Sferics rate in relation to thunderstorm dimensions

Abstract

On the basis of data from 3 days in 1978 this paper investigates the relation between thunderstorm dimensions and electrical activity to determine the relative importance of thunderstorm size and thunderstorm environment on the sferics rate. Sferics were recorded continuously by a wide-band (100 + 50 kHz) crossed-loop radio direction finder located at the radar site' precipitation data during the corresponding period came from the radar volume scan. The number of sferics associated with specific storms and recorded in 5-min intervals varied from a few tens to a few thousands, depending on the dimensions of the source storm and on its distance from the sferics receiver. For four thunderstorm situations this variation was reduced to a small scatter, with standard deviations corresponding to factors of 1.17-1.47, by fitting the data with the empirical relation' S oc A" x I(F n x 10 -:r, where S is the observed number of sferics per 5 min, A the cross-sectional area, H the height, and r the range of the storm. For three of the four sets of data analyzed, x was just over unity, giving a nearly linear relation between the sferics rate and the storm's area' for a supercell storm the relation was higher than linear. For thunderstorms between 50 and 300 km distant the number of recorded sferics decreased exponentially, by a factor of about 2 every 45 km, or with z = 0.0068 + 0.0011 per km. Thunderstorm height appeared the dominant parameter determining the rate of electrical activity' values of y for the 3 days were between 0.13 and 0.28 per km, the greater effect of thunderstorm height was found associated with a stronger wind shear in the 7.5-to 12-km layer. 1. INTRODUCTION Until recent years, few studies related the frequency of lightning to the radar-observed thunderstorm features. Vonnegut and Moore [1959] have reported the high rate of electrical activity associated with large thunderstorms, and results of Shackford [1960] indicate that the lightning rate increases significantly with the height reached by intense precipitation. More recently, Fitzgerald [1978] considered the existence of radar reflectivity levels between 25 and 30 dBZ the necessary threshold for the onset of lightning, and Grosh [1978] reported that the period of the maximum lightning rate coincided with the time of maximum storm height and the maximum of precipitation-filled volume. In a detailed analysis of severe thunderstorms, Rust et. al. [1981] found that the lightning rate coarsely followed the Doppler-derived updraft velocity, and Williams [1985] reported that the number of cloud-to-ground lightning flashes increased as the fifth power of thunderstorm size. [1973] and Larsen and $tansbury [1974] compared radio-located lightning to the radar-observed precipitation pattern and found cores of intense precipitation that reached over ? km height, associated with the bearings of the observed lightning. At the McGill Radar Weather Observatory, Larsen Results of Marshall and Radhakant [1978] support the strong correlation between the occurence of ...

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