Behaviour of recurving cyclonic storms in Arabian Sea as a response to atmospheric interactions

During the year 1998, Arabian sea witnessed two distinct and unusual cyc\onic systems. The very severe cyclonic storm of June 1998 was the most severe storm since last fifteen years as far as Arabian Sea systems are concerned; while October 1998 system was unique for, Its unusual and sudden easterly track after almost touching Saudi Arabia coast, (ii) Very high speed of movement and (iii) Interaction and merger with another vortex present in Arabian Sea off Saurashtra coast. This study is an attempt to understand the peculiar behaviour of the systems in Arabian sea which do not intensify beyond cyclone stage or even show a tendency to weaken before crossing coast as a response to atmospheric interactions. All the cyclonic systems in Arabian Sea since 1975 (Satellite era) have been examined in detail and the main findings of the study are : (i) Interaction between cyclonic disturbances of different intensities simultaneously present, appears to be the key factor in determining the further intensification or not of the systems. (ii) Track prediction for the systems upto marginal cyclone stage appears to be governed mainly by middle level atmospheric forcing. (iii) Upper level wind steering is found to be important for movement of systems from severe cyclonic storm stage onwards. The above results have direct relevance to operational cyclone forecasting.

The Efficacy of the WRF-ARW Model in the Genesis and Intensity Forecast of Tropical Cyclone Fani over the Bay of Bengal

An attempt has been carried out to assess the efficacy of the Weather Research and Forecasting (WRF) model in predicting the genesis and intensification events of Very Severe Cyclonic Storm (VSCS) Fani (26 April – 04 May 2019) over the Bay of Bengal (BoB). WRF model has been conducted on a single domain of 10 km horizontal resolution using the Global Data Assimilation System (GDAS) FNL (final) data (0.250 × 0.250). According to the model simulated outcome analysis, the model is capable of predicting the Minimum Sea Level Pressure (MSLP) and Maximum Sustainable Wind Speed (MSWS) pattern reasonably well, despite some deviations. The model has forecasted the Lowest Central Pressure (LCP) of 919 hPa and the MSWS of 70 ms-1 based on 0000 UTC of 26 April. Except for the model run based on 0000 UTC of 26 April, the simulated values of LCP are relatively higher than the observations. According to the statistical analysis, MSLP and MSWS at 850 hPa level demonstrate a significantly greater influence on Tropical Cyclone (TC) formation and intensification process than any other parameters. The model can predict the intensity features well enough, despite some uncertainty regarding the proper lead time of the model run. Reduced lead time model run, particularly 24 to 48 hr, can be chosen to forecast the genesis and intensification events of TC with minimum uncertainty. Journal of Engineering Science 12(3), 2021, 85-100

A diagnostic analysis of MONEX-1979 onset vortex over the Arabian Sea

Based on MONEX-,1979 data over the Arabian Sea, the paper analyses observationally the structure, development and movement of a vortex which formed during onset of the monsoon around mid-June near the coast of Kerala developed into a cyclonic storm at mid-sea and moved towards the coast of Oman to die out there Heat budget computations bring out the differential behaviour of the different quadrants of the disturbance and appear to highlight the contrasting features between the northwestern and the other quadrants in regard to vertical. distributions of diabatic heating, local temperature tendency thermal advection and adiabatic heating or cooling. The study reveals an interaction of the vortex with two eastward-propagating subtropical westerly troughs which might have contributed significantly to its explosive development (decay) through warm (cold) advection. Both barotropic and baroclinic energy conversions appear to supply energy to the storm; though there appears to be a dominance of one over the other at different stages of development and at different heights. It seems likely that condensation heating also contributed to development of the storm.

Numerical simulation of storm surge associated with severe cyclonic storms in the Bay of Bengal during 2008-11

tc Hkh m".kdfVca/kh; pØokr vkrk gS rc Hkkjr vkSj blds fudVorhZ {ks=ksa esa rwQkuh leqnzh rjaxksa dh vkinkvksa ds dkj.k tku vkSj eky dh Hkkjh gkfu] rVh; <k¡pksa dh {kfr vkSj —f"k dks gkfu igq¡prh gSA uoEcj 1970 esa caxykns’k ¼igys iwohZ ikfdLrku½ esa vk, ,d vR;ar iapaM pØokr dh otg ls yxHkx 3]00]000 yksxksa dh tkus xbZaA uoEcj 1977 esa vkU/kz esa vk, pØokr us Hkkjr ds iwohZ rV dks rgl ugl dj fn;k ftlesa yxHkx 10]000 yksxksa dh tkus xbZaA vDrwcj 1999 esa Hkkjr ds mM+hlk ds rV ij ,d izpaM pØokrh rwQku vk;k ftlls ml {ks= esa laifRr dh vR;kf/kd gkfu gksus ds vfrfjDr 15]000 ls Hkh vf/kd yksxksa dh tkus xbZaA gky gh esa ebZ 2008 esa vk, pØokr uxhZl ls E;kaekj esa yxHk.k 1]40]000 yksxksa dh tkusa xbZa vkSj laifRr dk vR;f/kd ek=k esa uqdlku gqvkA ;s fo’o dh lcls cM+h ekuoh; vkink;sa eq[;r% m".kdfVca/kh; pØokrksa ls lac) gaS o leqnzh rwQkuh rjaxksa ls izR;{k:i ls tqMh gSA vr% ml {ks= esa laf{kIr iwokZuqeku vkSj leqnzh rwQkuh rjaxksa dh iwoZ psrkouh nsus dk izko/kku ml {ks= ds fgr esa gksrk gSA bl 'kks/k i= dk eq[; mÌs’; caxky dh [kkM+h vkSj vjc lkxj esa mBus okyh leqnzh rwQkuh rjaxksa dk iwokZuqeku djus ds fy, gky gh esa fodflr fd, x, ekWMyksa dks izdk’k esa ykuk gSA bl 'kks/k&i= esa o"kZ 2008 ls 2011 ds nkSjku caxky dh [kkM+h esa cus izpaM pØokrksa ls tqM+h leqnzh rjaxksa dk iwokZuqeku [email protected] djus esa fun’kZ ds fu"iknu dk Hkh mYys[k fd;k x;k gSA Storm surge disasters cause heavy loss of life and property, damage to the coastal structures and the losses of agriculture in India and its neighborhood whenever a tropical cyclone approaches. About 3,00,000 lives were lost in one of the most severe cyclone that hit Bangladesh (then East Pakistan) in November 1970. The Andhra Cyclone devastated the eastern coast of India, killing about 10,000 persons in November 1977. Orissa coast of India was struck by a severe cyclonic storm in October 1999, killing more than 15000 people besides enormous loss to the property in the region. More recently the Nargis cyclone of May 2008 killed about 1,40,000 people in Myanmar as well as caused enormous property damage. These and most of the world's greatest human disasters associated with the tropical cyclones have been directly attributed to storm surges. Thus, provision of precise prediction and warning of storm surges is of great interest in the region. The main objective of the present paper is to highlight the recent developments in storm surge prediction model for the Bay of Bengal and the Arabian Sea. Paper also describes the performance of the model in forecasting/simulating the surges associated with severe cyclones formed in the Bay of Bengal during 2008 to 2011.

Doppler Weather Radar analysis of short term cyclonic storm

On the east coast of India, during South-West monsoon period severe cyclonic storms are very rare and if they are short term cyclones then their prediction becomes very difficult due to rapid change in the intensity of the system. Though synoptic observations failed and satellite observations also cannot give decisive picture about such systems, in that case timely warning can not be issued by the weather agencies. Such a system was formed on 19 September, 2006 at about 250 km South-East of Kolkata (India). Very heavy rainfall associated with the system caused several human casualties and extensive damage to the property. According to news agencies, more than 100 people died and a million people became homeless due to heavy rainfall and strong winds associated with the cyclone during 19 September -21, 2006. At 0600 UTC, Doppler Weather radar (DWR) at Kolkata observed initial signatures of the system like a depression. Subsequently at 0900 UTC the observations indicated that the intensification of the system has taken place to a higher stage of deep depression and at about 1200 UTC clear spiral bands with a circular eye recorded by DWR confirmed for a fully developed severe cyclonic storm. The system weakened in to a deep depression at 1630 UTC after the landfall but again became a cyclonic storm at 2100 UTC of 19 September, 2006. Present study establishes that DWR is very useful for prediction of this short term cyclonic storm, its direction of movement and heavy rainfall associated. The maximum radial winds of the magnitude 32 m/s (64 knots/115 km/h) were also recorded by DWR at an altitude of 2.5 km in the eye wall region of the system. The high wind speed and the well defined structure of the cyclone observed by DWR confirmed that the system was a Severe Cyclonic Storm of T number 3.5. Records are available with surface observatories in the region for strong winds of the order of 110 km/h. This study also revealed that an early warning for strong winds and heavy rainfall could have been issued for development of such a short duration tropical cyclone using DWR data well in advance.

Estimation of intensity of tropical cyclone over Bay of Bengal using microwave imagery

caxky dh [kkM+h esa o"kZ 2008&2010 esa ,Q- Mh- ih- vof/k ¼15 vDrwcj ls 30 uoEcj½ ds nkSjku vk, ik¡p pØokrksa ds lw{e rjaxh; es?k fcEckofy;ksa rFkk 85 fxxkgV~tZ vko`fÙk esa izkIr fd, x, mRiknksa dh tk¡p dh xbZ gS ftlls rkieku nhfIr] rkieku nhfIr esa vfu;ferrk] dsUnz dk LFkku] lrg ij vuojr cgus okyk vf/kdre iou ¼,e- ,l- MCY;w-½ rFkk pØokrksa ds fHkUu&fHkUu fLFkfr;ksa esa muds rhozhdj.k ls lacaf/kr djdksa tSls% vonkc ¼Mh-½] xgu vonkc ¼Mh- Mh-½] pØokrh; rwQku ¼lh- ,l-½] rhoz pØokrh; rwQku ¼,l-lh-,l-½] vfr rhoz pØokrh; rwQku ¼oh-,l-lh-,l-½ vkfn dk vkdfyr dsUnzh; nkc ¼bZ- lh- ih-½ dk vkdyu fd;k tk ldsA izf{kr fd, x, nhfIr rkieku vfu;ferrkvksa dh rqyuk lS)kafrd :i ls bZ-lh-ih- ds csLV VªSd vkdyu ij vk/kkfjr nhfIr rkieku vfu;ferrk ,oa bu pØokrksa ds ckgjh nkc ds lkFk Hkh dh xbZ gSA dsUnz ds LFkku] bZ-lh-ih- ,oa lw{erajxh; fcEckoyh ds vk/kkj ij vkdfyr ,e- ,l- MCY;w- dh rqyuk csLV VªSd ,oa Hkkjr ekSle foKku foHkkx ds Mh- oksjkWd ds vkdyu ls dh xbZ gS vkSj mldk fo’ys"k.k fd;k x;k gSA pØokrh; fo{kksHk ¼lh- Mh-½ ds dsUnz ds LFkku esa varZ tSlkfd lw{erjaxh fcEckofy;ksa rFkk csLV VªSd vkdyu ds }kjk vkdfyr fd;k x;k gS] fo{kksHkksa ds rhozhdj.k ds lkFk&lkFk de gksrk tkrk gS vkSj vonkc ¼Mh-½ dh fLFkfr esa yxHkx 25 fd-eh- ls vfr rhoz pØokrh; rwQku ¼oh-,l-lh-,l-½ dh fLFkfr esa 18 fd- eh ds chp cnyrk jgrk gSA tcfd ;g varj Mh oksjkWd ds vkdyu ls dkQh vf/kd gSA lw{erjaxh; vkdyuksa ij vk/kkfjr ,e- ,l- MCY;w- vkdyu oh-,l- lh- ,l- ds nkSjku csLV VªSd vkdyuksa ls yxHkx 28 ukWV~l vf/kd vkdfyr fd;k x;k gS vkSj vonkc ¼Mh-½@pØokrh; rwQku ¼lh-,l-½@rhoz pØokrh; rwQku ¼,l- lh- ,l-½ dh fLFkfr esa ;g 6&8 ukWV~l vkdfyr fd;k x;k gSA csLV VSªd vkdyuksa ls lkisf{kd varj dks ns[kus ls irk pyk gS fd lh-,l- vkSj ,l-lh- dh fLFkfr esa lw{e rajx esa ,e-,l-MCY;w- yxHkx 12&15 izfr’kr vkSj oh-,l-lh-,l- dh fLFkfr esa yxHkx 30 izfr’kr vf/kd vkdfyr gqvk gS tcfd Mh- oksjkWd dk ,e- ,l- MCY;w- vkdyu lh- ,l-] ,l- lh- ,l- vkSj oh- ,l- lh- ,l- dh fLFkfr;ksa esa 15&18 izfr’kr de gks x;k gSA caxky dh [kkM+h ds Åij 230 dsfYou dk nhfIr rkieku vonkc ds cuus ds fy, vuqdwy gksrk gS] 250 dsfYou dk rkieku bldks pØokrh rwQku esa 260 dsfYou rhoz pØokrh rwQku esa vkSj 270 dsfYou vfr izpaM+ pØokrh rwQku esa cny nsrk gSA nhfIr rkieku ds nsgyheku ¼FkszlksYM osY;w½ ds vfHkKku ¼fMVSD’ku½ ls bl iz.kkyh ds rhoz gksus dk iwokZuqeku nsus ds fy, iz;kIr vfxze le; fey ldrk gSA blh izdkj nhfIr rkieku folaxfr 3 dsfYou ls vf/kd gksus ij pØokrh; rwQku rhoz pØokrh; rwQku esa cny tkrk gS vkSj 8 dsfYou dk rkieku bls caxky dh [kkM+h esa vfr izpaM pØokrh; rwQku ds :i esa cny nsrk gSA Microwave cloud imageries and derived products in the frequency of 85 GHz have been examined for five cyclones that occurred during FDP period (15 October- 30 November) of 2008-2010 over the Bay of Bengal to estimate the brightness temperature, brightness temperature anomaly, location of centre, maximum sustained wind (MSW) at surface level and estimated central pressure (ECP) associated with cyclones in their different stages of intensification like depression (D), deep depression (DD), cyclonic storm (CS), severe cyclonic storm (SCS), very severe cyclonic storm (VSCS), etc. Also the observed brightness temperature anomalies are compared with theoretically derived brightness temperature anomalies based on the best track estimates of ECP and outermost pressure for these cyclones. The location of centre, ECP and MSW based on microwave imagery estimates have been compared with those available from the best track and Dvorak’s estimates of India Meteorological Department and analyzed. The difference in location of the centre of cyclonic disturbance (CD) as estimated by microwave imageries and best track estimates decreases with intensification of the disturbances and varies from about 25 km in depression (D) stage to 18 km in VSCS stage whereas the difference is significantly higher in case of Dvorak estimate compared to best track estimate. The MSW based on microwave estimates is higher than that of best track estimates by about 28 knots during VSCS and 6-8 knots during D, CS, SCS stage. Considering relative difference with respect to best track estimates, the MSW is overestimated in microwave by about 12-15% in case of CS and SCS stage and by about 30% in VSCS stage while Dvorak’s MSW overestimation reduced to 15-18% during CS, SCS and VSCS stages. Brightness temperature of the order of 230 K is favourable for genesis (formation of D), 250K for its intensification into CS, 260 K for intensification into SCS and 270K for its further intensification into VSCS stage over the Bay of Bengal. Detection of threshold value of brightness temperature may provide adequate lead time to forecast intensification of the system. Similarly, when brightness temperature anomaly exceeds 3K, CS intensify into SCS and 8K, it intensifies into a VSCS over Bay of Bengal.

A preliminary study about the prospects of extended range forecast of tropical cyclogenesis over the north Indian Ocean during 2010 post-monsoon season

o"kZ 2010 esa ekulwuksRrj _rq ds nkSjku nks pØokrh; rwQku fufeZr gq, FksA tSls ‘fxjh’ uked vfr izpaM pØokrh; rwQku ¼oh-,l-lh-,l-½ 19 vDrwcj dks fufeZr gqvk vkSj ;g 22 rkjh[k dks E;kaekj leqnz rV dks ikj dj x;k vkSj nwljk ‘tky’ uked izpaM pØokrh; rwQku ¼,l-lh-,l-½ 2 uoacj dks fufeZr gqvk vkSj ;g psUuS ds mRrjh Hkkx ds lehi mRrjh rfeyukMq & nf{k.kh vka/kz izns’k ds leqnzh rVksa dks 07 uoacj dks ikj dj x;k ftldh otg ls rfeyukMq vkSj nf{k.kh vka/kz izns’k ds leqnz rVh; {ks=ksa esa u dsoy rhoz iou ls cfYd mlls gqbZ Hkkjh o"kkZ ls Hkkjh {kfr gqbZA okLrfod le; foLr`r {ks= iwokZuqeku xR;kRed fHkUurkvksa ds lkIrkfgd vkSlr ds vk/kkj ij nks lIrkg ds fy, rS;kj fd, x, gSa tks- bZ- lh- ,e- MCY;w- ,Q-] ,u- bZ- lh- ih- rFkk nksuksa ds 2 ekWMYl vkSlr ¼2 ,e- ,- oh- bZ-½ ds ;qfXer ekWMy ifj.kke ij vk/kkfjr gSaA lkIrkfgd vkSlr] iou vkSj lkisf{kd Hkzfeyrk ds 5&11 fnuksa ds izpkyukRed iwokZuqeku 14 vDrwcj 2010 ds vkjafHkd fLFkfr ij vk/kkfjr gSa ftlls irk pyk gS fd 18&24 vDrwcj dh vof/k ds nkSjku e/; caxky dh [kkM+h ds Åij fuEu nkc dk pØokrh; ldqZys’ku Fkk tks vfr izpaM pØokrh; rwQku ‘fxjh’ ds leku FkkA ‘tky’ uked pØokr dh mRifRr dk 2 ,e- ,- oh- bZ- esa vPNh rjg irk yxk fy;k x;k FkkA bldk iwokZuqeku 12&18 fnuksa ds fy, oS/k Fkk vkSj ;g 21 vDrwcj 2010 dh vkjafHkd fLFkfr ij vk/kkfjr FkkA 2 ,e- ,- oh- bZ- iwokZuqeku 1&7 uoacj rd ds fy, oS/k Fkk tks 28 ,oa 21 vDrwcj dh vkjafHkd fLFkfr;ksa ij vk/kkfjr Fkk ¼buds iwokZuqeku dh vof/k Øe’k% 5&11 fnuksa rFkk 12&18 fnuksa dh Fkh½ ftlesa Li"V :i ls n’kkZ;k x;k gS fd rfeyukMq leqnz rV vkSj blls yxs gq, vka/kz izns’k ds {ks= esa izsf{kr dh xbZ folaxfr;ksa ls dkQh vf/kd ?kukRed o"kkZ folaxfr;k¡ ns[kh xbZ gSaA bl izkjafHkd v/;;u esa vkxs crk;k x;k gS fd lkIrkfgd pØokrh; Hkzfeyrk ds ekWMy iwokZuqekuksa dh vf/kdre folaxfr =qfV yxHkx &0-8 ls &1-0 × 10&5 izfr lSds.M dks fuEu LRkjh; vf“lj.k folaxfr yxHkx &0-8 ls &1-0 × 10&5 izfr lSds.M ds lkFk feyus ij m".kdfVca/kh pØokr cuus dh laHkkouk curh gSA rFkkfi bl flLVe ds pØokr ds :i esa rhozhdj.k gsrq Fkzs’kgksYM oSY;w dh igpku djus ds fy, vkSj vf/kd ekeyksa ds fo’ys"k.k djus dh vko’;drk gSA There were two cyclonic storms formed during the post monsoon season of 2010 viz., “Giri” a very severe cyclonic storm (VSCS) formed on 19th October and crossed the Myanmar coast on 22nd and the second system “Jal” a severe cyclonic storm (SCS) formed on 2nd November and crossed north Tamil Nadu-south Andhra Pradesh coasts, close to north of Chennai on 7th November, which caused lot of damage in Tamil Nadu and south Andhra Pradesh coast associated with not only strong wind but also due to associated heavy rainfall. The real time extended range forecasts in terms of weekly mean of dynamical variables are prepared for two weeks based on the coupled model outputs from ECMWF, NECP and the 2 models average (2MAVE) of both. The operational forecast for days 5-11 of weekly mean wind and relative vorticity based on 14th October, 2010 initial condition indicates cyclonic circulation at low level over the central Bay of Bengal during the period from 18-24 October associated with the very severe cyclone “Giri”. The genesis of the cyclone “Jal” was very much captured in the 2MAVE forecast valid for 12-18 days forecast based on the initial condition of 21st October, 2010. The 2MAVE forecast valid for 1-7 November based on 28 October and 21 October initial conditions (with forecast period of days 5-11 and days 12-18 respectively) also clearly indicated large positive rainfall anomalies over Tamil Nadu coast and adjoining coastal Andhra Pradesh region like that of observed rainfall anomalies. This preliminary study further indicates that the model forecasts anomaly of weekly cyclonic vorticity maximum of about 2.5´10-5 sec-1 combined with a low level convergence anomaly of about -0.8 to -1.0 ´ 10-5 sec-1 may lead to formation of a tropical cyclone. However, more number of cases required to be analysed for the proper identification of the threshold values for intensification of the system into a cyclone.

Influence of cyclonic disturbances from sea on rainfall over Eastern Ghats in Andhra Pradesh

Using data of 84 Years, the influence of depressions and cyclonic storms on the rainfall over Eastern Ghats in Andhra Pradesh is studied. The portions of the Ghats, which receive heavy rains due to a depression or cyclonic storm are identified and the rainfall distribution in corresponding months is studied. The results show that the depressions/cyclonic storms crossing not only the coastal parts of AndhraPradesh but also those of south Orissa north and TamilNadu can cause heavy rain over the Ghats in Andhra Pradesh.

Studies on VLF atmospherics during the tropical cyclone “AILA” and several monsoon period thunderstorms over north-east India

bl 'kks/k i= esa vizSy 2009 ls vDrwcj 2009 dh vof/k ds nkSjku HkkSfrdh foHkkx] f=iqjk fo’ofo|ky; }kjk oh- ,y- ,Q- ok;qeaMyh; vFkok LQsfjDl dh vfHkyk{kf.kd fofo/krk dks pkj vyx & vyx vko`fr;ksa esa ntZ fd;k x;k gSA bl 'kks/k i= esa mRrj iwoZ Hkkjr ds 76 lfØ; xtZ ds lkFk rwQku okys vf/kdk¡’k fnuksa ds vk¡dMs+ iz;qDr fd, x,A buls izkIr ifj.kkeksa esa ekulwu ds nkSjku LQsfjDl ifjorZu esa dbZ izdkj dh fo’ks"krk,¡ n’kkZbZ xbZ gSaA bu Øfed fxjkoV] Øfed o`f) ¼th- ,Q- th- vkj-½] Øfed o`f) vpkud fxjkoV ¼th- vkj- ,l- ,Q-½] Øfed o`f) Øfed fxjkoV ¼th- vkj th- ,Q-½] Øfed fxjkoV vpkud o`f) ¼th- ,Q-,l- vkj-½] vkdfLed o`f) Øfed fxjkoV ¼,l- vkj- th- ,Q-½] vpkud fxjkoV vpkud o`f) ¼,l- vkj- ,l- ,Q-½ vpkud fxjkoV vpkud o`f) ¼,l- ,Q- ,l- vkj-½ vpkud fxjkoV Øfed o`f) ¼,l- ,Q- th- vkj-½ rFkk dBksj ¼Spiky½ Lo:i esa n’kkZ;k x;k gSA ekulwuh lfØ; xtZ ds lkFk rwQkuksa okys vf/kdka’k fnuksa esa lHkh izdkj ds iSVuksaZ esa ¼th- vkj th- ,Q-½ dh vko`fr vf/kdrj izdj.kksa esa ntZ gqbZ gS ¼izR;sd vko`fr esa vkSlru yxHkx 37 izfr’kr ?kVuk,¡ ntZ gqbZ gSa½ gekjh izs{k.kkRed vof/k ds nkSjku 23&26 ebZ dh vof/k esa caxky dh [kkM+h esa ‘vkbZyk’ uked ,d rhoz m".kdfVca/kh; pØokr vkj-,l-,e-lh- inuke% ch- vks- ch- vks-2] ts- Vh- MCY;w- lh- inuke% vks- Vw- ch- vk;k FkkA 25 ebZ 2009 ds m".k ds m".kdfVca/kh; pØokr ds nkSjku lHkh izdkj ds lkekU; ekulwuh vof/k dh vfHkyk{kf.kd fo’ks"krkvksa esa ,l-vkj-,l-,Q- ¼izR;sd vko`fr esa 86 izfr’kr ds vkSlru nj ls ?kfVr½ vko`fr vf/kdrj izdj.kksa esa ns[kh xbZ gSA tc pØokr caxky dh [kkM+h ds rVksa ls Vdjk;k ml fnu ds LQsfjDl ds vyx&vyx vko`fr forj.kksa ds vuqlkj o`f) vkSj fxjkoV dk xaHkhjrk ls fo’ys"k.k fd;k x;kA ekulwuh fnuksa vkSj ‘vkbyk’ pØokr lfØ; fnol ds lHkh ds lHkh izdkj ds iSVuksaZ dk rqyukRed v/;;u fd;k x;kA gekjs fu"d"kZ ds vuqlkj izpaM pØokr ‘vkbyk’ vkSj vU; xtZ Hkjs rwQku okys ekulwuh fnuksa ds nkSjku mRrj iwoZ Hkkjr esa cknyksa dh lw{e lajpuk esa dkQh varj fn[kkA pØokrh xfrfof/k ds nkSjku LQSfjDl ds ns[ks x, cnyko dh laHkkfor O;k[;k dks xtZ Hkjs rwQku okys cknyksa dh rfM+r xfrfof/k ds vk/kkj ij le>k;k x;k gSA The present work reports the characteristic variations in VLF atmospherics or sferics at four discrete frequencies recorded at the Department of Physics, Tripura University, during the period from April 2009 to October 2009. Data from 76 active thunder days over North-East India is considered for the present investigation. Results show several types of features in the variation of sferics during the monsoon period. These are termed as gradual fall gradual rise (GFGR), gradual rise sudden fall (GRSF), gradual rise gradual fall (GRGF), gradual fall sudden rise (GFSR), sudden rise gradual fall (SRGF), sudden rise sudden fall (SRSF), sudden fall sudden rise (SFSR), sudden fall gradual rise (SFGR) and spiky. During the Monsoon thunder active days, amongst all the patterns, GRGF occurred in most of the cases in all frequencies (average occurrence rate around 37% in each frequency). During our observational period, a severe tropical cyclonic storm named “AILA” (RSMC Designation BOB02, JTWC Designation 02B) occurred over the Bay of Bengal during 23-26 May 2009. Among several characteristic features during normal Monsoon period, SRSF (average occurrence rate around 86 % in each frequency) dominated the sferics on the 25 May, 2009, when the cyclone struck the coastal areas of the Bay of Bengal. The sferics of that day has been analyzed critically with respect to discrete frequency distribution of rise rate and fall rate of the intensity of the sferics. A comparison is made for all the patterns for the Monsoon days and the AILA cyclone active day. Our findings show substantial difference in the microstructure of clouds producing severe cyclonic storms like AILA and other thunderstorms during Monsoon seasons over North-East India. The possible interpretation of the observed variations in sferics is explained on the basis of the electrical activity that occurs inside a thunder-cloud especially during cyclonic activity.