RESEARCH ARTICLES CURRENT SCIENCE, VOL. 118, NO. 3, 10 FEBRUARY 2020 421 *For correspondence. (e-mail: rajeshsing03@gmail.com) Electrical signature of the October 2013 very severe cyclonic storm Phailin Adarsh Dube 1 , Ajeet K. Maurya 2 and Rajesh Singh 1, * 1 Dr K. S. Krishnan Geomagnetic Research Laboratory, Indian Institute of Geomagnetism, Allahabad 221 505, India 2 Department of Physics, Doon University, Dehradun 248 001, India In this study we examine first of its kind from Indian sub-continent which concentrates the electrical signa- tures of lightning discharges associated with a very severe cyclonic storm (VSCS). Phailin cyclone during 8–14 October 2013 has been selected for the study. We have primarily used ground-based GLD360 network lightning data to understand the distribution, polarity and radiated peak current of lightning discharge asso- ciated with the inner core (~100 km radius) of Phailin. In the initial development stage of Phailin as a deep depression on 8–9 October, there were very few lightning discharged (>50) in the inner core, but when Phailin developed into a VSCS on 10 October, ~2300 lightning discharges were recorded in inner core. There was near-even distribution of positive cloud to ground and negative cloud to ground discharges in the core, and with strong opposite peak currents of ±150 kA prior to the cyclone landfall. The observa- tions show that monitoring of lightning discharges in eye of the cyclone is helpful in tracking its intensity changes and hence can serve as early warning systems. Keywords: Electrical signature, lightning discharges, peak current, tropical cyclone. TROPICAL cyclones are a potential source of turbulence, wind shear, storm surges, gusts and gale, thunderstorms, heavy rainfall, lightning discharges, etc. 1 . The intensity of tropical cyclones is estimated based on satellite imagery of clouds and cloud parameters 2 . A cyclone typically con- sists of a precipitation-free eye surrounded by an eyewall, with deep convection and spiral rain bands outside the eyewall 3 . According to their characteristics and locations, spiral rain bands can be classified into inner and outer rain bands 3,4 . Inner rain bands are usually invisible to space-based satellite imagery, but are evident on ground- based radar reflectivity. Outer spiral rain bands are defined as those rain bands that occur outside the inner core. They generally have larger horizontal scales than the inner rain bands. Although rain bands are distinct features of a tropical cyclone, they may interact with the eyewall thus modulating its structure and intensity 5,6 . In general, inner rain bands in the cyclone are always active immediately outside the eyewall up to a radius of about ~60 km. Although the outer spiral rain bands domi- nate outside ~60 km radius, their activity shows a quasi- periodic nature 6–8 . Previous studies have shown that the convection in the inner core of a tropical cyclone is intense and can be studied for tropical cyclone intensity forecasting and cyclone track changes 9,10 . Zhang et al. 10,11 reported details of different features of a tropical cyclone and related lightning activity study for the intensity change forecasting. These studies showed marked changes in lightning outbreak prior to storm intensifica- tion. Studies have also reported that eyewall lightning could be a potential predictor to changes of storm track and storm intensity 10–12 . They also observed that an increase in +CG (cloud-to-ground) lightning might be considered as a prediction for storm weakening. To analyse and track lightning discharge activity associated with large tropical cyclones has become more interesting over the years due to the latest ground- and space-based remote sensing techniques. Usage of lightning discharge records over the last 30 years for mapping the tropical cyclone intensity and its track fore- casting is a boon to society, considering the associated destructiveness 13 . In such scenario, ground-based lightning networks for the study of tropical cyclone evolution, structure and energetics become important. Kaplan et al. 14 showed that lightning input influences the identi- fication of rapidly intensifying and rapidly weakening cyclone cases as much as many other parameters that are currently utilized in an operational rapid intensification index. Lightning data from space based Optical Transient Detector (OTD) and Lightning Imaging Sensor (LIS) on-board the Tropical Rainfall Measuring Mission (TRMM) satellite have been used to study the relation- ship between lightning activity and cyclone intensity 15,16 . However, as the OTD and LIS do not provide a conti- nuous coverage of lightning within a cyclone, they are by design not the most suitable tools to examine the relation- ship between lightning activity and change in intensity of a cyclone. There are errors of false detections due to back- ground illuminations from bright cloud tops, high-energy particles, non-lightning optical signals such as solar glint reflections from the oceans and sensor noise 15,17 . One of the important reasons for choosing and relia- bility of GLD360 dataset over OTD/LIS is that because of the short viewing times, the detection efficiency of