Author
Jarno Schipper - ZAMG
Introduction
From a meteorological point of view this small case study is rather classical. The resulting effect it had however on public life was far from classical and in fact catastrophic. The resulting floods of three consecutive days of rain brought chaos to a large swathe of the Alpine region, triggering landslides and cutting roads and railway lines. Mudslides resulted in fatalities in both Switzerland and Austria. Thousands of people were urged to evacuate when water in nearby streams and rivers were rising and threatening their homes. Due to the torrential rain that lashed for nearly three days, efforts to clear debris and shore up flood defences were hampered making it difficult to bring relieve to those who were in aid.
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| Floods causing devastation in the Alpine region. Copyright © APA |
This case study will present a compilation of events in which the Genoa low that triggered these flooding events is discussed and presented using several satellite images of the various Meteosat 8 channels as well as RGB satellite images. The minicase study is further supplemented with a chapter "interactive overlays". In this chapter you can self overlay various numerical parameters on the corresponding satellite imagery. Through this interactive way you can reconstruct the case and exercise with it. If you think you can contribute to this Case study or to EUMeTrain by providing us dates of interesting case studies which happened in your region of interest, we encourage you to please write us using the above e-mail address.
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| Enhanced Meteosat 8 IR10.8 image - 22nd August 2005:12 UTC - The depression can be located over the Alps. Especially on the northside of the Alps the satellite image reveals several convective cells. |
Synoptic situation
A Genoa low is a low-pressure system developing south of the Alps in the Gulf of Genoa and northern Adriatic Sea. Genoa cyclones usually remain stationary (or at least leave a residual trough) south of the Alps throughout their life history. If the lows do move, they generally follow one of two tracks (source weatheronline):- The first track is a northeasterly to north-northeasterly direction across the Alps. This track is also known as the so-called '5b-path' bringing extensive rainfall and catastrophic flooding to Austria, Germany, the Czech Republic and Poland, as in Summer 2002 (see Flood Catastrophe Central Europe).
- The second track, associated with a strong anticyclone over the Balkans, Turkey and the Black Sea, is in a southeasterly direction from the Gulf of Genoa towards the Ionian Sea. In this case, a gale force Bora is extremely likely by the time the depression moves into the Ionian Sea.
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Synoptic chart - 22nd August 2005:06UTC |
The cyclone described in this case does not follow one of the paths described above. Instead it does move over the Alps but then remains stationary before it fills up. Due to the cyclonic movement on moist air from the Adriatic is transported into the Alpine region. The lifting of this moist air causes substantial rain on the nortside of the Alps. To the west of the cyclone the moist air again is slammed onto the north of the Alps providing more rain.
In the animation below the movement of the depression is shown. The blue arrow marks the movement of the moist air. This can also be derived when studying either the WV set of images or the set of Airmass images.
The track that this cyclone follows occurs several times a year. What made this case as catastrophic where the temperatures in the higher layers of the troposphere. Normally the precipitation associated to these cyclones bring snow in the mountains. This snow acts as a buffer as water is stored. The high temperatures however in August 2005 caused all precipitation to be in form of rain acting as an instantaneaous unit hydrograph triggering flash floods and mudslides.
IR Imagery (Meteosat 8 channel 10.8): time sequence
This chapter shows and briefly describes the development in a sequence of 3-hourly images.
Enhanced IR Imagery (Meteosat 8 channel 10.8): time sequence
This chapter shows the development in a sequence of 3-hourly images. Due to the artificial enhancement of the images the convective cells can better be recognised.
WV Imagery (Meteosat 8 channel 6.2): time sequence
In the following chapter the cloud configurations can be better related to the upper level dynamics using Meteosat 8 - WV6.2 channel.
Meteosat 8 - Airmass RGB: time sequence
The development and especially the physical background is pictured using the Airmass-RGB. The satellite images are each time 3-hours apart.