Upper-level low moving south - Cyclogenesis in Genoa Bay

At 00 UTC on 18th December a broad low over the Mediterranean and West Europe, with two centers – one over France combined with an upper-level low and the other over Greece and south Italy, can be seen in Fig 5.2. In the upper levels (at 500 hPa) the most striking feature is a zone of strong pressure gradient between the low over Europe and the high over the northern Atlantic. At 500 hPa there are two lows – one over Poland and Czech Republic and the other over the northern France.
The latter, earlier situated more to the north east, was the one responsible for the snowfall in the Netherlands.
In the next 24 hours that upper-level low moved southward. The other one over the Central Europe spread and deepened and at the end of the day there was only one broad low over Central Europe, with a trough axis extending towards Genoa Bay. During the same time in the lower level (1000 hPa) the low over Greece moved toward the Black sea. At the end of the 18th December (18 UTC) the Genoa low started to deepen and during the 19th December it moved across the Adriatic Sea further to the east.
The upper-level low moved to the south, towards Genoa Bay and north Italy and at the end of the 19th it was situated over Austria, Slovenia and Hungary. During that period the surface low and the upper-level low were not co-located which can be regarded as an indication of the ongoing cyclogenesis.

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Fig 5.2 - ECMWF Geopotential Height 1000 hPa (magenta) and 500 hPa (green), from 18 Dec 00 to 20 Dec 06 UTC

During the 18th December the strong north-easterly flow, most pronounced over the British Isles, as well as north-westerly flow over the Iberian peninsula brought very cold air over the Mediterranean. (Fig 5.3).

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Fig 5.3 - Meteosat 9 IR10.8 image overlayed with temperature advection at 700 hPa (red-warm advection, blue-cold advection), from 18 Dec 00 to 20 Dec 06 UTC

At 06 UTC a cold advection maximum reaches France and the second maximum is spreading over the eastern Mediterranean. At 12 UTC a strong northerly flow is still persisting over the British Isles, on the leading side of the extensive blocking high with center over Iceland and Greenland. The cold air reached the Mediterranean on the back side of the extensive and deep low with center over Central Europe. The upper level low at 500 hPa has moved southwards and at this point a cloud spiral is clearly seen over France. (Fig 5.4)


Fig 5.4 - Meteosat 8 IR 10.8 image, 18 Dec 12 UTC


In the next 12 hours the situation has rapidly changed. A strong westerly wind caused the low to move to north Italy, followed by a pronounced maximum of cold advection. The most important feature at this point is the presence of a warm advection maximum upstream the maximum of cold advection. This warm advection will be responsible for the formation of the cloud head. The strong dipole of cold and warm advection over Genoa Bay and the north Adriatic is a sign of the ongoing cyclogenesis.
At 00 UTC on 19th December the formation of the cloud head within the maximum of warm advection is seen over the north Adriatic. (Fig 5.5)


Fig 5.5 - Meteosat 8 IR 10.8 image, 19 Dec 00 UTC

Very strong rising motion due to positive vorticity advection maximum in the left exit region of the jet streak intensified the process. (Fig 5.6)

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Fig 5.6 - Meteosat 9 IR10.8 image overlayed with isotach at 300 hPa (yellow) and PVA300 (orange), from 19 Dec 00 to 20 Dec 06 UTC

Cyclogenesis in Genoa Bay resulted in heavy snowfall as recorded over north Italy and Istria in the northern Adriatic. The low over Italy and the north Adriatic was pronounced only in the lowest layers. In the upper levels the situation was characterized by the strong advection of warm air within the cloud head and strong cold advection behind, as we can follow in the loop of images in Fig 5.3.
In the loop of Meteosat 9 Airmass RGB (Fig 5.7) the reddish area indicates dry descending stratospheric air. This is another sign of the ongoing cyclogenesis and supports further development of the system.

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Fig 5.7 - Meteosat 9 Airmass RGB overlayed with geopotential height 500 hPa, from 18 Dec 12 UTC to 20 Dec 06 UTC

There was also a pronounced PVA maximum in both 300 and 500 hPa connected to the jet indicating further development of the depression (Fig. 5.6). At 06 UTC the flow started to turn to a southwesterly direction, bringing moist air over a cold surface. At this point snowfall was recorded along the northern and central Adriatic coast as well as inland. By noon the low moved to the south Adriatic with snow spreading also to the eastern parts of the Croatian inland. In the upper levels a deep low persisted over north Italy and central Europe. Strong gradient in geopotential height field at 500 hPa indicated a very strong southwesterly flow. Cold air was moving across the Adriatic and warm advection within the cloud-head was still very strong.

By the end of the day cold air covered the whole country with snow still falling over the most parts of the inland.