studies. using the best track data made available by the Joint Typhoon Warning Center (JTWC). This best track data is available on the JTWC website (https://metocph.nmci.navy.mil/jtwc.php). Then, 1 Bangladesh is divided into 64 districts. Moreover, each district is divided we defined the size of tropical cyclones by producing an analysis into a number of Upazilas (sub-district).