Medicane in northern Adriatic

By Djordje Gencic, Ivan Smiljanic, Jochen Kerkmann, Carla Barroso, Natasa Strelec Mahovic and Barbara Vodaric Surija (DHMZ)

In the days preceding the event, a vast area of low pressure was impacting the weather above the Mediterranean. A small disturbance in the southwesterly flow, associated with the low pressure, traversed the Apennine peninsula and entered the still relatively warm (for the season) Adriatic Sea, and the unstable air mass above it.

Due to the favourable conditions, a disturbance started strengthening and transitioned into a tropical-cyclone-type of whirl , known in the Mediterranean as a medicane (MEDIterranean hurriCANE). This disturbance had a short lifespan, since it moved west-northwest towards the coast of Italy, and made a landfall in the early hours of 22 January, leading to its dissipation.

On 21 January at 15:00 UTC, a major cyclonic system above the Mediterranean, both at 500hPa and at sea level can be seen on the Airmass RGB with 500hPa geopotential and mean sea level pressure overlaid (Figure 1).

Also, quite prominent tropopause folding is present in the area and can be seen as red hues in Airmass RGB. Such red hues are typically a sign of high potential vorticity values and, consequentially, a very good proxy for atmospheric instability, which significantly aids development/strengthening of the cyclones/disturbances.

At 15:00 UTC a centre of the surface low was positioned above the central Adriatic Sea. Throughout the following evening and overnight this smaller scale whirl started displaying characteristics of a tropical low (smaller radius, wall of cumulonimbus clouds, and highest wind speed just outside of the centre or the eye), rather than an extra-tropical low-pressure system (larger scale, maximum of the wind speed spread out more evenly outside of the centre of the system, less symetric associated cloud bands). As the medicane was progressing northwestwards through the northern Adriatic, the eye of the storm was occasionally very clearly visible, see the  Enhanced IR 10.8µm loop (Figure 2).

A loop of composite radar reflectivity (Figure 3) from the National Hydrometeorological Service of Croatia (DHMZ) confirms what we see in the IR 10.8 loop. Relatively strong convective cloudiness was rotating quickly around the eye of the medicane, while it was moving over the Adriatic sea. The medicane made landfall in the wider Rimini area, in Italy, and was gradually dissipating thereafter. The loop shows maximum reflectivity product (MAX) in the period from 21 January 21 00:00 UTC to 22 January 06:00 UTC. The product is the result of a radar volume scan performed at five-minute intervals, starting at an elevation of 0.5° and ending at 25°.

The second radar loop (Figure 4) shows the wind speed measured by radar at the Debeljak location, from 10:00 UTC on 21 January to 01:00 UTC on 22 January. Wind speed was measured using the Doppler effect. Blue and green shades show wind blowing towards the radar, and yellow, red, and pink shades show wind blowing away from the radar. The wind direction is perpendicular to the line where 0m/s is measured. The imagery clearly shows rotation signature in the cyclone.

Strong winds around the centre of the medicane were also measured by the ASCAT scatterometer on the Metop-B and Metop-C satellites (Figure 5). The left image clearly shows the wind rotating around the centre of the cyclone, whereas the right one shows very strong northeasterly wind on the western edge of the vortex and northwesterly and westerly along the Italian coast, with the approaching medicane.

A wider view on the cyclone dynamics and intensification of the core rotation is provided through the Airmass RGB loop from 20 January 00:00 UTC to 21 January 07:00 UTC (Figure 6). Early stages of the cyclone development are shown in the loop in Figure 7.

The high-resolution True Colour RGB composite imagery (at 1km horizontal resolution), from Sentinel-3 A and B overpasses, shows the details of the associated cloud structures (Figure 8).

Though microphysical signal and the phase of the clouds are not spectrally resolved through this RGB product, a higher resolution of this image provides better insight into the shape, geometry and the cloud-top topography, all of which are already important indicators of the cloud type and character. Similar the RGB product, at comparable horizontal resolution, will be possible to create using data from the MTG FCI instrument.

Additional content

Station pressure observed at Pesaro, Italy (MedCyclones/Twitter)