IJECT Vo l . 4, Is s u e Sp l - 1, Ja n - Ma r c h 2013 ISSN : 2230-7109 (Online) | ISSN : 2230-9543 (Print) Thermodynamics of the Atmosphere in Relation to Tornado Activity Over NE India and Bangladesh 1R. Bhattacharya, 2A. Bhattacharya, 3R. Das 1Dept. of Environmental Science, University of Kalyani, Kalyani, West Bengal, India 2Panighata UDM High School, Paglachandi, Nadia 3Biraballavpara High School, Biraballavpara, North 24 Parganas

Abstract develop an effective means for the prevention of these destructive The intensification of tornado depends mainly on the synoptic phenomena, we need to understand clearly the physical processes conditions of the troposphere. The main objective of this paper is involved. A complete statistical record of tornado distribution is to differentiate the prevalent atmosphere from thermodynamically highly needed in order to take control measures [7-8]. Present point of view. Atmospheric stability parameters such as Lifted Index analysis is a post mortem of tornado events with special emphasis (LI), Total Total Index (TT), Deep Convective Index (DCI), Wind on its distribution and thermodynamic structure. Index (WI), Severe Weather Threat Index (SWEAT), Convective Available Potential Energy (CAPE), Convective Inhibition (CIN) II. Methodology and Bulk Richardson Number (BRN) are computed on the tornado Tornadoes experienced over Eastern India and Bangladesh during events over NE India and Bangladesh from 1993 to 2010. It is 1993 to 2010 (www.bangladeshtornadoes.org) are analyzed observed that TT may be used as a best predictor of differentiating to examine their distributional pattern and thermodynamical prevalent atmosphere from fair one. characteristics particularly the stability parameters such as Lifted Index (LI), Total Total Index (TT), Deep Convective Index Keywords (DCI), Wind Index (WI), Severe Weather Threat Index (SWEAT), Atmospheric stability, Tornado, Lifted Index, Total Total Index, Convective Available Potential Energy (CAPE), Convective Deep Convective Index, Wind Index, Severe Weather Threat Index, Inhibition (CIN) and Bulk Richardson Number ( BRN). The Convective Available Potential Energy, Convective Inhibition, parameters are computed from the daily data of surface and radio- Bulk Richardson Number sonde observations of nearby India Meteorological Department [9-11]. Skill scores viz POD (probability of detection), CSI I. Introduction (critical success index), EI (efficiency index), FAR (False alarm Mesoscale convective systems are responsible for about 10 – 25 rate) are computed for each parameters. For good prediction the % of total rainfall in premonsoon months over West Bengal and scores CSI, EI and POD should be close to unity and FAR must at the same time severe atmospheric disturbances; viz. torrential be close to zero. Validity of success of (yes/no) forecasts are done rainfall, high winds, hail, thunderstorm, lightning, tornadoes etc by Equivalent Percentage of success (EPS) method [12-13]. [1-2]. Even a nearly accurate prediction of thunder squalls is a challenging problem. It is well known that the favourable synoptic conditions that trigger thunder squalls are conditional instability, low level convergence, upper level divergence, high moisture fed and strong vertical shear. Thunderstorms are generated, infact, from the interaction of synoptic scale motion and localized mesoscale convection [3]. In the premonsoon months the existence of low (L) to the north of 25°N, high (H) to the south of 20°N and large scale flow at 500 hPa are the precursor of atmospheric instability. In addition strong insolation and orogaphic lifting help to form convergence in Gangetic West Bengal. Sometimes thunderstorms may turn violent after getting much energy from strong surface heating. Such thunderstorms may associate with tornadoes. Tornadoes are originated from the base of a strong thundercloud and look like a ‘funnel cloud’ accompanied with a rotating air column with a speed which is varying from 100 to 200 mph [4]. Tornadoes present a violent threat to society and according to meteorologists. A tornado is a violently rotating column of air extending from a thunder storm cloud down to ground. Severe thunderstorms resulting into tornadoes present a serious problem since they cause deaths from lightning, fires, power plant damages, aircraft crashes and so on. Thus the tornado causes thousands of deaths and damage billions of dollars. These storms not only leave sign of destruction over the area it passes but also disturb microwave link by introducing atmospheric noise in the signal strength. Doppler radar or common weather radar can detect formation of vertical towering clouds but an alert to the potential of storm must be given otherwise valuable information regarding detection and occurrence would be lost [5-6]. To Fig. 1: Longitudinal and Latitudinal Distribution of Tornadoes

92 International Journal of Electronics & Communication Technology www.iject.org ISSN : 2230-7109 (Online) | ISSN : 2230-9543 (Print) IJECT Vo l . 4, Is s u e Sp l - 1, Ja n - Ma r c h 2013

III. Results and Discussion The number of occurrences of tornadoes over East India and Bangladesh are examined and noticed that the cluster is formed between 880 E to 920 E and 230 N to 250 N zones as shown in fig.1. The tornado events are mostly occurred during the nor’wester season March to May. The percentage occurrences of tornadoes in the months March, April and May are respectively 21%, 64% and 15%. The track of the tornadoes lies in the sector 320 - 350° azimuth. The topology of NE India and Bangladesh is sustainable to tornado genesis [14]. The time distribution of the occurrences of tornadoes is presented in fig. 2. Less intense tornadoes during monsoon months are excluded from our analysis. The figure exhibits maximum occurrences between 14:30 to 17:30 hrs IST. Moreover the geographic locations of West Bengal and Bangladesh are responsible for the formation of tornado in premonsoon months [6]. Tibetan Plateau on the North and Bay of Bengal on the South are the source of heat and moisture which make Eastern region

Fig. 2: Distribution of the Occurrences of Tornadoes With Respect to (a) Time and (b) Month

Fig. 3: Distribution of Stability Indices on Severe Tornado Days www.iject.org International Journal of Electronics & Communication Technology 93 IJECT Vo l . 4, Is s u e Sp l - 1, Ja n - Ma r c h 2013 ISSN : 2230-7109 (Online) | ISSN : 2230-9543 (Print) of India and Bangladesh sustainable for thunderstorms incidences. [4] The microphysical and electrical properties of thunder cells govern [5] Bhattacharya A.B., Bhattacharya R.,"Radar observations of the severity of the storms. Atmospheric convection depends on tornadoes and the field intensity of atmospheric", Arch. Met. stability parameters. Geophys. Biokl., Ser. A 1983, 32, pp. 173-179. The distribution of the stability indices LI, TT, DCI, WI, SWEAT, [6] Kar I, Bandopadhyay R.,"Climatological study of pre- CAPE, BRN and CIN are shown in fig. 3. It is observed that range monsoon thunderstorms over Sriniketan", Alipore and of variation of all the parameters are high. Usually the potential Kalaikunda. Mausam, 1998, 49, pp. 262-264. of thunderstorms are prevalent in the region where the stability [7] Tyagi A.,"Thunderstorm climatology over Indian region", parameters are above threshold limit [15]. Mausam, 2007, 58, pp. 189-212. [8] Farbush EJ, Miller RC, Starrett LG.,"An emperical method Table 1: Comparison of Stability Indices on Thunderstorm and for forecasting tornado development", Bull. Amer. Meteor. Tornado Days with Critical Values Soc., 1951, 32, 1-9. Parameters Thunderstorm Tornado Critical value [9] Rasmussen EN, Blanchard DO.,"A baseline climatology of sounding derived supercell and tornado forecast parameters", LI -1.5 -3.57 -3 Wea. Forecast., 1998, 13, pp. 1148-1164. TT 40.2 47.64 44 [10] Seshadri N, Jain PS.,"Study of role of Stability Index in DCI 30.1 42.40 Not specified forecasting of thunderstorms", Mausam, 1990, 41, pp. 102- WI 75.5 60.73 Not specified 114. SWEAT 140 243.03 300 [11] Huntrieser H, Schiesser HH, Schmid W, Waldvogel CAPE 1000 2244.34 1000 A.,"Comparison of traditional and newly developed BRN 75 528.94 50 thunderstorm indices for Switzerland", Wea. Forecasting, 1997, 12, pp. 108-125. CIN -150 -185.57 -100 [12] Khole M, Biswas HR.,"Role of total-totals stability index It is evident from the figures that most of the cases, the parameters in forecasting of thunderstorm/non-thunderstorm days over are above the threshold value. The values of eight stability Kolkata during pre-monsoon season", Mausam, 2007, 58, parameters on thunderstorm days and tornado days along with pp. 369-374. critical values are presented in Table 1. [13] Makridakis S, Wheelwright SC, Hyndman RJ,"Forecasting: In case of tornado events the average value of all the stability methods and applications", John Willy and sons, New parameters are much higher than the estimated threshold value York,1983. for thunderstorms [16]. [14] Wilks DS.,"Statistical methods in the atmospheric science", 2nd Edn Academic press, 627. IV. Conclusion [15] Karmakar S, Alam Md M,"Instability of the troposphere Among all the stability indices TT can be used as a best predictor of associated with thunderstorms/nor’westers over Bangladesh the degree of atmospheric instability. The probability of detection during the premonsoon season", Mausam, 2006, 57, pp. 629- using this parameter is 87 %. The parameters DCI and CAPE may 638. also have capability to differentiate tornadoes from thunderstorm [16] Doswell C. A., Schultz D.M.,"On the use of indices and days. A combined estimation of TT, DCI and CAPE may give parameters in forecasting severe storms", Electronic J. Severe better results for prediction of the intensity of stability. Some of Storms Meteor., 2006, 1, pp. 1-5. the tornado cases have high CAPE values (>2500). Most of the [17] Bhattacharya R, Bhattacharya A.,"Stability parameters and tornadoes have SWEAT value >300. Average value of BRN for their skill to forecast thunderstorm", Int. J. Phys., 2011, 4, the Tornado events is 528.94 which is very high in comparison pp. 21-30. with the standard threshold value (>50) of Bulk Richardson number. However a long term analysis of the stability indices on prevalent atmosphere is essential for optimizing the threshold limit beyond which the atmospheric condition is favourable for tornado formation.

V. Acknowledgement Authors are thankful to National Data Centre, Pune, India for providing some of the relevant data.

References [1] Sinha Roy S, Roy Bhowmik SK,"Evaluation of thermodynamics of the atmosphere in relation to premonsoon convective activity over north India", Mausam, 2003, 54, pp. 397-405. [2] Mohapatra M, Koppar AL, Thulasi Das A,"Some climatological aspects of thunderstorm activity over Bangalore city. Mausam", 2004, 55, pp. 184-189. [3] Cesar MB.,"Vertical profiles of winds and electric fields triggered by tropical storms under the hydrodynamic concept of air particle", Int. J. Phys. Sci., 2009, 4, pp. 242-246.

94 International Journal of Electronics & Communication Technology www.iject.org