Cold ring flash flood in Thessaloniki

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Two short-lived and small, but very rapidly developing, convective cells joined together to bring severe flash floods to Thessaloniki, Greece, for around an hour on 10 May 2018.

Date & Time
10 May 2018 06:00–18:00 UTC
High Resolution Visible (HRV), Infrared Channel

By Ivan Smiljanic (SCISYS)

The morning's blue skies in Thessaloniki on 10 May turned into grey, rainy and thundery skies around midday, pouring vast amounts of rain in only a couple of hours.

The short but intense life cycle of convective cells around the second largest city in Greece was directly responsible for torrential rain and flash floods.

In particular, two convective cells that grew into full-size cold ring shaped thunderstorms, merged almost above the city and brought heavy rain, lightning and hail episodes. The hilly terrain around the city contributed to the strength of associated flash floods.

They grew very quickly to reach maturity in less than an hour. Not long after merging the heavy downpours destroyed the merged system and it dissipated in less than two hours after genesis took place. In this case the lack of vertical shear in the wind field led to no separation between updraft and downdraft in the same convective cell.

Figure 1
Figure 1: Stages of convective system development annotated over infrared 10.8 µm channel, supported by HRV channel

This lifecycle is annotated in Figure 1 where in just a few steps the growth of the system is explained. Some 30 min after the last time step in Figure 1 the system started to disintegrate.

The full convective episode is captured on the two animated loops of SEVIRI HRV and the IR 10.8 µm channel (with enhanced colour scheme).

Figure 2: Meteosat-11 HRV animation, 10 May 06:00 UTC–18:00 UTC. Download animation (MP4, 5 MB)

Figure 3: Meteosat-11 infrared animation, 10 May 06:00 UTC–18:00 UTC. Download animation (MP4, 4 MB)

Confirmation for the existence of multiple updrafts (hinting two) can also be found in the cloud-top signature, namely radial waves seen in the High Resolution Visible channel at the last step in Figure 1 (10:15 UTC).

the animated gifs in Figure 4 and 5 illustrate the difference between present capabilities of current Meteosat Second Generation visible channels (nominally at 3 km resolution, 15 min time step) and the future FCI visible channels (nominally at 1 km resolution, 10 min time step), both in a full-disc scanning mode. It is obvious that most of the cloud-top dynamics are lost with reduced spatial resolution, although the difference in the time step is not crucial. In this case the two scenarios are simulated using MSG HRV channel data in rapid-scanning mode (nominally 1 km resolution, 5 min time step).

Figure 4: Simulated scenario showing 1 km, 10 min time step
Figure 5: Simulated scenario showing 3 km, 15 min time step
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