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

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.