*3.1. Thunderstorm on 10 June 2019*

Figure 2 shows the temporal evolution of rain rates (as derived from C-band weather radar data, Figure 2a) and lightning discharges (Figure 2b) during one hour of the thunderstorm on 10 June 2019, when most lightning activity was observed close to the Milešovka hill. The temporal evolution is displayed with a time step of 10 min. Based on rain rates (Figure 2a), it is clearly visible that the thunderstorm was severe, at least within the central European context.

Figure 2 also shows that the lightning discharges occurred (horizontally) not only within the precipitation areas but also outside of the precipitation cores; i.e., they may have originated in non-precipitating parts of the thundercloud as well. As the system moved in time towards east-northeast, CC flashes had a tendency to precede CG flashes, while lightning flashes (CC + CG together) tended to occur not only during the period of intense rainfall, but also prior to intense rainfall. This has been mentioned in other works as well, e.g., [40].

Figure 3 depicts precipitation totals with a time step of 1 min, as registered by a rain gauge with a resolution of 0.1 mm at the Milešovka observatory between 21 and 22 h UTC on 10 June 2019. Note that rain rates (i.e., precipitation intensities, Figure 2a) cannot be directly compared with recorded precipitation totals (Figure 3) as they do not represent the same information. Figure 3 shows that the highest 1-min precipitation total occurred between 21:39 and 21:40, while the closest lightning was recorded at 21:37 and 50.341 s. This confirms that lightning flashes may occur prior to heavy rain. Here, we note that measured precipitation totals by the rain gauge might be underestimated. The reason is that at the top of Milešovka hill, strong winds frequently appear during storms, which may result in an underestimation of rainfall totals due to the blowing away of the precipitation from the surface of the rain gauge.

**Figure 2.** Temporal evolution with a 10-min time step of (**a**) rain rates derived from C-band weather radar data (radar circle domains having a radius of 250 km) and (**b**) lightning discharges between 21 and 22 h UTC as registered by EUCLID network on 10 June 2019. Note that in (**b**), blue symbols represent Cloud-to-Cloud (CC) flashes while red symbols display Cloud-to-Ground (CG) flashes. The Milešovka position is shown in Figure 1.

Concerning data of the cloud radar, Figure 4 displays radar reflectivity together with NL (i.e., lightning discharges that occurred not farther than 1 km from the Milešovka observatory). It is obvious from the figure that NL were related to high reflectivity values, although high reflectivity values were also typical for the melting layer and below (i.e., below 2.5 km approximately).

**Figure 3.** Rainfall totals with a 1-min time step recorded by a rain gauge at the Milešovka observatory from 21 to 22 h UTC on 10 June, 2019.

**Figure 4.** Temporal evolution of radar reflectivity factor (Z [dBZ]) of the cloud radar at the Milešovka observatory during the thunderstorm on 10 June 2019 from 20 to 24 h UTC. Vertical magenta lines depict CC near-lightning (NL) discharges, while vertical black lines display CG NL discharges. Note that y-axis (z [m]) displays the height in meters above the cloud radar situated at an elevation of 837 m a.s.l.

It is worthy of note that between 21:30 and 21:40 approximately, there was a sudden decrease in the vertical span of the thundercloud, according to the cloud radar data (Figure 4). This is the time when most of rainfall was registered at the Milešovka observatory (Figure 3). This is probably caused by the fact that the received signal in the lowest gates was too strong due to heavy rain that the radar was unable to capture signal from higher gates.

Furthermore, it is interesting to check the temporal evolution of LDR values during the thunderstorm (Figure 5) that were not corrected using the integrated cross-polarization ratios [41]. In Figure 5, high values of LDR clearly show that the melting layer was around 2500 m above ground in the thunderstorm. Another zone of high LDR values is visible from 21:30 to 21:50 at higher altitudes, which is the time interval of intense rainfall (Figure 3) and lightning activity near the radar site (Figure 4). Contrary to very high LDR values in the melting layer, which are commonly associated with melting snow flakes, very high LDR values at higher altitudes, such as 4-7 km, can correspond to non-spherical shape of graupel and/or hail or to aligned ice crystals due to a strong electric field if the crystals are not aligned along with the co-channel, instead they are oriented at angle close to 45◦ with both the co- and cross-channels [25,42,43]. It is worthy of note that the elevation around 4-7 km, where we observed increased LDR, is also considered as the elevation where the main negatively charged area appears [40]. We discuss this finding further below.

**Figure 5.** Temporal evolution of linear depolarization ratio (LDR) [dB] during the thunderstorm on 10 June 2019 from 20 to 24 h UTC at the Milešovka observatory. Vertical (black) lines depict CG NL discharges and y-axis (z [m]) displays the height in meters above the cloud radar situated at an elevation of 837 m a.s.l.

It should be noted that the LDR data are not available at all gates where we obtained radar reflectivity factor data (for example, after 21:50). This is the consequence of the attenuation of the signal received in the plane perpendicular to the transmission plane.

Figure 6 shows the evolution of hydrometeor distribution (resulting from Hclass, Section 2.3) on 10 June 2019 from 21:00 to 21:59 UTC. Clearly, the lack of data in the vertical profiles between approximately 21:30 and 21:40 makes the interpretation of the obtained results difficult, especially because many NL discharges occurred at that time. Nevertheless, the majority of the ten closest discharges (Figure 6) occurred after 21:40, when we had data again, covering almost all the troposphere.

**Figure 6.** Hclass during the thunderstorm on 10 June 2019 from 21:00 to 21:59 UTC at the Milešovka observatory. Vertical lines depict NL discharges; blue lines depict CC NL discharges, red lines the CG NL discharges. Ten lightning discharges closest to the observatory are numbered in ascending order starting from the closest discharge denoted no. 1. Dashed lines represent cases, when more CG/CC discharges occurred around the same time (order of ms). Note that capital letters in the legend indicate first letter of classified hydrometeors (Table 2) and y-axis (z [m]) displays the height in meters above the cloud radar situated at an elevation of 837 m a.s.l.

The results of Hclass indicate that the highest LDR values at the elevation from 4 to 7 km (Figure 5) correspond to a mixture of several hydrometeor species with a predominance of ice and snow particles and graupel. These are the species which play major roles in the process of cloud electrification by collisions of hydrometeors according to currently accepted theories [44]. The mixture of many hydrometeor species is also evident during very close lightning activity (between 2400 s and almost 3000 s in Figure 6).

The electrification process by collisions of hydrometeors at an elevation of 4–7 km is also supported by Figure 7, which presents values of the Doppler spectrum width (σ). High values of σ, i.e., large variability of vertical velocities, just after 21:40 confirm the coexistence of various hydrometeors and support the existence of collisions of hydrometeors (light species collide with heavier species having larger terminal velocity). The obtained results of high LDR and sigma values together with the presence of diverse hydrometeor species may bring us to the conclusion that around 21:40, collisions of hydrometeors caused a strong electrification of the thundercloud near the radar site.
