Relationship between cloud-to-ground lightning ashes over the Iberian Peninsula and sea surface temperature

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1 Q. J. R. Meteorol. Soc. (2002), 128, pp Relationship between cloud-to-ground lightning ashes over the Iberian Peninsula and sea surface temperature By FERNANDO DE PABLO and LUIS RIVAS SORIANO University of Salamanca, Spain (Received 29 September 2000; revised 22 May 2001) SUMMARY The correlation between cloud-to-ground (CG) lightning anomalies over the Iberian peninsula and sea surface temperature (SST) anomalies in its bordering seas has been calculated. The results disclosed a signi cant link between them. The warmer SSTs in the Atlantic north-west of the Iberian Peninsula and in the Cantabrian Sea are related to increased CG lightning activity over the Iberian Peninsula. The warmer SSTs in the Atlantic south of the Iberian Peninsula are related to increased CG lightning over the northern half of the Iberian Peninsula and to decreased CG lightning over the southern half. The Mediterranean Sea does not show signi cant links with CG lightning. The way these links work seems to involve changes in the advection of moisture from the Atlantic Ocean and the Cantabrian Sea as well as changes in static instability. It was also observed that there is no signi cant link between CG lightning anomalies and land surface temperature anomalies over the Iberian Peninsula. KEYWORDS: Atlantic Ocean Cantabrian Sea Cumulonimbus Mediterranean Sea Land surface temperature Sea-level pressure Thunderstorms 1. INTRODUCTION Lightning location systems have been installed in many countries and these have resulted in numerous research projects. Lightning is a manifestation of cumulonimbus cloud dynamics and microphysics and is, therefore, physically related to convective development. Lightning can be used to estimate convective rainfall (e.g. Petersen and Rutledge 1998), to forecast precipitation in convective systems (Blanchard and Lopez 1985), and to gain further insight into synoptic systems (Reap 1994) or low-level wind ows (Lopez and Holle 1987). There is increasing evidence that present-day trends in atmospheric trace-gas concentrations will result in global warming. One possible impact of this warming could be on the frequency and intensity of thunderstorms, and hence on lightning (Price and Rind 1994). Conversely, owing to the sensitivity of the global circulation to changes in surface air temperature, lightning could also be used as an indicator of changes in the global temperature (Williams 1992, 1994; Reeve and Toumi 1999). There are further interests in lightning activity: it plays a fundamental role as a cause of natural forest res in mid latitudes (e.g. Krider et al. 1980); it is a source of nitrogen oxide and, hence, of tropospheric ozone (Franzblau and Popp 1989; Toumi et al. 1996); and it is a major cause of weather-related deaths, property damage and power-system breakdowns. Meteorologists have long considered the surface properties of oceans and seas (in particular the sea surface temperature (SST)) as a prime candidate for use in the analysis of the in uence of air sea interactions on atmospheric variability in different geographical areas. In particular, SST is strongly related to instability. For instance, the convergence ow in the planetary boundary layer is driven by the SST gradient in tropical zones and, also, non-neutral strati cation may be partially determined by the distribution of SST (Numagati 1995). Despite the above, the possible relationship between lightning and SSTs has not yet been investigated. Reeve and Toumi (1999) suggested that SST is poorly correlated with lightning activity over the sea. However, it is over land where lightning mainly occurs Corresponding author: Department of Atmospheric Physics, Faculty of Sciences, University of Salamanca, Pl. Merced s/n, Salamanca (Spain). fpd123@gugu.usal.es c Royal Meteorological Society,

2 174 F. DE PABLO and L. RIVAS SORIANO and it is sensitive to deep convection (Mackerras et al. 1998). Reason and Mulenga (1999), for example, have found that SST is signi cantly linked to rainfall over land, and have suggested that the increase in rainfall arises from an enhancement of moisture advection driven by the SST. Lightning activity is mostly affected by the moisture content (Petersen et al. 1996; Rivas Soriano et al. 2001a), and this suggests that it might be possible to nd a relationship between SSTs and lightning over land. In this work we analyse this issue, focusing our attention on CG lightning over the Iberian Peninsula (IP) and the SSTs of its bordering seas. It should be noted that the effects of extratropical SST anomalies on the atmospheric circulation are less well known than the effects of tropical SST anomalies. The paper is organized as follows. In section 2, a description of the datasets used is given. The relationship between SSTs and CG lightning is presented and discussed in section 3. Section 4 contains a brief analysis of the relationship between CG lightning and land surface temperature (LST). Conclusions are given in section DATA The lightning detection system installed on the IP (belonging to the Instituto Nacional de Meteorologia) consists of a network of 13 sensors distributed throughout Spanish territory (Fig. 1). Since there are no sensors located over Portugal, one would expect a lower detection ef ciency over this country than in Spain. This lightning detection system uses the ALDF model 141-T, manufactured by LLP Inc., and is similar to those used in other countries. It has been described in depth by several authors (Krider and Noggle 1976; Krider et al. 1980; Orville et al. 1983; Lopez and Holle 1986). The system can only detect CG lightning ashes. Thus, intracloud ashes are not available even though they are more frequent than CG ashes. The ratio between the frequencies of both types of lightning ash may depend on the geographical latitude. Although Prentice and Mackerras (1977) have suggested a ratio of 3 for the mid latitudes, Mackerras and Darveniza (1994) indicated a lower value of 1.3. We have examined the total (both positive and negative) CG ash number per month over each area considered (the whole IP, the northern and southern halves, and the inland area). We only had lightning data available for three years ( ) and, hence, all data and results presented in this work are related to this three-year period. Lightning activity over the IP is only signi cant during the warmest months in the year, (de Pablo et al. 2000; Rivas et al. 2001b) and, therefore, we focused our attention from May to September. Since little or no CG lightning occurs over much of the IP from October to April (about 20% of total annual number), a small variation in the number of ashes in one month would cause a strong anomaly that would not be signi cant. Consequently, these months were excluded from our analysis. The SST and sea-level pressure (SLP) data were obtained from the NCEP /NCAR 40-year reanalysis project (Kalnay et al. 1996). The NCAR collaborated to create a very broad reanalysis using a frozen state-of-the-art global data assimilation system and a database as complete as possible. This system was designed with advanced quality controls and a monitoring system. The SST data used refer to the monthly mean of each grid point on an 1 ± 1 ± grid for the domain extending from 12 ± W to 4 ± E and 36 ± N to 46 ± N (Reynolds and Smith 1994). This gave a total of 608 data points of monthly National Centers for Environmental Prediction. National Centre for Atmospheric Research. This analytical information is available on the Internet at

3 CLOUD-TO-GROUND LIGHTNING FLASHES OVER THE IBERIAN PENINSULA 175 Figure 1. Locations of the lightning detection ALDF sensors, (indicated by stars), the main orographic features of the Iberian Peninsula, and the names of its borderings seas. TABLE 1. THE LIGHTNING FLASH COUNTS OVER THE ENTIRE IBERIAN PENINSULA AND THE MONTHLY ANOMALIES OF THE SEA SURFACE TEMPERATURE AT SELECTED POINTS, THE LONGITUDINALLY AVER- AGE SEA-LEVEL PRESSURE ALONG SELECTED LATITUDES AND THE LAND SURFACE TEMPERATURE AT SELECTED METEOROLOGICAL STATIONS OVER THE 15 MONTHS INVESTIGATED Sea surface Longitude-average Land surface temperature sea-level pressure temperature anomalies anomalies anomalies Lightning ( ± C) (hpa) ( ± C) Month ash count A B C 42.5 ± N 40 ± N 37.5 ± N S L T 1992 May : : :36 June :42 1:20 2:45 2:12 1:45 1:15 1:63 2:80 July :08 0:85 0: :52 August : :62 September May :37 1:32 1: : June : July : : : :07 August :47 0:20 0: : September : :23 2:53 1:80 0:99 0:35 0:90 0: May :21 0:42 0:08 1:41 0:74 0:36 1:74 0:63 2:61 June : July : :33 1:12 0: : August : :98 1:07 1: : September : :26 0:75 0: :55 The selected sea surface temperature points are A: (44.5 ± N, 4.5 ± W); B: (38.5 ± N, 9.5 ± W); C: (40.5 ± N, 2.5 ± E). The selected meteorological stations are S: Santander (43.4 ± N, 3.8 ± W); L: Lisboa (38.7 ± N, 9.1 ± W); T: Toledo (39.8 ± N, 4.1 ± W).

4 176 F. DE PABLO and L. RIVAS SORIANO averaged SSTs. The SLP data used in this study were monthly means on a 2:5 ± 2:5 ± grid from 10 ± W to 2.5 ± E and 37.5 ± N to 42.5 ± N. The LST was obtained using quality-controlled data from the Spanish and Portuguese Meteorological Services. We used the monthly mean LST of 60 meteorological stations, 45 in Spain and 15 in Portugal, in order to obtain the monthly (May to September) time series for each locality. The SST, LST, SLP, and lightning activity in mid latitudes show a similar seasonal cycle, with a maximum during the warm season and a minimum during the cool season (e.g. Peixoto and Oort 1992; von Storch and Zwiers 1998; Finke and Hauf 1996; Rivas et al. 2001b). To assess how variations in these parameters might be related, the seasonal cycle must be removed from the dataset. This was done using anomalies instead of the original dataset. Each monthly anomaly was calculated by subtracting the monthly mean averaged over the three years from the monthly mean of each month. Thus, we obtained sets of 15 data points for each time series of anomalies (the months May to September over the three years, ). The correlation coef cients between the anomalies were obtained by linear regression established using the sets of 15 points. The time series of anomalies for some selected points are shown in Table SST AND CG LIGHTNING ACTIVITY OVER THE IBERIAN PENINSULA The IP lies within the region of prevailing westerly winds (Linés Escardó 1977), and the storms, therefore, tend to follow west east tracks. Both the northern and southern halves of the IP show very different types of climatological and electrical behaviour. The northern area is more humid and displays more electrical activity than the southern one (Rivas Soriano et al. 2001b). These two areas were, therefore, considered separately to analyse the relationships between SSTs and CG lightning activity over the IP. Figure 2 shows the correlation between SST anomalies in the seas that surround the IP and CG lightning anomalies in the northern and southern halves of the IP, respectively (the boundary between the two halves was chosen arbitrarily). Both areas appear to show a signi cant link between increased CG lightning activity and warmer SSTs in both the Atlantic north-west of the IP and the Cantabrian Sea, although signi cant differences were found between the two halves of the IP. The signal is stronger, comes closer to lower latitudes, and extends more along the Cantabrian Sea for the northern half. Other signals are also shown in Fig. 2. Warmer Gulf of Cadiz SSTs are also linked to an increase of CG lightning activity over the northern half of the IP. By contrast, the warmer SSTs in the Atlantic to the south-west and in the Mediterranean to the south-east of the IP are linked to decreased CG lightning activity over the southern half of the IP. In general, the Mediterranean Sea SST does not seem to show a signi cant relationship to CG lightning activity over the IP. This latter notion is in apparent agreement with the prevailing west east storm tracks. Figure 3(a) shows the correlation between the SST anomalies in the seas surrounding the IP and CG lightning anomalies over the whole IP. The pattern is similar to that seen in the northern half. This result is a consequence of the large number of ashes counted over the northern half of the IP; about 80% of all CG lightning ashes in the IP were counted over the northern half from May to September during the years It could be speculated that a large part of the correlation between SST anomalies and CG lightning anomalies could be due to coastal effects, since it seems clear that the SST exerts a direct in uence on coastal weather. We calculated the correlation between SST anomalies in the water masses surrounding the IP and CG lightning anomalies over its inland areas. The limits of the inland area were chosen to include only the northern

5 CLOUD-TO-GROUND LIGHTNING FLASHES OVER THE IBERIAN PENINSULA 177 Figure 2. Correlation coef cients between sea surface temperature anomalies and cloud-to-ground lightning anomalies for (a) the northern half and (b) the southern half of the Iberian Peninsula. The area considered in each case is shaded. Longitudes west of the Greenwich meridian are negative. and southern plateaux, which are free of direct maritime in uences. The result, shown in Fig. 3(b), was similar to those found for the northern half of the IP and the whole of the IP. Correlation coef cients between SSTs in the Atlantic to the north-west of the IP and in the Cantabrian Sea decreased, but the signal still remained signi cant. Before considering possible causes of the relationships between SSTs and CG lightning activity, it would be useful to discuss brie y the major synoptic systems over the IP during the warmer months of the year. For much of the IP the most important system is the Azores anticyclone, which is sometimes modi ed by the presence of a weak thermal low over the IP and northern Africa, inducing a cyclonic curvature. Goodes and Palukitof (1998) suggested a frequency of occurrence of 67% for this system in summer. In this situation, the associated low-level ow tends to be northnorth-westerly over much of the IP, thereby advecting moisture from the Atlantic

6 178 F. DE PABLO and L. RIVAS SORIANO Figure 3. As Fig. 2 but for (a) the whole Iberian Peninsula and (b) its inland areas. north-west of the IP and from the Cantabrian Sea. The warmer SSTs in the Atlantic north-west of the IP and in the Cantabrian Sea lead to the air masses located over these waters having a higher moisture content. Moisture advection over the IP will, consequently, be enhanced. CG lightning activity is mostly affected by the moisture content at low levels (Petersen and Rutledge 1998; Rivas Soriano et al. 2001a). In this way, CG lightning activity over the IP may respond to SST anomalies in the Atlantic north-west of the IP and in the Cantabrian Sea. The correlation coef cients are lower for the southern half of the IP than for the northern half because the moisture content of the air masses coming from those sea areas decreases as they reach lower latitudes over land. Several papers have indicated that warm SST anomalies force positive SLP anomalies on the poleward side of the SST anomalies, and also negative SLP anomalies on

7 CLOUD-TO-GROUND LIGHTNING FLASHES OVER THE IBERIAN PENINSULA 179 Figure 4. Correlation coef cients between sea surface temperature anomalies and longitudinally averaged sealevel pressure anomalies over the Iberian Peninsula at (a) 42.5 ± N; (b) 40 ± N and (c) 37.5 ± N. The horizontal lines indicate the latitudes considered and longitudes west of the Greenwich meridian are negative.

8 180 F. DE PABLO and L. RIVAS SORIANO the equatorward side (e.g. Kushnir and Held 1996; Reason and Mulenga 1999). This suggests that warmer SSTs in the Atlantic south-west of the IP and in the Mediterranean Sea south-east of the IP will force an SLP increment, thereby reinforcing the Azores anticyclone over the IP. This would contribute to enhancing low-level north-westerly winds over the IP. Furthermore, higher SLPs over the IP will tend to increase static stability. If the increase in SLP is stronger over the southern half of the IP than over the northern half, the enhancement of north-westerly winds would be the prevailing effect over the northern half of the IP and this would, consequently, explain the positive correlation observed between CG lightning anomalies and SST anomalies found in the Atlantic south-west of the IP and in the Mediterranean Sea south-east of the IP. On considering the southern half of the IP, the increase in static stability would be the dominant effect and, hence, a negative correlation with CG lightning activity anomalies would appear. This explanation, based on the relationship between SLP anomalies and SST anomalies, needs to be checked using a model experiment. Unfortunately, such information was not available for this study. However, in order to estimate this effect we calculated the correlation between SST anomalies and SLP anomalies over the IP, instead of using a model. The results are shown in Fig. 4 for longitudinally averaged SLP at three latitudes: 37.5 ± N, 40 ± N and 42.5 ± N (because the SLP data used by us are available on a 2.5 ± resolution grid (see section 2), there are no data at other latitudes within the IP). Each longitudinally averaged SLP was calculated using data points every 2.5 ± longitude ( 220 km) within the IP. A positive correlation was found between SLP anomalies and SST anomalies, pointing to a link between warmer SSTs and increased SLP over the IP. The correlation is strong for the Gulf of Cadiz and decreases northward and eastward in all longitudinally averaged SLPs. This correlation is stronger for SLP at low latitudes than for SLP at high latitudes within the IP. The same results appeared when the SLP of each point was considered instead of averages (not shown). In conclusion, warmer SSTs in the surrounding seas of the IP, especially in the Gulf of Cadiz, increase the SLP in the IP. This increase is more pronounced in the southern half. These results seem to support the above explanation about the correlation between CG lightning over the IP and SSTs of southern IP seas. 4. LAND SURFACE TEMPERATURE AND CG LIGHTNING ACTIVITY OVER THE IBERIAN PENINSULA It could be proposed that the correlation between SST anomalies and CG lightning anomalies might be the result of a general relationship between lightning and temperature, for example due to solar heating, and not speci cally between lightning and SSTs. If this were true, a signi cant correlation between LST anomalies and CG lightning anomalies would also be found. To analyse this topic, we considered the monthly-mean LST of 60 points distributed across the IP (see section 2) and the number of CG lightning ashes per month, in blocks of 0.1 ± longitude 0.1 ± latitude surrounding each point. The correlation between LST anomalies and CG lightning anomalies is shown in Fig. 5. All the points show correlation coef cients lower than j0:5j and the average absolute value was 0.2. This poor correlation shows that, unlike SSTs, there is no signi cant link between LST and CG lightning activity over the IP. The result shown in Fig. 5 is not surprising because, as mentioned above, the moisture content plays a major role in CG lightning activity. In fact, CG lightning activity over land is related to wet-bulb temperature over land instead of dry-bulb temperature, as reported by Reeve and Toumi (1999).

9 CLOUD-TO-GROUND LIGHTNING FLASHES OVER THE IBERIAN PENINSULA 181 Figure 5. Correlation coef cients between land surface temperature anomalies and cloud-to-ground lighning anomalies over the Iberian Peninsula. Longitudes west of the Greenwich meridian are negative. 5. CONCLUSIONS This paper analyses the relationships between SSTs and CG lightning activity over the IP. In order to do so, the northern and southern halves of the IP were considered separately. Both areas show a signi cant link between increased CG lightning activity and warmer SSTs in the Atlantic north-west of the IP and in the Cantabrian Sea. This may be explained on the basis of the north-north-west low-level ow, which prevails over the IP during the warmest months of the year and is caused by the Azores anticyclone. Warmer SSTs will contribute air masses located over those waters having higher moisture content and, hence, the moisture advected over the IP will be enhanced, helping to increase its CG lightning activity. Similar results were found when the whole IP and its inland area were considered. This shows that the relationships between SSTs and CG lightning are not a coastal effect. To test whether the correlation between SSTs and CG lightning is an effect of some other process affecting them, the relationship between LST and CG lightning over the IP was also analysed. A poor correlation (0.2 on the average) was found between LSTs and CG lightning anomalies, showing that there is no signi cant link. Currently, it is not known whether these conclusions, obtained over the threeyear period ( ), are representative of the long-term relationships between CG lightning over the IP and SSTs in its bordering seas. Given the few data used (for only three years), the climatological properties arising from this study should be taken with caution. The analysis of data for additional years will be necessary for this purpose. Future work is planned to address these issues and to include data pertaining to several more years. It would also be relevant to conduct analyses like this in other zones across world to verify whether the relationships between SSTs and lightning are a common phenomenon or, in contrast, merely a peculiarity of the IP.

10 182 F. DE PABLO and L. RIVAS SORIANO ACKNOWLEDGEMENTS This research was partially supported by the regional Government of Castilla y Leon and the European Union, through Contract SA011/01. The authors thank the Iberdrola Electric Company for help in obtaining data. REFERENCES Blanchard, D. O. and Lopez, R. E Spatial patterns of convection in south Florida. Mon. Weather Rev., 113, De Pablo, F., Rivas Soriano, L. J., Garcia Diez, E. and Alvarez, E Cloud-to-ground lightning ashes over Castilla-Leon (Spain). In Proceedings of the second Assembleia Luso-Espanhola de Geodesia e Geo sica, February 2000, Lagos, Portugal Finke, U. and Hauf, T 1996 The characteristics of lightning occurrence in southern Germany. Beitr. Phys. Atmos., 69, Franzblau, E. and Popp, C. J Nitric oxides produced from lightning. J. Geophys. Res., 94, Goodess, C. M. and Palutikof, J. P Development of daily rainfall scenarios for southeast Spain using a circulation-type approach to downscaling. Int. J. Climatol., Kalnay, E., Kanamitsu, M., Kistler, R., Collins, W., Deaven, D., Gandin, L., Iredell, M., Saha, S., White, G., Woollen, J., Zhu, Y., Chelliah, M., Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K. C., Ropelewski, C., Wang, J., Leetmaa, A., Reynolds, R., Jenne, R. and Dennis, J. 18, The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteorol. Soc., 77, Krider, E. P. and Noggle, R. C A gated, wideband magnetic direction nder for lightning return strokes. J. Appl. Meteor., 15, Krider, E. P., Noggle, R. C., Pifer, A. E. and Vance, D. L Lightning direction- nding system for forest re detection. Bull. Amer. Meteorol. Soc., 61, Kushnir, Y. and Held, I. M Equilibrium atmospheric response to North Atlantic SST anomalies. J. Climate, 9, Linés Escardó, A 1977 Climates of northern and western Europe. In World survey of climatology, 5, Ed. C. C. Wallen. World Meteorological Organization, Geneva, Switzerland Lopez, R. E. and Holle, R. L Diurnal and spatial variability of lightning activity in northeastern Colorado and central Florida during the summer. Mon. Weather Rev., 114, The distribution of summertime lightning as a function of lowlevel wind ow in central Florida. NOAA Tech. Memo ERL ESG-28. (Available from the National Severe Storms Laboratory, 1313 Halley Circle, Norman OK 73069, USA) Mackerras, D. and Darveniza, M Latitudinal variation of lightning occurrence characteristics. J. Geophys. Res., 99, Mackerras, D., Darveniza, M., Orville, R. E., Williams, E. R. and Goodman, S. J Global lightning: Total, cloud and ground ash estimates J. Geophy. Res., 109, Numaguti, A Dynamics and energy balance of Hadley circulation and the tropical precipitation zones. Part II: Sensitivity to meridional SST distribution. J. Atmos. Sci., 52, Orville, R. E., Henderson, R. W. and Bosart, L. F An east coast lightning detection network. Bull. Amer. Meteorol. Soc., 64, Peixoto, J. P. and Oort, A. H Physics of climate. American Institute of Physics, New York Petersen W. A. and Rutledge, S. A On the relationship between cloud-to-ground lightning and convective rainfall. J. Geophy Res., 103, Petersen W. A., Rutledge, S. A. and Orville, R. E Cloud-to-ground lightning observations from TOGA COARE: Selected results and lightning location algorithms. Mon. Weather Rev., 124, Prentice, S. A. and Mackerras, D The ratio of cloud to cloud ground lightning ashes in thunderstorms J. Appl. Meteorol., 16,

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