Concurrent Satellite and ground-based Lightning Observations from the Optical Lightning Imaging Sensor (ISS-LIS), the LF network Meteorage and the SAETTA LMA in the northwestern Mediterranean region

The new space-based Lightning Imager (LI) on board the Meteosat Third Generation (MTG) geostationary satellite will improve the observation of lightning over Europe, the Mediterranean Sea, Africa and the Atlantic Ocean from 2021 onwards. In preparation of the use of the upcoming MTG-LI data, we compare observations by the Lightning Imaging Sensor (LIS) on the International Space Station (ISS), which applies an optical technique similar to MTG-LI, to concurrent records of the Low Frequency (LF) ground-based network Meteorage. Data were analyzed over the northwestern Mediterranean Sea from 5 March 01, 2017 to March 20, 2018. Flashes are detected by ISS-LIS using illuminated pixels, also called events, within a given (2.0 ms) frame and during successive frames. Meteorage describes flashes as a suite of Intra-Cloud/cloud-to-cloud (IC) pulses and/or Cloud-to-Ground (CG) strokes. Both events and pulses/strokes are grouped to flashes using a novel in-house algorithm. In our study, ISS-LIS detects about 57 % of the flashes detected by Meteorage. The flash detection efficiency (DE) of Meteorage relative to ISS-LIS exceeds 80 %. Coincident matched flashes detected by the two instruments show a good spatial 10 and temporal agreement. Both peak and mean distance between matches are smaller than the ISS-LIS pixel resolution (about 5.0 km). The timing offset for matched ISS-LIS and Meteorage flashes is usually shorter than the ISS-LIS integration time frame (2.0 ms). The closest events and pulses/strokes of matched flashes achieve sub-millisecond offsets. Further analysis of flash characteristics reveals that longer lasting and more spatially extended flashes are more likely detected by both ISS-LIS and Meteorage than shorter duration and smaller extent flashes. ISS-LIS’ relative DE is lower for daytime versus nighttime as 15 well as for CG versus IC flashes. A second ground-based network, the Very High Frequency (VHF) SAETTA Lightning Mapping Array (LMA), further enhances and validates the lightning pairing between ISS-LIS and Meteorage. It also provides altitude information of the lightning discharges and adds a detailed lightning mapping to the comparison for verification and better understanding of the processes. Both ISS-LIS and Meteorage flash detections feature a high degree of correlation with the SAETTA observations 20 (without altitude information). In addition, Meteorage flashes with ISS-LIS match tend to be associated with discharges that

occur at significantly higher altitudes than unmatched flashes. Hence, ISS-LIS flash detection suffers degradation with low-level flashes resulting in lower DE.

Introduction
Lightning defines electrical discharges within the atmosphere. The discharges can happen within a cloud or between clouds 25 (IC) or between a cloud and the ground (CG). The total lightning activity (IC+CG) is of interest for e.g. the numerical weather prediction (NWP) as lightning serves as tracer for deep convection. The total lightning flash rate is associated to storm intensity features. For example,  found strong correlation between the updraft volume above the −5 • C level in clouds and total lightning activity.  show a fairly stable relationship and strong correlation between the precipitation ice mass flux, the non-precipitation ice mass flux and their product on the one hand and the total 30 lightning flash rate on the other. Especially graupel/small hail ice mass correlates well with the mean total lightning rate in their study. Among others Mattos et al. (2017) investigated the life cycle of thunderstorms and processes leading to the different discharge types. They found in their analysis of 46 isolated thunderstorms that in 98% of their cases, the first CG flash is preceded by IC lightning by approximately six minutes on average.
At this time, lightning observations in Europe are mainly made with ground-based sensors. To maximize the impact of 35 lightning data on the assimilation in NWP systems, total lightning should be observed continuously over large areas. In a few years, the new Lightning Imager (LI) on board the Meteosat Third Generation (MTG) satellite (Stuhlmann et al., 2005) will provide continuous lightning observation over Europe, the Mediterranean Sea, Africa, the Atlantic Ocean and parts of Brazil.
The satellite sensor will be able to detect the total lightning including CG and IC flashes when launched in the 2021 time frame. The Lightning Imaging Sensor (LIS) on the International Space Station (ISS) (Blakeslee and Koshak, 2016) creates 40 a unique opportunity to provide proxy data to help prepare research and operational applications for the MTG-LI data. It overpasses, among others, wide parts of Europe, including the entire Mediterranean region. ISS-LIS is in principle similar to the planned MTG-LI, so that ISS-LIS data can to some extent mimic the upcoming MTG-LI data. In addition, a comparison between European ground-based lightning observation networks and ISS-LIS should improve the understanding of groundand space-based lightning observations. All instruments and networks are hereafter simply referred to as lightning locating 45 systems (LLSs).
This study uses the term relative DE. It is defined as the ratio of the number of matched flashes to the number of flashes in the other (reference) LLS, expressed as a percentage.
A LIS instrument was previously operational on the Tropical Rainfall Measurement Mission (TRMM) satellite (e.g., Christian et al., 1999;Cecil et al., 2005). Several LLSs comparisons exist for regions covered by TRMM-LIS. The focus of the 50 following (not exhaustive) literature review is on observational analyses rather than laboratory experiments, e.g. Boccippio et al. (2002). Ground-based LLS observe different frequency ranges of the lightning radio signal. They are classified as e.g. very low frequency (VLF) and low frequency (LF) LLSs as well as very high frequency (VHF) LLSs (e.g. Figure 2, Cummins and Murphy, 2009). A summary of detection characteristics, (dis)advantages and the range of the various ground-based LLSs are provided in Nag et al. (2015). VLF/LF systems detect lightning on mid to long range. Their DE is somewhat limited. It 55 varies for different networks and flash types (CG flash DE is usually higher than IC flash DE for VLF/LF LLSs) but increases in general with lower baseline distance. Thompson et al. (2014) aimed at exploring suitable proxy data for the Geostationary Lightning Mapper (GLM) (Goodman et al., 2013). They report a pulse/stroke DE maximum for two long range LLSs, the World Wide Lightning Location Network (WWLLN) and the Earth Networks Total Lightning Location Network (ENTLN), of 18.9 % and 63.3 %, respectively, relative 60 to 18-months records of TRMM-LIS groups (a combination of adjacent illuminated pixels in the optical image that occur in the same 2 ms time frame). The maxima were found over the Pacific Ocean for WWLLN and near North America for ENTLN (within the analyzed region with the highest sensor density) in 2010 and 2011. They did not study how many WWLLN and ENTLN pulses/strokes had coincident TRMM-LIS groups. Rudlosky et al. (2017)  While the previous papers focused on the DE, Höller and Betz (2010) analyzed TRMM-LIS and a VLF/LF lightning location network (LINET) in order to generate random proxy optical data from a given set of LINET data using model distribution functions. The outcomes are of specific interest for proxy data for the MTG-LI. Besides the relative DEs (approximately 50 % 85 for both LLSs), they investigated distribution functions and correlations between TRMM-LIS group and LINET pulse/stroke number per flash, flash extent and duration and between LINET pulse/stroke amplitude and TRMM-LIS group radiance.
Although the Pearson correlation coefficients remained low, the approach can be further refined for high fidelity MTG-LI proxy data. VHF LLSs are sensitive to lightning channel formation and leader processes, which occur multiple times during a single 90 flash. Hence, VHF LLSs typically feature high DE performances and three-dimensional (3D) mapping of lightning channel propagation and spatial extent (Thomas et al., 2004). VHF LLSs depend on direct line-of-sight detection, and thus, the range suffers from the Earth's curvature and terrain shading effects. Thomas et al. (2000) presented a case study of a storm in Oklahoma, USA, at local nighttime. The storm was observed by both the local Lightning Mapping Array (LMA) and TRMM-LIS. 108 of the 128 LMA lightning discharges were detected by 95 TRMM-LIS and the LMA detected all TRMM-LIS flashes. The lightning missed by TRMM-LIS was mainly confined to low altitude discharges, i.e. below 7.0 km. Optical signals of lightning discharges that propagated via scattering to the upper part of the cloud were easily detected by TRMM-LIS. Blakeslee et al. (2002) studied the São Paulo LMA (SP-LMA) dataset and its capability to serve as GLM proxy data.
TRMM-LIS events were in good agreement with the concurrent SP-LMA, ENTLN and LINET observations regarding latitude,  (and reverse) is analyzed, while SAETTA is used to verify and understand the results. Indeed, the spatially and temporally high resolution of SAETTA's measurements capture the structure and the life cycle of each lightning flash and gather additional information, i.e. discharge altitude, to assess more thoroughly ISS-LIS and Meteorage strengths and weaknesses. Besides the commonly investigated relative DEs, distances and timing offsets, this work examines also specific characteristics of matched ISS-LIS 115 and Meteorage flashes. It aims at providing the basis for mimicking optical, satellite-based lightning data from a VLF/LF LLS.
In section 2 ISS-LIS, Meteorage and SAETTA are introduced as well as the data processing, developed algorithms and the investigation methodology. Results are presented in section 3. A brief summary and some discussion are given in section 4.

Instrumentation and Methodology
This paper aims at identifying the individual lightning detection characteristics by the satellite-based ISS-LIS, the VLF/LF

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The region was limited to 40.5°N to 44.0°N and 7.0°E to 11.0°E around the island of Corsica in the NW Mediterranean Sea. Figure 1 shows the domain with accumulated data of one overpass (a), an infrared (IR) satellite picture (b) and the example of one flash recorded by ISS-LIS, Meteorage and SAETTA (c). The three instruments are introduced within this section. In total, 125 ISS-LIS field of view (FOV) intersected the region of interest 851 times during the study period, with 26 of the overpasses exhibiting lightning activity. In this work, all times are given in Coordinated Universal Time (UTC). Altitudes are defined above sea level (ASL). Distances are calculated using Vincenty's formulae (Vincenty, 1975) based on the WGS 84 reference ellipsoid which are more accurate on Earth than for example great circle distances (assumes the Earth as oblate sphere rather than a sphere). The term detection efficiency (DE) means in the following the DE for flashes, not event or pulse/stroke DE.

ISS-LIS
The ISS operates in Low Earth Orbit (LEO) and overpasses one region on the surface up to three times a day (up to two times in the tropics). Lightning observation of a specific point lasts up to 90 seconds per overpass due to the ISS orbit characteristics and the LIS FOV of approximately 655 x 655 km 2 . The optical lightning detection is performed at a wavelength of 777.4 nm at the atomic oxygen line. ISS-LIS observes both IC and CG discharges but cannot distinguish the lightning type. ISS-LIS captures an 135 image of the Earth every 2 ms referred to as a frame. The LIS focal plane consists of a 128 x 128 pixel Charge Couple Device (CCD) that is read out every 2 ms. The pixel FOV ranges between 4.5 km (nadir) and 6.2 km at the edges (Dennis Buechler, personal communication 2019). Blakeslee and Koshak (2016) apply a four-step filtering approach, involving spatial, spectral, temporal and background subtraction filter, to identify pixels with lightning activity. This is required to detect the lightning during daytime when the sunlight reflected off the cloud tops otherwise overwhelms and masks the lightning signal (i.e., it is 140 daytime lightning detection that drives the design of space-based lightning detectors such as LIS and the new MTG-LI). An illuminated pixel that breaks a predefined threshold in a given 2 ms frame is identified as an event. Events define the smallest units of the optical signals in the ISS-LIS data set. Their latitude and longitude correspond to the pixel center. A group is the next unit of ISS-LIS data. An ISS-LIS group contains one or more events occurring within the same time frame and in adjacent pixels of the ISS-LIS image . Next, groups are organized into flashes, so that a flash can consist of one 145 or multiple groups. A Weighted Euclidean Distance (WED) employs spatial and temporal clustering with 330 ms and 5.5 km, respectively, to merge groups in flashes (Mach et al., 2007). The locations of groups and flashes are defined by the radiance weighted average positions of their events and groups, respectively. Finally, an area contains all flashes with distances of less than 16.5 km to each other. The National Aeronautics and Space Administration (NASA) provides the ISS-LIS in different postprocessing levels. In the latest available version, P0.2, the quality control is already close to its (expected) final stage, but 150 the data may contain some undetected minor errors (Blakeslee et al., 2017). The main difference will concern the detection efficiency. The fully validated flash density should not alter more than 5.0 % to 10.0 % from version P0.2 (R. Blakeslee, personal communication 2018). LIS data comprises the 2 ms scientific data, e.g. time, latitude, longitude and optical amplitude count of events and instrument, platform or external errors to verify the data quality, and housekeeping data. The available ISS-LIS P0.2 version data have not yet included the (background-)calibrated radiance. The strength of the optical signal is 155 defined by the raw amplitude count. It depends somewhat on the background value, but, in general, the radiance increases with  The original ISS-LIS data contains times in the International Atomic Time with reference to 01 Jan. 1993 (TAI93) format. 160 For the intercomparison of the LLSs, times are converted to UTC while taking the missing leap seconds into account. The ISS-LIS times include a time-of-flight (TOF) correction accounting for the time photons need to travel from the optical source at cloud-top to the satellite.

Meteorage
The Meteorage LF LLS uses Vaisala LS7002 sensors (Vaisala, 2013) at a frequency between 1 kHz and 350 kHz. It includes 165 21 ground sensors across France and contributes to the European Cooperation for Lightning Detection (EUCLID). EUCLID comprises lightning sensors all over Europe and helps to improve the performance of national LLSs (Schulz et al., 2016). The  The LMA technology was developed by New Mexico Tech (Rison et al., 1999). The SAETTA LMA operates in the 60-66 MHz VHF band, with an 80 µs analysis window (Coquillat et al., 2014), and consists of 12 LMA stations distributed over the island of Corsica. The distance between the network's northernmost and southernmost (westernmost and easternmost) stations approximates 180 km (70 km). The station altitude ranges from 3.8 mASL to 1950.2 mASL. SAETTA maps the total lightning activity. A minimum of six stations is needed to capture a lightning source in 3D. Redundant information from 185 more stations improves the location accuracy and consequently decreases the chance of mislocation and possible noise (e.g. single VHF sources in Figure 1(c)). As a drawback, less VHF sources and flashes are detected simultaneously by more than six stations. Aiming at a high flash DE, coincident signals at six stations are sufficient for the LMA data in this study.
SAETTA data include the time, latitude, longitude, altitude, amplitude of each lightning source. Lightning location reaches up to a radius of 350 km from the center of the network. The VHF LLS depends on the direct line-of-sight to a lightning 190 discharge. The altitude of the lowest detectable VHF source increases with the distance to the LMA due to Earth curvature. For example, sources in 100 (200) km distance to a station at sea level must be at least 0.8 (3.1) km in altitude to be visible to that station. Formula (6) of Koshak et al. (2018) is applied here. Their study also investigates effects of the LMA network geometry mainly on the altitude errors. For SAETTA, Coquillat et al. (2019) show that the displacement of 2 stations in 2016 markedly reduced the radial error and increased the altitude error over wide parts of the studied domain ( Figure 3 in Coquillat et al.,195 2019). Therefore when different sets of at least 6 stations are involved in the reconstruction of the VHF source position one would expect a different geometry of the network, which influences the location precision. In general the SAETTA location uncertainty increases with the distance to the network center. According to the theoretical model of Thomas et al. (2004), the radial, azimuthal and altitude errors are, at best for VHF sources at 10 km altitude bounded at 15.0 kmASL and the maximum reduced χ 2 , which defines a measure for the overall uncertainty of the time-ofarrival based system (Thomas et al., 2004), is set to 0.5.

Flash -Grouping algorithm
The NASA LIS flash clustering algorithm distinguishes events, groups and flashes (section 2.1). It makes use of a WED with maximum difference of 5.5 km in space and 330 ms in time. The WED analyzes group centroids and not the events 215 in the group to find if two groups are considered part of the same flash. One flash cannot last longer than 2.0 s . An analysis of the (P0.2) NASA LIS flash clustering algorithm revealed that it tends to separate flashes when compared to concurrent SAETTA observations. Similar results were observed by Defer et al. (2005). Consequently, a new algorithm is developed to merge the ISS-LIS events to flashes. It has the additional advantage of treating both ISS-LIS events and Meteorage pulses/strokes. The lightning elements sensed by each LLS, that are the smallest available lightning signals 220 (events and pulses/strokes), are merged into flashes. More explicitly, an event of ISS-LIS (pulse/stroke of Meteorage) should belong to exactly one flash and a flash is defined as a collection of events (pulses/strokes). Flash characteristics are derived from the underlying element characteristics, e.g. the positions of its elements are used instead of the mean flash location. This study makes use of the elementary ISS-LIS event data as provided by the NASA prior to any data merging. It is accepted that ISS-LIS events do not have a direct representation in the Meteorage-like data. Former studies claimed that LIS groups 225 roughly correspond to the physical processes detected by VLF/LF LLSs (e.g., Bitzer et al., 2016;Höller and Betz, 2010).
Nevertheless, those studies found significantly more groups than pulses/strokes within the same region and time period. Bitzer et al. (2016) found for the number of TRMM-LIS groups to ENTLN pulses/strokes a factor of about 28.4 globally and even 3.7 in North America in 2013. Höller and Betz (2010) analyzed 6.7 groups per pulse/stroke on average. Due to those results, LIS optical groups emerge from both discharge processes measured by VLF/LF sensors but also processes lacking significant 230 VLF/LF radiation. In addition, the detected lightning sources of the applied VHF LLS comply more with the LIS events than the groups. Using events rather than group centroids improve in particular the finding of the coincident LMA data. The analysis of flash extents profits from the use of events in that the extent of an ISS-LIS flash corresponds to the full illuminated area rather than the ISS-LIS group centroid locations. The representation of the flash extent (density) will influence the future assimilation of lightning data in NWP models. A statistical analysis of (ISS-LIS) events and LF strokes/pulses will also be of interest for 235 creating a proxy optical data set, e.g. for MTG-LI, derived from LF data.
Our grouping algorithm analyzes the elements (events or pulses/strokes) and groups them based on their relative location and time of occurrence to each other. First, the spatial and temporal constraints, ds merge and dt merge , for elements within one flash must be determined. Then, a combined space-time test merges the elements into flashes. It starts with the first available element (in the data of one LLS) and identifies all elements (of the same LLS' data) within the range of the constraints. Thereby, an 240 element can only belong to the same flash if both the distance to any element of the flash is less than ds merge and the time difference (to the same element) is shorter than dt merge . All elements identified for a flash (including the initial element) are classified as used. For each used element within a flash, the test is repeated until no unused element can be added to the flash.   The same algorithm is applied to group the Meteorage pulses/strokes into flashes. It needs, however, modified constraints ds merge and dt merge since physical processes producing Meteorage pulses/strokes do not always correspond to ISS-LIS events and occur with significantly lower counts. Meteorage pulses/strokes do not cover the full structure and duration of a lightning flash. Figure 2 The determination of ds merge and dt merge does not ensure a perfect arrangement of the elements in flashes though. The objective is to find constraints leading to statistical representations of flashes in the ISS-LIS and Meteorage data. Therefore, 275 all identified flashes are double-checked against concurrent 3D SAETTA observations. Even if it is sometimes challenging to separate the flashes in the SAETTA data, the detailed VHF mapping helps to understand the processes leading to the identification of the ISS-LIS and Meteorage flashes. The SAETTA data should also be used to find possible false alarms in the ISS-LIS and Meteorage data. correspond to exactly one matched flash. It is also possible that a flash meets the matching criteria of more than one given flash (and is collocated to more than one flash). Hence, the two categories LIS detected by both and Meteorage detected by both are expected to have different counts.
The detailed analysis of distances and timing offsets between matched flashes refines the matching algorithm further: The number of cases where one flash is matched to multiple flashes of the second LLS should be reduced. Therefore, the refined 305 algorithm initiates with finer matching criteria, i.e. one percent of both ds match and dt match . It seeks for one element detected by the second LLS that meets both the finer criteria to any element of the given flash. Only if no match is found, the allowed distance and time difference increase by one percent of ds match and dt match , respectively. The process repeats iteratively until either a match is found or the allowed distance (timing offset) exceeds the original ds match (dt match ). In the latter case, the algorithm stops and the flash is labeled unmatched (Note: The refined analysis is performed for matched flashes only, however, 310 the algorithm can treat the unmatched flashes, too.). One or more matches for the given flash are still possible because of the discrete increments from one iteration to the following. There might also be flashes within equal distance and equal time offset to the given flash.
The criteria ds match and dt match are determined through a sensitivity study of the relative DEs of ISS-LIS and Meteorage hampering effectively the finding of suitable elements, i.e. pulses/strokes, for a collocation. ISS-LIS relative DE decreases within the entire range of investigated times dt match . The most sensitive behavior occurs for dt match up to 1.5 s (Figure 4(b)).
Meteorage appears to be sensitive to dt match only up to 0.5 s (Figure 4(d)). Despite the differences in sensitivity to the criteria 320 between ISS-LIS and Meteorage, it is aimed at using the same ds match and dt match for both LLSs. Finally, ds match of 20 km and dt match of 1.0 s are chosen to balance the individual sensitivities of the LLSs to the criteria. They allow to identify matches if, for example, ISS-LIS detects primary IC discharges of a flash and Meteorage only detects a CG stroke occurring during the final stage of the same flash. Our criteria are relatively coarse compared to some former studies (section 1). Höller and Betz (2010) applied the same dt match but an even coarser ds match (i.e. 30 km) to match LINET VLF/LF flashes and TRMM-LIS 325 flashes. Further investigation of the matched flashes, e.g. the distributions of the distances and timing offsets, will demonstrate to which extent matches rely on the fairly coarse criteria.

Results
The different LLSs detect flashes in different ways and with distinct characteristics. In this section, flash observations are compared and analyzed. As an example, the ISS overpass with the corresponding observations of ISS-LIS, Meteorage and 330 SAETTA in Figure 1 comprises (almost) the entire study region. It lasted 169 seconds, from FOV entering to leaving the region. The effective viewtime per 0.5°x 0.5°grid box is indicated in grayscale in Figure 1(a). Wide parts of the domain have been seen for at least 60 seconds. Figure 1(b) shows additionally an IR satellite image indicating the cloud tops. The example of a single flash observed by all three LLSs during this overpass is given in Figure 1(c). SAETTA captures the most detail of the flash structure and there are significantly more ISS-LIS events than Meteorage pulses/strokes. All but the first Meteorage 335 signals indicate an IC pulse. Since the first stroke is of type CG, the entire flash is characterized as CG-flash.

Distances and timing offsets between collocated flashes
In this section, the matched ISS-LIS and Meteorage flashes are studied regarding their relative location and time of occurrence.
For each element of a flash detected by one LLS the closest (in time or in space, not a combined filter here) element of the 405 matched flash(es) accounts for the statistic. One element can be closest to multiple elements of the second LLS. The entirety of elements of flashes with matches is analyzed statistically. Figure 6 presents the results for distances (a) and timing offsets (b) between events and pulses/strokes. and secondarily at about 4.50 km with a median (mean) of 4.74 km (5.68 km). The distribution given a Meteorage pulse/stroke has a broad maximum from 0.75 km to 2.75 km with a median (mean) of 2.31 km (3.60 km). The Meteorage pulse/stroke distance distribution features a more pronounced (if wider) peak for less distance than the distribution given an ISS-LIS event.
This is due to the calculation method and the numbers of available events and pulses/strokes. The higher number of (and smaller distance between) ISS-LIS events allows in general for finding a closer event to a given Meteorage pulse/stroke than vice versa. distribution peaks for 1-2 km after the correction (5-6 km before the correction). The distribution peak distances before the correction were similar to those reported by Bitzer et al. (2016) and Rudlosky et al. (2017). Hence, it is assumed that distances between TRMM-LIS groups and VLF/LF LLS pulses/strokes are similar or slightly smaller than distances comparing ISS-LIS events and Meteorage LF pulses/strokes in this work. It should be mentioned that TRMM-LIS pixel size is slightly smaller than that of ISS-LIS, i.e. 4.3 (3.7) km nadir after (before) TRMM boost versus 4.5 km nadir.

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The optical ISS-LIS sensor might be affected by different lighting. Therefore, the accuracy of ISS-LIS flashes relative to ground-based LLSs is explicitly investigated during day and night (not shown as Figure). The overall median (mean) values yield 2.36 ms (54.60 ms) and −0.00 ms (2.70 ms) given an ISS-LIS event and Meteorage pulse/stroke, respectively. The mean for a given ISS-LIS event is an artifact of the skewed distribution (also in the tails).
Considering the ISS-LIS integration frame time of 2.0 ms, the remaining average statistics are close to the temporal accuracy of ISS-LIS. Both conditional distributions given ISS-LIS and given Meteorage show an overall similar shape (Figure 6(b)). The 465 matched element, considering both the ISS-LIS and Meteorage distributions, occurs with similar probability earlier or later (or simultaneously) than the element itself and the distribution peak is centered at zero time offset. This is an interesting finding since e.g. Höller and Betz (2010) and Bitzer et al. (2016) found that TRMM-LIS detected lightning on average one to two milliseconds later than the ground-based LLSs. This is not the case for ISS-LIS in our study (and again one must consider the ISS-LIS integration time frame of 2.0 ms). Although the order of magnitude of the time offsets agrees well with our results.

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Timing differences can in fact be directly compared to those studies as the closest event provides the same time as the closest group (groups merge several events within the same time frame and in adjacent pixels of ISS-LIS).
The distribution given an IC pulse is also symmetric around zero and shows a maximum between −1.0 ms and 1.0 ms ( Figure 6(b)). Its median (mean) is 0.00 ms (4.29 ms). For CG strokes, however, the distribution peaks between −1.0 ms and 0.0 ms. The negative distribution peak and median (mean) of −0.07 ms (−4.32 ms) indicate that ISS-LIS detected CG 475 lightning slightly later than Meteorage. It might account for the time the light of the CG lightning needs to propagate towards the higher parts of the cloud and to become visible from space.

Characteristics of detected flashes
The and minimum of the elements at the mean latitude (as that distance depends also on the latitude). Flash durations, or the times       (Table 2).

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The overall ISS-LIS flash mean altitude distribution, that is dominated by 83.3 % flashes with match, peaks at about 9.5 km, as shown in the histogram in Figure 13(a)(i). The daytime distribution has a second mode near 5.0 km of altitude. ISS-LIS flashes reach on average altitudes of 9.9 km and were observed up to almost 13 km of altitude (Table 2) (a noteworthy high value considering the tropopause in ten to twelve kilometers of altitude).  (Table 3). Hence, it is confirmed that ISS-LIS flash detection declines from 600 high to low altitude flashes. The result agrees with the case study of , who found significantly less skill of TRMM-LIS for (CG) discharges near the cloud base than for lightning channels propagating to near the top of the clouds.  currents. The flashes with minimum altitudes above 5.0 km exhibit statistically more positive than negative maximum currents.

Differences between matched and unmatched ISS
Further investigation reveals that about 94 % of the (absolute) currents above 22.5 kA belong to CG strokes. The strongest currents reach up to 150.0 kA (both negative and positive currents) and are related to CG strokes. CG strokes have almost exclusively negative currents in this study. IC pulse currents do not exceed 50 kA. About 90 % of pulses/strokes with amplitude 610 below 10.0 kA are IC pulses. A similar result is provided by Cummins and Murphy (2009). They found that 90 % of positive LF currents with less than 10.0 kA belong to IC pulses. Negative currents are observed for approximately 26 % of the IC pulses.
The Meteorage mean (maximum) flash absolute amplitude equals 8.0 kA (13.2 kA) and 11.6 kA (18.1 kA) for matched and unmatched flashes, respectively. The difference between matched and unmatched flashes is attributed to some low to mid-level 615 flashes producing strong currents and being not detected by ISS-LIS (compare (a) and (b) in Figures 14 and 15). However, the overall distributions of absolute flash amplitudes appear to be similar for matched and unmatched Meteorage flashes.
Flashes observed in this study show a statistical relationship between the polarity of the maximum (LF) current and the altitude. The relationship was detailed for the flash minimum altitudes and appears also for the flash maximum altitudes. In this study, flashes with maximum altitudes below 10.0 km exhibit mainly negative maximum currents. As it was found that 620 ISS-LIS' DE is 30 % higher for flashes with maximum altitudes above 10.0 km than for flashes restricted to lower levels, the polarity of the flash maximum current might provide a first information whether a flash is detected by ISS-LIS. This finding is probably specific for storm types and flashes analyzed in this study (and region). The observed relationship between the polarity of the maximum current of a flash and its altitude might change for inverted polarity storms or hybrid (IC+CG) flashes.