World Wide Lightning Location Network Research Articles

Variation of Lightning and Convective Rain Fraction in Mesoscale Convective Systems of the MJO

Abstract Characteristics of mesoscale convective systems (MCSs) in regions affected by the Madden–Julian oscillation (MJO) are investigated using a database of MCSs observed by the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E). Lightning occurrence detected by the World-Wide Lightning Location Network (WWLLN) is composited in a framework centered on the MCSs. During MJO active periods, MCSs are more numerous and larger, as the convective features persist and attain greater horizontal scales. Anomalies of the lifted index, derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim) fields, indicate that MCS environments are more stable during MJO active periods. Over the Indian Ocean, Maritime Continent, and western Pacific, lightning density in an MCS maximizes during the time that the total number of systems begins to increase as the MJO is beginning to be more active, implying both more vigorous convection and less extensive stratiform rain areas at this transitional time of the MJO. The peak in MJO precipitation coincides with peak occurrence of interconnected MCSs with larger stratiform rain fraction, shown by the Tropical Rainfall Measuring Mission satellite, while composites of lightning frequency show that during MJO active periods the zone of lightning is contracted around the centers of MCSs, and flashes are less frequent.

Climatology of global lightning classified by stroke energy using WWLLN data

ABSTRACTClimatological features of lightning, based on stroke energy, were studied by considering the hourly detection efficiency of the World Wide Lightning Location Network (WWLLN) data set from April 2009 to April 2014. Five regions were found in which the ratio of strong lightning (>90th percentile of stroke energy) to overall lightning exceeded 30% (high S‐ratio regions), and one region was determined in which the ratio of weak lightning (<10th percentile) to overall lightning exceeded 30% (high W‐ratio region). One region of high S‐ratio was the Tibetan Plateau. Lightning activity and the S‐ratio in this region increased during summer afternoons as rainfall increased; however, the large S‐ratio was observed in the grid where lightning activity was very weak. The four other areas of high S‐ratio were located over the ocean in the sub‐polar climatic zones of the Northern and Southern Hemispheres. In cold seasons of the two hemispheres, when rainfall amount increased, both the lightning activity and the S‐ratio increased, displaying nocturnal maxima; however, the amount of rainfall did not exhibit diurnal variation. Here, a large S‐ratio was also observed in the grid where lightning activity was very weak. Weak lightning activity dominated in the East Pacific. The activity of weak lightning and the W‐ratio showed a tendency to increase during daytime in the cold season. The seasonal variation of the W‐ratio was almost in accordance with rainfall amount; however, diurnal variation of the W‐ratio was not. A large value of the W‐ratio was also observed in the grid where the lightning activity was very weak.

Thunderstorm-related variations in the sporadic E layer around Rome

Superposed epoch analysis (SEA) was used to study possibly statistically significant variations of the critical frequency (foEs) and virtual height (h’Es) of the sporadic E layer (Es) related to thunderstorm activity generated in the troposphere. The reference time for the SEA was the time of lightning strokes measured by the World Wide Lightning Location Network at the ionosonde station of Rome (41.9 $$^\circ $$ N, 12.5 $$^\circ $$ E) during the year 2009. The results obtained reveal that: (a) a statistically significant decrease of foEs after the time of lightnings has been found for time windows of $$\pm $$ 100 h; (b) the effects of thunderstorms on the ionosphere is larger when the thunderstorm approaches from the opposite direction to the mean neutral stratosphere–mesosphere wind flow; (c). a statistically significant decrease of foEs related to thunderstorms during nighttime was observed. No significant changes in foEs and hEs over the seasonal time scale as well as in the latter parameter in the three (a–c) cases related to thunderstorms.

Analysis of the results of thunderstorm forecasting based on atmospheric instability indices using the WRF-ARW numerical model data

Presented are the statistical estimates of 26 indices of atmospheric instability widely used in the world practice for the thunderstorm prediction. Proposed is a new index for the thunderstorm forecasting that takes account of the vertical component of wind speed. The computation of indices is based on the data of forecasts obtained with the WRF-ARW hydrodynamic mesoscale model. The verification of thunderstorm forecasts is carried out using the observational data from weather stations and the data of the World Wide Lightning Location Network.

On the spatial and temporal distribution of global thunderstorm cells

Estimates of global thunderstorm activity have been made predominately by direct measurements of lightning discharges around the globe, either by optical measurements from satellites, or using ground-based radio antennas. In this paper we propose a new methodology in which thunderstorm clusters are constructed based on the lightning strokes detected by the World Wide Lightning Location Network (WWLLN) in the very low frequency range. We find that even with low lightning detection efficiency on a global scale, the spatial and temporal distribution of global thunderstorm cells is well reproduced. This is validated by comparing the global diurnal variations of the thunderstorm cells, and the currents produced by these storms, with the well-known Carnegie Curve, which represents the mean diurnal variability of the global atmospheric electric circuit, driven by thunderstorm activity. While the Carnegie Curve agrees well with our diurnal thunderstorm cluster variations, there is little agreement between the Carnegie Curve and the diurnal variation in the number of lightning strokes detected by the WWLLN. When multiplying the number of clusters we detect by the mean thunderstorm conduction current for land and ocean thunderstorms (Mach et al 2011 J. Geophys. Res. 116 D05201) we get a total average current of about 760 A. Our results show that thunderstorms alone explain more than 90% in the variability of the global electric circuit. However, while it has been previously shown that 90% of the global lightning occurs over continental landmasses, we show that around 50% of the thunderstorms are over the oceans, and from 00-09UTC there are more thunderstorm cells globally over the oceans than over the continents. Since the detection efficiency of the WWLLN system has increased over time, we estimate that the lower bound of the mean number of global thunderstorm cells in 2012 was around 1050 per hour, varying from around 840 at 03UTC to 1150 storms at 19UTC.

Remote sensing and modeling of lightning caused long recovery events within the lower ionosphere using VLF/LF radio wave propagation

Abstract. On the 4 November 2012 at 3:04:27 UT a strong lightning in the midst of the North Sea affected the propagation conditions of VLF/LF transmitter radio signals from NRK (Iceland, 37.5 kHz) and GBZ (UK, 19.58 kHz) received at 5246° N 8° E (NW Germany). The amplitude and phase dips show a recovery time of 6–12 min pointing to a LOng Recovery Early VLF (LORE) event. Clear assignment of the causative return stroke in space and time was possible with data from the WWLLN (Worldwide Lightning Location Network). Based on a return stroke current model the electric field is calculated and an excess electron density distribution which decays over time in the lower ionosphere is derived. Ionization, attachment and recombination processes are modeled in detail. Entering the electron density distribution in VLF/LF radio wave propagation calculations using the LWPC (Long Wavelength Propagation Capability) code allows to model the VLF/LF amplitude and phase behavior by adjusting the return stroke current moment. The results endorse and quantify the conception of lower ionosphere EMP heating by strong – but not necessarily extremely strong – return strokes of both polarities.

Subionospheric VLF perturbations observed at a low latitude station Varanasi (L = 1.07)

Initial results of subionospheric very low frequency (VLF) perturbations observed on NWC (19.8kHz) and NPM (21.4kHz) transmitter signals at low latitude ground station Varanasi, India, during the period January–June 2010, are presented. Characteristics and occurrence rates of these events mainly observed during nighttime are studied. Most of the early VLF events had slow recovery with amplitude perturbations of 0.5–4.5dB and phase changes of 3–12°. Temporal variation of the events is studied. World Wide Lightning Location Network data along with the broadband VLF data are analyzed to find the location of causative lightning discharge and/or the sferics associated with these early VLF events. Lightning induced changes in D-region ionospheric conductivity are attributed to the perturbations in the VLF signals.

The Convective Evolution and Rapid Intensification of Hurricane Earl (2010)

Abstract The relationship between an inner-core (r < 100 km) lightning outbreak and the subsequent rapid intensification (RI) of Hurricane Earl (2010) is examined using lightning strikes recorded by the World Wide Lightning Location Network (WWLLN) and in situ observations from various aircraft missions. Moderate (8.4 m s−1) northeasterly deep-layer (850–200 hPa) vertical wind shear, caused by outflow from Hurricane Danielle, existed over Earl at the beginning of a prolonged period of RI. Over 70% of the lightning strikes within a 500-km radius occurred downshear, with a preference toward downshear right in the outer rainbands, in agreement with previous studies. The location of inner-core strikes in Earl differed markedly from previous studies. The inner-core lightning activity precessed from left of shear to upshear, an extremely rare event, beginning just prior to the onset of RI. Diagnosis of the vortex tilt midway through the lightning precession showed this convection was occurring downtilt in the upshear-left quadrant; however, limited observations could not confirm if the vortex tilt was precessing with the lightning. Elevated values of low-level relative humidity and CAPE were also found upshear and supported the inner-core convection, which was found to occur within the radius of maximum wind (RMW). Previous studies have shown that convection located inside the RMW promotes intensification. It is hypothesized that intensification may have occurred in part because the vertical wind shear acted to reduce the upshear tilt, and the occurrence of convection inside the RMW helped to enhance the warm core.

Development of a hybrid variational-ensemble data assimilation technique for observed lightning tested in a mesoscale model

Abstract. Lightning measurements from the Geostationary Lightning Mapper (GLM) that will be aboard the Geostationary Operational Environmental Satellite – R Series will bring new information that can have the potential for improving the initialization of numerical weather prediction models by assisting in the detection of clouds and convection through data assimilation. In this study we focus on investigating the utility of lightning observations in mesoscale and regional applications suitable for current operational environments, in which convection cannot be explicitly resolved. Therefore, we examine the impact of lightning observations on storm environment. Preliminary steps in developing a lightning data assimilation capability suitable for mesoscale modeling are presented in this paper. World Wide Lightning Location Network (WWLLN) data was utilized as a proxy for GLM measurements and was assimilated with the Maximum Likelihood Ensemble Filter, interfaced with the Nonhydrostatic Mesoscale Model core of the Weather Research and Forecasting system (WRF-NMM). In order to test this methodology, regional data assimilation experiments were conducted. Results indicate that lightning data assimilation had a positive impact on the following: information content, influencing several dynamical variables in the model (e.g., moisture, temperature, and winds), and improving initial conditions during several data assimilation cycles. However, the 6 h forecast after the assimilation did not show a clear improvement in terms of root mean square (RMS) errors.

A Comparison of Two Ground-Based Lightning Detection Networks against the Satellite-Based Lightning Imaging Sensor (LIS)

Abstract Lightning stroke data from both the World Wide Lightning Location Network (WWLLN) and the Earth Networks Total Lightning Network (ENTLN) were compared to lightning group data from the Lightning Imaging Sensor (LIS) from 1 January 2010 through 30 June 2011. The region of study, from 39°S to 39°N latitude, chosen based on the orbit of LIS, and 164°E east to 17°W longitude, chosen to approximate the possible Geostationary Lightning Mapper (GLM) longitude, was considered in its entirety and then divided into geographical subregions. Over this 18-month time period, WWLLN had an 11.0% entire region, 13.2% North American, 6.2% South American, 16.4% Atlantic Ocean, and 18.9% Pacific Ocean coincidence percent (CP) value. The ENTLN CP values were 28.5%, 63.3%, 2.2%, 3.0%, and 2.5%, respectively. During the 18 months, WWLLN CP values remained rather consistent but low and often higher over ocean than land; ENTLN CP values showed large spatial and temporal variability. With both networks, North America had less variability during summer months than winter months and higher CP values during winter months than summer months. The highest ENTLN CP values were found in the southeastern United States, especially in a semicircle that extended from central Oklahoma, through Texas, along the northern Gulf of Mexico, across southern Florida, and along the U.S. East Coast. There was no significant change in CP values over time; the lowest monthly North American ENTLN CP value was found in June 2011 at 48.1%, the last month analyzed. These findings are consistent with most ENTLN sensors being located in the United States.

An altitude and distance correction to the source fluence distribution of TGFs.

The source fluence distribution of terrestrial gamma ray flashes (TGFs) has been extensively discussed in recent years, but few have considered how the TGF fluence distribution at the source, as estimated from satellite measurements, depends on the distance from satellite foot point and assumed production altitude. As the absorption of the TGF photons increases significantly with lower source altitude and larger distance between the source and the observing satellite, these might be important factors. We have addressed the issue by using the tropopause pressure distribution as an approximation of the TGF production altitude distribution and World Wide Lightning Location Network spheric measurements to determine the distance. The study is made possible by the increased number of Ramaty High Energy Solar Spectroscopic Imager (RHESSI) TGFs found in the second catalog of the RHESSI data. One find is that the TGF/lightning ratio for the tropics probably has an annual variability due to an annual variability in the Dobson-Brewer circulation. The main result is an indication that the altitude distribution and distance should be considered when investigating the source fluence distribution of TGFs, as this leads to a softening of the inferred distribution of source brightness.

Lightning activity within tropical cyclones in the South West Indian Ocean

Lightning activity is investigated within tropical convective events of the South-West Indian Ocean (SWIO). The World Wide Lightning Location Network (WWLLN) provides global lightning data since 2005 and is therefore a well suited network for the study of systems like tropical cyclones as they remain over ocean most of their life. Firstly, a 7-year lightning climatology over the South West Indian Ocean has been performed using the WWLLN data from 2005 to 2011. Results show the same main features presented in other studies: the hot spots in this region are found in Madagascar and over the Great Lakes in East Africa (> 10-20 fl. km 2 yr 1 ). Lightning flashes within tropical cyclones represent 50% to 100% of the total lightning activity in some oceanic areas (between 10 S and 20 S). Moreover, lightning activity is more intense during the November-to-April period which corresponds to the wet/cyclonic season. Then, lightning activity in tropical storms of the basin was studied using both WWLLN data and best- tracks from the Regional Specialized Meteorological Center La R´ eunion from January 2005 to May 2013. The South-West Indian Ocean is divided into three distinct regions (open ocean, Mozambique channel, and the oceanic region 400 km offshore the eastern coast of Madagascar) to account for the impact of land on the tropical storm structure. The tropical storms are divided into annular rings based on a radius of maximum wind criteria. We show that lightning activity is mainly found in the eyewall region whatever the system stage in the open ocean. The possible link between lightning activity and tropical cyclone intensity change is investigated using different thresholds for the intensification rate. We show that, whatever the intensity stage is, the proportion of 6-h period with lightning activity is higher during rapid intensifications. For systems rapidly intensifying, 20 to 50% of the 6-h periods are associated to lightning flashes.

CARIBIC DOAS observations of nitrous acid and formaldehyde in a large convective cloud

Abstract. The chemistry in large thunderstorm clouds is influenced by local lightning-NOx production and uplift of boundary layer air. Under these circumstances trace gases like nitrous acid (HONO) or formaldehyde (HCHO) are expected to be formed or to reach the tropopause region. However, up to now only few observations of HONO at this altitude have been reported. Here we report on a case study where enhancements in HONO, HCHO and nitrogen oxides (NOx) were observed by the CARIBIC flying laboratory (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container). The event took place in a convective system over the Caribbean Sea in August 2011. Inside the cloud the light path reaches up to 100 km. Therefore the DOAS instrument on CARIBIC was very sensitive to the tracers inside the cloud. Based on the enhanced slant column densities of HONO, HCHO and NO2, average mixing ratios of 37, 468 and 210 ppt, respectively, were calculated. These data represent averages for constant mixing ratios inside the cloud. However, a large dependency on the assumed profile is found; for HONO a mixing ratio of 160 ppt is retrieved if the total amount is assumed to be situated in the uppermost 2 km of the cloud. The NO in situ instrument measured peaks up to 5 ppb NO inside the cloud; the background in the cloud was about 1.3 ppb, and hence clearly above the average outside the cloud (≈ 150 ppt). The high variability and the fact that the enhancements were observed over a pristine marine area led to the conclusion that, in all likelihood, the high NO concentrations were caused by lighting. This assumption is supported by the number of flashes that the World Wide Lightning Location Network (WWLLN) counted in this area before and during the overpass. The chemical box model CAABA is used to estimate the NO and HCHO source strengths which are necessary to explain our measurements. For NO a source strength of 10 × 109 molec cm−2 s−1 km−1 is found, which corresponds to the lightning activity as observed by the World Wide Lightning Location network, and lightning emissions of 5 × 1025 NO molec flash−1 (2.3–6.4 × 1025). The uncertainties are determined by a change of the input parameters in the box model, the cloud top height and the flash density. The emission rate per flash is scaled up to a global scale and 1.9 (1.4–2.5) tg N a−1 is estimated. The HCHO updraught is of the order of 120 × 109 molec cm−2 s−1 km−1. Also isoprene and CH3OOH as possible HCHO sources are discussed.

Lightning in Western Patagonia

AbstractOn the basis of 8 years (2005–2012) of stroke data from the World Wide Lightning Location Network we describe the spatial distribution and temporal variability of lightning activity over Western Patagonia. This region extends from ~40°S to 55°S along the west coast of South America, is limited to the east by the austral Andes, and features a hyper‐humid, maritime climate. Stroke density exhibits a sharp maximum along the coast of southern Chile. Although precipitation there is largely produced by cold nimbostratus, days with more than one stroke occur up to a third of the time somewhere along the coastal strip. Disperse strokes are also observed off southern Chile. In contrast, strokes are virtually nonexistent over the austral Andes—where precipitation is maximum—and farther east over the dry lowlands of Argentina. Atmospheric reanalysis and satellite imagery are used to characterize the synoptic environment of lightning‐producing storms, exemplified by a case study and generalized by a compositing analysis. Lightning activity tends to occur when Western Patagonia is immersed in a pool of cold air behind a front that has reached the coast at ~40°S. Under these circumstances, midlevel cooling occurs before and is more prominent than near‐surface cooling, leading to a weakly unstable postfrontal condition. Forced uplift of the strong westerlies impinging on the coastal mountains can trigger convection and produces significant lightning activity in this zone. Farther offshore, large‐scale ascent near the cyclone's center may lift near‐surface air parcels, fostering shallow convection and dispersing lightning activity.

Lightning activity and its relation to the intensity of typhoons over the Northwest Pacific Ocean

Data from the World Wide Lightning Location Network (WWLLN) were used to analyze the lightning activity and the relationship between maximum sustained wind and lightning rate in 69 tropical cyclones over the Northwest Pacific Ocean from 2005 to 2009. The minimum lightning density was observed in the category 2 typhoon Kong-Rey (2007), with a value of only 1.15 d−1 (100 km)−2. The maximum lightning density occurred in the category 2 typhoon Mitag (2007), with a value of 510.42 d−1 (100 km)−2. The average lightning density decreased with radius from the typhoon center in both weak (categories 1–3) and super (categories 4–5) typhoons. The average lightning density in the inner core of super typhoons was more than twice as large as that for weak typhoons. Both groups of typhoons showed a near-monotonic decrease in lightning density with radius. Results also showed that lightning activity was more active in typhoons that made landfall than in those that did not. The mean correlation coefficient between the accumulated flashes within a 600-km radius and the maximum wind speed in the weak typhoons and super typhoons was 0.81 and 0.74, respectively. For more than 78% (56%) of the super (weak) typhoons, the lightning activity peaked before the maximum sustained wind speed, with the most common leading time being 30 (60) h. The results suggest that, for the Northwest Pacific Ocean, lightning activity might be used as a measurement of the intensification of typhoons.

Whistlers detected and analyzed by Automatic Whistler Detector (AWD) at low latitude Indian stations

Recently, at three Indian low latitude stations: Varanasi (geomag. lat. 14°55′N, geomag. long. 153°54′E, L: 1.078), Allahabad (geomag. lat. 16.05°N; geomag. long. 155.34°E, L: 1.081) and Lucknow (geomag. lat. 17.6°N, geomag. long. 154.5°E, L: 1.104) an Automatic Whistler Detector (AWD) has been installed in December, 2010 for detection and analysis of whistlers. This instrument automatically detects and collects statistical whistlers data for the investigation of whistlers generation and propagation. Large numbers of whistlers have been recorded at Varanasi and Allahabad during the year 2011 which is analyzed in the present study. Different types of whistlers have been recorded at Varanasi and Allahabad. The correlation between recorded whistlers and causative lightning strikes were analyzed using data provided by World-Wide Lightning Location Network (WWLLN). We observed that for both the stations more than 50% of causative sferics of whistlers were observed to match closely with the times of WWLLN detected lightning strikes within the propagation times of causative tweeks. All of these lightning strikes originated from the region within 500–600km radius circle from the conjugate point of Varanasi and Allahabad supports the ducted propagation at low latitude stations. The dispersion of the observed whistlers varies between 8 and 18s1/2, which shows that the observed whistlers have propagated in ducted mode and whole propagation path of whistlers lies in the ionosphere. The ionospheric columnar electron contents of these observed whistlers vary between 13.21 TECU and 56.57 TECU. The ionospheric parameters derived from whistler data at Varanasi compare well with the other measurements made by other techniques.

Properties of terrestrial gamma ray flashes detected by AGILE MCAL below 30 MeV

We present the characteristics of 308 terrestrial gamma ray flashes (TGFs) detected by the Minicalorimeter (MCAL) instrument on board the AGILE satellite during the period March 2009–July 2012 in the ±2.5° latitude band and selected to have the maximum photon energy up to 30 MeV. The characteristics of the AGILE events are analyzed and compared to the observational framework established by the two other currently active missions capable of detecting TGFs from space, RHESSI and Fermi. A detailed model of the MCAL dead time is presented, which is fundamental to properly interpret our observations. The most significant contribution to dead time is due to the anticoincidence shield in its current configuration and not to the MCAL detector itself. Longitude and local time distributions are compatible with previous observations, while the duration distribution is biased toward longer values because of dead time. The intensity distribution is compatible with previous observations, when dead time is taken into account. The TGFs cumulative spectrum supports a low production altitude, in agreement with previous measurements. We also compare our sample to lightning sferics detected by the World Wide Lightning Location Network and suggest a new method to assess quantitatively the consistency of two TGF populations based on the comparison of the associated lightning activity. According to this method, AGILE and RHESSI samples are compatible with the same parent population. The AGILE TGF catalog below 30 MeV is accessible online at the website of the ASI Science Data Center http://www.asdc.asi.it/mcaltgfcat/.

Energetics and geographic distribution of elve‐producing discharges

AbstractThe major types of transient luminous events (TLEs) are believed to be directly triggered by cloud‐to‐ground discharges. Intense lightning generally seems to have a higher production efficiency for TLEs, but this observation has not yet been statistically investigated. Two data sets, the upgraded World Wide Lightning Location Network (WWLLN) lightning stroke data and the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) TLE data, were used to investigate the energetics and the geographic distribution of TLE‐producing lightning. The global median energy of the strokes that produced the TLEs is at least an order of magnitude higher than the global median stroke energy for WWLLN lightning in the same data window. Furthermore, the energy distributions of the elve‐producing strokes exhibit no oceanic and land disparity. These results reveal that the elves are indeed triggered by energetic lightning and the production efficiency of elves with respect to the stroke energy of the causative lightning was insensitive to the underlying landform. Analysis of the spatial correlation between the ISUAL elves and the WWLLN lightning reveals that the geographic distribution of the ISUAL elves agreed well with that for the most energetic 10% of the WWLLN lightning strokes, better than for total lighting. We also found that elve occurrence rates in the apparently reduced detection regions behind the Earth's limb may have been greatly underestimated, partially due to the failure in providing triggers to initiate ISUAL recording or the severity of atmospheric attenuation to the elve emissions that may have caused them to be undetected.

Diurnal variation of the global electric circuit from clustered thunderstorms

The diurnal variation of the global electric circuit is investigated using the World Wide Lightning Location Network (WWLLN), which has been shown to identify nearly all thunderstorms (using WWLLN data from 2005). To create an estimate of global electric circuit activity, a clustering algorithm is applied to the WWLLN data set to identify global thunderstorms from 2010 to 2013. Annual, seasonal, and regional thunderstorm activity is investigated in this new WWLLN thunderstorm data set in order to estimate the source behavior of the global electric circuit. Through the clustering algorithm, the total number of active thunderstorms are counted every 30 min creating a measure of the global electric circuit source function. The thunderstorm clusters are compared to precipitation radar data from the Tropical Rainfall Measurement Mission satellite and with case studies of thunderstorm evolution. The clustering algorithm reveals an average of 660±70 thunderstorms active at any given time with a peak‐to‐peak variation of 36%. The highest number of thunderstorms occurs in November (720±90), and the lowest number occurs in January (610±80). Thunderstorm cluster and electrified storm cloud activity are combined with thunderstorm overflight current measurements to estimate the global electric circuit thunderstorm contribution current to be 1090±70 A with a variation of 24%. By utilizing the global coverage and high time resolution of WWLLN, the total active thunderstorm count and current is shown to be less than previous estimates based on compiled climatologies.

Diurnal Lightning Variability over the Maritime Continent: Impact of Low-Level Winds, Cloudiness, and the MJO

Abstract Lightning over the Maritime Continent exhibits a pronounced diurnal cycle. Daytime and evening lightning occurs near coastlines and over mountain slopes, driven by sea and valley breezes. Nocturnal and morning thunderstorms are touched off where land breezes or mountain breezes converge or by gravity waves propagating away from regions of vigorous afternoon convection. In this study, the modulation of the diurnal cycle of lightning and precipitation by 850-hPa winds, cloudiness, and the Madden–Julian oscillation (MJO) is investigated using observations from the World Wide Lightning Location Network (WWLLN) and the Tropical Rainfall Measuring Mission (TRMM) satellite. The 850-hPa wind speed and area-averaged cloudiness are shown to be negatively correlated with day-to-day lightning frequency over land, and thunderstorm occurrence is suppressed windward of, and enhanced leeward of, mountain ranges. Lightning and environmental conditions are similarly related in the MJO. During break periods, the regular diurnal cycle of lightning is enhanced where ambient low-level winds are easterly but abnormally weak—in the Strait of Malacca, over western and southern Borneo and the adjacent seas, and in the region of nocturnal thunderstorms to the west of Sumatra and Java. When the active, cloudy phase of the MJO, accompanied by low-level westerly winds, passes over the Maritime Continent, the regular diurnal cycle of lightning is enhanced leeward (to the east) of the mountains of Java, Borneo, and the Malay Peninsula. The spatial patterns of lightning and rainfall anomalies are broadly similar, but lightning anomalies tend to be more concentrated near coastlines.