Indian Journal of Radio & Space Physics Vol. 13, June 1984, pp. 98-102

Spectral Characteristics of the 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 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. , 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). The second peak has been observed peak (0--0) and end (0-·-0)time of the tornadic activity at 81 kHz with 15dB amplitude rise. This indicates E 90 that the tornado radiated its energy mostly around 15 "- and 81 kHz, i.e. in the VLF and LF. ~ >CI> Fig. 5 shows the frequency spectra of ARNFS o .Q070 during the start, peak and end time of the tornadic lD activity. We see that the spectra at start and end times ~ J: are nearly similar at all frequencies from 1 to 81 kHz. l• e>z ELF field strength is somewhat greater than that in the w 50 I•a: VLF and LF zone. A prominent peak is seen in the LF en a zone, in all the three phases of the tornado. ...J w A further series of spectra is shown in Fig. 6 which t;: 30 shows the values of ARNFS at all frequencies at 1

Fig. 6-Spectra of ARNFS prior to parent thunderstorm onset at 1000 hrs (0--- 0) and during steady thunderstorm activity at 200' (e) 1400 hrs (0-'-0) on 12 Apr. 1983 [Monthly median values 2000 (0--0) at 1930 hrs for April 1983 is also shown.]

1946 (b) -;:: !! 1942 1000hrs and 1400hrs 1ST when (i) the parent " thunderstorm was not even in its formative stage and ~ 1938 ~.w (ii) the parent thunderstorm was in its steady active i= 1920 stage. Absence of any peak in the LF is noticed. The t916 spectra of monthly median values at 1930hrs during the month of April at different freqeencies are also 19,2 shown. Median values at 1and 3 kHz are absent due to 1908 , 8 9 t5 2"--~4- 81 a few observations as these two receivers are FREQUENCY ( kHz) deliberately kept less sensitive to distinguish between Fig. 3-Spectra of start (a), peak (b) and end (c) timtklf the tornadic local and distant thunderstorms. activity 5 Discussion The hopping course of the Gaighata tornado can be .6 Iii explained from simple meteorological point of view. A .::! ::£ lIJ o tornado usually originates in the central part of a a '" ::> ~ powerful storm cloud wherein strong vertical air f- B :; 20 ~~ a. currents exist with sharp irregularities both in ~ «;::£ « a: direction and force of the wind. Whenever the axis of a::> this vertical current is hit by a strong horizontal o o CD 1 3 6- -9--~ 27 5'-81 'tl r<) current, the uprushing flow is overturned producing a FREQUENCY (kHz) vortex with horizontal axis that rolls forward and • Fig. 4-Spectra of 3 dB duration (0---0) and amplitude gradually emerges from the cloud and bends into a (0--0) of the tornado generated RNFS vortical ring. This whirlwind, in the form of tiny

100 1" SARA et at:: GAIGRATA TORNADO OF 12 APR. 1983 vortexes, moves down until it comes in contact with the reached peak activity earlier. This may be due to that ground forming the funnel of the tornado. If the vortex CG discharges gradually diminished while intracloud loses contact with the ground, its funnel is pulled back discharg~s started dominating. From Fig. 4 also we see into the cloud; but if there is a fresh influx of energy that the tornado radiated most part of its energy in the from the storm cloud, the funnel again dips towards VLF and LF but the VLF activity was for a shorter earth. This explains the hopping course of a tornado. period, indicating a dominance of intracloud The upper part of the tornado spire usually becomes discharges: linked with the main updraft of the mother storm The spectral nature of the tornado is very peculiar. cloud thereby causing rapid removal of air from the From Fig. 5 it appears that there is one peak around interior of the funnel and allowing a violent decrease of 15kHz. The return stroke current which produces pressure at the surface causing houses to blow up. intense electromagnetic signals' usually attains a The power available from a thunderstorm cell is of spectral peak in the VLF region. The peak around the order of 1022 ergs and it is sufficient to provide 15kHz may be assumed to be due to return strokes. power to a tornado. Vonnegut10 estimated the power But it is a well established fact that the tornadoes are (P) required to maintain a tornado of funnel radius r associated With a decrease in the CO discharges; using the relation instead, intracloud discharges dominate. Vonnegut P == pnr2 V3/2 and Moore 1 found almost continuous intracloud where p is the density of air (10 -3 gm cm -3) and V is lightning in a storm which produced a tornado. the Wind velocity at the periphery. In the present case Anderson et al.13 from the measurements of electric the meteorological observatory reported the wind field and relaxation time of the air during a few velocity to be 120 m/sec. According to thiS, the power tornadoes found that the discharges were mainly involved in the Gaighata tornado is 22000 MW. Such a within the cloud. During the course of a cloud discharge huge.. amount of power we cannot think of in normal one or more streamer channels are established between practice. the positive and negative charge· centres. Along these The steps in Fig. 1 during the tornadic activity are channels, which are highly conducting, large pulses of nothing but the electrical signatures of strong active current flow and constitute the so-called K-changes thundercells. Usually the average duration of which have a spectral peak around 10kHz. Spectra of precipitation and/or electrical activity from a single composite flashes are also found to have a peak around cell is of the order of 10 to 30 min. In our Gaighata 10kHz. This explains the peak around 15kHz during case, the lifetime of cells in the parent thunderstorm on the tornadic activity. It has also been found that there the average is less than 10min. Five such electrically is a shift of the peak in the frequency spectrum from active cells were present in the Gaighata tornadic 10kHz to higher values during tornadoes. Jones 7.8 storm. found· that the inner-cloud discharge ~tres do not Fig. 2 clearly indicates that the discharges were of produce sferic activity on 10kHz band and spectral CG nature and major portion of the radiated energy peak is attained in the LF. Hughes and Pybus6 also was confined to around 15kHz. This may have obtained similar results. In the case of the Gaighata triggered the thunderstorm to form a funnel which tornado the second peak appears to be around 54 kHz. ultimately touched ground at 1916hrs. Hillll found During both the start and end times, the peak is • out the average power per lightning stroke to be prominently higher in the LF than that in the VLF. 1010 kW. A large portion of this energy is released as However, during the peak activity period of the heat and produce hot air surrounding the arc within a tornado, the VLF peak is almost equal to the LF peak. very short time and may cause intense winds by But it is to be noted that in this matuce phase the convection process. Many investigators found that intensity is m\lch more higher, indicating tremendous during and prior to a tornado there is a marked energy radiation. At this point it must be mentioned increase in the lightning impulses 3.6. 12. Vonnegut10 that the tornado was much more electrically active found that if 10 strokes per second strike through the than the local thunderstorm which took place at channel then the surrounding air will be heated 1745hrs, as seen in Fig. 1. continuously by more than 100°C and estimated that From Fig. 6 it is seen that the spectra of the median the theoretical efficiency of conversion of thermal values at 1930 hrs have a single peak around 9 kHz. energy released by lightning into kinetic energ~1ofwind There was no prominent peak in the LF quiet period may be as high as 40%. (1000 hrs) and steady thunderstorm activity period It is interesting to note that ELF activity, as evident (1400 hrs), prior to the tornado funnel formation. from Fig. 3 always started later than the VLF and LF Comparing Figs 5 and 6 it is seen that the form of band during the start and peak phase of the tornadic spectra started changing from time to time and activity. However, the LF activity started a bit later but resulted in a multipeak spectra during the tornadic

101 INDIAN J RADIO & SPACE PHYS, VOL 13, JUNE 1984 activity. This peculiar nature of spectra is not seen Acknowledgement during other type of disturbed weather. The The authors wish to thank Mr R N Sen Sharma of observation in Figs 5 and 6 regarding large values at the Indian Meteorological Department, Alipore, the lowest frequency is not unlikely since about 30% of Calcutta, for some very stimulating discussions related the observed sferics possess an ELF but not a VLF to this paper and for the meteorological data. Thanks component14. This is also consistent with the i'indings are also due to Prof SN Ghosal, Head of the of Holzer and DeaF 5 who found that at least a very Department of Physics, University of Calcutta, high percentage of the energy in the ELF band Calcutta, for laboratory facilities. originates in the lightning discharges. From all these findings, it can be definitely said that the tornado References generator radiates its energy in a wide range of 'I Vonnegut B & Moore C B, Recent advances in atmospheric frequencies and there appears to be three peaks in the electricity, edited by L G Smith (Pergamon Press, London), spectrum, one in the ELF, the other two being in the 1958,399. 2 Anderson F J & Frier G D,J Geophys Res(USA), 70(1965)2781. VLF and LF region. This is contrary to the results of · 3 Taylor W L, Preprints of papers on the ninth conference on severe Jones7•8. local storms, American Meteorological Society, Norman, Okla, 1975, 311. 6 Conclusion • 4 Johnson (Jr) H L, Hart R D, Lind M A, Powell R E & Stanford J Tornadoes are supercell storms of high electrical L, Mon Weather Rev (USA), 105 (1977) 734. activity. They radiate intense electromagnetic noise • 5 Scouten D C, Stephenson D T & Biggs W G, J Atmos Sci(USA), which is distinguishable from radiation from other 29 (1972) 929. •.6 Hughes W L & Pybus E J, Preprints ofpapers of the cOliferenceon type of storms. The frequency spectra of radio noise cloud physics, American Meteorological Society, Ft. Collins, from tornadoes have multiple peaks while other Colorado, 1970, 181. storms have one peak around 10 kHz. Intrac10ud -7 Jones H L, Bull Amer Meteorol Soc (USA), 32 (1951) 380. discharges playa dominant role in tornadoes and it •.8 Jones H L, Recent advances in atmospheric electricity, edited by L shifts the spectral peak at a higher frequency. Abrupt G Smith (Pergamon Press, London), 1958,543. stepped rise of the VLF and LF atmospherics preceded • 9 Pierce E T, Problems of atmospheric electricity, edited by Samuel by steady thunderstorm activity for a few hours appear C Coronity (Elsevier Publishing Company, Amsterdam), to be the precursor of a tornadic activity. Hence, 1965, 156. . 10 Vonnegut B, J Geophys Res (USA), 65 (1960) 203. round-the-clock observations of the direction of Il Hill R D, Naval Research Reviews (USA), 1975, I. arrival of sferics along with the field strength vis-a-vis sferics count rate and the continuous measurement of -12 Stanford J L, Lind M A & Takle G S, J Atmos Sci (USA), 28 (1971) 436. important meteorological parameters like low-level • 13 Anderson F J, Frier G D, Liu C C & Tam F M, J Geophys Res temperature, moisture gradients, wind velocities at (USA), 71 (1966) 4279. different levels may help in forecasting tornadoes at a ~ 14 Tepley L R, J Geophys Res (USA), 64 (1959) 2315. place. -IS Holzer R E & Deal 0 E, Nature (G8), 177 (1956)536.

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