Circulation weather types
The synoptic meteorological analysis resulted in the categorization of each day to one of 10 PXE circulation weather types over Europe and the Eastern Mediterranean with regard to patterns of sea-level pressure (SLP), relative vorticity at 500 hPa, geopotential height at 500 hPa, and thickness of the layer between the 850 and 500 hPa isobaric surfaces, which are shown in Fig. 1 and described below. The seasonal frequency of these CTs and the mean values of the parameters involved in the PXE CT computation are shown in Fig. 2. Most CTs appear about 10% of the time each, except for CT10 which appears 6.9% of the time and CT9 and CT6 which appear for 12.5% and 15.7% of the time, respectively (see also Fig. 2).
CT1
This type is associated with a deep low over N Europe and results in W-SW surface flow over the site. This type of weather is encountered predominantly during winter but also during spring. Lightning activity over continental Europe, the Balkans, and Eastern Mediterranean is low (Fig. 7).
CT2
This type is associated with a high over the Balkans and results in NW flow over the site. Its seasonal frequency is the same as for CT1. Lightning activity over continental Europe, the Balkans, and Eastern Mediterranean is low (Fig. 7).
CT3
This type is associated with a high over Russia and a low over Italy, resulting in S surface flow over the site. This type of weather is encountered predominantly during winter but also during autumn and spring. The surface flow is such that it can result in Saharan dust transported to the site. It is known that intense Saharan dust outbreaks over Greece are associated with a low located in Central Europe with the south periphery of the low at Libya (e.g., Michaelides et al. 1999; Kaskaoutis et al. 2008). This is associated with an extended trough at 500 hPa geopotential height centered in Italy, which results in southwestern circulation over Greece (Kaskaoutis et al. 2008). This is the situation with CT3 (Fig. 1a, c). Additionally, CT3 is associated with high AOD over the Xanthi site as evident by mean MODIS AOD per CT 2011–2017 (figure not shown).
CT4
This type is associated with a relatively shallow low over most of continental Europe, resulting in W-WSW surface flow over the site. Compared with CT1, the center of high vorticity and the centers of shallow 500 hPa isobaric heights and shallow thickness of the 850 and 500 hPa isobaric surfaces are moved to the East, from N Europe to Russia. This type of weather is encountered predominantly during winter and spring but also during autumn.
CT5
This type is associated with lows over the Middle East and N Atlantic and high over S Russia and is encountered predominantly during autumn but also during the other seasons. The height of the 500 hPa isobaric and the thickness of the 850 and 500 hPa isobaric surfaces decrease gradually from N Africa to N Europe. Vorticity has negative values over most of Europe.
CT6
This type is associated with a high over NE Europe and a low over middle East, resulting in NE surface flow over the site. This type of weather is encountered predominantly during summer, and the resulting synoptic flow in the Aegean basin is called Etesians (e.g., Aristotle (330 BC); Repapis et al. 1978, Tyrlis and Lelieveld 2013, Anagnostopoulou et al. 2014). This is the most frequent CT, occurring about 15% of the time. Lightning activity over continental Europe, the Balkans, and Eastern Mediterranean is high (Fig. 7).
CT7
This type of weather is encountered during summer and autumn but also during spring. The height of the 500 hPa isobaric and the thickness of the 850 and 500 hPa isobaric surfaces decrease gradually from N Africa to N Europe. There is a NW to SE high to low vorticity gradient, the vorticity changing sign over the Balkans. Lightning activity over continental Europe, the Balkans, and Eastern Mediterranean is high (Fig. 7).
CT8
This type of weather is encountered during summer and autumn but also during spring. The height of the 500 hPa isobaric and the thickness of the 850 and 500 hPa isobaric surfaces decrease gradually from N Africa to N Europe. Lightning activity over continental Europe, the Balkans, and Eastern Mediterranean is high (Fig. 7).
CT9
For 80% of the cases, this type of weather is encountered during spring and summer. The height of the 500 hPa isobaric and the thickness of the 850 and 500 hPa isobaric surfaces decrease gradually from N Africa to N Europe. Vorticity has high positive values over the Balkans, with the center of the high vorticity area located over Greece. Surface flow is eastern. Lightning activity over continental Europe, the Balkans, and Eastern Mediterranean is high (Fig. 7).
CT10
This type is associated with a high over W Europe and a shallow low over Eastern Mediterranean, resulting in E-NE surface flow over the site. This type of weather is encountered about 7% of the time, predominantly during winter but also during spring. It is associated with high positive vorticity at 500 hPa, a low 500 hPa isobaric surface, and a thin 850–500 hPa thickness as well as steep gradients of the latter two from N to S. The high positive vorticity over Greece, the Balkans, and Eastern Mediterranean (Fig. 1) means that there will be considerable updraft during CT10. However, temperature and humidity (absolute and relative) are low at Xanthi (Fig. 3), and hence, thundercloud formation over the site will be limited.
The most frequent weather type over Europe and the Eastern Mediterranean is CT6. Weather types 1–4 and 10 occur predominantly in winter, CT5 in autumn and winter, while CTs 6–9 occur mostly in spring, summer, and autumn. The former are associated to lower absolute humidity (Fig. 3).
Statistics on the occurrence of the 10 CT classes are given in Fig. 2 which presents the mean values over Xanthi of the parameters per CT involved in the PXE CT computation. It is found that the height of the 500 hPa isobaric and the thickness of the 800–500 hPa layer are higher during CTs 6–9 (are mainly summer, autumn, and spring cases) and lower during winter cases. The vorticity seems to increase for CTs 8–10 when surface pressure is almost stable.
Meteorological variables
The means of the daily values of meteorological parameters 2011–2017 during each CT are shown in Fig. 3. The mean temperature, dew point, and humidity, absolute and specific, of each CT correlate very well with the mean thickness of the 850–500 hPa layer over Xanthi for this CT (r2 = 0.68, 0.87, 0.86, and 0.83, respectively). The mean wind velocity exhibits some moderate correlation with mean vorticity over Xanthi at 500 hPa (r2 = 0.34). The correlation holds also for the time series of the respective variables, although with lower r2. Considerably more rain is associated with CT10 than with any other CT.
Potential gradient
The mean daily values per CT of all-weather (AW) PG 2011–2017 and the corresponding daily mean standard deviation, the latter calculated from the hourly mean AW PG values for each day, as well as the mean daily values of fair-weather (FW) PG 2011–2017, and the corresponding daily mean standard deviation calculated from the hourly mean FW PG values for each day, are shown in Fig. 4. We also split the values of hourly PG in bands of 8000, 4000, 2000, and 1000 V/m, both positive and negative, which are also shown in Fig. 4. If we split our measuring period in two, the observed changes in mean PG per CT still hold (Fig. 4).
The mean FW PG per CT anticorrelates strongly with the mean height of the 500 hPa isobaric (r2 = 0.74) per CT. These anticorrelations hold also for the respective time series, although with a much lower r2 (r2 = 0.13).
The highest positive excursions of hourly PG above + 1000 V/m and even + 8000 V/m occur for CT3 followed by CT9. CT10 also is associated with high positive/negative PG excursions (is a mainly winter CWT characterized by strong advection and E-NE flow in the surface). This possibly means transportation of particles from the neighboring countries. Finally, the most frequent regime, CWT 6, is associated with smaller PG excursions, mainly negative, indicating a healthier impact to the people. CT3 can result in Saharan dust transport over the site, and Saharan dust is known to be charged (Ulanowski et al. 2007; Nicoll et al. 2011; Yair et al. 2016). Yair et al. (2016) also observed that during a dust storm in the Negev desert, PG values fluctuated between + 1000 and + 8000 V/m. Given the human health significance of desert dust in many countries and the increased efficiency of deposition to the lungs of charged particles (e.g., Bailey 1997) as well as that surface charge is a key factor influencing lung inflammation (Kim et al. 2016), this observation bears also bioclimatological significance.
As abrupt changes in weather parameters are known to influence health and well-being, we examined the changes in the mean daily values of all-weather and fair-weather PG when the CT changes from one type to another (Fig. 4, lower panels). As can be seen, the most frequent daily changes are from one weather type to the same (diagonal in the figures), and these are not associated with large changes in the PG. However, changes from one weather type to some other do occur, and some of these are associated with large changes in the PG (either in the positive or in the negative direction). Furthermore, CTs are found to also influence the daily pattern of PG (Fig. 5).
Schumann resonances
SR frequencies (Fig. 6) of the first and second modes are shifted to higher values in case of CTs 5–9 and 6–9, respectively. Regarding the third mode, frequencies of only the CT6 and CT9 are somewhat higher than those for the other CTs, while the frequencies are low with CT5 and CT8. SR amplitudes are higher for the 5, 7–9 CTs in case of the second and third mode, but not for the first mode. Note that CTs 2 and 4 have utterly small amplitudes in case of all 3 considered modes. SR frequencies of the first and second modes and amplitudes of all 3 modes tend to be higher for CTs with higher 500 hPa isobaric and thicker 500–850 hPa layer. CTs that produce higher first SR frequencies over Sopron have generally lower PG over Xanthi.
Comparing these tendencies with the distributions of meteorological parameters among CTs, the average temperature shows similar features. High frequencies in the first and second SR modes appear in CTs where the average temperature is relatively high. Additionally, relatively low temperature in CT4 matches with low values of the SR amplitudes in all three considered modes. Some similarity with the special humidity and dew point distributions can also be observed. These parameters have very low values in CT2 similarly to the SR amplitudes. The relation of SRs to temperature has been suggested by Williams (1992).
The interpretation of the amplitude and frequency shifts in connection with CTs in Europe is challenging, because the intensity and global distribution of thunderstorm activity as well as the location of NCK station in the structure of the nodal lines of the SR resonance modes need to be taken into account (Price 2016). The results above are nevertheless interesting since they hint to a lower-upper atmosphere coupling.
The variations in SR peak frequencies and intensity between different CTs is quite interesting for biometeorology, as SR appears to affect human condition (Cherry 2002; Mitsutake et al. 2005; Elhalel et al. 2019). Since various biometeorological studies have linked weather types to blood pressure and other manifestations of cardiac functioning (e.g. Morabito et al. 2008) and some authors have found possible coupling between SR and cardiac function (Mitsutake et al. 2005; Elhalel et al. 2019), in view of the results presented here, it appears that biometeorological studies may benefit from the combined use of meteorological and electric atmospheric variables.
Lightning
Weather types with higher 500 hPa isobaric altitudes over Xanthi and low winter occurrence produce more lightning over the site.
Regarding lightning over the whole European domain, it appears (Figs. 7and 1) that weather types produce more lightning over regions with higher 500 hPa isobaric height and thicker 500–850 hPa layer.