Measurements of Radio Frequency Noise from Severe and Nonsevere Thunderstorms

Abstract

Thunderstorm radio noise measurements at several frequencies in the range 0.01?74 MHz have been made with specially designed remote recording stations in Iowa. The data were recorded during the spring and summer of 1974 when a series of severe storm systems produced a great number of large hail and tornado reports in Iowa. Computer analyses were made of nearly a billion bits of data, corresponding to 170 h of real-time recordings. Careful compilations of surface severe weather reports, hail damage information from insurance companies, and studies on the Des Moines WSR-57 radar echoes were compared with the analyzed radio noise data. The results include the following: 1) In agreement with earlier work, large?amplitude radio noise impulse rates were found to he generally good indicators of thunderstorm severity. Although the majority of the radio energy radiated from major lightning strokes occurs in the 0.01 MHz range, this frequency was found to be a poor indicator of storm severity; the higher frequencies (megahertz range) were considerably better. The character of the noise appears similar at 2.5 and 74 MHz. 2) In at least five cases, tornadic events correlated in time with radio noise count rate peaks. One funnel cloud was reported equidistant at 60 km from two recording stations and coincident with count rate peaks at both stations, lending credence to the idea that the peak was associated with the storm occurrence, rather than with corona or other local effects. 3) No unusual radio noise was recorded during the lifetime of a small, verified tornado at 19 km range. In addition, the count rates for its parent thunderstorm would not have indicated severity. In spite of inherent atmospheric variableness, the radio noise technique is a useful complementary indicator of storm severity.