In the late afternoon of 24 June 2021 a tornado hit south-eastern parts of the Czech Republic resulting in casualties and huge material damage.
15 July 2021
02 July 2021
By Natasa Strelec Mahovic (EUMETSAT) and Ivan Smiljanic and Alen Berta (CGI)
This study presents the view, from the satellites, of the atmospheric conditions, and follows the development of the storms that caused the tornado.
The tornado occurred in the South Moravia region of the Czech Republic between 7 pm and 8 pm local time (17:00-18:00 UTC), destroying several villages and towns within a track more than 25 km long and 700 m wide. It was classified as a F4 tornado, according to International Fujita scale, making it one of the strongest European tornadoes on record.
It formed near Hrusky village (Figure 1- lower left part of the images) and went through the southern part of the village moving north-eastwards.
In its path (the trace in the satellite image was calculated to be approximately 21 km long and 300–500 m wide) (Figure 2), it devastated parts of six settlements until, near the village Ratiskovice, the destruction ended.
Detection of the tornado's path was based on NDVI (Normalized Difference Vegetation Index) difference analyses from clear Sentinel-2 images (16 and 29 June). The NDVI was calculated by the normalised difference of the NIR (band 8) and Red (Band 4) channels. As the tornado damaged the vegetation in its path, the negative difference in vegetation photosynthetic activity is detectable when pre- and post-event NDVI values are subtracted. The point of formation is not clearly visible in this NDVI difference image, because it probably formed over the village itself, but the path and end point are easily detectable.
The storm which caused tornado, plus very large hail (up to 7 cm in diameter), developed over north eastern Austria, close to the border with the Czech Republic, in the afternoon of 24 June. This tornadic storm was just one in a series of severe convective storms that developed over western and central Europe during the week starting 21 June. Many storms produced very large hail, severe wind gusts, and torrential rain. Several tornado reports were received in the European Severe Weather Database (ESWD) (Figure 3), with the Czech tornado being the most devastating. In total, 1,395 reports were received for those five days.
On the day of the event, the synoptic situation over central Europe showed a frontal system on the leading edge of a slowly moving upper-level trough, within a non-gradient surface pressure field (Figure 4). The situation was influenced by a quite strong south-westerly jet stream on the leading side of the trough, and, additionally, there was a small shortwave trough forecast to pass over the Alpine area in the afternoon.
The areas of central and eastern Europe were still under strong warm advection ahead of the front, whereas western Europe was already experiencing cold advection from the north west. In the humid and warm air, undergoing a strong synoptic forcing on the leading side of the trough, convective development was expected over large areas of southern Germany, north and north east Austria, the Czech Republic, Poland, and further east over east and south east Europe and the Black Sea.
In the pre-convective environment, satellite products showed high values of Total and Medium Layer Precipitable Water (Figure 5), indicating sufficient humidity content as one of the ingredients for convective development.
Stability indices, Lifted index and K index, shown in Figure 6, pinpointed the area of highest instability as the border between Austria, Hungary, Slovakia and the Czech Republic.
High instability in that area was forecast by the ECMWF model, and also revealed by the IASI hyperspectral sounder measurements, as seen in Figure 7.
Once the development of convective cells was initiated it could be followed in the NWC SAF Rapidly Developing Thunderstorms product (Figure 8), which showed multiple initiation and growing storms in the area.
In the growing and mature stage multiple storms in the affected area exhibited all of the typical cloud-top signatures — overshooting tops, above-anvil cirrus ejections, gravity waves, cold-U/V, and cold-ring temperature signature (Figures 9 and 10). Gravity waves seemed to be shadowed by the veil of above-anvil cirrus material (not very many waves detected), even though the cirrus plumes themselves were not very pronounced and not closely pinned to the updraft areas.
What was typical for the supercell that gave birth to the tornado (and for few other right-moving supercells in the area) was a persistent updraft associated with overshooting tops/domes, best seen by the High Resolution Visible (HRV) channel (Figure 11). Westward movement of the supercells (or right movement, relative to the main south west wind) is best seen through the SEVIRI five-minute rapid scan loop (Figure 12). Towards the end of the loop, the whole mesoscale systems appears to be picking up the cyclonic rotation (signature of a Mesoscale Convective Vortex).
It is interesting to notice from the temperature field in Figure 12 that the system over south west Poland (top right part for observed domain) existed in the environment of higher equilibrium level, e.g. the anvils of the associated cumulonimbus clouds were roughly 10 °C colder (red shades in the Sandwich product). This is confirmed also in the NWC SAF Cloud Top Temperature and Height products (CTTH) in Figure 13, where it is clearly seen that the clouds over Poland are higher that the ones within the tornadic system over the Czech Republic.
The loop of CAPPI 2 km non-filtered data (Figure 14) shows characteristic signatures indicating tornado occurrence — a hook echo was clearly visible in several time steps; very high reflectivity values were present at the time just before the tornado touch-down, and even a 'flying eagle feature' was seen in several images from 17:20 UTC onwards, during the time of the tornado. The type of severe thunderstorm with this 'flying eagle' feature in the radar data occurs when the updraft of the storm is very strong and very tall. The winds aloft are forced to go around the core of the storm and the result is a flying eagle shape appearance in radar reflectivity.
High precipitation intensities, shown in the radar reflectivity values, were mirrored also in satellite precipitation product. Figure 15 shows NWC SAF Convective Rainfall Rate hourly accumulation loop, with clear precipitation maxima over south eastern parts of the Czech Republic, after 17:30 UTC. Note again how the precipitation intensity was higher over Poland in the same period, where many cases of very large hail were recorded.
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