Marcelo M. F. Saba

Positive leader characteristics from high-speed video observations

values range from 0.3 to 6.0 10 5 ms 1 with a mean of 2.7 10 5 ms 1 . Contrary to what is usually assumed, downward +CG leader speeds are similar to downward CG 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., K. L. Cummins, T. A. Warner, E. P. Krider, L. Z. S. Campos, M. G. Ballarotti, O. Pinto Jr., and S. A. Fleenor (2008), Positive leader characteristics from high-speed video observations, Geophys. Res. Lett., 35, L07802, doi:10.1029/2007GL033000.

Thunderstorm and lightning characteristics associated with sprites in Brazil

[1] Astudy ofthe thunderstorm andcloud-ground lightning characteristics associated with sprite events observed in Brazil is presented. The study is based on ground and aircraft sprite observations with high sensitivity intensified CCD cameras of six different thunderstorms, GOES satellite infrared images, radar and lightning network data. A total of eighteen transient optical events were recorded at three different days in 2002 and 2003, sixteen of which exhibited vertical structures typically associated with sprites. Four thunderstorms were associated with two different cold fronts, one with a Mesoscale Convective System, and one was a local isolated thunderstorm. The sprites occurred during time periods when the percentage of positive flashes was higher than the average percentage for the storm lifetime. The lightning associated with the sprite events was all positive flashes with a mean peak current higher than the mean value for all flashes in the storms. INDEX TERMS: 3304 Meteorology and Atmospheric Dynamics: Atmospheric electricity; 3314 Meteorology and Atmospheric Dynamics: Convective processes; 3324 Meteorology and Atmospheric Dynamics: Lightning; 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342). Citation: Pinto, O., Jr., M. M. F. Saba, I. R. C. A. Pinto, F. S. S. Tavares, K. P. Naccarato, N. N. Solorzano, M. J. Taylor, P. D. Pautet, and R. H. Holzworth (2004), Thunderstorm and lightning characteristics associated with sprites in Brazil, Geophys. Res. Lett., 31, L13103, doi:10.1029/2004GL020264.

A study of the time interval between return strokes and K-changes of negative cloud-to-ground lightning flashes in Brazil

Abstract This paper presents the results about the time interval between return strokes and K-changes of negative cloud-to-ground lightning flashes in Brazil based on millisecond scale observations of electric field waveforms. They were obtained in Sao Jose dos Campos (45.864° W , 23.215° S ) , Brazil, in the period from October 1998 to February 1999. It was found that the interstroke time interval has an average value of 69.0 ms , a geometric mean value of 49.6 ms and does not depend on the return stroke order. The time interval between K-changes has an average value of 18.5 ms , a geometric mean value of 12.0 ms , and also does not depend on the return stroke order in which they occur between. These values are in reasonable agreement with the values obtained in other regions of the world.

Upward lightning flashes characteristics from high‐speed videos

One hundred high-speed video recordings (72 cases in Brazil and 28 cases in USA) of negative upward lightning flashes were analyzed. All upward flashes were triggered by another discharge, most of them positive CG flashes. A negative leader passing over the tower(s) was frequently seen in the high-speed video recordings before the initiation of the upward leader. One triggering component can sometimes initiate upward leader in several towers. Characteristics of leader branching, ICC pulses, recoil leader incidence, and interpulse interval are presented in this work. A comparison of the results is done for data obtained in Brazil and USA. The duration of ICC and the total flash duration are on average longer in Brazil than in USA. Only one fourth of all upward leaders are followed by any return strokes both in Brazil and USA, and the average number of return strokes following each upward leader is very low. The presence and duration of CC following return strokes in Brazil is more than two times larger than in USA. Several parameters of upward flashes were compared with similar ones from cloud-to-ground flashes.

Stratospheric conductivity measurements in Brazil

On January 26, 1994, a coordinated campaign was conducted to study the stratospheric conductivity in Brazil. The measurements were obtained, each 10 min, by a double probe electric field detector during a stratospheric balloon flight launched at 0720 LT from Cachoeira Paulista, Brazil (geographic coordinates 22°44′S, 44°56′W). The data were supported by radar and Meteosat satellite images. The balloon reached the ceiling altitude of 29.2 km in 1 hour and 20 min after launch and then floated for about 7 hours. For the first time, positive and negative conductivity vertical profiles were obtained in Brazil. During the flight, at an average altitude of 27.5 km, the balloon passed over two thunderclouds as identified by onboard video images and photographs, as well as by radar data. The simultaneous conductivity measurements and thundercloud observations allowed to determine that the presence of clouds affects the stratospheric negative conductivity, producing an extended and cumulative drop after the balloon passed over each cloud. The positive conductivity, in turn, was found to be predominantly influenced by the photoelectric effect.

CIGRE technical brochure on lightning parameters for engineering applications

CIGRE TB 549 (2013) is an update on previous CIGRE documents on the subject, published in Electra more than three decades ago. Lightning parameters needed in different engineering applications are reviewed. New experimental data, as well as the old data, are evaluated. Additional lightning parameters, previously not considered by CIGRE, are included. Possible geographical and seasonal variations in lightning parameters are examined. Specific applications are considered and recommendations are made.

Continuing current intensity in positive ground flashes

The highest directly measured lightning currents and the largest charge transfers to ground are thought to be associated with positive lightning. Brook at al. [1] for one positive lightning in a winter storm in Japan, inferred a charge transfer in excess of 300 coulomb (C) during de first 4ms. But charge transfers to beyond 3000C were reported from direct current measurements, by Miyake et al.[1] for positive winter lightning in Japan. Positive strokes may have high peak currents followed by long continuing current (CC), and thus combine these two threatening features for lightning protection. Although positive flashes are usually less frequent than negative lightning, the special characteristics of their CC make the understanding of positive lightning an important issue. Positive lightning were recorded in southeastern Brazil during the summers of 2009-2011. This study presents some CC intensity estimates obtained from an electric field capacitive antenna. Most CC intensities were much higher than the usual intensity values of CC in negative flashes.

Cloud-to-Ground Lightning Observations in Brazil

A comprehensive review of cloud-to-ground (CG) lightning observations in Brazil is presented. Brazil is the largest country in the tropical region and it is believed to have the largest cloud-to-ground (CG) lightning activity in the world, estimated from thunderstorm days and satellite data in about 50 millions CG lightning flashes every year. Emphasis is given to recent observations made by lightning location systems and high-speed cameras in the Southeast region of the country. At the present time, they correspond to the largest CG lightning data set available for the tropics. Some specific parameters emerging from these observations are quantified. The observations are compared with similar ones made in other tropical and temperate countries, as well as with past observations obtained by other techniques in Brazil.

Observations of bidirectional lightning leader initiation and development near positive leader channels

Based on the analysis of high-speed optical and electric field change data, we present three observed cases in which a naturally occurring bidirectional lightning leader initiated and developed in virgin air near a previous established positive leader channel. Twice a new leader formed near an upward propagating positive leader that had initiated from a tower during an upward flash and once a new leader formed near a downward propagating positive leader prior to a positive cloud-to-ground return stroke. There were clear and consistent behavioral differences between the positive and negative leader ends of the newly formed bidirectional leader, and the positive end grew more slowly than the negative end in each case. In all three cases, the negative end of the bipolar leader connected with the previously formed positive leader channel creating a new positive leader branch. These rare observations show the bidirectional nature of naturally occurring lightning and suggest that positive leaders can gain branches by connection with newly formed bipolar leaders.

On β2 stepped leaders in negative cloud‐to‐ground lightning

In their seminal lightning studies using streak cameras, Schonland et al. (1938) identified four negative stepped leader events that they term “β2,” a “rather rare variant of the type β leader”, and in it, “the second and slower stage of the leader is associated with the appearance of one or more fast dart streamers, which travel rapidly down from the cloud along the previously formed track and cease when they have caught up with the slower leader tip.” Seven negative downward leaders that agreed with the description given by Schonland et al. for type β2 were recorded in Tucson, Arizona, USA, and in Sao Jose dos Campos, Sao Paulo, Brazil. All cases were recorded by a high-speed camera operating at 4000 frames per second, and electric field changes were measured for three of them. Their “dart streamers” had speeds between 106 and 107 m s−1, compatible with previous observations of recoil leaders (RLs). Also, during the development of the three cases with correlated electric field changes, it was possible to identify sequences of microsecond-scale pulses preceding the propagation of a dart streamer in the channel. It is proposed that the luminous process that occurs during the development of a type β2 stepped leader is the visible manifestation of one or more RLs that begin inside the cloud and connect to the in-cloud, positive portion of the bipolar, bidirectional leader, and then travel downward to the lower end of the negative stepped leader path.