Brazilian Lightning Detection Network Research Articles

The influence of urban effect on lightning activity: Evidence of weekly cycle

A cloud-to-ground (CG) lightning data provided by the Brazilian lightning detection network (BrasilDAT) for a ten year period (1999–2008), and particulate matter (PM10), sulfur dioxide (SO2) and air temperature data from the environmental agency of São Paulo State (CETESB) for the same period, were examined in order to look for the influence of urban effect and weekly cycles in lightning activity over metropolitan region of São Paulo (MRSP). The results show that there is a significant weekly cycle in PM10 over the MRSP and lightning activity (CG lightning) on the area involving the MRSP, Campinas and São Jose dos Campos regions, both with reduction on the weekend. The average number of flash per storm and air temperature in three different ranges of PM10 concentrations was also examined. For the first one, there is an increase of the average number of flash per storm from low to intermediate range and a trend to decrease from intermediate to high range. However, the air temperature tends to stay the same in all ranges. These results suggest that pollution tends to saturate the intensification of storms and lightning activity in a specific level. After that, the increase of aerosols provided by pollution tends to decrease the lightning activity.

Methods to Overcome Lightning Location System Performance Limitations on Spatial and Temporal Analysis: Brazilian Case

Abstract One of the most interesting attributes of Lightning Location Systems (LLSs) data is that they can be analyzed in several ways according to the objectives of the study. However, the quality of the data is governed by the system performance and has some limitations when analyzed at different temporal/spatial scales, and these limitations will depend on the analysis method. This work focuses on approaches to minimize the variations associated with LLS performance. In this way, specific network configurations for the Brazilian Lightning Detection Network (BLDN) were obtained through the reprocessing of selected sensor data, resulting in three distinct datasets. Each dataset was then evaluated using different procedures: trimmed flash (exclusion of low current discharges), thunderstorm days (TDs), and thunderstorm hours (THs). The comparison between TDs obtained from the LLS and TDs available from surface stations shows consistent results with a good correlation of those datasets. An 11-yr analysis of BLDN data also shows that improvement (over time) of the system sensitivity has led to the detection of an increasing number of low peak current events. By eliminating low peak current discharges (less than 19 kA), the sensitivity variation was significantly reduced, partially “normalizing” long-term performance. TDs and THs were the most effective method to normalize temporal variations of the lightning activity, overcoming most of the network performance variations. From the spatial perspective, TDs and THs also seem to produce the most reliable lightning distribution. These results might guide long-term temporal and spatial analysis of lightning data, providing a more stable approach that is independent of system performance.

Lightning risk warnings based on atmospheric electric field measurements in Brazil

This paper presents a methodology that employs the electrostatic field variations caused by thundercloud formation or displacement to generate lightning warnings over a region of interest in Southeastern Brazil. These warnings can be used to prevent accidents during hazardous operations, such as the manufacturing, loading, and test of motor-rockets. In these cases, certain equipment may be moved into covered facilities and personnel are required to take shelter. It is also possible to avoid the threat of natural and triggered lightning to launches. The atmospheric electric field database, including the summer seasons of 2007/2008 and 2008/2009 (from November to February), and, for the same period and region, the cloud-to-ground lightning data provided by the Brazilian lightning detection network - BrasilDAT - were used in order to perform a comparative analysis between the lightning warnings and the cloud-to- ground lightning strikes that effectively occurred inside the area of concern. The analysis was done for three areas surrounding the sensor installation defined as circles with 5, 10 and 15 km of radius to determine the most effective detection range. For each area it was done using several critical electric field thresholds: +/- 0.5; +/- 0.8; +/- 0.9; +/- 1.0; +/- 1.2; and +/- 1.5 kV/m. As a result of the reduction of atmospheric electric field data provided by the sensor installed in area of concern and lightning provided by BrasilDAT, it was possible, for each of the areas of alert proposals, to obtain the following parameters: the number of effective alarms; the number of false alarms; and the number of failure to warning. From the analysis of these parameters, it was possible to conclude that, apparently, the most interesting critical electric field threshold to be used is the level of 0.9 kV/m in association with a distance range of 10 km around the point where the sensor is installed.

Cloud-to-ground lightning and Mesoscale Convective Systems

This work analyzes some physical and microphysical properties of Mesoscale Convective Systems (MCSs) and cloud-to-ground lightning. Satellite data from the GOES-10 infrared and NOAA-18 and TRMM microwave channels and lightning information from the Brazilian lightning detection network (BrasilDAT) were utilized for the period from 2007 to 2009. Based on an automatic MCSs detection method, 720 MCSs life cycles were identified during the period and in the region of study, with a lightning detection efficiency of over 90%. During the diurnal cycle, maximum electrical activity occurred close to the time of maximum convective cloud fraction (18 UTC), and 3h after the maximum normalized area expansion rate. Diurnal cycles of both properties were modulated by diurnal heating, and thus could be used to monitor diurnal variability of lightning occurrence. The electrical activity was more intense for the widest (Pearson’s correlation of 0.96) and deeper (Pearson’s correlation of 0.84) clouds, which reached 390 km size and 17 km maximum cloud top height. Area growth during the initial phase of MCSs exerted a strong influence on their size and duration, and thus also showed a potential for defining the possibility of electrical activity during their life cycle. The average lightning life cycle exhibited a maximum close to MCSs maturation, while the maximum average lightning density occurred in the MCSs initial life cycle stage. The growth rate of electrical activity during the early stages can indicate the strength of convection and the possible duration of systems with lightning occurrence. Strong condensation processes and mass flux during the growth phase of the systems can provide favorable conditions for cloud electrification and lightning occurrence. A comparison of high microwave frequencies with lightning data showed a strong relationship of vertically integrated ice content and particle size with lightning occurrence, with Pearson's correlation of 0.86 and 0.96, respectively. The polarization difference in the 85GHz channel showed that electrical activity increases linearly with polarization reduction, associated with a high value of Pearson's correlation coefficient (above 0.90). This suggests that regions with more intense electrical activity are predominantly located in areas with a high concentration of larger ice particles that are vertically oriented, due to the existence of intense updrafts and the electric field. These results demonstrate the potential use of thermodynamic, dynamic and microphysical characteristics for analyzing thunderstorms severity, and as additional information for nowcasting and monitoring electrical activity over large regions that lack ground-based lightning sensors.

Climatology of large peak current cloud‐to‐ground lightning flashes in southeastern Brazil

The goal of this article is to present the first climatology of large peak current cloud‐to‐ground (LPCCG) flashes in southeastern Brazil, in terms of flash density, percentage of positive LPCCG flashes, peak current, and diurnal distributions. The results are based on data provided by the Brazilian Lightning Detection Network (BrasilDat) from 1999 to 2006, and only flashes with peak currents >75 kA recorded during the months of December, January, February, and March (the approximate summer season in the Southern Hemisphere) were considered in order to compare with similar data obtained by Lyons et al. (1998) in the contiguous United States. The LPCCG data set, consisting of approximately 122,000 flashes, is currently the largest data set available for the tropical region. The LPCCG flashes represent about 3% of all flashes recorded during this period. All LPCCG distributions were obtained for both negative and positive flashes. The flash density distributions for negative and positive LPCCG flashes are different and furthermore differ from the flash density distributions for all negative and positive cloud‐to‐ground (CG) flashes. This suggests that the flash density distribution of CG flashes is peak current dependent. While the negative LPCCG flash density distribution has no significant dependence on any specific geographical or meteorological feature, the positive LPCCG flash density distribution and the distribution of the percentage of positive LPCCG flashes are closely related to the occurrence of mesoscale convective systems. The peak current and diurnal distributions of LPCCG flashes were found to be similar to those obtained in the contiguous United States.

On the sensitivity of cloud‐to‐ground lightning activity to surface air temperature changes at different timescales in São Paulo, Brazil

This paper presents a study about the sensitivity of cloud‐to‐ground (CG) lightning activity to changes in surface air temperature at daily, monthly, yearly, and decadal timescales in the city of São Paulo (Brazil). Lightning data collected in the city by the Brazilian Lightning Detection Network (BrasilDat) from 1999 to 2006 and thunderstorm day data obtained from 1951 to 2006 were analyzed and compared with surface air temperature data. The lightning activity increases significantly with increasing temperature, with a sensitivity of approximately 40% per 1°C for daily and monthly timescales and approximately 30% per 1°C for decadal timescale. For the yearly timescale, the increase is not statistically significant. The lower sensitivity for the decadal timescale suggests that the lightning sensitivity to changes in surface air temperature decreases for larger timescales, in agreement with what is expected on the basis of convective adjustment. The decadal lightning sensitivity found in this study is in reasonable agreement with the increase in the global lightning activity estimated by most climate models. The study is the first to investigate in detail this relationship in a large urban area inside the tropics and should contribute to the effort to understand the impact of the global warming on lightning activity.

Improvements in the detection efficiency model for the Brazilian lightning detection network (BrasilDAT)

The detection efficiency (DE) is the most important performance gauge of a lightning detection network (LDN). Moreover, the main motivation for evaluating the DE of a LDN is to separate the geographical variations of the CG lightning parameters from the variations regarding the network performance. A review of previous relative DE techniques and simple methods to correct the cloud-to-ground (CG) lightning flash density maps is presented. In addition, recent improvements in the flash DE model for the Brazilian lightning detection network (BrasilDAT) are discussed. The DE estimated values are based on the sensor individual DE probability functions, which are derived from a large amount of CG stroke data provided by the network considering different distances from the sensor and specific peak current ranges. The new approach provides better results when compared with the previous developments, since the calculation of the sensor DE probability functions neglects the lightning data provided by the minimum number of reporting sensors. Hence it is possible to minimize the unrealistic enhancement of the DE closer to the network boundaries (“border effect”) without affecting significantly the performance inside the network. The main result is a more realistic correction of the CG flash density maps, particularly at the outermost network areas, leading to an improvement in the model sensitivity.

The intracloud/cloud-to-ground lightning ratio in Southeastern Brazil

The intracloud to cloud-to-ground lightning flash ratio ( Z) has been estimated for the first time in Southeastern Brazil and in the tropical region using the Lightning Imaging Sensor (LIS) and Brazilian lightning detection network (BrasilDat) lightning data obtained from 1999 to 2005. Geographical variations of Z and their relation to elevation, latitude, precipitation, total lightning density and percentage of positive CG lightning will be discussed. Daily variations of Z will also be presented. The results suggest that Z values are similar to studies outside the tropics and that are influenced by orographic features.

Anomalous lightning activity over the Metropolitan Region of São Paulo due to urban effects

A significant enhancement in the number of negative cloud-to-ground (CG) lightning and a decrease in the percentage of positive CG flashes are observed over the city of São Paulo, similar to observations in other large urban areas. Strong evidence indicates that this anomalous behavior results from several mechanisms related to the urban effect. In this paper, we investigated the importance of the air pollution using CG lightning data provided by the Brazilian lightning detection network (BrasilDAT) for a 6-year period (1999–2004). Due to the large variations in the CG lightning activity in response to different meteorological processes, it is not an easy task to infer the contribution of air pollution to the enhancement in the lightning activity. In order to overcome such difficulty, two approaches were considered: (1) the weekly variation of the number of days with lightning in comparison to the mean concentration of particulate matter (PM 10), as well as other thermodynamical parameters; (2) the variation of the number of CG flashes and the maximum storm flash rate per individual thunderstorm for different levels of pollution. The results of both analyses suggest that: first, the enhancement in the CG lightning activity during the week days over São Paulo metropolitan region is related to the PM 10 concentration (pollution); second, the PM 10 concentration tends to increase the lifetime of the storms and, in consequence, the number of flashes per storm, and not the flash rate of the thunderstorm; and third, the effect of the pollution in the enhancement of the CG lightning activity is probably less significant compared to the effect of the urban heat island.

The influence of topography on the cloud-to-ground lightning density in South Brazil

A comparative analysis between cloud-to-ground (CG) lightning density and terrain parameters (altitude and terrain slope) in South Brazil is presented. This region is characterized by a contrasting topography, where a mountain chain separates lower (depression) and higher (plateau) landscapes. The altitude and terrain slope data were obtained from the Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM). Two years of CG lightning data (from June 2005 to May 2007) obtained by the Brazilian Lightning Detection Network (BrasilDAT) were used in the study. To avoid intracloud contamination, positive CG flashes with peak currents less than 20 kA were removed from the data set. A relative detection efficiency model was used to correct the lightning data. The results indicate that, for localized areas (following the mountain shape) within this region of Brazil, the CG lightning density is correlated with the terrain slope and not the altitude. This suggests that terrain slope has more influence than altitude on the thunderstorm occurrence and lightning activity. In addition, a temporal analysis shows that over high altitude regions the diurnal variation (amplitude) of lightning activity is stronger and the peak occurs 1 h earlier than over low altitude regions.

Relation between lightning return stroke peak current and following continuing current

The magnitude of the return stroke peak current preceding continuing current (CC) in ground flashes was investigated. This study was performed using a high‐speed camera (1,000 frames/s), an electric field flat‐plate antenna and data provided by the Brazilian Lightning Detection Network (RINDAT). The observation of 454 negative strokes followed by CC with durations from 4 ms to 542 ms revealed that negative strokes combining both peak current greater than 20 kA and CC greater than 40 ms are highly unlikely to occur. However, this was found to be possible for positive strokes. In addition, we found that on average the longer the CC, the lower the return stroke peak current preceding it. Therefore, the average detection efficiency of RINDAT was found to decrease from 62% for negative strokes followed by very‐short CC (less than 10 ms) to 36% for strokes followed by long CC, with an intermediate value of 57% for strokes followed by short (from 10 to 40 ms) CC.

Intraseasonal Forcing of Convection and Lightning Activity in the Southern Amazon as a Function of Cross-Equatorial Flow

Abstract This study focuses on modulation of lightning and convective vertical structure in the southern Amazon as a function of the South American monsoon V index (VI). Four wet seasons (December–March 1998–2001) of Tropical Rainfall Measuring Mission (TRMM) Lightning Imaging Sensor (LIS) and Precipitation Radar (PR) data are examined together with two wet seasons (2000–01) of ground-based Brazilian Lightning Detection Network (BLDN) data. These observations are composited by VI phase (northerly or southerly) for a region of the southern Amazon and discussed relative to VI-regime environmental characteristics such as thermodynamic instability and wind shear. Relative comparisons of VI-regime convective properties reveal 1) slightly larger (20%–25%) PR pixel-mean rainfall during periods of northerly VI due to increased stratiform precipitation, 2) a factor of 2 or more increase in lightning flash density and the lightning diurnal cycle amplitude during periods of southerly VI, 3) a factor of 1.5–2 increase in the conditional probability of any PR radar reflectivity pixel exceeding 30 dBZ above the −10°C level during periods of southerly VI, and 4) an associated factor of 2 or more increase in southerly VI pixel-mean ice water path, with the ice water path being highly correlated to trends in lightning activity. During periods of southerly VI, convection occurs in an environment of increased thermodynamic instability, weak southeasterly low-level, and deep upper-tropospheric easterly wind shear. During periods of northerly VI, low-level westerly shear opposes stronger deep tropospheric easterly shear in a relatively moist environment of weaker thermodynamic instability, consistent with the occurrence of more widespread stratiform precipitation. The composite results of this study point to 1) regime differences in convective forcing that alter the prevalence of ice processes and, by inference, the vertical profile of latent heating and 2) the utility of lightning observations in delineating convective regime changes.

Evidence of thermal and aerosol effects on the cloud‐to‐ground lightning density and polarity over large urban areas of Southeastern Brazil

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 PM10 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.