Lightning activity in the eastern Mediterranean region

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2004jd005710, 2005 Lightning activity in the eastern Mediterranean region E. Defer, K. Lagouvardos, and V. Kotroni National Observatory of Athens, Institute for Environmental Research, Athens, Greece Received 17 December 2004; revised 5 August 2005; accepted 22 September 2005; published 28 December [1] Twenty days of lightning activity recorded during winter in the Mediterranean region with significant rain accumulation in Greece were studied on the basis of the measurements of the UK Met Office long-range VLF sferics Arrival Time Difference (ATD) system and the spaceborne NASA Lightning Imaging Sensor (LIS) sensor onboard the Tropical Rainfall Measurement Mission (TRMM) satellite. Cloud-to-Ground (CG) flash density as well as history of CG activity were investigated on the basis of the sferics fixes. LIS observations were studied to document both intracloud (IC) and CG flashes in terms of extension, duration and development during different TRMM passes over the Mediterranean Sea. The analysis revealed that more than 266,000 CG flashes occurred during the 20 studied days. CG flashes were mostly located over sea, and CG density was found to follow the North African and Turkish shorelines. About 85% of the CG flashes were composed of a single sferics fix while the number of sferics fixes per CG flash was found ranging up to 15. Flash duration was found ranging from a few milliseconds to 2 s on the basis of LIS observations. Citation: Defer, E., K. Lagouvardos, and V. Kotroni (2005), Lightning activity in the eastern Mediterranean region, J. Geophys. Res., 110,, doi: /2004jd Introduction [2] Lightning activity in Europe as recorded by longrange VLF sferics systems occurs predominantly over land during summer while during winter thunderstorms are usually located over the Mediterranean Sea [Holt et al., 2001]. During spring the geographical distribution of lightning activity is more spread while during autumn the lightning flashes are mainly observed over the sea as well as in North Africa, the east coast of Spain and along the Italian and Balkan coasts [Holt et al., 2001]. Adamo [2004] reported similar results for the Mediterranean region based on 3-year Tropical Rainfall Measurement Mission (TRMM) Lightning imaging Sensor (LIS) observations. Altaratz et al. [2003] also reported higher cloud-to-ground (CG) flash density over the sea than over the land during winter in the eastern Mediterranean Sea while Holt et al. [2001] reported a maximum number of days of thunder near the coasts of Italy and Greece. Adamo [2004] also reported that the LIS flash rates over the Mediterranean Sea are significantly smaller than those recorded at similar latitudes in the United States. This finding is consistent with the fact that convection and consequently lightning activity are significantly stronger over land than over sea [Christian et al., 2003]. Additionally Adamo [2004] analyzed TRMM Precipitation Radar (PR) data and showed that 70% of the rain recorded during summer in the Mediterranean region is associated with thunderstorms while during winter only 30% of the rain events are recorded during thunderstorms. Copyright 2005 by the American Geophysical Union /05/2004JD However, because TRMM-PR sampling area is relatively limited, higher percentages can possibly be found especially for latitudes above 39 N in the Mediterranean region and more specifically along the Adriatic Sea where lightning activity and precipitation are often recorded. [3] Winter storms in the eastern Mediterranean region are often driven by low-pressure systems [Alpert et al., 1990; Kotroni et al., 1999; Jansà et al., 2000]. Lightning activity is often recorded during those weather situations but is poorly documented. It then seems relevant to investigate the lightning activity in order to characterize the convective and electrical processes occurring in the Mediterranean region during winter with the aim to use lightning information for nowcasting and forecasting applications. [4] Different techniques have been developed to record CG and intracloud (IC) lightning activities. Some techniques are based on acoustic or optical methods while others sense the electromagnetic radiation emitted by flashes at specific wavelength [MacGorman and Rust, 1998, Instruments chapter, pp ]. Ground-based long-range VLF or/and spaceborne optical and electromagnetic sensors are quite suitable to cover large regions such as the Mediterranean Basin. For instance, the long-range sferics VLF sensor such as the UK Met Office Arrival Time Difference (ATD) system monitors the CG activity in Europe for more than a decade [Lee, 1986; Holt et al., 2001] and can document the CG history of the storms during their entire life. In addition even if TRMM LIS optical measurements provide snapshots of the total (IC and CG) lightning activity it seems pertinent to use both data sets to initiate some analysis of the lightning activity in the Mediterranean region and to document flash characteristics such as flash extension or flash duration. 1of12

2 [5] The present paper is one of the few contributions documenting the lightning activity over the eastern Mediterranean Sea and summarizes the results of our investigations on the lightning activity as sensed by the ATD system and the LIS sensor for twenty days. The lightning sensors and the measurements that have been analyzed are presented in section 2. In the same section the algorithms developed for the present study are described. Section 3 is devoted to the documentation of a significant weather event while section 4 presents an overview of the different cases investigated for the winter period Finally section 5 contains some concluding remarks of this study. 2. Instruments and Methodology 2.1. UK Met Office Long-Range VLF Sferics ATD System [6] The UK Met Office long-range VLF sferics ATD system is designed to record CG activity. It covers all Europe and the Mediterranean Sea. Each of the 7 stations of the ATD system senses the VLF signal radiated when CG flashes connect to the ground [Lee, 1986]. Each station records accurately the time of the lightning events. After identifying the same lightning event from its waveform, the location of the event, called ATD fix, is computed on the basis of equations that depend on the difference of time arrival of the VLF signals at the different ATD stations [Lee, 1986]. [7] The ATD data used in the present study consist of a list of fixes defined by their occurrence time and their geolocations. Because a CG flash can be composed of many ground connections (i.e., ATD fixes), we apply a basic algorithm in order to combine the ATD fixes into flashes. This algorithm uses the criteria defined to retrieve CG flashes from the measurements of the U.S. National Lightning Detection Network (NLDN) [Cummins et al., 1998]. Two ATD fixes are part of the same flash when (1) the time interval between two fixes is less than 500 ms, (2) the distance between the two fixes is less than 10 km, (3) a flash can be composed no more than 15 strokes, and (4) the flash duration cannot exceed 1 s. On the basis of the aforementioned criteria we defined the CG multiplicity as the number of ATD fixes being part of the same flash NASA LIS Sensor [8] We also used the observations from the NASA LIS sensor to document the total lightning activity that occurred during each TRMM overpass. The spaceborne LIS sensor onboard TRMM satellite is designed to sense at 777-nm wavelength the light radiated by the lightning flashes [Christian et al., 1999]. LIS is composed of a wide-angle lens coupled with CCD array with 2 ms time frame. LIS can observe in its 500 km 500 km field of view a point on Earth during only 90 s. LIS can record optical radiation during both CG and IC flashes [Thomas et al., 2000]. [9] Different levels of LIS data are available. For the present study we used the raw LIS data sets, called Science data sets that consist in the succession of all LIS frames recorded during each TRMM orbit. Each 2-ms LIS frame is composed of illuminated pixels called hereafter LIS optical pulses, which map the cloud region where light has been radiated from flashes. Typically, a flash is sensed by LIS as a succession of frames, not necessarily continuous in time, with a given number of illuminated pixels. Because of the relatively high time resolution of LIS it is sometimes possible to study the development of flashes especially for spatially extensive flashes as we show in the next paragraphs. [10] LIS optical pulses are combined to form flashes on the basis of algorithms dependent on time and space criteria [Christian et al., 2000]. The NASA algorithm combines the optical pulses or LIS events in groups, and the groups in flashes on the basis of temporal and spatial criteria. A LIS event corresponds to a single pixel of the CCD array with a magnitude exceeding the background threshold and recorded during a single LIS frame. The events recorded during the same LIS frame are combined in groups. A group corresponds to multiple adjacent events recorded during the same LIS frame. A group can be composed of one or more events. Groups are then combined together to form flashes on the basis of time and space criteria. Two groups form the same flash when they are separated in time by no more than 330 ms and in space by no more than 5.5 km [see Christian et al., 2000, pp ]. Finally the NASA algorithm does not limit in time the flash duration. [11] Analysis of flash characteristics as retrieved from the current NASA algorithm revealed that sometimes the NASA algorithm artificially separates extensive and longduration flashes in many flashes. We developed a new algorithm, called hereafter National Observatory of Athens (NOA) algorithm, to combine the LIS optical pulses in flashes. The NOA algorithm processes the LIS data twice. It first applies the following time and space criteria: (1) A flash cannot last more than 2 s; (2) two LIS optical pulses are part of the same flash only when the time interval between them is less than 0.5 s; (3) finally, a LIS optical pulse is part of a flash when the distance between its location and the location of any optical pulse already part of the flash is less than 15 km. [12] It sometimes appears that some illuminated pixels can be spatially far enough that the spatial criterion (15 km) is not met, while later during the life of the flash the lightning branches illuminate those pixels. In such configuration, the isolated pixels would be considered as the origin of new flashes if only the first step of the NOA algorithm was applied. To solve this problem the NOA algorithm scans backward the initial flash list and determines if any optical pulse of two flashes is such that the space and time criteria defined previously are met. If this happens the optical events of both flashes are then merged together to form a single flash. Finally because there is no limit on the flash duration during this second phase of the NOA algorithm, the flash duration can exceed 2 s. [13] Figure 1 shows two flashes recorded during the same TRMM overpass over the region of Antalya (Turkey) on the 5 December The first flash occurred at 1807:55 UT. Optical signal was sensed by intermittence during 500 ms. LIS recorded subsequent bursts of optical radiation during short periods (Figures 1a and 1c). The locations of the LIS events suggest that the flash was compact (Figure 1b). At 1808:27 UT, LIS recorded a very widespread flash, propagating probably outside of the upper limit of LIS field of view (Figure 1e). This flash lasted for more than 2 s and 2of12

3 Figure 1. Cases of two flashes recorded during the same pass over the Antalya region (Turkey) on 5 December (a) Latitude time series (color coded in gray scale) of LIS events, (b) geolocations of the LIS events, and (c) time series of the maximum radiance per LIS frame recorded during the flash stamped at 1807:55 UT. (d f) Same as Figures 1a 1c but for the second flash (1808:27 UT). Triangle in Figure 1f shows the time of the ATD stroke recorded during the flash while the intersection of the solid lines in Figure 1e shows the stroke location. See color version of this figure in the HTML. was composed of relatively continuous bright components (Figures 1d and 1f). During the same flash ATD system detected a ground connection (Figures 1e and 1f ). The time and location of the ground connection suggest that the extensive flash connected to the ground almost at the end of its life. Krehbiel et al. [2000] have already reported ground connections during the final part of long-duration flashes within a continental storm. [14] Both NASA and NOA algorithms combined the optical pulses recorded during the first flash as being part of a single flash. While for the second flash NASA algorithm identified 12 flashes from the observations plotted in Figures 1d 1f, the NOA algorithm determined only one single flash. [15] In order to have an idea on the impact of the NOA algorithm on the flash retrieval process we studied in detail some LIS raw data for several flashes sensed over the Mediterranean Sea and applied our algorithm. The NOA algorithm has more impact for extensive flashes and longduration flashes as shown in the previous paragraphs. We extended our analysis for 36 LIS full orbits. For those orbits NASA algorithm identified originally 9889 flashes while the NOA algorithm reported only 7531 flashes (76% of the flash population retrieved by NASA algorithm). The discrepancy in the retrieval of the number of flashes per orbit (and implicitly the flash characteristics such a flash duration, number of components, etc...) is higher than the 15% indicated by Boccippio et al. [2002]. For the present work 3of12

4 Figure 2. Geolocations of CG flashes based on ATD observations per 3-hour period starting at the time indicated on the top right corner of each plot from 7 November UT to 9 November UT. LIS data sets were reprocessed and new flash data sets were produced and used for the following investigations Methodology [16] The analysis of the lightning measurements was first performed on the regional scale, mostly on the Mediterranean basin on the basis of the ATD data. CG flash rate and CG flash density were computed first. The flash density was determined by counting the number of flashes located in a regular grid mesh of grid box. The time series of the multiplicity were plotted as well as the time interval between the last and first strokes for a given flash with a multiplicity greater or equal to 2. [17] LIS observations were then studied in detail in order to determine some typical characteristics (flash extension and development, magnitude of the light radiated) of the lightning flashes over the Mediterranean Sea. Additionally microwave, visible and infrared observations respectively from TRMM Microwave Imager (TMI) and TRMM Visible and Infrared Scanner (VIRS) [Kummerow et al., 1998] were plotted in order to document the cloud content and cloud extension. [18] Finally we completed the analysis by comparing the measurements of the two lightning sensors in order to document the CG/(IC+CG) ratio. Because LIS cannot observe more than 90 s an Earth point in its field of view, only ATD observations located in the field of view and during the period of observations were considered. LIS science data sets were then compared to ATD records. [19] The algorithm to match the lightning flashes consists in checking if the ATD flash time was sensed during the duration of a given flash as derived from LIS measurements and also during a period of 0.5 s before the first LIS optical pulse of the flash as well as during a period of 0.5 s after the last LIS optical pulse of the flash. We use the 0.5 s criterion in order to take into account cases where LIS did not record the downward leader-return stroke processes while later or earlier LIS records IC components of the CG flash such as the case documented by Thomas et al. [2000]. A second computation determines the distance between the ATD flash location and any of the optical pulses recorded during the given flash. If the distance is less than 10 km and if the time criteria are both met, the measurements of the two lightning sensors are considered as being part of the same flash. The flash is then identified as a CG flash. 3. Analysis of the Lightning Activity for 7 8 November Regional Scale [20] During the period of 7 8 November 2002, Greece was affected by a low-pressure system that moved over the southern part of continental Greece and deepened to 1002 hpa at 0000 UTC 8 November. At the 500 hpa level a short wave trough with its axis oriented from north to south was evident over the central Mediterranean (not shown). As discussed in the following paragraphs, this system was accompanied with significant lightning activity, mainly along the frontal discontinuities. Twenty-four hour accumulated precipitation ending at 0600 UTC on 8 November exceeded 100 mm at one station in northeastern Greece and 90 mm at two island stations in the northern Aegean Sea, while several stations in the southern part of Greece have reported more than 60 mm of precipitation within 24 hours. [21] Figure 2 shows the location of the lightning activity recorded by ATD per 3-hour period during 7 8 November 4 of 12

5 Figure 3. (a) CG flash rate and (b d) multistroke CG flash ratio per 5-min period for the period 7 November UT to 9 November UT. No data were available after 2200 UT on 8 November Data were also missing for a few minutes during the study period During the first hours on 7 November 2002 lightning flashes were recorded east of Spain as well as over Greece and Tunisia. The electrical system that was located east of Spain moved southeast over the sea, died on the Algerian coastline and stopped producing CG flashes at about 0900 UT. In the central Mediterranean Sea the lightning activity was distributed along a very well defined line. This line rotated slowly to the east about a pivot located over the Aegean Sea. Later the southern part of the line started moving faster while electrical activity was still sensed at its pivot. After 2100 UT on 7 November 2002 the line of electrical activity started moving toward the western coastline of Turkey and finally died along the southern Turkish coastline. On 8 November 2002 new electrical activities were sensed in the central Mediterranean Sea and moved eastward. [22] For the entire region the CG flash rate computed per 5-min period peaked at about 350 CG flashes per 5 min at 1300 UT, 7 November 2002 (Figure 3a). The CG flash rate recorded on 8 November barely exceeded one third of the peak of the flash rate recorded on 7 November. More than 62,000 CG flashes were retrieved from the ATD observations on the basis of the algorithm described in the previous section. About 80% of the CG flashes were composed of a single fix, while 13% and 4% of CG flashes were composed of two and three fixes respectively. Multiplicity was found to range from 1 to 10 for the studied period and for the entire region. It should be reminded that subsequent strokes might have been missed because of limited (and unknown) stroke detection efficiency of the ATD system and consequently the multiplicity found here could be probably underestimated. [23] The number of fixes per flash found in the present study is different to the one reported by Rivas Soriano and De Pablo [2002] in the western part of the Mediterranean. reports from an Advanced Lightning Direction Finder (ALDF) network deployed along the Mediterranean coast of the Iberian Peninsula and on the Balearic islands they reported that 48% (91%) of the negative (positive) CG flashes were single stroke and also that the multiplicity exhibits minimum values in winter for negative CG flashes. Orville et al. [1997] reported an annual average of 2.3 strokes per negative CG flash in the region of the Tropical Ocean Global Atmosphere (TOGA)-Coupled Ocean-Atmosphere Response Experiment (COARE) while the present analyzed data set exhibits an average of 1.2 fix per flash for the 20 days of the study over the Mediterranean Sea. The difference in percentage can be mainly explained by different stroke detection efficiency between the different sensors. [24] Figures 3b 3d show the time series of the percentage of CG population (computed every 5 min) composed of 2, 3 and more than 3 strokes. The percentages were computed relative to the total number of CG flashes recorded during each 5-min period. CG flashes with two strokes were recorded during the entire studied period but at different ratio. CG flashes with multiplicity greater than 2 occurred at a much smaller percentage and only during the periods of the highest CG flash rates (Figures 3c and 3d) During the LIS Overpasses [25] Figure 4a shows the location of the lightning activity over the Mediterranean Basin recorded during the five TRMM overpasses that were performed during 7 8 Novem- 5of12

6 Figure 4. Concurrent observations of lightning activity recorded over the Mediterranean region on 7 and 8 November (a) Geolocations of the LIS observations (shaded dots for 7 and triangles for 8) and ATD CG flash locations (crosses for 7 and pluses for 8), (b) time series of the latitudes of the observations, (c) distribution of the flash duration derived from LIS observations, (d) distribution of CG multiplicity, (e) distribution of the time interval between the first LIS observations and the first CG stroke (negative values of the distribution mean that the stroke event was recorded before any LIS event), and (f ) distribution of the distance between the first CG stroke and the average locations of the LIS observations. ber 2002 (Figure 4b). Only ATD fixes (indicated with crosses and plus signs for 7 and 8 November respectively in Figures 4a and 4b) that were located in LIS temporal and spatial field of view are considered in the following section. In general both lightning sensors reported consistent measurements of the lightning activity in terms of spatial locations and temporal information. The two first LIS overpasses plotted in Figure 4 document the lightning activity located at the southern tip of the front geographically located over Peloponnese (Greece). [26] Figure 4c shows the histogram of the flash duration as determined by the time interval between the first and the last LIS frames for a given flash. During the five overpasses 358 flashes were identified from LIS observations. The distribution of the flash duration peaked at ms and ranged from 2 ms (one single LIS frame) to 2 s. Less than 5% of the flashes were characterized by a duration less than 10 ms while 3% flashes lasted more than 1 s. The duration deduced from the optical observations might not necessarily correspond to the real flash duration or to the duration deduced from VHF mapping systems. For instance Thomas et al. [2000] showed a case of a horizontal hybrid CG flash with no optical radiation during the first 600 ms of the flash while VHF radiation was recorded. However, flash duration as deduced from LIS observations can bring additional information in terms of flash characteristics. [27] During the overpasses, 60 CG flashes were identified from the ATD reports. Multiplicity ranged from 1 to 4 while 6of12

7 77% of the CG flashes were composed of a single stroke (Figure 4d). Analysis of the lightning data revealed the two lightning sensors recorded simultaneously 39 CG flashes (65% of the CG flashes recorded by ATD) during the five overpasses. A detailed investigation of the observations revealed that 16 out of the 21 not common flashes (76%) were actually located at the spatial or/and temporal edges of LIS field of view with an ATD location error ranging from 4 (for most of the cases) to 42 km (for one single flash). Only 2 out of the 21 flashes were completely missed by LIS or at least they did not radiate enough to be recorded by LIS, while 3 out of the 21 flashes were in the LIS field of view but they were not associated with any LIS flashes because they were located too far according to the criteria applied for the analysis of the observations from both lightning sensors. [28] Considering all overpasses and excluding all CG flashes outside LIS field of view about 11% of the flashes connected to the ground (IC/CG ratio of 7.8). When considering independently the two days that are studied it was found that during the first (second) day about 14% (3%) of the flashes connected to the ground. Different IC/CG regimes were recorded within LIS sampling time windows suggesting that different convective stages and/ or structures of charged regions were sampled. The first regime is composed of CG flashes predominantly located over the mountains of Peloponnese that were recorded during the overpasses on 7 November The second regime is characterized by significant IC activity recorded during the TRMM overpasses and refers to some cells over the sea on 7 November and to the storm located northwest of Cyprus on the 8 November The IC/CG ratio obtained here are just for the times of the overpasses and depends of course of the detection efficiency of both lightning sensors. [29] The time interval between the first LIS frame and the first stroke for a given CG flash ranged from few milliseconds up to 1.2 s (Figure 4e). About 52% of the first connection to the ground occurred during the first 20 ms of the flash duration while 13% of the CG flashes reported simultaneously by the two lightning sensors were first recorded by ATD and later by LIS with a delay ranging from 100 to 500 ms. This population of flashes is presented with negative percentages in Figure 4e. [30] Finally the distance between the average location of all LIS optical events and the location of the first stroke ranged from few kilometers to 29 km (Figure 4f). Eighty percent of the first CG strokes were located at less than 10 km away from the average location of LIS optical pulses. [31] Figure 5 shows observations from the TRMM-TMI at 85 GHz (vertical polarization), visible and infrared TRMM-VIRS images and also LIS flash locations recorded during the two successive overpasses over Greece at 07:56 and 0933 UT (0956 and 1133 local time) on 7 November Brightness temperatures (TB) at 85 GHz were measured as low as 110 K during the two successive overpasses. Additionally polarization difference at 85 GHz (TB85V TB85H) exhibited negative values (not shown) where simultaneously relative cold 85 GHz TB and lightning activity were measured. Prigent et al. [2005] show that the negative polarization difference at low brightness tem- Figure 5. TRMM-VIRS 0.63 and 10.8 mm, TRMM-TMI 85 GHz V observations and locations of LIS events recorded during the 90-s time window during the two passes over Peloponnese (Greece) on 7 November See color version of this figure in the HTML. perature can be related to large, mostly vertically oriented, nonspherical particles (prolate elongation or cone-like type of graupel) mostly located in the strongest convective updrafts. The current hypothesis on electrification processes, which are thought to be driven by ice-ice interactions and measured with low TB at 85 GHz, can then explain that LIS sensed lightning flashes in these cells. [32] A comparison of VIRS visible and infrared observations with the locations of LIS optical pulses showed that the lightning flashes did not spread within the entire clouds 7 of 12

8 Table 1. List of the Days Investigated in the Present Study With the Associated Number of ATD Fixes and Flashes, the Maximum Flash Rate per 5-min and the Maximum Flash Density in 24 Hours Recorded in the Mediterranean Region Days Number of ATD Fixes Number of CG Flashes Maximum Flash Rate (per 5 min) Maximum Flash Density (in 24 hours, per ) 7 Nov ,516 48, Nov ,327 14, Dec ,716 19, Dec ,998 26, Dec ,478 14, Dec ,925 12, Dec ,523 11, Dec ,103 13, Jan ,099 11, Jan. 03 5,249 4, Jan ,947 27, Jan ,820 9, Jan ,542 11, Feb ,873 10, Feb. 03 6,712 5, Feb. 03 3,190 2, March 03 5,289 4, March 03 14,448 11, Apr. 03 4,323 3, Apr. 03 6,812 5, Total 350, ,721 suggesting that the electrified regions were (relatively) compact, probably within the main cores of the storms at the time of the overpasses (Figure 5). 4. Overview of 20-Day Lightning Activity [33] Twenty days recorded during winter and characterized by significant 24-hour rain accumulation in Greece were studied to document the lightning activity associated with cold period weather situations in the Mediterranean Region (see Table 1 for the list of days investigated in the present study). All these cases were associated with low-pressure systems propagating eastward through Greece. The total number of CG flashes in the entire Mediterranean basin for the studied days exceeded 266,000 flashes. From the algorithm associating ATD fixes in flashes it was found that about 14% of the CG flashes was composed of two or more ATD fixes (Figure 6). The multiplicity was found to range from 1 to 12 and only one flash was composed of 15 ATD fixes. About 1% of the flashes exhibited a multiplicity greater than 3. [34] Figure 7 shows the distribution of the time interval between the first and the last fixes of each flashes for all CG flashes with a multiplicity >1 as well as separately for each value of multiplicity from 2 to 15. The time interval ranges from 8 ms to 1 s for the entire CG flash population with a multiplicity >1. The clear cutoff of the distribution at 1 s is due to the applied criterion of flash duration (see section 2) which sometimes may artificially separate a single flash in two flashes. The same distribution was computed as function of CG multiplicity. It was found that the higher the number of fixes per flash is, the longer the flash lasts. For flashes composed of two ATD fixes, the distribution cuts off at 0.5 s, which corresponds to the time threshold criterion between two ATD fixes to be part of the same flash (see section 2). [35] Figure 8 shows the flash density per as determined from the ATD data for the 20 days. For the entire CG population the flash density peaks at 470 flashes per (Figure 8). The flash density covers almost the entire Mediterranean region. The edges of the flash distribution follow the shorelines especially in North Africa and Turkey (Figure 8). [36] In the present study the CG density exhibits high values in the southern Adriatic Sea, in the Gulf of Antalya (southern Turkey) and along a line over southern Greece and the Aegean Sea (Figure 8). This line was recorded during the period 7 8 November and it was described in detail in the previous section. As it concerns the high CG density located in the Gulf of Antalya, it was mainly Figure 6. Distribution of number of flashes for a given number of ATD fixes per flash for the studied period. The number above each bar gives the percentage of flashes (from a total of 266,721 flashes) determined for the given number of ATD fixes per flash. 8of12

9 Figure 7. Distribution of the time interval between the first and last ATD fixes of multifix flashes, in logarithmic scale. Distribution bin is set at 0.1. The number of multifix flashes is 37,571 for the 20 days of the study. The number in the top left corner of each plot gives the number of fixes per flash considered. recorded during a heavy precipitation event on 5 December During this event the lightning activity was stationary over the Antalya region for 12 hours and rain accumulation in Antalya exceeded 230 mm in 24 hours. Flashes shown in Figure 1 were recorded during that heavy rainstorm. Finally the hot spot located in the southern part of the Adriatic Sea was associated with a single storm here again with significant lightning activity. [37] Finally for the 20 days of the present study, 39 LIS overpasses were investigated (Figure 9). From the algorithm applied on the LIS data (and described in section 2), 1671 flashes were identified from LIS optical pulses. Most of the flashes sensed by LIS during the overpasses were located in the eastern Mediterranean Sea (Figure 9a). Others were located in Morocco, on the coastlines of Algeria and Tunisia. [38] Flash duration based on LIS observations was found to range from 2 ms to 2.3 s, with a peak at ms (Figure 9c). About 6% of the flashes lasted less than 10 ms while the duration of 4% of the flashes exceeded 1 s. For comparison Defer et al. [2001] reported for a continental storm a flash duration ranging from 23 ms to 1.8 s with mean flash duration of about 240 ms. Noble et al. [2004] reported flash duration up to 1.6 s with a peak around 200 ms based on the observations of 190 flashes (during 2 min 45 s) from the Lightning Mapping Array (LMA) during the STEPS experiment. In the present study the mean flash duration (as derived from LIS observations) was found to be of 360 ms. This value is between the mean flash duration found by Defer et al. [2001] and the value of 0.5 s reported by Kitagawa and Brooks [1960]. It also ranges in the mean or typical values synthesized by Rakov and Uman [2003] for CG ( ms, page 7) and for IC (245 to 660 ms, page 325) flashes. [39] During the 39 TRMM overpasses, 425 flashes connected to the ground according to ATD measurements Figure 8. Spatial distribution of CG flashes per for the 20 days of the study. See color version of this figure in the HTML. 9of12

10 Figure 9. (a f) Concurrent observations of the lightning activity over the Mediterranean Sea during the 20 days of the study. See Figure 4 for details on the plots. (Figure 9d). This number represents about 25% of the flash population recorded during the TRMM overpasses. Monostroke CG flashes represented 86% of the CG flashes and multiplicity ranged from 1 to 6. Only 208 flashes, 12% of the LIS flashes, were simultaneously recorded by LIS and ATD. The time interval between the first LIS observation and the first CG stroke ranged from few milliseconds up to 1.5 s. About 42% (54%) of the CG flashes connected to the ground less than 20 ms (100 ms) after the record of first optical radiation by LIS (Figure 9e). Only 1% of the CG flashes connected 1 s or more after the beginning of the flashes. Finally 12% of the CG flashes were sensed first by ATD and then later by LIS with a time interval ranging from 20 to 400 ms (indicated with negative percentage in Figure 9e). [40] Finally the distance between the first CG stroke and the average location of the flash as reported by LIS was found ranging from few kilometers up to 47 km (Figure 9f). Fifty percent (92%) of the flashes recorded simultaneously by the two sensors had a distance lower than 10 km (20 km). 5. Concluding Remarks [41] We have conducted a preliminary study of the lightning activity over the Mediterranean Sea based on observations from two different techniques. Indeed, history of the CG activity was investigated from ground based VLF ATD records while snapshots of the total lighting activity were studied on the basis of LIS observations. Especially for the analysis of LIS observations a new algorithm was developed and applied in order to combine the LIS optical pulses in flashes. The studied period consists of 20 days within winter that were associated with important rainfall accumulations and lightning activity in the eastern part of the Mediterranean sea and mainly over Greece (see Table 2 for a summary of the results). 10 of 12

11 Table 2. Summary of the Results for the 20-Day Study Parameter Sensor Results CG activity ATD 20 days of study >266,000 flashes sensed 86% flashes with 1 single fix 1% flashes with more than 3 fixes Total activity LIS 1,671 flashes recorded during a total of 39 TRMM overpasses flash duration ranging from 2 ms to 2.3 s with a peak between 400 and 500 ms 6% of flashes with duration < 10 ms 4% of flashes with duration > 1 s CG activity ATD + LIS 12% of the LIS flashes were sensed by ATD delay before first connection to the ground ranged from few ms up to 1.5 s 42% (54%) of the ground connections occurred less than 20 (100) ms after the first LIS observation 50% (92%) of the ground connections occurred less than 10 (20) km away from the average LIS flash location [42] For the analyzed period the vast majority of the lightning activity was observed over the Mediterranean waters. The multiplicity of the flashes was found to range from 1 to 12 while on average 85% of the flashes were identified with a single fix. Analysis of LIS optical measurements revealed that the flash duration ranges from few milliseconds up to 2 s. Additionally, from this analysis it was found that for the studied TRMM overpasses most flashes can be separated in two categories. The first one, called regular flashes, is presented in Figure 1. It is often composed of bursts of optical radiation with short duration. The flashes of that category do not propagate over long distance. The second category is composed of very extensive flashes with duration greater than 1 s and often exhibits bright components with duration up to few hundreds of milliseconds. Those two categories are recorded within different storm types. The regular flashes are usually recorded within isolated storms or multicell storms, while flashes from the second category are often recorded within organized mesoscale systems. Note that regular flashes can also be recorded in mesoscale systems with embedded convective cells. [43] Analysis of additional measurements of clouds as provided from both TRMM TMI (cloud water and ice content) and PR (reflectivity profiles) can provide significant insights on the nature of the microphysics for the Mediterranean storms studied in the present work. Indeed, it is in the authors plans to investigate the relationship between the microphysical properties of these storms and the lightning activity. [44] Further analysis of lightning activity associated with Mediterranean storms will be performed based also on the long-range VLF lightning sensing system ZEUS that has recently become operational at the National Observatory of Athens. [45] Acknowledgments. 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