Severe storms as an example of a natural hazard in the urban area – case studies of the area of Warsaw, Poland

On 17 June 2016 Poland was under the influence of an active low-pressure system. The low appeared on 16 June in the morning, as turbulence on a quasi-stationary front crossing Austria and started moving north (fig. 2a). Within the next few hours, the low moved across Poland, deepening as it did so. At 12:00 UTC, its centre was located over northern Poland and the unstable tropic air mass was observed over the eastern part of Poland (fig. 2b). Concurrently, a cold front was moving from the south to north-eastern Poland and, just a few hours later, it left Poland.

Figure 1

Figure 2

On 17 June 2016 at 11:00 UTC at the Warsaw-Okęcie synoptic station the temperature dropped to 26.1°C but the dew point increased to 19.0°C. An increase in air humidity and a decrease in air temperature were observed due to the convergence zone passing before the cold front, to the convective clouds with rain showers, and a nearby thunderstorm.

Unfortunately, the sounding (from 12:00 UTC from Legionowo) was made after the passing of the convergence zone. In consequence, the real conditions before the storm could not be registered. According to that sounding, the total SBCAPE value was only 160 Jkg⁻¹ and, what is more, the intensive CIN (−140 Jkg⁻¹) was also measured. At the same time a significant increased airflow was observed, with a deep layer shear (0–6 km, DLS, the wind speed difference between surface and layer at 6 km) equal to 30.6 ms⁻¹ and a low-level wind shear (0–1 km, LLS) up to 18.5 ms⁻¹, which is rare in this area. This was the main reason the storm was so powerful and could induce destructive wind gusts.

Concurrently, the data from Era5 showed even higher values of the thermodynamic and convective indices before the sounding time. At 10:00 UTC the SBCAPE value was 1,901 Jkg⁻¹, while CIN was about −48 Jkg⁻¹. At 11:00 UTC the values were 1,516 Jkg⁻¹ and −22 Jkg⁻¹, respectively. These CAPE values could be considered as moderately high and, with high wind shear values, they could cause severe storms. Values of Storm Relative Helicity were also high at 0–1 km: 213 m²s⁻² and 0–3 km: 299 m²s⁻². In connection with a wind shear of 0–6 km around 30 ms⁻¹ and a wind shear of 0–1 km above 18 ms⁻¹, strong supercell storms or a mesoscale convective system (MCS, system of storm cells which lasts a few hours or more and is more than 100 km in length) with squall lines could develop in this environment.

The first storm cells developed at 8:00 UTC on the convergence zone. The zone was moving towards central Poland and after 10:00 UTC the next storms developed – more than an hour before moving over Warsaw. At 10:23 UTC on the PPI 0.5° product a bounded weak echo region signature (BWER) was detected (fig. 3). The updraft was strong, and features of a supercell storm were clearly visible. Several dozen minutes later, the first information about damage caused by strong wind gusts was reported. At 11:00 UTC the convergence zone passed over Warsaw and the supercell moved to south-western districts of Warsaw (fig. 4). Only four minutes later (11:04 UTC) the storm core moved to the centre of the city (the area of Warsaw Central Railway Station) and at 11:08 UTC the storm was noticed in the eastern district of the city. Most of the damage reports came from the south-western, central and eastern parts of the city. At about 11:30 UTC, storm cells formed a squall line, moving in a north-easterly direction.

Figure 3

Figure 4

Video recorded next to the Central Railway Station (fig. 1) allowed the progress of the storm to be analysed. The storm lasted only about 4.5 minutes and two zones of strong wind and heavy rainfall were observed. The first zone of strong wind was related to the forward flank downdraft and the second was caused by the rear flank downdraft, which is consistent with radar data. The wind gust recorded at the meteorological station Warszawa-Filtry was 28.0 ms⁻¹. The affected area was narrow, along the path from south-west to north-east, across the centre.

Most of the damage was caused by destructive wind speed due to a downburst (fig. 5). This was related to strong forward flank and rear flank downdrafts. Many trees and branches were broken and some roofs, walls of buildings and cars were damaged (fig. 6). The wind also damaged some infrastructure: traffic lights, road signs, billboards and power lines. A few hours after the storm, there was still some disruption to public transport, especially because of damaged conductor rails and the stoppage of tram traffic. The type of damage allows us to suppose that the wind speed exceeded the recorded 28 ms⁻¹ (101 kmh⁻¹) locally. Possible significant causes were the shape of the urban layout and the microburst (violent downdraft reaching the ground with a high wind speed in a short time, with the diameter of the phenomena of less than 4 km) related to the supercell storm. In a few places, the damages may have been caused by wind exceeding 35 ms⁻¹ (126 kmh⁻¹).

Figure 5

Figure 6

As mentioned above, the dispersion of the damage was related to buildings (fig. 7a, b, c). The most serious damage was noticed between buildings, in narrow passageways, parallel to the dominant wind direction. The buildings had an impact on the wind direction as well as on the angle of fallen trees (fig. 7a, b). The angle was different from the wind direction by about 25–45°. This confirms that the tunnel effect was accountable for the direction and maximum wind speed that day. In an area without buildings (fig. 7c), the damage dispersion was in accordance with the dominant wind direction.

Figure 7

On 4 September 2016, the area of Poland was under a low-pressure influence. At 00:00 UTC, its centre was located over Great Britain and at 12:00 UTC, it moved over the German–Danish border (fig. 8). Concurrently, the advection of a warm air mass was observed in the south-eastern part of the low, with temperatures reaching 28–29°C. The cold front with a convergence zone was moving eastwards and it was located near the western border of Poland in the evening. The pace of the front was high and, at about 19:00 UTC, the convergence passed over Warsaw. On 5 September at 12:00 UTC the low was located over northern Ukraine and the occluded front crossed Poland.

Figure 8

Warsaw was under the influence of a warm, polar air mass with a maximum air temperature of 28.6°C. At 14:00 UTC, a shower was reported, and this influenced air humidity. Unfortunately, it was impossible to get information about real conditions in the troposphere from soundings – the storm passed over Warsaw between the sounding times. The surface observations before the storm at Warsaw-Okęcie indicated an air temperature of 20.4°C and a dew point at 15.6°C. Both soundings – at 12:00 UTC and at 00:00 UTC – detected moderate wind shear. The deep layer shear (0–6 km, DLS) was 16.7 ms⁻¹ and 15.1 ms⁻¹. At Era5 sounding the MUCAPE (the most unstable layer CAPE) was only a little above 300 Jkg⁻¹, but there were moderate and high SRH values of 0–3 km: 165 m²s⁻², 0–1 km: 140 m²s⁻². The second value was especially high, making it favourable for supercell storms to develop. Nevertheless, the forecasts and reanalysis did not show conditions for such an intensive event that evening in central Poland, despite moderate wind shear in the layer 0–6 km, the values being considered as moderate.

Deep, moist convection started to develop at 14:30 UTC over south-western Poland. At 15:10 UTC first convective cells were observed over southern Greater Poland. At 17:20 UTC the storm was located about 100 km west of Warsaw and its intense development was observed up to the supercell form. At PPI 0.5° product, at 17:33 UTC, an intense, coherent storm core was visible (fig. 9), with an intense, high updraft region inside it. Before 18:00 UTC a high precipitation supercell storm structure was observed and the storm was located over western Mazovia – a strong, locally destructive wind gust and hail were reported. At 18:50 UTC, the supercell reached Warsaw. Despite the night hours, the supercell shelf cloud was observed (fig. 10). It is very characteristic that the damage path across Warsaw was not wider than 2–4 km, with some parts not wider than 1 km. At 19:20 UTC, the storm passed over Warsaw and was still very active.

Figure 9

Figure 10

On the cross-section product (18:53 UTC), there was the structure of a weak echo region up to 4 km high with a massive overhang structure (fig. 11). This suggests that the updraft was really strong. Moreover, Doppler data and the right-moving path of the storm confirmed the rotation. That is also why the storm was so active in those conditions and why it caused severe phenomena, with strong microbursts. Along the storm path, strong wind gusts connected to downburst were reported, which was the main reason for very concentrated damage, with a path consistent with the storm core trajectory. Locally, heavy rainfall and large hailstones (with a diameter of 2.5–3.5 cm) were also observed. Most of the National Fire Service damage reports concerned broken trees or branches and interrupted power lines but there were also damaged cars and parts of roofs and some reports about flooded streets, but most of these were not dangerous.

Figure 11

The damage path was parallel to the movement of the supercell core (fig. 12). The first reports came from the western districts. Next, the southern part of the central district was affected (fig. 13) and the storm passed over the Vistula river. To the right of the river, the most damaged parts were those of eastern Warsaw (especially the residential areas). It was also very interesting to notice that the path of the core was really concentrated. The maximum wind gust at the Filtry meteorological station was 23.3 ms⁻¹ but, locally, damages were characteristic of a wind of over 30 ms⁻¹.

Figure 12

Figure 13