TOTAL LIGHTNING ACTIVITY AND ELECTROSTATIC FIELD IN A HAIL-BEARING THUNDERSTORM IN CATALONIA

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1 th International Lightning Detection Conference April Tucson, Arizona, USA 1st International Lightning Meteorology Conference April Tucson, Arizona, USA TOTAL LIGHTNING ACTIVITY AND ELECTROSTATIC FIELD IN A HAIL-BEARING THUNDERSTORM IN CATALONIA J. Montanyà 1, N. Pineda 2, S. Soula 3 and V. March 1 1 Technological University of Catalonia. Terrassa (Barcelona), Spain. 2 Catalan Meteorological Service (Meteocat). Barcelona, Spain. 3 Laboratoire d Aérologie, UMR 5560 UPS/CNRS, OMP, Toulouse, France Abstract: A thunderstorm occurred in North-Eastern Spain on 10 th July, 2005, is analyzed. This storm produced hailstones up to 15 mm. The total lightning activity was monitored by a regional VHF interferometer network (SAFIR system) and the field mill network (PREVISTORM) of the city of Terrassa. The cell was not strong vertically developed, it reached a maximum altitude up to 10.5 km and had a duration of 3h48. The storm presented low cloud-to-ground rates (lower than 2 min -1 ) during its mature phase while Intra- Cloud (IC) flash rate reached 20.1 min -1. The ratio IC/CG was very high: 57 of maximum and 22 average. Peak currents for negative CG averaged ka (median value 9.35 ka). Not positive flashes were produced. 1. Introduction Several works investigated the relationships between lightning activity and microphysics or dynamics in convective storms. For most of thunderstorms following a typical behavior, the lightning activity starts with intracloud (IC) flashes after the ice phase develops in the cloud (Williams et al., 1989; Carey and Rutledge, 1996). The cloud-to-ground (CG) flashes occur when the main core of the convective cell descends to lower altitudes and their rate peak after that of IC flashes, when the rainfall and the dynamical phenomena are stronger at the ground (Goodman et al., 1988; Williams et al., 1989 ; MacGorman et al., 1989). For this kind of storm, the total lightning flash rate does not exceed generally 10 min-1 and the CG flash rate averaging around 2 min-1 can reach this value of 10 min-1 at a given moment of the storm lifetime (Williams et al., 1989; Carey and Rutledge, 1996). In this case the CG flashes are largely dominated by negative ones. In severe storms, lightning flash rate can be huge, with values greater than 30 min-1, with low CG flash proportions and sometimes no CG at all during several-minute periods (MacGorman et al., 1989 ; Williams et al., 1999 ; Lang et al., 2000 ; Williams, 2001). However, as indicated by Williams (2001) for example, a large variability is observed in the relationships between lightning activity and severe weather characteristics, especially tornadoes and large hail (diameter higher than 19 mm).

Few studies considered the total lightning activity (IC+CG). Recent studies show a tendency for total lightning activity to lead severe weather on the ground (Williams et al.

(1999) showed that an abrupt increase in the total flash rate systematically preceded the severe weather at the ground by 5 to 20 minutes.

2 Few studies considered the total lightning activity (IC+CG). Recent studies show a tendency for total lightning activity to lead severe weather on the ground (Williams et al., 1999) but high flash rates (> 30 min-1) are not guarantee of severe weather (Williams, 2001). A study of 32 severe storms cases by Williams et al. (1999) showed that an abrupt increase in the total flash rate systematically preceded the severe weather at the ground by 5 to 20 minutes. The total lightning activity and the radar-inferred precipitation structure of a severe hailstorm were investigated by Carey and Rutledge (1998). This thunderstorm exhibited a very high IC-to-CG ratio after it became severe and produced large hail and weak tornadoes. The initial production of hail aloft corresponded very well with a rapid increase of the ratio IC/CG. In the study by Lang et al. (2000) devoted to two long-lived storms of STERAO-A, low negative CG flash rates (< 1 min -1 ) and relatively high IC flash rates (> 30 min -1 ) were observed, and even long periods without any CG flashes (from 10 to 30 min) occurred. This paper presents a detailed study of a thunderstorm that occurred on July 10th of 2005 over Catalonia (North-Eastern of Spain) which was a hail-bearing cell (Figure 1). The studied cell occurred over the city of Terrassa and was stationary during the initial and mature phase where hail was present at ground. The Catalan region, and specifically the city of Terrassa is covered by the Catalan Meteorological Service (Meteocat) total lightning detection network (SAFIR system), a field mill network (PREVISTORM) and three volumetric C-band radars. Thus, the observations made can allow a comparison with other works and a discussion about the mechanisms proposed to explain some characteristics of the lightning activity of hailstorms. a) b) Figure 1. a) Location of the studied cell and b) detail of reported hailstones.

3 2. Data Intra-cloud (IC) VHF sources and Cloud-to-ground (CG) lightning flash characteristics (time, location, polarity, peak current and multiplicity) were obtained the Catalan Lightning Location Network (XDDE) operated by the Catalan Meteorological Service (Montanyà and March, 2004). This network covers the Catalan region (North-Eastern of Spain) and is composed by three VHF interferometers (Richard and Lojou, 1996) SAFIR type. The expected flash detection efficiency for the XDDE is 93 % with maximum location accuracy of 0.5 km (Montanyà and March, 2004). Three C-band radars in the region of study provided volumetric reflectivity. These radars operate at long (240 km) and short (130 km) ranges, which allow Doppler filtering. Every 6 minutes, at short range, is made a series of 14 scans with elevations sweeping of 0.6º. More details of the radar network are given in Bech et al. [2004]. 3. Meteorology Surface high pressures (1020 hpa) on 10th July 2005 at 12 UTC over Catalonia are consequence of a wedge, which generates northerly winds at the northeast and mistral at the Ebro valley. Thus, these strong winds at the north and south coasts had generate a convergence in the central coast, with an intake of humidity from the sea to the studied region where thunderstorms occurred. Surface temperatures in the region reached a maximum of 28ºC.At 500 hpa, a trough was affecting Catalonia. The study area of Terrassa was situated, at 12:00 UTC, at the frontal part of the trough, where the activity is higher. The presence of this trough brought cold air intakes, reaching the -16ºC at 500 hpa, quite cold temperatures at July for the region. Such cold temperatures, in combination with the warm surface temperatures are conditions may favour convection. Rawinsonde data can be useful to confirm the favourable conditions for convection in the area. From the Barcelona (20 km from Terrassa) Meteocat rawinsonde from 10th July at 12:00 UT, some thunderstorm index can be established, that are represented in Table X. Table 1. Rawinsonde parameters from the 10th July :00 UT Barcelona rawinsonde Parameter Weak Moderate Strong LI (Lifted Index) -3.7 TT (Total Totals Index) 49.2 CAPE Total 843 KO Index 857 WBZ (WetBulb Zero Height (feet) 8829 Hail (inches) 0.01 The Lifted Index (LI) is a measure of atmospheric stability, giving a thunderstorm potential. From -2 ºC to - 5 ºC moderate unstable, and below -5 ºC is very unstable. The Total Totals index (TT), is commonly used as a severe weather indicator. The thunderstorm potential becomes moderate above 45 ºC and strong above 55 ºC.

4 The Convective Available Potential Energy (CAPE) represents the amount of buoyant energy available to accelerate a parcel vertically, and is an indicator of instability. A CAPE below 0 J/Kg indicates stability. From 0 J/Kg to 1000 J/Kg corresponds to marginally unstable, from 1000 J/Kg to 2500 J/Kg moderate unstable and above 2500 J/Kg very unstable. However, the thresholds of the previous indices have been generally obtained for USA areas. Previous studies (p.e. Sairouni et al., 2000) performed in the Western Mediterranean show that those thresholds can differ substantially in the analyzed region. For instance, CAPE values closed to 1000 J/Kg have been obtained for some severe events. The KO Index was designed by the German Weather Service to account for low and mid-level potential instability. KO is used to estimate thunderstorm potential in Europe, and becomes moderate below 6 and strong below 2. The storm-relative helicity (srh) measures the potential rotation that can be performed by a storm moving through a vertically sheared environment (Davies-Jones et al. 1990). The srh is used to forecast severe with intense lightning activity. A threshold of 60 indicates the possibility of storm development. The KO Index (Andersson et al., 1989) was designed by the German Weather Service to account for low and midlevel potential instability by comparing the values at low (1000 to 850 hpa) and mid (700 to 500 hpa) levels. KO is used to estimate thunderstorm potential in Europe, and becomes moderate below 6 and strong below 2. Only case B presented low thunderstorm potential for this parameter. The WetBulb Zero Height (WBZ) is that level on the sounding whereby the lowest temperature attainable is zero degrees ºC. In general, WBZ heights from 5000 to ft AGL are associated with hal at the ground. The last index on the Table X, hail, estimates the diameter of the hail. The hail estimated by the rawinsonde is quite lower than the observed this day (see figure 1b) Finally, from the rawinsonde data, for the 10 th july 2005 the estimated height of the -10ºC isotherm (-10ºC H) is 4880 meters. Such isotherm is considered a rough lower boundary for the central dipole region of the cloud (Williams et al. 1989). Another source of meteorological data is the Meteocat Automatic Weather Stations Network. From the Sabadell AWS, values of relative humidity, temperature and precipitation are represented in figure X. It cab be seen that the temperature in the thunderstorm area has lowered almost 8ºC in less than 4 hours, as the relative humidity has increased a 40% in the same 4 hours period. Rain gauge intensity register is around the 50mm/hour.

5 :00 10:30 11:00 11:30 12:00 12:30 13:00 13:30 14:00 14:30 15:00 15:30 16:00 16:30 17:00 17:30 18:00 18:30 19:00 RH (%) Pcp Rel.Humidity Temp Precipitation (mm), Temperature ºC 14 Time UT (hh:mm) Figure 2. Relative Humidity, Temperature and precipitation at a 30 min. intervals registered by a Meteocat Automatic Weather Station (Sabadell AWS, nearby Terrassa) that was affected by the 10 th July thunderstorm episode. 3. Analysis method In the analysis, the cell was primarily identified from radar maximum reflectivity plots and total lightning activity images. The radar image frequency of 6 minutes allows describing correctly the lifetime of these cells. The different reflectivity displays used are the maximum reflectivity horizontal distribution and vertical scans reconstituted from the scanning at different elevations performed during the 6-minute period. For the lightning activity, the locations of the strokes associated with the CG flashes are superimposed in the radar images, by distinguishing the positive and negative ones. One of the interesting characteristics in hail-thunderstorms is the presence of positive CG activity. As indicated by many authors, positive CG flashes detected by the DF sensors can be misidentified in-cloud lightning flashes (Cummins et al., 1998 ; Wacker and Orville, 1999a,b ; Orville and Huffines, 2001 ; Carey et al., 2003). In order to have an accurate study, several positive flashes reported by the LF network were eliminated from the original dataset. The criterion to validate a positive CG flash was based on a threshold minimum for the peak current value fixed to 10 ka. For the representation of total lightning issued from the interferometer, all of VHF sources related to each flash are fitted into an ellipse. The large axis of the ellipse is oriented according to the main direction of the cluster of VHF sources included in the flash. Large and small axes of the ellipse are obtained by a least square method in a linear regression where the ellipse contains the maximum number of flash detections with a minimum size. In order to characterize the lightning activity, several rates are calculated with a 6-minute resolution corresponding with that of the radar images. So, the CG flash rate and the stroke rate for both polarities, the IC flash rate including the isolated VHF sources are calculated with this time resolution.

6 Although several hailstones were in-situ observed, a criterion has been adopted for the probability of hail (POH). It is based on radar observations and consists in exceeding a radar reflectivity value of 55 dbz. This criterion agrees with Mason (1971) and Auer (1994). In order to analyze high reflectivity, the maximum reflectivity of all radar elevation sweeps for each cycle is adopted. Then, cumulative areas where the reflectivity exceeds 55 dbz and 40 dbz (A(55) and A(40), respectively) are calculated. While A(55) corresponds to an indicator of probability of hail, A(40) is chosen arbitrarily to represent the convective cloud size. Both cumulative areas are related in order to obtain the percentage of fractional areas Af(55): A f (55) = A(55) A(40) (1) 4. Cell overview This cell occurred at 1248 UT and lived until 1636 UT. With such lifetime and its hail production this cell approached to be classified as a supercell. However this cell was not strongly vertical developed. The dbz echo tops reached an altitude up to 10.5 km while the dbz echo tops reached an altitude of 9 km. This cell produced 114 CG flashes; 0 +CG flashes and 1031 IC flashes including 1-VHF source events. Figure 2 displays time series of negative flash rates, IC flash rate, maximum multiplicity in negative CG flashes, the fractional area A f (55) and the quasi-stratic electric field at ground level. Figure 3. Negative CG rate (CG/min), total IC rate (IC/min), fractional 60 dbz aera (%Af(60)), maximum negative CG flash multiplicity and electric field (kv/m in potential gradient).

The cell exhibited reflectivity in the range of 55-60 dbz for 56 minutes. The ratio of A f (55) reached a maximum value of 15.

7 The cell exhibited reflectivity in the range of dbz for 56 minutes. The ratio of A f (55) reached a maximum value of 15.7 % at 1442 UT but it kept high (over 5 %) during the most part of the cell lifetime. Radar reflectivity cross-sections shows a core of reflectivity up to 60 dbz at 4.8 km of altitude at 1406 UT and 1435 UT; and at 6 km at 1442 UT. Then hail appeared at ground level between 1500 UT and 1520 UT. During the presence of high reflectivity CG rate had a maximum of 1.5 min -1. The maximum IC rate was 20.1 min-1 at 1406 UT when appeared a 60 dbz core for first time. After the apparition of this core IC maximum rate decreased to 14.6 min -1 in a progressively decreasing tendency. Peak currents for negative CG averaged ka (median value of 9.35 ka). After a second period of reflectivity up to 60 dbz at 1530 UT, peak currents, multiplicity and CG rate peaked. From 1500 UT and 1520 UT hailstones where observed at ground level. For this period of time, CG activity decreased practically to zero. The electrostatic field showed an inversion of field just before the presence of hail at ground level. In terms of potential gradient, it changed from negative field (negative charge above) to positive (positive charge above) during the presence of hail. Figure 3. Radar reflectivity and cross-sections (defined by the discontinuous line and arrows) at 1406 UT. Each IC is represented by an ellipse; 1-VHF IC source ( singletons ) is represented by small black square; white squares are negative CG stroke locations.

8 Figure 4. Radar reflectivity and cross-sections (defined by the discontinuous line and arrows) at 1506 UT. Each IC is represented by an ellipse; 1-VHF IC source is represented by small black square; white squares are negative CG stroke locations. 5. Discussion The studied cell exhibited low CG rate during their whole lifetime. CG remained lower than 2 min -1 during more than 2h42 and exceeded this value in the last 40 minutes of its lifetime. Several cases of severe storms characterized by hail, displayed low values of CG flash rates during the whole lifetime (Carey and Rutledge, 1998 ; Lang et al., 2000 ; Soula et al., 2004). In all cases of these hailstorms, the CG flash rate was lower than 2 or 3 min -1. In term of CG lightning activity, the cells considered in this study confirm some observations made about hailstorms by other authors. The average multiplicity for CG flashes was 2.07 while peak currents averaged ka (median 9.35 ka). Rivas Soriano et al. (2005) found an average multiplicity of 2 and an average peak current of 27.3 ka (median 23.5 ka) for CG flashes in the analysis of ten years of CG flashes in the Iberian Peninsula. By analyzing 21 typical stormy days in the south-western part of France, Seity et al. (2001) found average values of 2.39 (over sea) and 2.52 (over land) for the multiplicity, and 32.1 ka (over sea) and 26.9 ka (over land) for the peak current. The average peak current calculated in this study is therefore low. A large proportion of IC flashes were therefore short events, with only one source, i.e. "singletons" according to Williams et al. (1999). In this previous study of severe storms in Florida, the proportion of singletons depended on the distance of the storm and could reach about 30 at large distances. In the present study, this proportion could be in a maximum of 57 % and an average of 22 %. At 1406 UT, when updrafts would be strong, IC locations are mainly on the higher reflectivity region but avoiding the highest reflectivity region. In a different way, at 1506 UT, when hail descended to ground IC locations drastically avoids the high reflectivity regions.

9 6. Conclusion The analysis of the total lightning activity in a hail-bearing thunderstorm is presented. The most important characteristics are summarized as follows: CG flash rates lower than 2 min -1 during their mature phase. This is a common characteristic in some severe storms characterized by hail. The cell presented high IC rate of 20.1 min -1 and high proportion IC/CG of 25. These values are typical for severe storms but differences in the detection techniques and network capabilities make comparisons difficult The cell ended its lifetime with an increase of the CG flash rate and a decrease of IC flash rate. The behavior observed of high IC rates with low CG rates is consistent with the elevated dipole theory. But still this theory could not explain high IC rates during the mature phase. The large amount of singletons VHF sources suggests that the charged region resides in higher regions where flash are easily triggered due to a lower electric field threshold. References Auer, A. H. Jr., Hail recognition through the combined use of radar reflectivity and cloud-top temperatures, Mon. Wea. Rev. 122, , Bech, J., Vilaclara, E., Pineda, N., Rigo, T., López, J., O'Hora, F., Lorente, J., Sempere, D., Fàbregas, F. X., The weather radar network of the Catalan Meteorological Service description and applications, in Proceedings of the European Conference on Radar in Meteorology and Hydrology (ERAD) - COST 717 Final Seminar, ERAD Publication Series Vol 2. Copernicus GmbH (c) 2004 ISBN , pp , Carey, L. D., and S. A. Rutledge, A Multiparameter Radar Case Study of the Microphysical and Kinematic Evolution of a Lightning Producing Storm, Meteorol. Atmos. Phys., 59, 33-64, Carey, L.D., and S.A. Rutledge, Electrical and multiparameter radar observations of a severe hailstorm, J. Geophy. Res., 103, NO D12, 13,979-14,000, Carey, L. D., S. A. Rutledge, and W. A. Petersen, The relationship between severe storm reports and cloud-to-ground lightning polarity in the contiguous United Sates from 1989 to 1998, Mon. Wea. Rev., 131, , 2003.

10 Cummins, K.L., M.J. Murphy, E.A. Bardo, W.L. Hiscox, R.B. Pyle, and A.E. Pifer, NLDN 95, A combined TOA/MDF technology upgrade of the US National Lightning Detection Network, J. Geophys. Res., 103, 9,035-9,044, Goodman, S. J., D. E. Buechler, P. D. Wright, and W. D. Rust, Lightning and precipitation history of a microburst-producing storm, Geophys. Res. Lett., 15, , Lang, T.J., S.A. Rutledge, J.E. Dye, M. Venticinque, P. Laroche, and E. Defer, Anomalously low negative cloud-to-ground lightning flash rates in intense convective storms observed during STERAO-A, Mon. Wea. Rev., 128, , MacGorman, D. R., D.W. Burgess, V. Mazur, W.D. Rust, W.L. Taylor, and B.C. Johnson, Lightning rates relative to tornadic storm evolution on 22 May 1981, J. Atmos. Sci., 46, , Mason, B. J., The Physics of Clouds, Clarendon Press, Oxford, UK, Montanyà, J and V. March, Electromagnetic fields measurement campaign 2004, in Proceedings of the European Lightning Detection Workshop 2005, Inst. Tecnológico de Canarias, Tenerife, Spain, Orville, R. E., and G. R. Huffines, Cloud-to-ground lightning in the United States: NLDN results in the first decade, , Mon. Wea. Rev., 129, , Richard, P., and J.Y. Lojou, 1996, Assessment of application of storm cell electrical activity monitoring to intense precipitation forecast, In Proceedings of the 10 th International Conference on Atmospheric Electricity, June 10-14, Osaka, Japan, pp Rivas Soriano L., F. de Pablo, and C. Tomas, Ten-year study of cloud-to-ground lightning activity in the Iberian Peninsula, J. of Atmos. and solar-terrestrial Phys., 67, (16), , Sairouni A., J. Aymamí, J. Vidal, B. Codina and A. Redaño, Hacia un nuevo índice de estabilidad en el Mediterráneo Occidental: primeros resultados, II Spanish-Portuguese General Assembly, pp , Seity, Y., S. Soula, and H. Sauvageot, Lightning and precipitation activities in coastal thunderstorms, J. Geophys. Res., 106(D19), 22, , Soula, S., Y. Seity, L. Feral, and H. Sauvageot, Cloud-to-ground lightning activity in hail-bearing storms, J. Geophys. Res., 109, No. D2, D02101, /2003JD003669, Wacker, R. S., R. E. Orville, Changes in measured lightning flash count and return stroke peak current after the 1994 U.S. National Lightning Detection Network upgrade: 1. Observations, J. Geophys. Res., 104, , 1999a. Wacker, R. S., R. E. Orville, Changes in measured lightning flash count and return stroke peak current after the 1994 U.S. National Lightning Detection Network upgrade: 2. Theory, J. Geophys. Res., 104, , 1999b.

11 Williams, E.R., The electrification of severe storms, Meteorological Monographs, Vol. 28, N 50, , Williams, E.R., M. E. Weber, and R. E. Orville, The relationship between lightning type and convective state of thunderclouds. J. Geophys. Res., 94, , Williams, E., B. Boldi, A. Matlin, M. Weber et al., The behavior of total lightning activity in severe Florida thunderstorms, Atmos. Res., 51, , 1999.

A study of the total lightning activity in two hailstorms

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi:10.1029/2006jd007203, 2007 A study of the total lightning activity in two hailstorms Joan Montanyà, 1 Serge Soula, 2 and Nicolau Pineda 3 Received 15 February