Indian Journal of Radio & Space Physics Vol. 13, June 1984, pp. 98-102 Spectral Characteristics of the Gaighata Tornado of 12 Apr. 1983 ANUPAM SAHA, B K DE & S K SARKAR Department of Physics, University of Calcutta, Calcutta 700 009 l I Received 13 October 1983 Results obtained from analysis of the records of atmospheric radio noise field strength (ARNFS) at 1,3,6,9, 15,27,54 and 81 kHz due to a severe tornado which ripped the Gaighata area in West Bengal on 12 Apr. 1983 are reported. Some of the important results are as follows. (i) A sudden stepped rise in ARNFS occurred in the VLF and LF bands. (ii) Tornado is preceded by a uniform and high thunderstorm activity for a long time ( - 4 hr) and discharges during this period show the usual spectral peak around 10kHz. (iii) Starting time and time of peak tornadic activity have been found to occur later in the ~ ELF band. (iv) Electrical discharges during the tornadic activity shift the spectral peak to a higher frequency at around 54 kHz >- besides the usual peak around 10kHz. (v) The tornado radiates most part of its energy in the VLF and LF bands but the VLF activity was present for a shorter period. (vi) The rise and fall of ARNFS during the tornadic activity have been found to be in steps indicating presence of several strong and active thundercells in the parent storm. (vii) The frequency spectra of the tornadic radiation show three peaks, one each at VLF and LF bands (at 15 kHz and 54 kHz) and the other at ELF band. The results contradict the findings of H L Jones [Bull Arner Meteorol Soc (USA), 32 (1951) 380; and Recent advailces in atmospheric 1 Introductionelectricity, edited by L G Smith (Pergamon Press, London), 1958,to have 543] and a peak suggest around intensive 10kHz. investigation But in it this has regard. been foun({ /'/ ~ The exact mechanism of the formation of a tornado that there is a shift of the spectral peak from is still unknown though much work has been done 10 kHz to higher values during tornadoes. Hughes during and prior to the formation of its funnel. and Pybus6 also investigated the emission spectrum However, most of the meteorologists think that from tornadoes from 10 to 250 kHz and observed that tornadoes are a result of excessive instability and the the upper end of the spectrum is the best indicator of ~ steep lapse rates in the atmosphere. They find that tornadic activity. Jones 7,8 reported this spectral peak tornadoes are the ultimate manifestation of severe to be around 150 kHz from the observations on two local storms. It has its characteristic acoustical, optical frequencies (10 and 150kHz) only, which is and electrical features. Acoustically, the interesting controversial. Pierce9, however, pointed out the feature of the tornado is the loud roar it generates. requirement of a series of simultaneous observations at Vonnegut and Moore1 associated this sound with different frequencies, from ELF to LF band to intense point discharges. Anderson and Frier2 found ascertain the frequency where the peak lies. Our that circulating acoustic waves can exist in a tornado records of ARNFS during a tornadic activity are being vortex and produce intense sound. Optically, apart reported here to show its electrical characteristics, from the very frequent lightning Rashes within the spectral nature in particular. cloud, ball lightning sometimes accompany tornadoes. Because the ball lightning is a rare phenomenon, till 2 The Event • now no detailed knowledge about its formation A tornado ripped the Gaighata area on 12Apr. 1983 j" mechanism is known. Another view is that tornadoes leaving behind horrible destruction along its 30 km are nothing but a conductor formed out of the clouds hopping route with barely a width of 180m including which serves as a passage of electrical charges from the human lives within a very short span of time. mother storm to the earth. Local reports are that just prior to the tornado The electrical characteristics of tornadoes are very funnel touchdown (at about 1915hrs 1ST) villagers complicated and explanations are scanty. Taylor~ heard a deafening crash and observed a revolving ball reported that tornadoes emit noise bursts which are of fire during the storm. Dum Dum, the nearest short-lived bunch of pulses. Johnson et al.4 found that observatory to Gaighata, reported sharp dew point pulse rates at higher frequencies and at high thresholds discontinuity over a large region surrounding are better discriminators between severe and non- Gaighata. Stratocumulus and altocumulus clouds severe thunderstorm activity at ranges less than 40 km. were present throughout the day. There was inflow of Tornado funnels are reported to produce RF radio moisture into the affected area up to 0.9 km above sea noises. Spectra of composite fla-shesare usually found level (asl) at 1730 hrs. Beyond this level, the wind was 98 SAHA et al.: GAIGHATA TORNADO OF 12 APR. 1983 westerlies. At 2.1 km asl, the wind was 270°/25 knots I ••• , •• I I ••••• I ••• I I I I •• i IIIIII while at 3.1 km asl, it w~ 270°/65 knots and the shear . ·r vorticity was cyclonic. Sharp gradient of mixing ratio at 975 and 890 mbar level was observed. 3 Experimental Technique Atmospheric radio noise field strength (ARNFS) at 1,3,6,9,15,27, 54and 81 kHz is being recorded in our laboratory round the clock. The receivers are of similar type. Inputs of all the receivers are derived from a master cathode follower connected to an inverted-L antenna. A four-section n-filter (low pass) is used to keep the local broadcasting stations away from interference. Each receiver consists of one L-C frequency selective network sandwiched between two linear IC buffers followed by two high gain tuned voltage amplifiers the output of which is fed to the detector via a buffer stage. The detection time constant I!. is I sec. The detected output is fed to a dc amplifier which drives the I mA fsd pen recorder. All the data are corrected for 300 Hz bandwidth (within 3 dB points) while calibrating . •' •.. I, ". \"" • ':~""'~;........ .. 4 Results Thunderstorm activity started with a sharp rate of increase of ARNFS at 1112hrs 1STon 12Apr. 1983,as ". "~~E"~r~-i'i evident in Fig. I. An almost steady level was observed ···'··················" ·~~~I. "-'.,:;Li: ..'···················L '. "i-I . q from about 1200hrs to 1545hrs. Then the radio noise , •••••••••••••••••••••••••• II •••••••••••••••• level again started to increase, this time with a low rate. A thunderstorm took place over the receiving site and lasted from 1745 to 1846hrs, as evident from I kHz record in Fig. I. The prominence in I kHz is due to that the thunderstorm was just overhead and the dynamic range of this particular receiver"is elaborate. Fig. I clearly shows a large spike-like increase of ARNFS at 1906hrs (about 10min before the tornado onset). This 1200 1600 20000000 0400 spike has been identified as a series of terrible cloud-to• TIME (hrs 1ST) ground (CG) discharges which seems to be intimately associated with the funnel formation of the tornado. Fig. I-Photograph of ARNFS of the Gaighata tornado on 12Apr. 1983 (The arrow mark indicates the onset of tornadic activity.) The electrical nature of these discharges in terms of E 40 their actual amplitude rise at all frequencies is drawn in "-> .5 Fig. 2. The peak at 15kHz is to be noticed . w The tornado started at 1916hrs 1ST(arrow mark in 3 :s 20 Fig. I) with a steep rate of rise of ARNFS(0.5 dB/min . ..J zf'" at 15kHz) and reached the peak value at 1939hrs, ~ w 0 remained steady there for about 6 min, started falling '" 1 and ended at 2000 hrs. Short steps were found during ~ FREqUENCY (kHz) both rise and fall stages of the ARNFS during the Fig. 2-Spectrum of the cloud-to-ground discharges at 1906 hrs tornadic activity. During rise two steps have been 1ST observed at 1925 and 1933 hrs followed by steady levels having durations of 6 and 3 min, respectively. thunderstorm, which gave rise to the Gaighata During fall similar two steps have been noticed at 1946 tornado, lasted up to 0445 hrs on the next day . • and 1943hrs with steady levels for 4 and 3 min, The time of starting (a), time of peak attainment (b) respectively. Interestingly, fadings due to gravity and the end time (c) of the tornadic activity at waves were found after the tornadic event. The parent frequencies ranging from 1 to 81 kHz are shown in 99 INDIAN J RADIO & SPACE PHYS, VOL 13, JUNE 1984 Fig. 3. The entire electrical nature of the event was ~ ::>:l, noticed except a part at 9 kHz which was disrupted due - 100 to circuit malfunction. Fig. 3 clearly shows that the ~CI> .Q electrical activity of the tornado starts and attains the o CD peak activity later in the ELF band than those in the 'tl VLF and LF bands. l-J: 80 ze> Fig. 4 shows the 3dB duration from the peak time of w I•a: the electrical activity of tornado at all frequencies and en a 60 the corresponding rise in amplitude in dB from the ...J W mean value of starting and end of the tornadic activity. iL: The 3 dB duration is maximum at 1kHz (39 min) and 3 6 9 15 27 54 81 FREQUENCY (kHzl minimum (7.5 min) at 15kHz. Amplitude rise is minimum at 1kHz, being 4.5 dB while it is maximum Fig. 5-Frequency spectra of ARNFS during the start (0--- 0), at 15kHz (16 dB).