Intense Mediterranean cyclone's Alpine crossing

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A cyclone that intensified over Mediterranean waters took a very direct path over the Alps, causing different impacts during its journey.

Date & Time
27 October 00:00 UTC–02 November 00:00 UTC
Satellites
Meteosat-11, Aqua, Suomi-NPP
Instruments
SEVIRI, MODIS, VIIRS
Channels/Products
Airmass RGB, Natural Colour RGB, True Color RGB, 24h Microphysics RGB

By Ivan Smiljanic (SISYS), Michal Žák (CHMI) and Yasmin Markl (ZAMG) and Sancha Lancaster (Pactum)

An exceptionally strong cyclogenesis took place in the western Mediterranean during the last five days of October 2018. The cyclone (named as Adrian by Météo-France and Vaia by the Freie Universität Berlin (FUB)) brought extreme weather, especially to Italy where intense thunderstorms, high winds and very heavy rainfalls lead to flash floods and landslides. More than 30 fatalities were reported.

Impacts in Italy

The violent storms battered Italy for days, causing 29 deaths. The storms, combined with very high tides, caused the worst flooding in Venice for a decade.

  • In Venice water levels rose by 156 cm (more than 5 ft), flooding many parts, including St Mark's Square and the Basilica.
  • In Sicily 12 people died after swollen rivers burst their banks.
  • In Rapallo, north-west Italy, the breakwater walls, yachts and the port area were destroyed by a storm surge.
  • The resort town of Portofino was cut off by a landslide.
  • The fishing village of Vernazza was flooded with seawater.
  • High winds toppled trees that killed passersby in three accidents in Naples and Lazio.
  • Around 300,000 trees were flattened, after winds swept through the Val d'Assa in the Asiago plateau.
  • Two people were killed when a tree hit a car they were travelling in near Frosinone, south of Rome.
 

Synoptic development

The catalyst for this event was a pronounced dip in the polar jet stream that extended into northwest Africa. A very cold air mass was advected over southern parts of the UK on 26 October.

This cold and dry air can be seen as the red/magenta areas on the Airmass RGB satellite imagery (Figure 1).

Following this cold air intrusion, an intense cyclogenesis occurred over Spain and the western Mediterranean during 28 October.


Figure 1: ECMWF model Geopotential height and Temperature at 500 hPa (Credit: ZAMG). Download animation (MP4, 1 MB)

On the eastern side of the cyclone (which was also very well developed in the upper levels) a strong flow of unseasonable warm air extended over the Balkans up to southern Scandinavia, while the cold air flowed on the western side to north-western Africa.

A very large temperature gradient (about 15 °C) and the very warm waters of the Mediterranean Sea contributed to the cyclone's intense development.


Figure 2: Meteosat-11 Airmass RGB, 27 October 00:00 UTC–02 November 00:00 UTC. Download animation (MP4, 33 MB)

The cyclone's maximum intensity was reached on 29 October, when severe storms formed on the cold front. These crossed over almost all of Italy, then moved on to Croatia and Slovenia during the evening.

The cumulonimbus clouds reached very high levels, as can be seen from the bright white colour of their tops in the Meteosat-11 Airmass RGB (Figure 2) — indicating dangerous weather phenomena were occurring.

A high resolution view of the storm, perhaps when it was at peak intensity, is provided by the Suomi-NPP VIIRS True Color RGB (Figure 3).

Figure 3
 
Figure 3: Suomi-NPP VIIRS True Color RGB, 29 Oct 12:48 UTC
 

As well as the wrapped cyclone centre visible west of Corsica, dust intrusion was also apparent (brown shades) in the warm conveyor belt along the associated cold front.

During the following night, the cyclone moved across the Alps, taking an unusual track directly from south to north. The satellite imagery shows how the cloudiness became disorganised in the vicinity of the low centre during this mountain crossing.

Later, on 30 October, the cyclone continued its track north, over Germany, where it was already slowly filling up.

The flow of dry air, originating from stratosphere (red colour), led to almost clear skies in the belt stretching from the Adriatic Sea, over Czechia, to eastern Germany.

Finally, on 31 October, the cyclone moved over Scandinavia, reaching its final stage of development, filling up and merging with an older cyclonic system.

In the meantime, another cyclogenesis was starting over the western Mediterranean region. Although its development was not so impressive, the convection connected with the low brought further heavy and deadly thunderstorms to Italy.

High winds, waves and sea levels

Intense and persistent wind forcing associated with the cyclone was responsible for the build up of the waves and sea levels. This led to the destruction of coastal areas, and heavy flooding, mostly in the downwind areas.

The northwest coast of Italy suffered the most from the high waves meeting coastal infrastructures and communities. From the wave model it is obvious that significant wave heights exceeded 6 m. (Figure 4)

Figure 4
 
Figure 4: ECMWF Significant Wave Height plus Wind direction at 850 hPa, 29 Oct 18:00 UTC
 

Looking at Figure 4 it is also obvious that south-easterly ‘Sirocco’ winds assisted with the ‘pumping up’ of the sea levels in the northwest Adriatic, bringing a level rise of more than 150 cm  in Venice.

High winds also caused a lot of damage, along the path of the cyclone:

  • On the Mediterranean island of Corsica powerful winds left 21,000 homes without power, and ports and airports were closed.
  • In Croatia gales caused flooding and loss of some power.
  • Storms also swept into the Ticino region of Switzerland causing flooding, fallen trees and damage to properties.
  • In Austria rivers burst their banks and high winds caused damage to a section of the Medieval ramparts of Salzburg.

South Foehn wind on October 29/30 in a widespread Alpine region

Due to a strong pressure gradients, well-pronounced wind fields directed towards the southern side of the Alps produced the perfect conditions for Foehn winds north of the main alpine ridge.

The ECMWF chart for wind gusts and surface pressure forecast wind speeds up to almost 200 km/h. (Figure 5).

Figure 5: ECMWF Geopotential and Temperature field, with Wind Barbs at 500hPa level 4 (Credit: ZAMG)
Figure 6: ECMWF furcated surface level pressure and wind speed and wind direction (Credit: ZAMG)

The special feature of this Foehn case was that the cold front reached the southern side of the Alps, before it reached the northern side. This had an important influence on the temperature gradient and the static pressure field, and triggered strong wind gusts at the exact moment when the cold front passed through.

This could be observed in weather station data, by the typical increase of temperature, and a rapid decrease of humidity (dew point), with a synchronous increase of wind speed and a change of wind direction to a mostly southerly direction.

Figure 7: Meteosat-11 24 hours microphysics RGB, 29 Oct 21:00 UTC with weather station data from Brenner, Tyrol, Austria (Credit: ZAMG)
Figure 8: Meteosat-11 24 hours microphysics RGB, 29 Oct 23:00 UTC with weather station data from Zell am See, Salzburg, Austria (Credit: ZAMG)

Also with different satellite images it was possible to observe interesting clouds structures, such as the lee clouds northeast of the Alps, and an almost cloud-free area north of the Alps. (Figure 9).


Figure 9: Meteosat-11 HRV Fog RGB, 30 October 07:00 UTC–XX?? UTC Download animation (MP4, 7 MB)

Heavy rainfall October 29/30 in a widespread Alpine region

Due to a passage of the cyclone directly over the Alps, many clouds built up on the southern slopes of the Alps, producing heavy and long-lasting rain events in the region.

These clouds are seen well on the 24h Microphysics RGB sequence as a red shaded features (high and thick clouds). (Figure 10).


Figure 10: Meteosat-11 24h Microphysics RGB and Geopotential Field at 500 hPa, XXXDATE/TIME. Download animation (MP4, 8 MB)

The structure of these clouds could be better seen in the the higher resolution MODIS Natural Color RGB (Figure 11).

Figure 11
 
Figure 11: Aqua MODIS Natural Color RGB at 1 km, 29 Oct 12:40 UTC
 

The weather system brought wet conditions to the south of the Alpine ridge and dry conditions to the north of ridge. This can be clearly seen on the overview of the 48 h cumulative rainfall amounts in Figure 12 (red to pink shades indicate more rain), with some direct reading of these rainfall amounts from the weather stations.

Figure 12
 
Figure 12: 48h accumulated rain amounts (28–30 October) (Credit: ZAMG)
 

In some places, accumulated rainfall exceeded 400 mm during the passage of the cyclone (see the graph of 40 days of rainfall at Lago Maggiore. Credit: Swiss Federal Office of the Environment). Under such conditions water levels rose by a few metres in many of the lakes in the alpine region, causing them to overflow.

Snowfall in southern France

Looking at the Natural Color RGB (Figure 13) over France a huge area of snow-covered ground was visible in south-eastern France. The snow areas appear as cyan (same colour for ice clouds, although there are not many in this imagery).

Figure 13
 
Figure 13: Suomi-NPP VIIRS Natural Color RGB, 30 Oct 12:30 UTC
 

Thick snow, unusual in that part of France at that time of the year, brought chaos. One person died. Up to 15 cm (5.9 in) of snow was reported to have fallen over a wide area and up to 50 cm (20 in) over higher ground.

  • More than 1,000 drivers were trapped in their cars for the night in the mountains of the Massif Central region, as snowstorms engulfed the roads.
  • Another 400 had to spend the night in train carriages at the main station in the eastern city of Lyon after heavy snow blocked the tracks.
  • More than 100,000 homes were left without power across mainland France, most of them in eastern and central regions.
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