@InProceedings{CamposSaWaCuKrOr:2010:DoAvDo,
               author = "Campos, Leandro Zanella de Souza and Saba, Marcelo Magalh{\~a}es 
                         Fares and Warner, T. A and Cummins, K. L and Krider, E. Philip and 
                         Orville, R. E",
          affiliation = "{Instituto Nacional de Pesquisas Espaciais (INPE)} and {Instituto 
                         Nacional de Pesquisas Espaciais (INPE)} and {South Dakota School 
                         of Mines and Technology} and {University of Arizona} and 
                         {University of Arizona} and {Texas A\&M University}",
                title = "Does the average downward speed of a lightning leader change as it 
                         approaches the ground? An observational approach",
            booktitle = "Proceedings...",
                 year = "2010",
         organization = "International Lightning Detection Conference, 21.",
             keywords = "Lightning Physics, Atmospheric Electricity, High-speed video 
                         observations, Stepped-leader velocities, Positive leaders, 
                         Positive lightning.",
             abstract = "In the lightning literature, it is commonly assumed that the 
                         stepped-leaders that initiate the first return stroke in negative 
                         cloud-to-ground lightning flashes accelerate during the final 
                         stages of their development, but there have been only a few 
                         observational studies supporting this claim. Other studies have 
                         reported a tendency for positive leaders to accelerate and 
                         negative dart-leaders to decelerate. The aim of this investigation 
                         is to determine how the average two-dimensional (2-D) downward 
                         speed of lightning leaders changes as they approach the ground and 
                         to see if any such changes depend on the leader type, polarity, or 
                         instrumentation used to measure the speed. We have examined the 
                         optical properties of leaders that were recorded using high-speed 
                         video cameras at different sites in south and southeastern Brazil, 
                         southern Arizona, and South Dakota, in conjunction with data 
                         obtained by lightning locating systems. The GPS time-stamped 
                         cameras were operated at frame rates ranging from 1000 to 11,854 
                         frames per second. Our dataset consisted of 46 negative 
                         steppedleaders, 53 negative dart-leaders, and 28 positive leaders. 
                         Two different approaches were used in the data analysis; first, 
                         the average downward speed of individual leaders was computed over 
                         preset distance intervals above the ground, i.e. from 0 to 500 m, 
                         500 to 1000 m, etc, and then the geometric mean (GM) was 
                         calculated for all the cases in a given interval. We computed and 
                         compared the GMs since the speeds of positive leaders and negative 
                         dart-leaders follow a lognormal distribution (at the 0.05 level, 
                         according to the Shapiro-Wilk test), and the negative 
                         stepped-leaders have a similar distribution (according to the 
                         Kolmogorov-Smirnov test). The GM of the positive leader speeds 
                         increases by about a factor of 10 as they get closer to the ground 
                         (i.e. from 104 to 105 m/s), and the GM speed of negative 
                         stepped-leaders varies between 2.5x105 and 3.0x105 m/s. On the 
                         other hand, the GM speed of negative dart-leaders shows a clear 
                         decrease as they approach the ground. The second approach involved 
                         analyzing the leaders individually over their measurable length, 
                         and we found that: a) 82% of the 20 positive leaders accelerated, 
                         7% decelerated, and 11% oscillated around an average speed; b) 46% 
                         of the 46 negative stepped-leaders accelerated, 6% decelerated and 
                         48% oscillated around an average speed; and c) 32.1% of the 53 
                         negative dart-leaders accelerated, 54.7% decelerated, and 13.2% 
                         oscillated around an average speed. We conclude that the 
                         acceleration of negative stepped-leaders is not as strong as some 
                         authors have assumed, but the tendencies for positive leaders to 
                         accelerate and for negative dart-leaders to decelerate are clear 
                         and agree with previous studies. We believe that the presented 
                         behavior asymmetry between negative stepped- and positive leaders 
                         might suggest microphysical differences in their propagation which 
                         should be investigated further.",
  conference-location = "Orlando",
      conference-year = "19 - 20 Apr.",
           targetfile = "campos_does.pdf",
        urlaccessdate = "28 abr. 2024"
}