MULTITECHNIQUE STUDIES OF NORWESTER USING ELECTRICAL AND METEOROLOGICAL PARAMETERS

Radar observations of nor'wester (premonsoon thunderstorm) as experien ced over the Gangetic West Bengal and the integrated field intensity o f atmospherics (IFIA) at 10, 20 and 30 kHz recorded over the tropical station at Calcutta have been analysed. The records if IFIA reveal tha t, during nor'wester, the enhancement in the signal level occurs in th ree different stages; (i) gradual rise of atmospherics (GRA), (ii) fir st sudden enhancement of atmospherics (SEA1) and (iii) second sudden e nhancement of atmospherics (SEA2). Their respective occurrences have b een found to be prevalent in pre-noon, latenoon and afternoon hours of a day. Of the three stages, GRA and SEA1 have been found to occur bef ore the onset of thunderstorm, while SEA2 may even occur after its ons et. A comparative study reveals that the precipitation during premonso on thunderstorms is associated with the second sudden enhancement, whi le it is associated with the steady recovery of IFIA during the monsoo n and postmonsoon thunderstorms. The characteristics of the first nor' wester of every year have been critically examined with an additional consideration of simultaneous radiosonde records. A probable explanati on has been made in terms of the distribution of the effective sources at such times around the observing site.