[1] An experimental lightning detection network, the World Wide Lightning Location network (WWLL), is being developed to provide real time global coverage with 10 km location accuracy and at least 50% detection efficiency. This paper provides a ''worst case'' analysis of WWLL location accuracy in Brazil where the VLF lightning receivers that make up the network are very distant (>7000 km). Through comparison to a local lightning detection network, we analyze the detection accuracy in Brazil with respect to time, location, and peak current of lightning strokes. In this study, we find that WWLL detection is highly dependent upon the peak return stroke current, resulting in detection of about 0.3% of the total lightning strokes. However, the detected strokes have a location accuracy of 20.25 ± 13.5 km and a temporal accuracy of 0.06 ± 0.2 ms, providing a good overview of regions of overall global lightning activity in real time.

[1] Based on analyses of high-speed video recordings of cloud-to-ground lightning in Brazil and the USA, the characteristics of positive cloud-to-ground (+CG) leaders are presented. The high frame rates permitted the average, 2-dimensional speeds of development along the paths of the channels to be resolved with good accuracy. The values range from 0.3 to 6.0 Â 10 5 ms À1 with a mean of 2.7 Â 10 5 ms À1. Contrary to what is usually assumed, downward +CG leader speeds are similar to downward ÀCG leader speeds. Our observations also show that the speeds tend to increase by a factor of 1.1 to 6.5 as they approach the ground. The presence of short duration, recoil leaders (RLs) during the development of positive leaders reveal a highly branched structure that is not usually recorded when using conventional photographic and video cameras. The existence of the RLs may help to explain observations of UHF-VHF radiation during the development of +CG flashes. Citation: Saba, M. M. F.,

[1] From analysis of digital high-speed video records of 233 negative cloud-to-ground (CG) lightning flashes associated with 27 thunderstorms in southeastern Brazil, various lightning properties have been determined. The analysis of the video records showed that although 20% of them were single-stroke flashes and the average number of strokes per flash was 3.8, a significant variation was observed in these parameters from storm to storm. In a smaller subset containing 138 flashes, 70 (51%) had multiple terminations on the ground. As 138 flashes produced 235 different strike points, the average number of strike points per CG flash was 1.70. Considering that in this study the missing of strokes is practically negligible, we can say from the average multiplicity and from the average number of strike points per flash that each ground contact point is, on average, struck 2.2 times. The 608 time intervals between strokes of 186 negative multiple-stroke flashes presented a geometric mean value of 61 ms. Although time intervals preceding subsequent strokes that create a new termination tend to be greater than intervals between subsequent strokes that follow the previously formed channel, the difference was not statistically significant. A strong positive correlation between the number of subsequent strokes in a flash and the flash minimum duration may indicate that some processes concerning the time requisite for the channel decay and for the positive leader in the cloud to provide more charges for the next stroke do not permit multiple strokes to occur under a certain minimum time duration.

[1] The Brazilian lightning detection network detected about 310,000 cloud-to-ground flashes over three large metropolitan areas in Southeastern Brazil during three summer seasons (from 2000 to 2002). It was observed an enhancement of 60-100% in the flash density over the urban areas compared to their surroundings. Conversely, a decrease of 7 -8% in the percentage of positive flashes was also observed. The spatial distribution of the flashes follows closely the shape of the São Paulo city heat island, thereby supporting the thermal hypothesis. The PM 10 concentration showed a positive linear correlation with the number of cloud-to-ground flashes and a negative correlation with the percentage of positive flashes, supporting the aerosol hypothesis. These results indicate that both hypotheses should be considered to explain the CG lightning enhancements over the urban areas. However, only the aerosol hypothesis could better support the decrease of the percentage of positive flashes.

No abstract