Electromagnetic Noise Studies of Severe Convective Storms in Iowa: The 1970 Storm Season

Abstract

Electromagnetic noise from six convective storms in Iowa has been studied at a variety of frequencies from 0.67 to 144 MHz, with the majority of the data being recorded at 53 MHz. The quasi-static atmospheric electric field was also studied. Twelve tornadoes, numerous funnel clouds and several hailstorms occurred during these storms. Eleven of the tornadoes appear to correlate with some type of enhancement of the recorded electromagnetic pulse rate. A spectacular peak in pulse rate during Storm No. 5 is attributed to a brief but destructive tornado at 38 km; the event is discussed in detail. One tornado at long range, the longest lived of the season, showed no correlation and is thought to represent a minority class of tornadoes which exhibit little electromagnetic noise generation. Good time correlation is also achieved between data events and several hailstorms and funnel clouds. Except for the closest events (<20 km), the region above 1 MHZ appears to be a better indicator of tornadic activity than that portion of the radio spectrum below 1 MHz. The data also reveal a number of pulse-rate peaks which could not be correlated with known severe weather events. Four and possibly five types of radio noise observed from severe storms are described, along with some initial suggestions about the possible mechanisms involved. The lack of understanding of the basic physics involved is indicated and the need for more observational data is emphasized.