Publications

The development of a new observational system called LISDAD (Lightning Imaging Sensor Demonstration and Display) has enabled a study of severe weather in central Florida. The total flash rates for storms verified to be severe are found to exceed 60 fpm, with some values reaching 500 fpm. Similar to earlier results for thunderstorm microbursts, the peak flash rate preceeds the severe weather at the ground by 5-20 min. A distinguishing feature of severe storms is the presence of lightning 'jumps' -- abrupt increases in flash rate in advance of the maximum rate for the storm. The systematic total lightning precursor to severe weather of all kinds -- wind, hail, tornadoes -- is interpreted in terms of the updraft that sows the seeds aloft for severe weather at the surface and simultaneously stimulates the ice microphysics that drives the intracloud lightning activity.

Severe storms often have high flash rates (in excess of one flash per second) and are dominated by intracloud lightning activity. In addition to the extraordinary flash rates, there is a second distinguishing lightning characteristic of severe storms that seems to be important. When the total lightning history is examined, one finds sudden increases in the lightning rate, which we refer to as lightning "jumps", that precede the occurrence of severe weather by ten or more minutes. These jumps are typically 30-60 flashes/min, and are easily identified as anomalously large derivatives in the flash rate. This relationship is associated with updraft intensification and updraft strength is an important factor in storm severity (through the accumulation of condensate aloft and the stretching of vorticity). In several cases, evidence for diminishment of midlevel rotation and the descent of angular momentum from aloft is present prior to the appearance of the surface tornado. Based on our experience with severe and tornadic storms in Central Florida, we believe the total lightning may augment the more traditional use of NEXRAD radars and storm spotters. However, a more rigorous relation of these jumps to storm kinematics is needed if we are to apply total lightning in a decision tree that leads to improved warning lead times and decreased false alarm rates.

Severe weather is defined by specific thresholds in wind. hail size and vorticity. All of these phenomena have close physical connections with vertical drafts in deep convection, which are themselves not directly measured with scanning Doppler radars of the NEXRAD type. Cloud electrification and lightning are particularly sensitive to these drafts because they modulate the supply of supercooled water which is the growth agent for the ice particles (ice crystals, graupel and hail) believed essential for electrical charge separation. For these reasons, one can expect correlations at the outset between total lightning activity and the development of severe weather which may aid in the understanding and prediction of these extreme weather conditions. The exploration of these ideas has historically been impeded by lack of good quantitative observations. A recent review of results on severe storm electrification (Williams, 1998) indicates a general absence of cases for which total lightning activity is documented over the lifetime of a severe storm. The recent development of LISDAD (Lightning Imaging Sensor Data Application Display) (Boldi, et aI., 1998) has largely remedied this problem. This paper is concerned with the use of LISDAD to quantify the behavior of total lightning in all types of severe weather, with a focus on a pair of extraordinarily electrified supercells in the Florida dry season.

A number of prior studies have examined the association of lightning activity with the occurrence of severe weather and tornadoes, in particular. High flash rates are often observed in tornadic storms but not always. Taylor found that 23% of nontornadic storms and I% of non-severe storms had sferics rates comparable to the tornadic storms. MacGorman (1993) found that storms with mesocyclones produced more frequent intracloud (lC) lightning than cloud-ta-ground (CG) lightning. MacGorman (1993) and others suggest that the lightning activity accompanying tornadic storms will be dominated by intracloud lightning- with an increase in intracloud and total flash rates as the updraft increases in depth, size, and velocity. In a recent study, Perez et aI. (1998) found that CG flash rates alone are too variable to be a useful predictor of (F4, F5) tornado formation. Studies of non-tornadic storms have also shown that total lightning flash rates track the updraft, with rates increasing as the updraft intensifies and decreasing rapidly with cessation of vertical growth or downburst onset (Goodman et aI., 1988; Williams et aI., 1989). Such relationships result from the development of mixed phase precipitation and increased hydrometeor collisions that lead to the efficient separation of charge. Correlations between updraft strength and other variables such as cloud-top height, cloud water mass, and hail size have also been observed. In this paper we examine the total lightning activity (with high time resolution), and the associated Doppler radar time history of weaker (FO, Fl) tornadic storms in Florida. Much of the prior work has focused on tornadic supercells in the Great Plains.