Local Severe Storms in Bangladesh and adjoining Indian Territory: A Case Study of 13th-15th April, 2010

Local Severe Storms (LSS) are very hazardous weather phenomenon that frequently develops in the pre-monsoon season (March - May) over Bangladesh and adjoining Indian territory. LSS are consist of cumulonimbus clouds, gusty wind, squall and torrential rainfall accompanying with lighting and thunder in this region. The systems generally migrate from north-west to south-east direction and are called as Nor'wester (locally known as Kalbaishakhi). Sometimes violent tornado also develops in LSS and cause great　loss of lives and properties. A case study is conducted to understand the environmental features of LSS from of 13th April – 15th April, 2010. Three violent storms struck parts of Bangladesh and Northeastern India at 17 UTC (11pm Bangladesh local time) April 13, 2010.Some tornadoes associated with the system. The devastation were lasted as long as 90 minutes, with the most intense portion spanning 30–40 minutes. More than 140 deaths have been reported in India. In Bangladesh, 5 deaths and 200 injuries were reported. Over 91,000 dwellings were completely destroyed in India and several thousand in Bangladesh; power and telecommunication lines were snapped, trees were uprooted, and massive damage was caused to crops and livestock. Again LSS were reported on subsequent days 14th and 15th in the same area. The major purposes of this study are to identify the environmental characteristics of LSS that occurred in the study region.

The high resolution 20km analysis data produced by using Global Spectral Model (GSM) of Japan Meteorological Agency (JMA) are available to see the detail environmental conditions. On the events occurring days, in the lower level sufficient southerly/south-westerly warm moist air flowed from the Bay of Bengal to the land. Dry warm westerly blew down from the Indian high lands to Bangladesh. At middle troposphere westerly or north-westerly prevailed over North India. It advected cold air mass towards Bangladesh. At the mid of the day rapid rise of temperature was found at the lower level over India. Stationary low pressure was observed at the central part of West Bengal. The specific humidity was found very high (20g/kg) all over Bangladesh. Strong horizontal gradient of Specific humidity, 14 g/kg over 3° longitude, were observed between humid airmass from the Bay of Bengal and dry airmass of Indian territory. Sometimes this situation mentioned as dryline (Weston, 1971). At the event days as the time advances, low level temperature and specific humidity started to increase, which enhanced the instability condition of the atmosphere. Strong instability was observed 10 to 6 hours before the event occurrence. On the 13th of April, in Bogra (latitude 24°N, longitude 89.5°E) 5 hours before the event occurrence, the instability parameters calculated as K-Index (KI)= 30, Lifted Index (LI) =-6, Precipitable Water (PW) = 42 kg/m2 ,Total Total Index (TTI)=71, Convective Available Potential Energy (CAPE) = 3800j/kg and Energy Helicity Index (EHI)=1.6. At the same time West Bengal (latitude 23°N, 88° longitude) showed, KI=28, LI =-6, PW 40 kg/m2, TTI=65 and CAPE =4100 J/kg, which also suggested very high instability. On 14th April and 15th April, the instability parameters were calculated as, LI = -4.5, PW = 39kg/m2, TTI=77, CAPE = 3300J/kg, EHI = 1.4, and LI = -5.5, TTI=67, PW = 36kg/m2, CAPE = 3000J/kg respectively.

LSS are locally generated meso-scale phenomena. It is also necessary to investigate the large scale features to see the disturbances. The satellite images are able to see these features. The IR imageries of Multi-functional Transport Satellite (MTSAT) of JMA and Fuyang 2 Chinese satellite are investigated to see the cloud structures and movements. The images of 17UTC, 17.30UTC of 13th April showed eastward moving high convective clouds over Bihar, Northeastern India, northwest and northeast of Bangladesh. During the growing stage of thundercloud systems the surface temperature became very high in central West Bengal. DWR Dhaka and DWR Kolkata images collected from Severe Thunderstorm Observation and Regional Modeling (STORM) project's archive that also showed the clear signatures of multiple cells and which subsequently converted in to a squall line structure. The radar images showed the similar multiple convective cells and those were moving very slowly to east direction. Since upper air sounding is only once or twice in a day and the location is only at Dhaka and Calcutta in the study region, it is difficult to compute stability parameters from the sounding data for forecasting purpose. The use of objective analysis data is quite promising in place of sounding in such data sparse region. Further studies are, however needed.