Atmospheric Research 85 (2007) 159 – 170 www.elsevier.com/locate/atmos
Lightning and precipitation relationship in summer thunderstorms: Case studies in the North Western Mediterranean region Nicolau Pineda ⁎, Tomeu Rigo, Joan Bech, Xavier Soler Servei Meteorològic de Catalunya (Meteorological Service of Catalonia), Berlin 38, Barcelona E-08029, Spain Received 12 June 2006; received in revised form 1 December 2006; accepted 1 December 2006
Abstract This study analyzes the relationship between lightning and precipitation in nine convective events. They occurred during the summer season of 2004 in Catalonia (NE Spain) and its coastal area, in the North Western Mediterranean Sea. The data examined were issued from C-band volumetric radar observations, from radiosoundings, and total lightning detection records, including both cloud-to-ground (CG) and intra-cloud flashes. The overall Rainfall–Lightning Ratio (RLR) found was 38.9 103 m3/CG flash, which is a value closer to those found in the Southeastern United States than in the Atlantic coast of France. Moreover, the range of variation found in the studied episodes goes from 10.8 to 87.2 103 m3/CG flash. These variations are analyzed in terms of the synoptic conditions of the events and regarding their spatial distribution, comparing land and sea domains. © 2006 Elsevier B.V. All rights reserved. Keywords: Total lightning activity; Convective precipitation; Rainfall–Lightning Ratio; North Western Mediterranean region
1. Introduction Interest in improving the understanding of convective precipitation systems has led many authors to study relationships between lightning and rainfall. A better knowledge of local thunderstorm phenomenology can lead to a better estimation of convective rainfall in areas with poor radar coverage and may also be useful to assess weather surveillance tasks. Considering previous studies of convective precipitation and lightning, two types of approaches can be distinguished. The first one compares rain yields with ⁎ Corresponding author. Remote Sensing Unit, Servei Meteorològic de Catalunya, Spain. Tel.: +34 93 567 60 90; fax: +34 93 567 61 02. E-mail addresses:
[email protected] (N. Pineda),
[email protected] (T. Rigo),
[email protected] (J. Bech),
[email protected] (X. Soler). 0169-8095/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2006.12.004
cloud-to-ground lightning flashes (the ratio of rain mass to cloud-to-ground lightning flash) for long temporal and spatial domains (Petersen and Rutledge, 1998; Rivas Soriano et al., 2001). The second approach focuses on local thunderstorm case studies, calculating a Rainfall–Lightning Ratio, and reporting a correlation between convective precipitation and lightning (such as Piepgrass et al., 1982; Sheridan et al., 1997; Tapia et al., 1998; Soula et al., 1998; Zhou et al., 2002). The relation between rainfall and lightning is usually quantified as the Rainfall–Lightning ratio (RLR). This ratio estimates the convective rainfall volume per cloudto-ground (CG) lightning flash (Tapia et al., 1998). According to a summary provided by Kempf and Krider (2003) concerning studies where precipitation was estimated using radar, RLR ranged from 38 to 72 103 m3/CG flash for isolated thunderstorms (Southeastern and Central United States, Florida, France and Spain).
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Other studies found values for isolated thunderstorms ranging from 14 to 35 103 m3/CG flash in China (Zhou et al., 2002) and from 3 to 33 103 m3/CG flash in the Pyrenees (Molinie et al., 1999). Intense storms tend to produce lower RLR values than moderate storms, but the range of the RLR found in diverse studies is quite wide (Rakov and Uman, 2003). This relationship apparently depends on thunderstorm type, local climatology and other meteorological factors like the convective regime (Williams et al., 1989; Tapia et al., 1998; Seity et al., 2001; Lang and Rutledge, 2002). Seity et al. (2001) compared the lightning and precipitation relationships in the SW Atlantic French area, finding differences between the land and sea domains. Thunderstorm activity offshore was much lower than inland, and a lower land–sea contrast for rainfall was observed. On the other hand, larger percentages of CG flashes were observed over sea, which were associated with lower cloud vertical development. Williams et al. (1992) and Zipser (1994) found that considerably more precipitation fell per CG flash from storms embedded in monsoonal convection than from continental storms in Darwin (Australia). It has been shown that cloud electrification is closely associated with the relative motions responsible for collisions of the different types of hydrometeors in the cloud (Saunders, 1993). For example, Coquillat and Chauzy (1994) studied how liquid hydrometeors facilitate initiation of lightning discharges. It is not clear which parameter associated with cloud electrification is more closely related to rainfall. According to Williams et al. (1989) the mechanisms leading to the generation of lightning strikes are related to strong vertical air flows present in the storm. Zipser and Lutz (1994) related the presence of lightning to the vertical updraft speeds: lightning is absent or highly unlikely if the vertical updraft speed does not exceed a threshold roughly 6–7 m s− 1 (mean) or 10–12 m s− 1 (peak), regardless of cloud depth. Moreover, Backer et al. (1999) indicated that lightning flash rate was proportional to the fourth power of vertical velocity. Perhaps the most important variable linking surface air temperature and lightning activity is the vertical air motion (Williams et al., 2005). The Convective Available Potential Energy (CAPE) can be used to assess vertical air motion in moist convection. Several studies have shown an inverse correlation between RLR and CAPE (Buechler and Goodman, 1990; Williams et al., 1992). According to these authors, the decrease in the RLR is due to the increase in updraft strength observed on days of higher CAPE. Sheridan et al. (1997), after examining CG observations and precipi-
tation in the South-Central US, concluded that on days with high RLR, atmospheric stability indices tended to be low. However, Tapia et al. (1998) reported that no clear relationship could be established between RLR and CAPE in their study of Florida thunderstorms. The objective of this paper is to analyze the relationship between precipitation and lightning over Catalonia (NE Spain) in the North Western Mediterranean region. Nine summer convective events were selected for the analysis. The comparison of lightning and precipitation was performed through a eulerian spatial analysis of each episode, considering separately land and sea domains. The present study is a case-by-
Fig. 1. (a) Region of interest in NE Spain. (b) Location of the Vallirana weather radar (white dot), the radar coverage area when operating in volumetric mode (lighter circle), the SAFIR detection sensors (black dots) and the limits of the 99% (white line) and 90% (grey line) SAFIR detection efficiency areas.
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case study, using total lightning information (intra-cloud and cloud-to-ground flashes). 2. Data sources and processing The studied data are composed of meteorological radar products, lightning detections, and radiosonde data from the Meteorological Service of Catalonia (Meteocat). The region of interest covers approximately an area of 53,000 km2 in the NE of the Iberian Peninsula and the Mediterranean Sea (Fig. 1). 2.1. Data used The data used in the present paper correspond to nine episodes observed in the summer of 2004 in the region of interest. Such episodes were selected according to their lightning activity: they totaled more than 77,000 CG flashes, which represent almost 60% of the CG flashes for the year 2004 over the region. These episodes are periods of continuous convective activity (up to 48 h) in the studied region, where different convective structures can coexist. Lightning flash rates were not always elevated, but rather continuous through each episode. Convective precipitation structures observed during these episodes include multi-cell structures and isolated thunderstorm cells. Such structures presented six-minute averaged CG flash min− 1 rates between 2.4 and 16.5, with peak CG flash rates greater than 15 CG flash min− 1 in most of the cases, reaching sometimes values greater than 40 CG flash min− 1. These high values were associated with multi-cell well-organized structures. According to Lang and Rutledge (2002) such rate values suggest intense convection. No periods of high IC/CG ratio were observed, neither situations of positive CG flash predominance were there. Hail at ground was reported in six of the nine episodes, but diameters greater than 19 mm were not observed. The cumulative rainfall was estimated from weather radar observations recorded with the Meteocat Vallirana radar, a C-band Doppler radar with 1.5° beam-width. Fig. 1 shows the area covered by the radar operating in volumetric mode, and its location 25 km W of Barcelona city. This area is defined as the region of interest in this study. An operational version of the EHIMI system was used to generate post-processed rain rate fields from volumetric radar observations (Bech et al., 2005). The EHIMI system applies different algorithms (ground echoes mask and topographical beam blockage corrections) in order to improve the quality of the radar quantitative precipitation estimates.
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Lightning information was collected by the Meteocat SAFIR lightning detection system (hereafter XDDE). The XDDE is composed of three stations, covering the region of Catalonia (NE of Spain), and its contiguous sea (Fig. 1). The SAFIR system (Richard and Lojou, 1996) uses an interferometric technique in the VHF range (108–116 MHz) to detect total lightning flashes. The VHF sources detected are associated with a flash when they are separated by less than 7 km and less than 100 ms. The detection of CG return strokes is performed in the LF range. Strokes are grouped into CG flashes considering a multiplicity delay of 0.5 s in a radius of 7 km. The SAFIR system then discriminates IC and CG flashes by comparing VHF and LF records. The XDDE spatial accuracy for the whole region studied (Fig. 1) is around 2–3 km. The detection efficiency of the XDDE is between 86 and 92%, according to a field campaign performed in summer 2004 (Montanyà et al., 2006). These values agree with the SAFIR manufacturer specifications (around 90%). In this study an efficiency of 90% has been assumed to estimate the real number of CG flashes in the studied episodes. Such a correction is necessary to facilitate the comparison of lightning-precipitation with prior literature (i.e. Kempf and Krider, 2003). Radiosonde data from the Meteocat Barcelona station were used to assess the thunderstorm probability in each episode through several stability indices. Table 1 shows the stability indices calculated from radiosonde data for the nine episodes, which include: (a) the Lifted Index (LI) a measure of atmospheric stability which is used as a thunderstorm potential indicator (Johns and Doswell, 1992); (b) the Total Totals index (TT), commonly used as a severe weather indicator (Miller, 1972); (c) the Convective Available Potential Energy (CAPE) that represents the amount of buoyant energy available to accelerate an air parcel vertically (Weisman and Klemp, 1986); (d) the Precipitable Water (PW), a measurement of the water vapor content that is susceptible to precipitate, usually in a layer between the surface and the 850 hPa level. Meteorological conditions of the studied events are characterized according to Global Model analysis maps of Deutscher Wetterdienst (sea-level air pressure, geopotential height and temperature at 500 hPa and 850 hPa). 2.2. Lightning characteristics in the region The studied region is one of the most active areas in terms of lightning in the Iberian Peninsula (Rivas Soriano et al., 2005; Rivas Soriano and De Pablo, 2002). Thunderstorms may take place any time of the year, but
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Table 1 Radiosonde derived quantities and intra-cloud (IC) and cloud-to-ground (CG) flash numbers for selected episodes: Lifted Index (LI), Total Totals Index (TT), Convective Available Potential Energy (CAPE), and Precipitable Water (PW) Episode E 1 2 3 4 5 6 7 8 9
Date 29/07/2004 01/08/2004 05/08/2004 09/08/2004 17/08/2004 29–30/08/2004 01–03/09/2004 06–07/09/2004 14–15/09/2004
Radiosonde hh:mm 29/07 00:00 01/08 12:00 05/08 00:00 09/08 12:00 17/08 00:00 29/08 12:00 02/09 12:00 06/09 12:00 14/09 12:00
LI
TT
Celsius − 7.2 − 4.2 − 6.3 − 7.9 −11.8 − 6.0 − 7.3 − 6.6 − 5.3
⁎⁎ ⁎ ⁎⁎ ⁎⁎ ⁎⁎ ⁎⁎ ⁎⁎ ⁎⁎ ⁎⁎
CAPE
Celsius
J Kg
⁎
1018 2164 1795 3033 3135 2376 2414 1672 1504
50 44 52 46 49 49 48 48 50
⁎ ⁎ ⁎ ⁎ ⁎ ⁎ ⁎
PW
−1
Total flashes −2
⁎ ⁎⁎ ⁎⁎ ⁎ ⁎
kg m
IC
CG
28.8 25.6 34.8 31.0 30.7 38.8 31.9 27.3 28.8
14238 4204 132756 15882 32037 55324 64252 73994 155523
2431 973 23841 3439 3380 8171 5774 8338 20816
Associated thunderstorm probability (except for PW) is indicated by asterisks: low (no asterisk), moderate (⁎) and high (⁎⁎).
the main thunderstorm season starts at the end of May and lasts until the end of October. According to a preliminary analysis examining all 2004 XDDE data, the thunderstorm distribution had its maximum in August (35% of the annual CG flashes), followed by September (30%) and June (12%). Although the CG flash percentage for August and September were similar, the distribution patterns were very different. While in August 74% of CG flashes were inland, in September thunderstorms were mainly offshore, with only 39% of CG flashes inland. The annual thunderstorm season starts inland in May and June, affecting mainly the Pyrenees region. During these months, there is little thunderstorm activity above the sea, with only 10% of CG flashes recorded by the XDDE. At the end of August, there is a change in the thunderstorm activity pattern: thunderstorms affect the entire region and not only the mountain area. In September, October and November, thunderstorms occur mainly offshore, concentrating near the coast. CG flashes over the sea in September and October represented 61% of the total sea CG flashes during 2004. Rivas Soriano and De Pablo (2002) studied the same sea region (period 1992–1994), and obtained a similar CG flash proportion for September and October over the sea (60%). They related the annual CG flash distribution to the annual Mediterranean Sea surface temperature distribution, which presents its maximum in September. The maximum lightning activity appears after the summer, when the thermodynamic atmospheric conditions over the Mediterranean Sea are more appropriate for convection. During this season considerable moisture supply and low static stability exist at low tropospheric levels (Romero et al., 2001). Concerning the hourly distribution of lightning; in May, June and July, thunderstorms occur mainly in the
afternoon (13 to 16 UTC), while in September and October thunderstorms over the sea occur mainly at night (22 to 07 UTC). It should be noted here that in the region of interest, UTC equals Local Solar Time (LST). For these months, thunderstorms inland still have the maximum in the afternoon, while the occurrence is more distributed during the day. Rivas Soriano and De Pablo (2002) found a similar diurnal variation for sea lightning, with a maximum around 5 UTC and a minimum around 15 UTC. The diurnal cycle inland for the year 2004 in the region of study is also similar to the diurnal cycle reported in Rivas Soriano et al. (2005) for the whole Iberian Peninsula (years 1992 to 2001), with a maximum at 17 UTC and a minimum between 9 and 11 UTC. 2.3. Synoptic situations of the studied episodes Most summer 2004 convective episodes occurred under three characteristic synoptic situations. The selection of the studied episodes took into account this fact and three episodes of each situation were chosen. The Iberian Mediterranean coasts normally experience high temperatures, great amounts of sunshine and few rainy days during summer. The pressure system that dominates the region is the Azores Islands anticyclone, contributing to the development of large thermally driven convective systems over the Peninsula and land–sea breezes over the coasts. The complex orography of the Iberian Peninsula easterly coast, characterized by a large range of coastal mountains with heights between 500–700 m a few kilometers inland, contributes to the development of an intricate air flow circulation. A combination of sea breezes with upslope winds contribute to the advection and injection inland of the seacoastal air masses. In this sense, the topography of the region plays an important role inducing the orographic
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with an observed maximum of 21.3 CG flash min− 1. For the whole episode, 8171 CG flashes were registered over the region of interest.
forcing of the flows that are injected aloft (Romero et al., 1997). The nine events studied have been classified into three main groups, according to their sea-level pressure synoptic pattern over the region of interest. The three groups considered were: a) absence of marked synoptic circulation due to sea-level Low Pressure Gradient (LPG), b) Iberian Summer Thermal Low (ISTL) and c) Frontal System (FS). Other authors in previous studies of lightning data performed similar classifications. For example, Tapia et al. (1998) related the RLR variability in Central Florida with the direction of the prevailing synoptic wind and Seity et al. (2001) studied the daily diurnal evolution of lightning activity in cases of frontal activity, SW flow and convection due only to diurnal evolution. The category of each episode is shown in the last column of Table 2.
2.3.2. Iberian Summer Thermal Low (ISTL) The position of the axis of an upper-level trough regarding the area of interest was found to be well related to lightning activity. In particular, if the region of study was in front (relative to the axis movement) or right below the upper-level trough, then the lightning activity was higher than in other situations. Therefore the location of the upper-level trough is a key feature in the second group of the classification of surface pressure characteristics: the Iberian Summer Thermal Low (ISTL). The ISTL is a surface thermal low that develops typically in summer from intense heating at surface level in the center of the Iberian Peninsula. This phenomenon favors the air vertical ascent and therefore produces a decreasing air pressure with respect to neighboring areas (Gaertner et al., 1993). To initiate convection, similarly to the LPG class, relative thermal lows also require the arrival of an upperlevel thermal and/or dynamical trough, typically visible in the 500 hPa topography. This situation occurred for example on 05/08/2004 (episode 3) that shows surface values of 24 °C and − 12 °C at 500 hPa. During this episode three multi-cell structures were observed, and the three of them presented high CG flash rates, with a mean value above 7 CG flash min− 1. All of them registered maximums above 25 CG flashes min− 1. This episode is the one with more CG flashes; up to 23,841 CG flashes. Another event within this group shows special distinctive characteristics. It took place on the 06/09/ 2004 (episode 8): on the synoptic scale, clearly a
2.3.1. Sea-level Low Pressure Gradient (LPG) The absence of marked synoptic surface circulation due to a sea-level Low Pressure Gradient (LPG) was the dominant feature in three events assigned to this group. LPG is a typical summertime situation in the Iberian Peninsula, with a total absence of sea-level pressure gradient in the surface synoptic chart. Under these conditions, upper-level cold air advections favor convective activity. One example of this category is episode 6 (29/08/ 2004). On that day, the circulation of an upper-level trough with − 14 °C at 500 hPa, combined with surface temperatures over 25 °C at 12 UTC triggered the convection and subsequent intense lightning activity. Isolated thunderstorms and multi-cell structures were observed during the episode. The mean CG flash rate of the multi-cells was between 2.4 and 6.2 CG flash min− 1,
Table 2 Intra-cloud (IC) and cloud-to-ground (CG) lightning flashes over land and sea, Rainfall–Lightning Ratio (expressed as rainfall volume per CG flash, 103 m3/CG flash) and synoptic class for the nine selected episodes Episode
Flashes over land
Flashes over sea
Rainfall– Lightning Ratio
Synoptic
E
Date
IC
CG
%CG
IC
CG
%CG
Land
Sea
Class
1 6 7 2 3 8 4 5 9
29/07/2004 29–30/08/2004 01–03/09/2004 01/08/2004 05/08/2004 06–07/09/2004 09/08/2004 17/08/2004 14–15/09/2004
12788 45256 15619 4204 93442 14391 15882 26687 55086
2103 6742 2114 973 16854 1604 3439 2746 9576
14.1 13.0 11.9 18.8 15.3 10.0 17.8 9.3 14.8
1450 10068 48633 0 39314 59603 0 5350 100437
328 1429 3660 0 6987 6734 0 634 11240
18.4 12.4 7.0 – 15.1 10.2 – 10.6 10.1
58.5 30.5 87.2 15.5 10.8 65.7 12.0 24.2 25.7
54.2 34.4 38.5 – 33.9 78.1 – 28.2 24.2
LPG LPG LPG ISTL ISTL ISTL FS FS FS
Synoptic classes are: sea-level Low Pressure Gradient (LPG), Iberian Summer Thermal Low (ISTL) and Frontal System (FS).
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2.3.3. Frontal System (FS) The third group of synoptic situations corresponds to events associated with Frontal Systems. The events were generally linked to surface frontal cyclones, cold fronts and upper-level thermal and dynamical troughs circulating from west to east of the Iberian Peninsula, and favoring diffluent conditions over the area of study. The great thermal contrast between low level warm air and mid-level well defined troughs is responsible for a high degree of atmospheric instability and subsequent convection. Three different events were assigned to this category. In fact this situation is relatively rare during the period examined. However, it generated some of the more active lightning events, like episode 9 (14–15/09/2004), with a total of 20,816 CG flashes. Another interesting episode occurred on 17/08/2004 (Fig. 2). On this day the 12:00 UTC Barcelona radiosonde indicated a surface temperature of 29.0 °C, 18.4 °C at 850 hPa and − 12.7 °C at 500 hPa. Under this situation, a multi-cell structure crossed the region from SW to NE, lasting for more than 8 h. The mean and the maximum CG flash rates were 8.5 and 27.7 CG flash min− 1, respectively. 2.4. Radar and lightning data processing
Fig. 2. Synoptic maps corresponding to episode 5 (17/08/2004 12 UTC): (a) mean sea-level presure (hPa) and (b) 500 hPa temperature (°C) and geopotential (m) fields.
relative thermal low dominated but within the region of interest there was SE advection, where most lightning activity occurred. Therefore, some of the characteristics observed on this date may differ from other events of this group. The coexistence of a depression located south of the region of study with an ISTL is quite a rare coincidence during this time of the year. These two factors probably impinged on the concentration of most lightning activity on the southern coast of Catalonia, where the surface warm and moist southern flux was more evident and continuous. Moreover, at higher levels there was diffluence with a cold air pool. The lightning activity of the six hour multi-cell observed during this episode was concentrated on the southern coastal area of the region, presenting a mean flash rate of 15.3 CG flash min− 1 and a maximum of 30.0 CG flash min− 1 .
The estimation of rainfall rate R from radar reflectivity factor Z was obtained by means of the Z–R relation Z = 200 R 1.6 (Marshall and Palmer, 1948). The radar data corresponded to the lowest CAPPI (1 km) of the Vallirana radar. The conversion process from Z to R and the subsequent accumulation P of rain rates was done considering only pixels with P N 0.1 mm in 1 h. Regarding lightning data, as indicated by many authors, positive CG flashes detected by LF sensors can be misidentified as in-cloud lightning flashes (Cummins et al., 1998; Wacker and Orville, 1999a,b; Orville and Huffines, 2001; Carey et al., 2003). To take into account this effect, positive CG flashes with peak current values below 10 kA were removed. To perform the RLR calculation, the region of interest was divided in pixels using a regular mesh of 0.1°, which in this latitude corresponds to approximately 91 km2. Each pixel was classified as land or sea, resulting in 59% of land pixels and 41% of sea pixels. Mixed pixels from the coastal zone were assigned to the class with a higher percent of surface within the pixel. Once the pixels were classified, for each event the rainfall accumulation and the number of IC and CG flashes was computed for each pixel. In order to avoid stratiform rain in the RLR calculation, pixels with no CG flashes were not included in the calculation of the
N. Pineda et al. / Atmospheric Research 85 (2007) 159–170 Table 3 Number of episodes in each synoptic class, duration and Rainfall– Lightning Ratio (103 m3/CG flash) and CG flash percent over land and sea Synoptic class
Episodes Hours RLR
%CG
Land Sea Sea-level Low Pressure 3 Gradient Iberian Summer 3 Thermal Low Frontal System 3
Land Sea
80
58.7
42.4 13.0
12.6
45
30.7
56.0 14.7
12.6
50
20.6
26.2 14.0
10.3
rainfall accumulation. Finally, the RLR for each event was calculated for land and sea, as well as for the whole region (Table 2). 3. Results 3.1. Lightning type and characteristics The Meteocat SAFIR lightning detection network computes separately IC and CG. Both flash types have been examined in the analyzed episodes. A summary of lightning and rainfall activity of the studied episodes is presented in Table 2. It may be seen that the average CG flash percentage (CG / (IC + CG)) of the different episodes ranges between 9% and 24%. Soula and Chauzy (2001) found CG percentage values ranging from 9 to 40% in the region of Paris, and Seity et al. (2001) reported mean values of 16% over land and 20% over sea on the French Atlantic region. In the present work, the total mean CG percentage is larger over land (15%) than offshore (13%). While in some individual episodes the CG percentage is higher over sea, for the three synoptic groups (Table 3) their mean value is higher over land. Table 4 shows several characteristics of the lightning flashes examined in the present study. It lists flash multiplicity, positive CG flash percentage, and negative
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and positive CG first stroke currents. The values presented are the range and mean values over land and sea for the whole data set of events considered. Moreover, additional columns include results referred to warm season averages over land and sea presented previously by Rivas Soriano et al. (2005) – hereafter RS05 – and Rivas Soriano and De Pablo (2002) – hereafter RS02–, respectively. Table 4 indicates that mean values over land and sea are similar both in the case of strokes per flash (2.6 and 2.7, respectively), and percentages of single stroke flashes (48.6% and 47.4%, respectively). These results are in very good agreement with RS02 and RS05. However, Seity et al. (2001) found lower stroke per flash multiplicity values in the SW of France, with a higher multiplicity inland than over sea. Parameters such as polarity and stroke intensity show a remarkable contrast over land and sea: CG positive flash percentage is higher inland (4.3%) than offshore (3.6%), and the currents of CG first strokes are higher over sea, both for negative and positive strokes. A similar pattern but with lower values was reported by RS02 and RS05. Unlike those studies, a 10 kA threshold was applied here to CG flashes, which may explain the higher positive CG flash percentage they found. Mean values of negative and positive CG first stroke currents found over land are in good agreement with those reported by RS02 and RS05. RS02 found a median value of 23.8 kA over sea which is within the range observed in this study. 3.2. Comparison of precipitation and lightning The Rainfall–Lightning Ratio found in the studied region has a mean value of 38.9 103 m3/CG flash, with a wide range of variation between episodes that goes from 10.8 to 87.2 103 m3/CG flash. These results agree in magnitude with other case studies (see a summary in Kempf and Krider, 2003). The mean RLR value for the studied episodes is similar to that found over Florida
Table 4 Multiplicity and polarity flash characteristics over land and sea Over land Multiplicity Strokes per flash Single stroke flashes (%) Positive CG flash percentage (%) Negative CG first stroke current (kA) Positive CG first stroke current (kA)
Range 1–15 1.6–9.4 16.2–42.3 31.9–88.5
Iberian P.
Over sea
Mean
Mean
Range
2.6 48.6 4.3 24.9 40.8
2.5 51.0 7.9 25.4 42.3
1–12 2.1–7.7 26.6–56.9 43.9–95.6
NW Med. sea Mean
Mean
2.7 47.4 3.6 32.8 64.4
2.2 48.0 6.6 – –
Third column corresponds to the Iberian Pensinsula averages for the warm season of years 1992–2001 reported by Rivas Soriano et al. (2005) and last column corresponds to averages over the North Western Mediterranean Sea for the warm season of years 1992–1994 (Rivas Soriano and De Pablo, 2002).
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Fig. 3. Total number of CG flashes vs. rainfall volume in the nine selected episodes, showing land (solid symbols) and sea (open symbols) samples. Synoptic classes are: low sea-level pressure Gradient (square), Iberian Summer Thermal Low (diamond), and Frontal System (triangle). Note that only 7 samples over sea are plotted as all lightning was over land in episodes 2 and 4. The correlation coefficient of the linear fitting line is R2 = 0.558.
with a RLR of 43 103 m3/CG flash (Tapia et al., 1998), or the Southeastern United States where Buechler and Goodman (1990) found 38 103 m3/CG flash. In France, Soula and Chauzy (2001) reported 72 103 m3/CG flash over the Paris area and Seity et al. (2001), 68 103 m3/CG flash over the SW Atlantic coast; higher values than those found in the present study despite the proximity to those French areas. Comparing the rainfall volume and the total number of CG flashes, separating the land and sea areas (Fig. 3), a positive correlation has been found. The linear fit for
Fig. 4. Cloud-to-ground flash density (CG flash km− 2) for the studied episodes.
the 16 samples (9 over land, 7 over sea) has a correlation coefficient R2 = 0.560. As pointed out before, Seity et al. (2001) found differences between inland and offshore thunderstorms on the SW coast of France. In the present study a different pattern has been found. It can be observed in Fig. 4 that the areas with high CG flash density are larger over the sea, and local density maxima are offshore near the coastline. Higher densities over the sea are concentrated in the south west coastal region, decreasing eastward. Such a pattern is similar to that found in the same region by Rivas Soriano and De Pablo (2002). Regarding the land–sea contrast, the ratio found between precipitation over land and over sea (1.32) was slightly lower than that observed for the CG flashes over land and sea (1.43). Seity et al. (2001) found higher contrasts: 1.78 for rainfall and over 2 for CG flashes. 3.3. Atmospheric stability and RLR Atmospheric stability conditions of the analyzed episodes are summarized in Table 1. As expected, the values of the indices shown indicate conditions favourable for thunderstorm development for all episodes. Most of them present a Lifted Index (LI) under − 5 °C, which suggests high instability. Total Totals (TT) indices (all above 44 °C) suggest a moderate thunderstorm potential. From the point of view of CAPE values (1000 to 3200 J kg− 1), the environment varies from moderate to very unstable. Besides the qualitative information given by the indices, a quantitative relationship between them and the
Fig. 5. Rainfall–Lightning Ratio (RLR) vs. Convective Available Potential Energy (CAPE) for the nine selected episodes for each synoptic class: low sea-level pressure gradient (square), Iberian Summer Thermal Low (diamond), and Frontal System (triangle). Correlation coefficient of the linear fit is R2 = 0.187.
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Rainfall–Lightning Ratio has been examined. In the data set considered there is a decrease in the RLR as CAPE grows (Fig. 5), but the negative correlation is relatively poor (linear correlation coefficient R2 = 0.187). The rest of the indices presented in Table 1 show lower correlation coefficients with the RLR. Correlations are higher only if the most intense episodes (with a RLR below 40) are considered. For example the Total Totals and the Precipitable Water present a correlation with a linear fit with R2 N 0.340 in both cases. However, there is no improvement in the correlation of the RLR with the CAPE and the Lifted Index. 3.4. Differences according to synoptic situations A large database is required to obtain proper climatological descriptions of precipitation vs. lightning characteristics under different synoptic situations. Therefore, the differences highlighted in this section should be necessarily taken as representative only for the period examined and further data covering more years would be required to determine average climatological characteristics. The densities of lightning discharges in the three synoptic situations considered were examined by plotting maps with grid sizes of 0.1° × 0.1° (Fig. 6). In LPG and FS situations two areas with maximum CG flash densities (one inland and another over the sea) are found while ISTL situations exhibit a less defined pattern with many more local maximum areas. The common inland and offshore maxima are located around (42.0°N, 1.5°E) and (41.0°N, 1.0°E), respectively. The values found in those maxima range from 3.6 to 6.1 CG flashes km− 2 (inland) and 1.7 to 6.4 CG flashes km− 2 (offshore) depending on the synoptic class considered. Examining the typical evolution and trajectories of the studied thunderstorms (not shown) it may be concluded that the two maxima have different origins. The inland maximum is the result of existing storms coming from the west. The complex topography of the Iberian Peninsula, with high mountains such as the Iberic Range, combined with the proper meteorological conditions, favors the development of the thunderstorms that later, following the general westerly flow, reach the region of study (Martín, 1999). The offshore maximum is caused by cells formed approximately in-situ, where the coastal mountain ranges may act as a triggering effect of convection. In fact, the high degree of atmospheric instability found typically in summer events in the region does not require important orographic obstacles as starting mechanisms of convection, unlike in mid and late fall when they may play a crucial role (Pascual, 1999).
Fig. 6. Cloud-to-ground flash density (flash km− 2) for the three synoptic classes: (a) sea-level Low Pressure Gradient (LPG), (b) Iberian Summer Thermal Low (ISTL), (c) Frontal System (FS).
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Fig. 7. Hourly evolution of mean cloud-to-ground flash rate for each synoptic class: sea-level Low Pressure Gradient (LPG), Iberian Summer Thermal Low (ISTL) and Frontal System (FS).
Some differences were found between land and sea areas regarding other parameters. For example, Table 3 shows some characteristics of the lightning data where different patterns are observed on land and sea. In particular, the rainfall to lightning ratio (RLR) is very similar in the first synoptic class for land and sea areas but is clearly higher for sea areas than the other two. Similarly, the percentage of CG flashes with respect to the total flashes is higher for the three synoptic classes over land. Fig. 7 shows the hourly temporal evolution of the three different synoptic situations. All of them show a clear peak between 13 and 16 UTC, associated with inland summer afternoon convection. ISTL and Frontal System events also show a nocturnal peak of activity (around 23 UTC) which, according to previous studies (Terradelles, 1999), may be associated with thunderstorms over the sea. 4. Summary and conclusions This study analyzes the relationship between lightning and precipitation in nine convective events that occurred during the summer of 2004 in Catalonia (NE Spain) and its coastal area, in the NW Mediterranean Sea. The data examined were issued from C-band volumetric radar observations, radiosonde and total lightning records, including both cloud-to-ground and intra-cloud flashes. In the selected episodes, which were grouped according to the synoptic situation, more than 77,000 CG flashes were observed. This amount corresponds to almost 60% of the CG flashes registered during the whole year in the region.
Some parameters like the percentage of CG flashes, the CG flash polarity and multiplicity and the Rainfall– Lightning Ratio (RLR) were analyzed inland and offshore. Main results include the following: • Average percentages of CG with respect to total lightning range from 9.3 to 23.5%, the mean value for the nine episodes being higher over land (14.9%) than over sea (13.0%). • Atmospheric stability indices, though useful to indicate the probability of thunderstorms, do not allow diagnosing robustly the lightning activity, neither the RLR. This result might be attributed to the small size of the data set and to limitations of the radiosonde representativeness. • No substantial differences are observed among the CG spatial distributions according to different synoptic environments. However, there is a different pattern in the daily evolution of the CG flash rate for the synoptic groups considered. • Flash multiplicity shows no notable differences between land and sea domains. The mean positive CG percentage is higher inland than offshore. Otherwise, the peak currents of the CG flashes are higher over sea for both positive and negative CG flashes. Mean negative and positive CG peak currents are higher offshore than inland by 38% and 59%, respectively. Cloud-to-ground flash characteristics like multiplicity, polarity and CG first stroke peak currents do not show major differences with the results found in longer periods for the warm season in the Iberian Peninsula
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(Rivas Soriano et al., 2005) or the NW Mediterranean sea (Rivas Soriano and De Pablo, 2002). Comparing these results with those obtained by Seity et al. (2001) in SW France, the present study reports more lightning activity over the area of study. The influence of the Mediterranean Sea, providing a supply of moist and warm air, seems to be instrumental in producing more lightning activity in the region during the summer season. Concerning the land/sea comparison, the lightning distribution pattern found by Seity et al. (2001) with a 2.36 land/sea ratio in the CG flash number was not observed in the North Eastern Mediterranean region, where such a ratio has a value of 1.08. Thus, no remarkable differences are found between land and sea domains. The differences in the atmospheric conditions, largely influenced by the warm Mediterranean Sea, favored maximum thunderstorm activity offshore or near the coastline. The lower CG percentage found – that can indicate a higher severity – can also be explained by the warm sea influence in the area. According to the RLR values found, these results are more similar to those obtained in semitropical areas such as Florida than in oceanic mid latitude regions (West Atlantic French coast). Although the present analysis shows consistent results with previous studies, it has been performed with only nine episodes. A larger data set should be considered in order to have a better knowledge of local thunderstorm phenomenology and a better estimation of convective rainfall which could enhance weather surveillance tasks. References Backer, M.B., Blyth, A.M., Christian, H.J., Gadian, A.M., Latham, J., Miller, K., 1999. Relationships between lightning activity and various thundercloud parameters: satellite and modelling studies. Atmos. Res. 51, 221–236. Bech, J., Rigo, T., Pineda, N., Segalà, S., Vilaclara, E., SánchezDiezma, R., Sempere-Torres, D., 2005. Implementation of the EHIMI software package in the weather radar operational chain of the Catalan Meteorological Service. 32nd Conf. on Radar Meteorology, American Meteorological Society. Buechler, D.E., Goodman, S.J., 1990. Echo size and asymmetry: impact on NEXRAD storm identification. J. Appl. Meteorol. 29, 962–969. Carey, L.D., Rutledge, S.A., Petersen, W.A., 2003. The relationship between severe storm reports and cloud-to-ground lightning polarity in the contiguous United Sates from 1989 to 1998. Mon. Weather Rev. 131, 1211–1288. Coquillat, S., Chauzy, S., 1994. Computed conditions of corona emission from raindrops. J. Geophys. Res. 99 (D8), 16897–16906. Cummins, K.L., Murphy, M.J., Bardo, E.A., Hiscox, W.L., Pyle, R.B., Pifer, A.E., 1998. NLDN'95, a combined TOA/MDF technology upgrade of the US National Lightning Detection Network. J. Geophys. Res. 103, 9035–9044.
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