In February 2018, parts of Europe were affected by a cold wave which brought widespread unusually low temperatures and heavy snowfall to large areas.
By Ivan Smiljanic (SCISYS) and Djordje Gencic (RHMSS)
The cold front, nicknamed the Beast from the East, brought the coldest end of February and start of March in years for Europe. Among those countries worst affected were Bulgaria, Croatia, the UK and Ireland, but even Rome had a rare snow storm.
In early February, long-term model runs suggested that during mid-February the phenomenon known as Sudden Stratospheric Warming (SSW) could occur.
SSW is a stratospheric event, but very often is a sign of incoming tropospheric polar-vortex deformation or split, and can greatly increase the chances of very wintry weather. During the event of polar vortex shifting or splitting, the jet stream is meandering enhancing the meridional heat exchange in general by mixing the colder air masses in the north with the warmer air masses in the south.
On Figure 1 an intensive positive anomaly of mean geopotential height in polar regions, related to temperature, is noticeable during mid-February, especially in higher altitudes, above 100 milibars (SSW). Later, this positive anomaly also spreads into the troposphere. The pressure systems configuration for polar areas is reflected well using the Arctic Oscillation index (AO), which can be seen in the lower part of the image. A pronounced drop of this index implies polar vortex dissolution.
The positive anomaly of geopotential corresponds well with high pressure in polar regions which is another sign that the polar vortex is disturbed. In the case of late February 2018, the polar vortex split produced two smaller areas of cold air, one was above parts of Europe and Asia.
On Figure 2 it clear that the mostly rounded polar vortex (left hand image) experienced significant change, with the strengthening ridge of high pressure (middle image) over the Atlantic Ocean and Arctic. This caused further formation of a jet stream branch with a southwest flow, resulting in colder air from the east travelling further west than usual (Figure 3).
Later on, with slow deformation of the high, the jet stream turned in an almost east-west direction (Figure 2, right hand panel), advecting a very cold and dry continental airmass from Asia towards western Europe, see the Meteosat-10 Natural Colour RGB animation (Figure 4).
Figure 4: Meteosat-10 Natural Colour RGB, 27 February, 05:00–18:00 UTC
Meanwhile, on the front side of this trough, a series of cyclones were bringing moist air to Italy and southeastern Europe causing significant snowfall.
Looking at the snow scene over the Dalmatia, Croatia region it is possible to spot the snow signature even over the remote islands (Figure 5). It is easier to distingish the snow signature on the MODIS Natural Color RGB image (Figure 5, left), than it is on the Meteosat-11 SEVIRI Natural Colour Colour RGB (right), because the SEVIRI instrument has a nominal resolution of 3 km, while the polar-orbiting MODIS instrument has a nominal resolution 1 km. The snow signature becomes apparent with typical cyan shades in the MODIS Natural Color RGB (green to cyan with only a traces of snow). Note: the resolution difference becomes even bigger since the SEVIRI instrument is looking at the scene from much higher viewing angle than the MODIS instrument (SEVIRI pixel lat/long size would be roughly 5.1 x 3.3 km in this region).
Zooming in to the advanced resolution of 500 m on the MODIS Natural Color RGB the real contours of the snow covered areas show up (Figure 6. Note: white stripes are image processing artefacts). These areas of snow normally follow higher orography in the region. In the future, with certain image processing being similar, Natural Colour RGB images will also be possible with data from the Meteosat Third Generation Flexible Combined Imager (FCI) at a resolution of 500 m, since the two important channels constituting this RGB (0.64 µm and 2.25 µm) are already at the desired resolution. Until then, the polar-orbiting MODIS instrument provides a good proxy information for what is expected from the future MTG geostationary imager.
This resolution will be the same on the two solar channels of future Meteosat Third Generation FCI instrument, .
Cumulus cloud fields seen in the Meteosat-10 Natural Colour RGB imagery portrayed nice circulation of the easterly polar air flow (Figure 6).
It is interesting to observe areas where these cloud fields have more intense development with ice cloud tops, seen in the cyan colour on the Natural Colour RGB combination (encircled areas in Figure 7). What is typical for these areas is that the cold air moves longer distances over the relatively warm water surfaces, picking up more moisture necessary for cloud formation.
What is also specific is that these areas are channeled through higher orography, leading to more concentrated moisture fields with more intense vertical development. This not the case for the cloud fields over the Bay of Biscay, where the channeling does not happen (hence dashed lines).
Such clouds streets are responsible for heavy snow showers, typically on the downwind, in this case the east shores of the land masses (so-called sea-effect snow).