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An Initial assessment of flash density and peak current characteristics of lightning flashes to ground in South Florida

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Abstract

Cloud-to-ground (CG) lightning flash density and first stroke peak current characteristics were determined during three months of observation in South Florida. The resultant distributions of these parameters are presented and compared with previous studies. The data are presented in the context of estimating the probability of lightning strikes to surface structures. The limitations of these estimates are described as well as additional research necessary to refine such estimates on a national scale.
... After the current cease to flow through the leader created channel, another type of leader, called dart leader, propagate down the same ionized channel followed by a subsequent return stroke. (Prentice and Mackerras, 1969;Mackerras, 1978;Maier et al., 1979;Anderson, 1980;Anderson and Eriksson, 1980;Lopez and Holle, 1986). The relation between the thunder days and thunder hours with CG flash density established by Prentice (1977) and by MacGorman et al., (1984) respectively as provided below = ( . ) = . ...
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A study of electrical and lightning properties of the premonsoon thunderclouds is carried out over the Northeastern and Eastern parts of India with the following objectives (i) To investigate the regional variation of the electrical and lightning properties of thunderclouds, in association with their cloud top temperature, vertical structure and cloud microphysical properties, (ii) To simulate the lightning and associated cloud microphysical, dynamical and thermodynamical properties of thunderclouds, (iii) To investigate the optimum atmospheric stability indices for detection of thundercloud day. For the present study, ground observations are carried out at Kohima (25.67o N, 94.08o E) in the Northeastern India and at Rampurhat (24.17° N, 87.78° E) in Eastern India. Kohima and Rampurhat are situated at an altitude of 1500 m and 40 m from the mean sea level respectively. The present study is a comprehensive multi-sensor approach. The observations from the ground-based electric field mills (EFM-100) and lightning detectors (LD-350) are utilized in synergy with the satellite onboard radiance observations from INSAT-3D. Apart from this, the observations, from Global Precipitation Measurement (GPM) and CloudSat satellites are also utilized to study the characteristics of vertical structure of reflectivity and cloud microphysics. Simulation study is carried out with the help of Weather Research and Forecasting (WRF) model. The electrical and lightning observations reveal that, Kohima experiences more number of, thunderclouds, albeit of smaller size, compared to Rampurhat. Over Rampurhat, there is a relatively strong diurnal cycle of the occurrence of thunderclouds compared to Kohima. The electric field observations show larger values of step change in electric field (±ΔE) over Rampurhat compared to Kohima, which appeaently suggest larger charge transfer over Rampurhat. The lightning type distribution from the LD-350 shows significant regional variation in lightning characteristics, which signify the variability in the charge structure of the thunderclouds. Overall percentage occurrence of Cloud-to-Ground (CG) lightning is more over Kohima compared to Rampurhat. On the contrary, occurrence of Intra-Cloud (IC) lightning is more over Rampurhat compared to Kohima. It is also observed that, over both the stations, the occurrence of –CG lightning is more compared to +CG lightning. Overall, negatives discharges dominated over Kohima, whereas the opposite is observed over Rampurhat. The prevalence of positive as well as negative discharge from the thunderclouds suggests the presence of tripole charge structure of the thunderclouds over both the stations, with distinct characteristics. The result is broadly in agreement with the EFM-100 observations. The thunderclouds over Rampurhat are associated with higher lightning flash density compared to Kohima, suggesting that, the thunderclouds over Rampurhat are more severe compared to Kohima. Satellite radiance observations shows thunderclouds with colder clouds over Rampurhat compared to Kohima. Thunderclouds over Rampurhat are associated with stronger mixed phase processes compared to Kohima. The simulation of thundercloud lightning and associated cloud and atmospheric properties is carried out with the help of WRF model in the month of April. It is found that, over both the region, though up to the median value, the simulated CG flash density are reasonably in good agreement with the observations, but over both the regions, the model simulation is not able to capture the extreme CG flash density. Nevertheless, the regional variability of the CG flash density is well captured by WRF simulation, with higher flash density over Rampurhat compared to Kohima, consistent with the observations. Simulation is able to capture the strong and weak diurnal variation of thundercloud occurrence over Rampurhat and Kohima respectively. The results also suggest that the performance of simulation of space-time evolution is better over Rampurhat compared to Kohima, suggesting more challenge to forecast the impending thunderclouds over the Northeastern part of India. The mean values of ice, graupel and cloud water mixing ratio are higher in the mixed phase region over Rampurhat compared to Kohima, suggesting a prevalence of stronger microphysical processes in the mixed phase region, a favorable condition for the lightning, consistent with the observations. The simulation results are able to capture the regional variation of total wind shear, with higher value over Rampurhat compared to Kohima. The performance of six stability indices namely, Lifting index (LI), K-Index, Total Total (TT) index, Severe Weather Threat (SWEAT) index, Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN) were analyzed to find the optimum stability index to detect the thundercloud days over each region. Over Kohima, the optimum stability index TT ≥ 38o C shows the best skill scores, whereas over Rampurhat, the optimum stability index CAPE ≥1680 J kg-1 shows the best skill scores. The overall performance of the optimum stability indices were found to be better over Rampurhat compared to Kohima. Overall, the present study provides a comprehensive understanding of the regional variability of the electrical and lightning properties of thunderclouds and associated microphysical, dynamical and thermodynamical properties over the Northeastern and Eastern parts of India. The simulated results are reasonably in good agreement with the observations. The present study will help to address the issues related to the lightning hazards and their mitigation in a better way.
... Maier [14] found that flash density may vary by an order of magnitude over distance of 20-30 km. Therefore, the data were organized into 0.2 0 ×0.2 0 latitude and longitude grids. ...
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The variation of the lightning activities over Sri Lanka and surrounded costal belt (5.75ºN-10.00ºN and 79.50ºE-89.00ºE) is studied using lightning flash data of Lightning Imaging Sensor (LIS) which was launched in November 1997 for NASA’s Tropical Rainfall Measuring Mission (TRMM). The LIS data in the period of 1998 to 2014 are based on this study and there were 23838 lightning flashes over the landmass covered by Sri Lanka during considered period. The data show that average flash density has an increasing trend of 0.093 flashes km-2 year-1. The linear trend is shown in annual flash count with the fitted straight line. Although there is a slight variation between years, there is a clearly increasing trend with 24 flashes per year. There is an incremental trend in distributing of lightning activities all over the country with time. Warming trends of surface temperature and atmosphere may have a correlation with increasing trend of the lightning activities over Sri Lanka and surrounded costal belt.
... Maier [16] found that flash density may vary by an order of magnitude over distance of 20-30 km. Therefore, the data were organized into respective 0.2 0 ×0.2 0 latitude and longitude grids. ...
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The variation of the lightning activities over Sri Lanka and surrounded costal belt (5.75 ⁰ N-10.00 ⁰ N and 79.5 ⁰ E-89.00 ⁰ E) is studied using lightning flash data of Lightning Imaging Sensor (LIS) which was launched in November 1997 for NASA’s Tropical Rainfall Measuring Mission (TRMM). The LIS data for the period of 1998 to 2014 are considered for this study. The spatial and temporal variation of lightning activities is investigated and respective results are presented. The diurnal variation over the studied area presents that maximum and minimum flash count recorded at 1530-1630 Local Time (10-11UTC) and 0530-0630LT (00-01UTC) respectively. Maximum lightning activities over the observed area have occurred after the 1330LT (08UTC) in every year during the considered time period. The seasonal variation of the lightning activities shows that the maximum lightning activities happened in First inter monsoon season (March to April) with 30.90% total lightning flashes and minimum lightning activities recorded in Northeast monsoon season (December to February) with 8.51% of total lightning flashes. Maximum flash density of 14.37fl km ⁻² year ⁻¹ was observed at 6.98 ⁰ N/80.16 ⁰ E in First inter monsoon season. These seasonal lighting activities are agree with seasonal convective activities and temperature variation base on propagation of Intra-Tropical Convection Zone over the studied particular area. Mean monthly flash count presents a maximum in the month of April with 29.12% of lightning flashes. Variation pattern of number of lightning activities in month of April shows a tiny increment during the time period of 1998 to 2014. Maximum annual flash density of 28.09fl km ⁻² yr ⁻¹ was observed at 6.98 ⁰ N/80.17 ⁰ E. The latitudinal variation of the lightning flash density is depicted that extreme lightning activities have happened at the southern part of the county and results show that there is a noticeable lack of lightning activities over the surrounded costal belt relatively landmass.
... In later years, Piepgrass et al. (1982) used a network of 26 electric field mills at the Kennedy Space Center, Florida, to estimate total flash densities for the summer months June and July for the years 1974-80. A few years later, flash density measurements were obtained using early versions of the lightning detection networks based on wideband magnetic direction finders installed in Florida (Maier et al. 1979;Peckham et al. 1984) and northeastern Colorado (López and Holle 1986). FIG. 2. The distance between the nearest pair of sensors with a direction finder capability (IMPACT sensor) is calculated and contoured. ...
Article
The physical and geographical characteristics of over 216 million cloud-to-ground lightning flashes recorded during the first decade (1989-98) of operation of the National Lightning Detection Network (NLDN) covering the entire continental United States are presented. These characteristics include the total cloud-to-ground flash density, the positive flash density, the percentage of positive flashes, the first stroke negative and positive peak currents, and the multiplicity for negative and positive flashes. All analyses were done with a spatial resolution of 0.2° corresponding to an approximate resolution of 20 km. Flash densities were not corrected for detection efficiency; the measured values are presented. The maximum measured flash density is found to exceed 9 flashes km-2 across Florida in the Tampa-Orlando-Cape Canaveral corridor, near Fort Myers, and between Lake Okeechobee and the Atlantic Ocean. The mean monthly flash count peaks in July at approximately 5.5 million flashes. Positive flash density maxima, greater than 0.4 flashes km-2 occur in southern Florida: Houston, Texas; and along the Texas-Louisiana border. A broad region of relatively high positive density also occurs throughout the Midwest. The mean monthly positive flash count peaks in June and July at approximately 240 000 flashes in each month. The annual mean percentage of lightning that lowered positive charge was highest in the upper Midwest, exceeding 10% or 20% throughout most of the region. High percentages are also characteristic along the West Coast. The annual percentage of positive lightning has increased from 3% in 1989 to approximately 9% in 1998. The authors believe the increase is the result of improved sensor detection capability in the past decade. The mean monthly percentage of positive lightning flashes ranged from 4% in August to 17% in December for the decade. The annual median negative peak current ranged from 30 kA in 1989, decreasing steadily to about 20 kA in 1998. The annual median positive peak current ranged from 55 kA in 1989 decreasing to about 22 kA in 1998. The annual median peak negative and positive currents have approximately the same value since 1995, the first year after the NLDN upgrade. The monthly median first stroke peak currents for the decade peak in the winter and reach a minimum in May (positive current) and July (negative current). The mean monthly negative multiplicity for the decade ranges from 2.1 in February to 2.5 from June to October. The mean monthly positive multiplicity is approximately 1.2 throughout the year. The diurnal variation of the maximum flash rate over land was examined and found to peak during 1200-2000 local time (LT) with an exception for the upper Midwest, which peaked during 2000-0400 LT. Over water surrounding the continental United States, the lightning flash rate peaks primarily in the morning hours from 0400 to 1200 LT.
... Large fires in the southeast historically appear to have occurred primarily during the transition from the nonlightning/dry season (hereafter "nonlightning season") to the lightning/wet season (hereafter "lightning season") (Komarek 1964;Doren et al. 1993;Platt 1999). At these times lightning strikes increase in frequency (Maier et al. 1979;Snyder 1991;Goodman & Christian 1993;Hodanish et al. 1997). The dried fuels at this time are easily ignited, potentially producing large landscape-level fires given certain synoptic weather conditions. ...
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Abstract We investigated effects of fire frequency, seasonal timing, and plant community on patchiness and intensity of prescribed fires in subtropical savannas in the Long Pine Key region of Everglades National Park, Florida (U.S.A.). We measured patchiness and intensity in different plant communities along elevation gradients in “fire blocks.” These blocks were prescribed burned at varying times during the lightning season and at different frequencies between 1995 and 2000. Fire frequency, seasonal timing, and plant community all influenced the patchiness and intensity of prescribed fires. Fires were less patchy and more intense, probably because of drier conditions and pyrogenic fuels, in higher elevation plant communities (e.g., high pine savannas) than in lower elevation communities (e.g., long-hydroperiod prairies). In all plant communities fires became increasingly patchy and less intense as the wet season progressed and moisture accumulated in fuels. Frequent prescribed fire resulted in increased patchiness but a wider range of intensities; higher intensities appeared to result from regrowth of more flammable vegetation. Our study suggests that frequent early lightning season prescribed fires produce a wider range of post-fire conditions than less frequent late lightning season prescribed fires. Our study also suggests that natural early lightning season fires readily carried through pine savannas and short-hydroperiod prairies, but lower elevation long-hydroperiod prairies functioned as firebreaks. Natural fires probably crossed these firebreaks only during drier years, potentially producing large landscape-level fires. Knowledge of how patchily and intensely fires burn across a savanna landscape should be useful for developing landscape-level fire management.
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Petersen, S.M. and P.B. Drewa (Case Western Reserve University, Cleveland, OH 44106). Did lightning-initiated growing season fires characterize oak-dominated ecosystems of southern Ohio? J. Torrey Bot. Soc. 133(2): 217–224. 2006.—The persistence of oak-dominated ecosystems throughout the Central Hardwoods Region has been attributed to chronic, non-catastrophic fire regimes prior to Euro-American settlement. The nature of these fires is not well understood. We used lightning strike, thunderstorm activity, and precipitation data to elucidate the seasonal timing of natural fire regimes for southern Ohio. Our results suggest that fires are most likely to be initiated during the growing season (especially August) when there is a high frequency of lightning strikes and thunderstorm activity that occurs simultaneously with dry environmental conditions. We hypothesize that the use of prescribed growing season fires would effectively deter encroachment of shade tolerant hardwood vegetation and foster regeneration of oaks. Such fires would provide an evolutionary basis for the conservation of oak-dominated ecosystems in southern Ohio.
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