Comparison of the signal strength of positive and negative cloud‐to‐ground lightning flashes in northeastern Colorado

Abstract

The frequency distribution of the peak signal strength associated with the first detected return stroke of positive and negative cloud‐to‐ground flashes was studied using lightning data from northeastern Colorado. These data were obtained during 1985 with a network of medium‐high gain direction finders (DFs) having a nominal range of 300 km. The median normalized signal strength of positive flashes was almost 2 times that of the negatives for flashes within 300 km of the DFs (170 versus 96 Lightning Location and Protection, Inc. (LLP), units). These DFs have an inherent detection threshold bias that tends to discriminate against weak signals. This bias increases with range and affects the detection of positive and negative flashes in different ways because of the differing character of their distributions. Positive flashes appear to have a large percentage (12 times larger than that of negative flashes) of signals clustered around very weak values (below 30 units) that are lost to the Colorado detection system of medium‐high gain very quickly with increasing range (most of these are undetected by 125 km). The resulting median for positive signals thus could appear to be much larger than the median for negative signals, which are more clustered around intermediate values (80 units). When only flashes very close (20–60 km) to the DFs are considered, however, the two distributions have almost identical medians (70 units). The large percentage of weak positive signals (below 30 units) detected close to the DFs (about 10 times larger than that of negative signals) has not been previously explored. It has been suggested that they come from intracloud discharges and are improperly classified as coming from CG flashes. The evidence in hand, however, points to their being real positive, albeit weak, cloud‐to‐ground flashes. Whether or not they are real positive ground flashes, it is important to be aware of their presence in data from magnetic DF networks. Even if the weak positive signals are eliminated by considering only distant flashes (beyond about 125 km), the frequency distributions of positive and negative signal strengths are very different, the positives lacking the large percentage of intermediate signals (around 80 units) of the negatives but having a somewhat larger relative frequency of large and very large signals (beyond 130 units).