In mid-September 2017 a huge mesoscale convective system (MCS) in Croatia caused flash floods, widespread material damage and even jeopardised human lives.
By Ana Petrov, Petra Mikuš Jurković and Tanja Renko (DHMZ) and Ivan Smiljanić (SCISYS)
On the morning of 11 September, Northern Dalmatia was under the influence of severe thunderstorms, and extremely large amounts of precipitation caused flash floods and significant material damage in the cities of Zadar and Nin.
Between Sunday evening (10 Sept) and Monday evening (11 Sept) 285 mm of rain was measured in Zadar, and around 325 mm at Zadar airport, Zemunik.
Synoptic analysis (Figure 2) shows that a well developed low formed on the previous day in the Gulf of Genoa, which moved towards the Adriatic.
Evolution of the system can be seen in the animated Airmass RGB imagery, 01:00–09:30 UTC (MP4, 433 KB).
In the sequence an area of red can be seen in the centre of the low. This is dry descending stratospheric air and represents the maximum in the Potential Vorticity field or the PV anomaly.
This convective system can be classified as a mesoscale convective system (MCS) since it covered the area of around 15,000 km, with the diameter of more than 400 km.
IR10.8 animated gif
IR 10.8 colour-enhanced loop (Figure 3) shows that the cloud tops were very cold, in the range of -70 to -65 °C. Also at 02:45 UTC (Figure 4) the storm exhibited a distinct enclosed warm spot and cold-U shape. Usually storms with cold/warm couplets on their tops penetrate though the tropopause into the warmer lower stratosphere.
This once again points to the possible severe nature of the convection, since there is a significant correspondence between these cloud-top patterns and a convective storm's severity.
This feature is also closely related to another similar cloud-top phenomenon, the cold-ring feature, from which it may evolve. The only difference is that cold-ring shaped storms develop in the environment with much less shear.
The sandwich product (Figure 5) is one of the best products for observing the tops of convective clouds. It consists of an underlying HRV image which is overlaid with a semi-transparent colour-enhanced IR 10.8 µm image.
Such a combination provides information about the 3D structure of the cloud, but also about the temperature of the cloud top. It is the most appropriate combination for detection and study of the characteristics of the overshooting tops, which form when a very strong thunderstorm's updraft and very high instability are present. In that case the cloud protrudes past its equilibrium level and reaches into the tropopause.
The Severe Convection RGB animation, 11 Sept 05:00–10:45 UTC (MP4, 1 MB) shows yellow reflections above the Zadar area. Yellow pixels in a severe convection image indicate that small ice particles are present on top of the convective cells, which also help us identify intense updrafts.
Convective clouds usually have large ice crystals on their top, when small ice crystals are present on the cloud top it indicates that the convective updrafts were so strong and fast that ice particles from the lower parts of the cloud didn't have time to grow.
It should be mentioned that not all yellow pixels indicate small ice particles. Some of them are a consequence of very low brightness temperature in IR10.8 µm (app. -70 °C) and relatively low reflectance in IR3.9 µm, indicating only very cold cloud tops with updraft of usual speed and strength and relatively large ice particles.
The bright yellow colours turn more orange as the convective activity dissipates and the ice crystals in the top of the cloud become larger.
The reasons for the formation of MCS which caused such extreme precipitation in very short time were the following:
- Continuous flow of moist air from the south in the lower layers of the atmosphere (see Figure 6).
- Very warm Adriatic sea, surface temperature was 25 °C, consequently high low-level moisture, warm advection in lower layers and cold advection in higher layers, all contributing to very high instability (see Figure 7).
- Strong deep layer wind shear.
- Convergence of southeast and southwest winds in the Adriatic, which helped lift warm, moist air (see Figure 8).
- Intrusion of the PV anomaly. The vertical cross section (Figure 9) shows that the value of two PVU (potential vorticity units), associated with thermal tropopause, reached quite low layers, between 600 and 700 hPa. As expected, convection developed at the leading edge of the PVA.
All the parameters which favour organised convective development were present and it resulted in the formation of a huge MCS, which was almost stationary above the area of northern Dalmatia.
Images from the web cam on the Zadar near Ugljan island revealed the severity of the rainfall that filled the island's Bay of Sutomišćica with the dirty run-off water. From altimetry measurements, it can be seen that the sea level oscillated — possibly a consequence of both the excessive rainfall and local pressure minima connected to the mesoscale system.
It should also be mentioned that on the previous day, 10 September, a huge convective system had hit Italy.
The infrared animation (Figure 10) shows the long-lived back-building convective system with extremely cold cloud tops, impressive OTs and very distinct enclosed 'warm stripe'.
Considering synoptic situation and the mesoscale conditions Croatia issued the highest degree of warning for the next day, a red Meteoalarm.