Total Flash Density Research Articles

Impact of the Mediterranean oscillation on total lightning activity

For the first time, the relationship between total (cloud-to-ground and intracloud) lightning activity and the Mediterranean Oscillation (MO) is addressed in this work. Data from optical transient detector (OTD) and lightning imaging sensor (LIS) were used to calculate the total flash density, and the MO was described using the Mediterranean Oscillation index (MOi). The study intended to analyse the monthly values of the variables, for the 19-year period from October 1995 to February 2015 and for the region between 31.25 N and 46.25 N, and between 3.75 W and 38.75 E (the Mediterranean Sea and bordering land areas). Moreover, the study was carried out for the whole year and a comparative analysis was also conducted with the results obtained for the cold periods (October to March) and warm periods (April to September), both for the positive and negative phases of the MOi. The results revealed that the lightning activity in the annual and cold periods was clearly similar, and the maximum values tended to concentrate over southern Italy and the central part of the Mediterranean Sea, while in the warm period lightning was scattered over the whole study area. When analysing the data from the cold period for the positive phase, significant negative correlations were observed in the west and central area. On the contrary, for the warm period, the significant negative correlation values tend to be concentrated in the eastern limit of the studied territory. During the negative phase of the MOi, most of the studied area showed significant positive correlation values during the cold period, whereas only a small area with negative correlation tended to concentrate on the eastern boundary during the warm period. The results are explained in terms of the circulation dynamics and temperature changes recorded for the MO.

Climatological analyses of LMA data with an open‐source lightning flash‐clustering algorithm

AbstractApproximately 63 million lightning flashes have been identified and analyzed from multiple years of Washington, D. C., northern Alabama, and northeast Colorado lightning mapping array (LMA) data using an open‐source flash‐clustering algorithm. LMA networks detect radiation produced by lightning breakdown processes, allowing for high‐resolution mapping of lightning flashes. Similar to other existing clustering algorithms, the algorithm described herein groups lightning‐produced radiation sources by space and time to estimate total flash counts and information about each detected flash. Various flash characteristics and their sensitivity to detection efficiency are investigated to elucidate biases in the algorithm, detail detection efficiencies of various LMAs, and guide future improvements. Furthermore, flash density values in each region are compared to corresponding satellite estimates. While total flash density values produced by the algorithm in Washington, D. C. (~20 flashes km−2 yr−1), and Alabama (~35 flashes km−2 yr−1) are within 50% of satellite estimates, LMA‐based estimates are approximately a factor of 3 larger (50 flashes km−2 yr−1) than satellite estimates in northeast Colorado. Accordingly, estimates of the ratio of in‐cloud to cloud‐to‐ground flashes near the LMA network (~20) are approximately a factor of 3 larger than satellite estimates in Colorado. These large differences between estimates may be related to the distinct environment conducive to intense convection, low‐altitude flashes, and unique charge structures in northeast Colorado.

Using cloud ice flux to parametrise large-scale lightning

Abstract. Lightning is an important natural source of nitrogen oxide especially in the middle and upper troposphere. Hence, it is essential to represent lightning in chemistry transport and coupled chemistry–climate models. Using ERA-Interim meteorological reanalysis data we compare the lightning flash density distributions produced using several existing lightning parametrisations, as well as a new parametrisation developed on the basis of upward cloud ice flux at 440 hPa. The use of ice flux forms a link to the non-inductive charging mechanism of thunderstorms. Spatial and temporal distributions of lightning flash density are compared to tropical and subtropical observations for 2007–2011 from the Lightning Imaging Sensor (LIS) on the Tropical Rainfall Measuring Mission (TRMM) satellite. The well-used lightning flash parametrisation based on cloud-top height has large biases but the derived annual total flash density has a better spatial correlation with the LIS observations than other existing parametrisations. A comparison of flash density simulated by the different schemes shows that the cloud-top height parametrisation has many more instances of moderate flash densities and fewer low and high extremes compared to the other parametrisations. Other studies in the literature have shown that this feature of the cloud-top height parametrisation is in contrast to lightning observations over certain regions. Our new ice flux parametrisation shows a clear improvement over all the existing parametrisations with lower root mean square errors (RMSEs) and better spatial correlations with the observations for distributions of annual total, and seasonal and interannual variations. The greatest improvement with the new parametrisation is a more realistic representation of the zonal distribution with a better balance between tropical and subtropical lightning flash estimates. The new parametrisation is appropriate for testing in chemistry transport and chemistry–climate models that use a lightning parametrisation.

Lightning flash density 1995-2010 in Brisbane, Australia

Observations of nearby lightning have been made in Brisbane, Australia (27.3 degrees S, 153.0 degrees E) from 1995 to 2010 using CGR3 and CGR4 lightning sensors and a 500 Hz CIGRE lightning flash counter, with the objective of measuring long-term lightning flash densities and their diurnal and annual variations. The observations with the CGR4 sensor covered a circular area around the sensor of about 400 km(2). The 16-year average flash densities based on direct measurements were:ground flash density = 2.29 km(-2) yr(-1), cloud flash density = 2.81 km(-2) yr(-1) and total flash density = 5.1 km(-2) yr(-1) and average of 16 annual measurements of the cloud flash-to-ground flash ratio (denoted Z) = 1.56. The most probable long-term ratio of positive to all ground flashes is about 0.04 based on the period 2006 to 2010 when the more reliable measurements were made. The range of values was about 0.02 to 0.17 with a 16-year average of about 0.06. The average annual variation of total flash density shows that about 35 per cent of all lightning occurs from January to June and about 65 per cent from July to December. The diurnal variation of total flash density has the expected peak between 1700 and 1800 hours when 16 per cent of all lightning occurs and a secondary peak between 2000 and 2100 hours when 13 per cent of all lightning occurs. The inter-annual variation in total flash density is large with about a 9:1 range from 1.4 to 12.2 km(-2) yr(-1). The average annual thunder day level was 25 with a range from 14 to 31 per year, about a 2:1 range. There is a weak correlation between annual flash densities and thunder days, but attempting to predict a flash density from the thunder day level is subject to large uncertainty. It was found that, averaged over the 16-year study period, 50 per cent of all local lightning occurs on about two days per year.

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.

Spatial distribution and frequency of lightning activity and lightning flash density maps for Australia

The spatial distribution and frequency of lightning activity in Australia have been analyzed using lightning data obtained by ground‐based lightning detection instruments denoted CIGRE‐500 and CGR3 and by NASA satellite‐based instruments denoted OTD and LIS. The geographical distribution of lightning incidence is described by a map of total lightning flash density, Nt (i.e., cloud‐to‐ground and intracloud flashes). A high level of lightning activity, Nt > 10 km−2yr−1, is observed in the northern parts of Australia, and a decrease in total flash density occurs southward to Nt < 5 km−2yr−1 in the central and southern parts of Australia. The peak lightning occurrence is in the northwestern part of the Australian continent with Nt values up to about 35 km−2yr−1 centered around 16°S 126°E. A reduction in Nt by a factor of about 10 for a change in latitude from 10°S to 40°S was found, which is in agreement with the earlier studies. The data from all the sources were used to estimate the cloud flash‐to‐ground flash ratio, Z, which at the studied localities was found to be in a range of values from 0.75 to 7.7. We concluded that for the range of latitude over Australia the most representative value of Z is about 2 ± 30%, and it is relatively independent of latitude. We used this to develop a map of average annual lightning ground flash density, Ng, the first for Australia. Ng varies from over 6 km−2yr−1 in the northern parts of Australia to about 1 km−2yr−1 and below in the southern parts.

Cloud‐to‐ground lightning in southeastern Brazil in 1993: 1. Geographical distribution

About 1.1 million cloud‐to‐ground lightning flashes were recorded by a lightning positioning and tracking system in southeastern Brazil in 1993. The 1‐year continuous lightning data set is the first obtained in Brazil. It has been analyzed for geographical distribution of total flash density, percentage of positive flashes, negative and positive flash densities, and negative and positive flash peak currents. The dependence of the flash density and peak current on latitude, altitude, and soil resistivity was investigated. Negative flash peak current was found to be inversely correlated with latitude, but no other significant correlation was found for flash density and peak current with these parameters. Positive flashes were found to be contaminated by intracloud flashes. The maximum total, negative, and positive flash densities were 15.5, 9.1, and 7.7 flashes/km2 per year, respectively. The average percentage of positive flashes was 36.5%. The geometric means of negative and positive peak current were 30.9 kA and 17.8 kA. The high density, high percentage and low average peak current of positive flashes found in this study are probably a result of such a contamination. Neglecting positive flashes below 15 kA, assuming that they correspond to intracloud flashes erroneously identified by the system, the maximum positive and total flash densities would be 3.9 flashes/km2 per year and 11.7 flashes/km2 per year. The percentage and geometric mean peak current of positive flash would be 23% and 38.7 kA, respectively. The results are discussed in the context of other similar measurements made at different parts of the world.

Global lightning: Total, cloud and ground flash estimates

A worldwide survey of the ratio of cloud flashes to ground flashes in thunderstorms using CGR3 (Cloud‐Ground Ratio #3) instruments at sites covering a latitude range from 59.9°N to 27.3°S between 1986 and 1991 inclusive has provided information on the latitudinal variation of total flash density and the ratio of cloud flashes to total flashes. Two surveys of global lightning occurrence using satellites, the Defense Meteorological Satellite Program (DMSP) survey using optical sensors to detect lightning at near local midnight and the Ionospheric Sounding Satellite‐b (ISS‐b) survey using radio‐frequency lightning detection equipment, were used in conjunction with the CGR3 observations to obtain the latitudinal variation of total flash density over each major land mass and each major ocean. These three surveys and other sources provided data on the annual and diurnal variation in lightning occurrence at various latitudes. We have combined the above information in a computational model of global lightning occurrence and have separated total flashes into cloud flash and ground flash components. Using this model, we estimate the global annual number of total flashes to be 2.05×109 yr−1, corresponding to a mean rate of 65 s−1. This estimate is probably low because of inadequate detection of weak cloud flashes. Depending on the assumption made concerning the latitudinal variation of the ratio of cloud flashes to total flashes, the mean rates of occurrence are between about 51 s−1 and 55 s−1 for cloud flashes and between about 10 s−1 and 14 s−1 for ground flashes. The uncertainty in these estimates is a factor of 2 in either direction. The calculated global diurnal variation has a peak rate of occurrence of about 1.6 times the mean rate at about 1400 universal time (UT) in the northern hemisphere spring and has its lowest rate of about 0.5 times the mean rate at about 0300 UT in the northern hemisphere summer. About 54% of all lightning occurs in the northern hemisphere. The mean global land total flash density is about 8.3 km−2 yr−1. This is about 3.4 times the mean global total flash density over oceans.

Latitudinal variation of lightning occurrence characteristics

Lightning occurrence has been recorded at 14 sites in 11 countries covering latitudes from 60°N to 27°S between 1987 and 1991 inclusive using lightning detection instruments denoted CGR3. These instruments have an effective range between about 12 and 16 km depending upon site latitude and type of lightning and have separate registers for negative ground flashes, positive ground flashes, and cloud flashes. From the registrations we have obtained approximate values of the annual lightning total flash density, Nt, and the cloud flash‐to‐ground flash ratio, z, at each site. We have observed a well‐marked variation of Nt with latitude from 61 km−2 yr−1 at 1° to 0.66 km−2 yr−1 at 60° latitude. Nt falls by a factor of about 10 for a 30° increase in latitude, in reasonable agreement with estimates based on satellite observations. Earlier studies concerning the latitudinal variation of z showed that z was high (about 6 to 9) in the tropics, was about 3 to 4 in the subtropics and temperate regions, and was about 1 to 1.5 at high latitudes. However, we have found tropical values of z in the range 0.5 to 3.4 (weighted mean 2.3 for latitude range 0° to 20°) and similar values in subtropical and temperate regions (range from 1.1 to 3.8 and weighted mean 2.2 for the latitude range 20° to 40°). At high latitudes (40° to 60°) our measured values (range 1.0 to 1.5 and weighted mean 1.3) agree with the earlier studies. The weighting was by years of observation at the site. The mean of the values of z measured at 14 sites was 1.9. Values of z at 13 of the sites were within a factor of 2 of this mean. We estimate the global mean value of z to be 2.3, based on weighting site z values by site total flash density. We have compared lightning occurrence characteristics at eight sites between 60°N and 27°S.

The cloud flash‐to‐ground flash ratio and other lightning occurrence characteristics in Kathmandu thunderstorms

Lightning and thunderstorm observations have been made in Kathmandu, Nepal (27.4°N, 85.2°E) over the 21‐month period March 1987 to November 1988 inclusive, using direct observations of thunder and lightning, and the registrations of a CGR3 lightning flash counter that provided records of the approximate numbers of cloud flashes, positive ground flashes, and negative ground flashes with an effective range of about 14 km. The observation period effectively covered two thunderstorm seasons as almost no lightning occurred during the winter months. During 1987 and 1988 the mean thunderdays per year was 42. The principal use of the CGR3 registrations was to estimate the long‐term ratio of cloud flashes to ground flashes, denoted z; it was found that z was 2.1 in 1987 and 3.0 in 1988, with a two‐season mean of 2.7. The uncertainty in ratio values is about 30%. The registrations also enabled lightning flash densities to be estimated with an uncertainty of about 40%. The two‐season mean ground flash, cloud flash, and total flash densities were 2.4, 6.6, and 9.0 km−2 yr−1, respectively. There were well‐marked seasonal variations in occurrence, with about 82% of all lightning occurring between mid‐February and the end of May. During this premonsoon period there were steady rises to maximum values in May of the thunderdays per month (to 10), the monthly total flash density (to 2.9 km−2 month−1), and the monthly value of z (to 3.6). The range of values of z calculated from the records for individual thunderstorm days was 0.17 to infinity, but very little lightning was associated with the extreme values. A distribution of values of the ratio with respect to cumulative lightning occurrence shows that 80% of all lightning was associated with values of z (calculated on a thunderstorm day basis) between 1.3 and 6.7. There is reasonably good agreement between the value of z for Kathmandu and values of z for other places at similar latitudes. However, major differences have been found for low latitudes between recently measured values of z and earlier predictions of the latitudinal variation of z.