Percentage Of Positive Flashes Research Articles

A decade of high‐latitude lightning location: Effects of the evolving location network in Finland

The present lightning location system of the Finnish Meteorological Institute has been in operation since 1998. We present statistics from the 10‐year period 1998–2007. The main emphasis of this study is to give a comprehensive analysis of the development of the location system. Our results complement previous studies by providing 10‐year lightning statistics from high latitudes in Europe (60–70°N). The changes in the detection efficiency have also been discussed. Our results indicate that in Finland (1) the median (average) peak currents in 2007 were −11.3 (−16.0) kA and +13.3 (+20.7) kA for negative and positive flashes, (2) the percentage of positive flashes in 2007 was 17%, (3) the average multiplicity for negative (positive) flashes in 2007 was 2.1 (1.2), (4) the median semimajor axis of the 50% location error ellipse in 2007 was 600 m (for 92% of flashes it was below 5 km), and (5) the average number of sensors reporting a flash in 2007 was 6.3. The network has experienced several configuration changes, leading to a more stable and better performing system, but these technical changes have also caused significant changes in the statistics of the measured lightning data. Clearly, lightning statistics over a 10‐year period cannot be handled without considering the impact of technical changes on the detection network.

Aerosol effects on the enhancement of cloud-to-ground lightning over major urban areas of South Korea

A long term (1989–1999) investigation has been made using the cloud-to-ground (CG) lightning flash data collected to study the aerosol effect on lightning activity over five major urban areas of South Korea. The cloud-to-ground (CG) lightning data were collected from the Korean Meteorological Administration (KMA) of South Korea. The results reveal that an enhancement of around 40–64% in the negative flash density and 26–49% in the positive flash density is observed over the urban areas compared to their surroundings. On the other hand a percentage decrease of around 7–19% in positive flashes occurs over the urban area. The results are in good agreement with those available in the literature. The enhancement of lightning is examined in relation to the PM10 (particulate matter with aerodynamic diameter smaller than 10 μm) and SO 2 concentrations. The PM10 and SO 2 concentrations exhibit a positive linear correlation with the number of cloud-to-ground flashes, while a negative correlation is observed between those concentrations and the percentage of positive flashes. Positive correlations of 0.795 and 0.801 are found for the PM10 and SO 2 concentrations, respectively, when compared separately with the number of CG flashes, establishing the effect of aerosols on urban CG lightning enhancement. However, negative correlations of − 0.577 and − 0.548 are obtained for the PM10 and SO 2 concentrations, respectively, when compared separately with the percentage of positive flashes.

Characteristics of cloud-to-ground lightning activity over Seoul, South Korea in relation to an urban effect

Abstract. Cloud-to-ground (CG) lightning flash data collected by the lightning detection network installed at the Korean Meteorological Administration (KMA) have been used to study the urban effect on lightning activity over and around Seoul, the largest metropolitan city of South Korea, for the period of 1989–1999. Negative and positive flash density and the percentage of positive flashes have been calculated. Calculation reveals that an enhancement of approximately 60% and 42% are observed, respectively, for negative and positive flash density over and downwind of the city. The percentage decrease of positive flashes occurs over and downwind of Seoul and the amount of decrease is nearly 20% compared to upwind values. The results are in good agreement with those obtained by Steiger et al. (2002) and Westcott (1995). CG lightning activities have also been considered in relation to annual averages of PM10 (particulate matter with an aerodynamic diameter smaller than 10 μm) and sulphur dioxide (SO2) concentrations. Interesting results are found, indicating that the higher concentration of SO2 contributes to the enhancement of CG lightning flashes. On the other hand, the contribution from PM10 concentration has not appeared in this study to be as significant as SO2 in the enhancement of CG lightning flashes. Correlation coefficients of 0.33 and 0.64 are found between the change in CG lightning flashes and the PM10 and SO2, respectively, for upwind to downwind areas, suggesting a significant influence of the increased concentration of SO2 on the enhancement of CG flashes.

Total flash density and the intracloud/cloud‐to‐ground lightning ratio over the Iberian Peninsula

Analysis of the total, intracloud and cloud‐to‐ground lightning flash density and the intracloud (IC) to cloud‐to‐ground (CG) ratio (Z = IC/CG) over the Iberian Peninsula (IP) has been performed using 5 years of combined data from the Spanish lightning detection network and the NASA Optical Transient Detector (OTD). The spatial distributions of total flash density and CG flash density show significant differences, especially on the northwest of the IP, the southern island of the Balearic Islands, and the southern/southeastern Mediterranean coastline. The spatial average of the annual value of Z is 3.48. A correlation was found between Z and latitude (correlation coefficient: −0.9). A relationship between Z and the total flash density was also observed (Z ∼ f0.5; correlation coefficient: 0.48). The spatial pattern of Z is reminiscent of that of the percentage of positive CG‐flashes, and high Z values seem to occur where the percentage of positive flashes is high.

Characteristics of cloud-to-ground lightning flashes over Sweden

A detailed study of the characteristics of cloud-to-ground (CG) lightning flashes over Sweden was carried out for the period 1987–2000 using data obtained from the Swedish lightning-locating network. Results are presented by analysing over half a million lightning ground flashes. The average variation of the annual mean of the total number of flashes was found to be 37%. About 12% of the CG flashes were positive flashes and their average variation about the annual mean was 26%. The average peak currents were fairly constant over the years with variations as little as 4% for negative flashes and 5% for positive flashes. The average peak current values for negative and positive flashes were −29.90 and +63.97 kA respectively. A correlation between the mean monthly flash count and percentage of positive flashes was seen. A similar relationship is seen with the mean monthly flash count and the peak currents for both polarities. In general, high flash density and high peak currents were observed in the southern part of Sweden where most of the major cities are located. Flash densities exceeding 0.4 flashes km−2 were observed for several large cities. The maximum flash rate of 32 flashes h−1 within a 10 km radius was seen in Jönköping (14.18°E, 57.78°N) in the province of Småland.

Ten-year study of cloud-to-ground lightning activity in the Iberian Peninsula

The characteristics (annual and diurnal cycle, polarity, multiplicity and first stroke peak current) of ∼4.3×10 6 cloud-to-ground flashes recorded in the Iberian Peninsula during the first decade of measurements of the lightning detection network installed in Spain are analyzed. The mean monthly variation shows maximum lightning activity between May and September, while minimum values are observed in January and February. The mean diurnal cycle shows maximum values at 1700 LT and minimum values at around 1000 LT. The average maximum flash density (not corrected for detection efficiency) is 2.1 flashes km −2 year −1. Maximum lightning activity is associated with mountainous areas. The effect of the Mediterranean Sea is also seen. The percentage of positive flashes is 9%, although this changes over the year from 6.5% in June to 22.6% in January. The average multiplicity is found to be 2.0 for the negative flashes and 1.1 for the positive flashes, and the percentages of single-stroke flashes are 53.6% and 89%, respectively. The monthly distribution of multiplicity for negative flashes peaks in the summer and minimum values are found in the winter. The multiplicity of the positive flashes does not seem to be function of the month. The median (mean) first stroke peak currents are found to be 23.5 kA (27.3 kA) for the negative flashes and 35.3 kA (47.1 kA) for the positive flashes. For both polarities, the peak current is higher in the summer than in the winter. The percentage of positive flashes and the mean peak currents for negative flashes are higher over the sea areas than over land.

Thunderstorm and lightning characteristics associated with sprites in Brazil

A study of the thunderstorm and cloud‐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.

Urban effect on the characteristics of cloud-to-groundlightning over Belo Horizonte-Brazil

Abstract. An 8-year climatological analysis (1989–1996) of lightning network data over Belo Horizonte, a large city located in Southeastern Brazil, and nearby surrounding areas has indicated a significant enhancement of approximately 100% in the negative flash density and of 50% in the positive flash density over and downwind of the city, compared with the other adjacent areas. A decrease of 25% in the percentage of positive flashes was also observed over and downwind of the city. No urban effect was evident in the peak current of both negative and positive flashes. These results are in agreement with the recent results obtained by Steiger et al. (2002) for Houston, except that the strength of the effect is twice larger than in Houston. The reason for this difference is not clear. Key words. Meteorology and atmospheric dynamics (atmospheric electricity; convective processes; lightning)

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.

Maritime cloud‐to‐ground lightning: The western Mediterranean Sea

For the first time, the temporal and spatial distribution of cloud‐to‐ground lightning over the western Mediterranean Sea have been studied. The monthly variation shows a single peak in September and October while minimum values are observed in January to March. The diurnal cycle shows a maximum on 0600–0700 LT and a minimum around 1600–1700 LT. The percentage of positive flashes is 8%, although it is higher in the winter than in the summer. The average multiplicity is 2.1 for the negative flashes and 1.1 for the positive flashes, and the percentage of single‐stroke flashes is 48% and 91%, respectively. The annual distribution of multiplicity reveals a maximum in the summer and a minimum in the winter for the negative flashes. The median intensity was found to be 27.5 kA for the negative flashes. Positive flashes show maximum intensities in April and minima in August. The average maximum flash density is 1.5 flashes km−2 yr−1. The geographical distribution of density shows that the highest values are associated with the coastline and decrease eastward. The lightning activity over the island of Mallorca shows characteristics associated with both the land and the sea. The results are discussed in the context of the measurements taken for the land area of the Iberian Peninsula and for other oceanic areas.

Cloud‐to‐ground lightning characteristics over Houston, Texas: 1989–2000

Cloud‐to‐ground (CG) lightning detected by the National Lightning Detection Network (NLDN) indicates a relatively high flash density over Houston, Texas, for the 12‐year period 1989–2000. A significant enhancement of 45% in the flash density is observed compared to the nearby surrounding areas. The strength of the enhancement varies on the basis of season and time of day, with the greatest increases occurring during the summer (58%) and during the 0900–1800 LT time periods in each season. Observations indicate that large lightning events (defined as days with >100 flashes in a geographic region that includes Houston and nearby rural areas) were responsible for the climatological lightning anomaly and that increased thunderstorm initiation was not the most significant cause of the enhancement. A decrease (−12%) in the percentage of positive flashes is observed over the city. Higher negative median peak currents along the coast and well into the Gulf of Mexico were also discovered. Several explanations for our observations are suggested. The urban heat island and increased cloud condensation nuclei concentrations, especially from industrial pollution, are speculated to be significant factors in creating lightning enhancement. Pollution effects are speculated to cause a change in a thunderstorm's charge distribution, which can affect the polarity of CG flashes. The potential effect of the nearby coastal Gulf salt water on the calculated peak current is examined. Variations in multiplicity values across the region are observed but not explained.

Activity of cloud-to-ground lightning observed in Sri Lanka and in surrounding area of the Indian Ocean

Birthday UNIVERSITY OF COLOMBO Birthday 11.6568 Normal 0 false false false MicrosoftInternetExplorer4 st1\:*{behavior:url(#ieooui) } /* Style Definitions */ table.MsoNormalTable {mso-style-name:Table Normal; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:Times New Roman; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;} Activity of cloud-to-ground lightning flashes observed during the monsoon thunderstorms over Sri Lanka with a lightning locating system consisting of two direction finders (DF) is presented. The Northeast monsoon produced over 884 cloud-to-ground flashes with a peak lightning rate of 96 flashes per hour whereas Southwest monsoon produced 3,294 flashes with a peak rate of 104 flashes per hour. A relative flash density maximum of 198 flashes per 28×28 km 2 was observed over Ratnapura area (6.68N, 80.40E). The percentage of positive flashes and average peak lightning current values for negative flashes are found to be 6.4% and 36 kA respectively for the Northeast monsoon period and 1.7% and 40 kA respectively for the Southwest monsoon period for the flashes that struck within 10-250 km range. No significant difference in average peak lightning current values were observed for the negative and positive flashes in both monsoons. doi:10.4038/sljp.v2i0.177 Sri Lankan Journal of Physics, Vol.2 (2001) 13-19

Cloud-to-Ground Lightning in the United States: NLDN Results in the First Decade, 1989–98

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.

Diurnal Variations in the Characteristics of Cloud-to-Ground Lightning Activity in the Great Lakes Region of the United States

Cloud-to-ground (CG) lightning observations for the warm seasons (May-September) of 1989 and 1990 were analyzed for the Great Lakes region in order to assess the diurnal variations in the lightning characteristics. Several parameters, including flash rate, the spatial extent of lightning activity, first stroke peak current, and the percentage of positive flashes, varied markedly over the course of the day. In contrast, other parameters, such as the frequency of lightning periods and the multiplicity of both negative and positive CG flashes, displayed little diurnal variation. A large degree of intraseasonal (i.e., month-to-month) variation was observed in the diurnal patterns for several of the lightning parameters.