In April 2017 Meteosat-10 observed the merging of two cut off lows that moved from central Europe to the Mediterranean area.
13 November 2020
04 April 2017
By Jochen Kerkmann (EUMETSAT) and Djordje Gencic (RHMSS)
At the beginning of April, two distinct cold upper-level lows were sitting (hovering) over Germany and France, as shown by the Meteosat-10 Airmass RGB image from 4 April, 12:00 UTC (Figure 1).
The low over France came from the west, a result of a cut process that took place over Ireland and the UK, the low over Germany came from the east, from Poland and the Czech Republic, where it formed on a deformation zone.
The magenta-coloured overlay shows the height where the PV (Potential Vorticity) equals 1.5 PVU (PV Units), as taken from the ECMWF global model. This parameter is usually used to identify upper-level PV anomalies related to cold, upper-level lows. In this case, the ECMWF model field well matches the PV anomalies, as indicated by the satellite image (red-coloured circular areas over France and Germany) — some deviations can be seen over France where the ECMWF model shows the upper-level low slightly further west than the satellite image.
As predicted by the GFS and ECMWF models, both lows moved southward getting closer to each other, until they finally merged to one bigger low in the area of the Balearic Islands. The merging process can be clearly seen in the zoomed-in Airmass RGB animation, 5 April 00:00 UTC–6 April 06:00 UTC, or in the longer Airmass RGB animation from 4 April 00:00 UTC–6 April 08:00 UTC that gives a larger picture over Europe.
The merging effect is called 'Fujiwhara effect', sometimes referred to as 'Fujiwhara interaction', which says that "binary interaction of smaller circulations can cause the development of a larger cyclone, or cause two cyclones to merge into one.".
The effect is named after Sakuhei Fujiwhara, the Japanese meteorologist who initially described the effect. According to Wikipedia "Extratropical cyclones typically engage in binary interaction when within 2,000 kilometres of one another, while tropical cyclones typically interact within 1,400 kilometres of each other".
This animation from the previous case study Cold pool moves from Sea of Norway to Alps (17 April 2015), shows a cut-off low over Northern Europe, which, over three days, makes a full circle from Denmark over the UK to the North Atlantic and back to Denmark.
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