A vivid cyclone-anticyclone duo dominated the synoptic situation over Europe on 18-20 September 2018.
By Ivan Smiljanic (SCISYS)
Synoptic dynamics over Europe on 19 September led to a west-east dipole pattern between cyclonic and anticyclonic systems, with the so-called ‘deformation zone’ in between.
This case is a vivid example of synoptic scale forcing inside these type of systems — convective forcing associated with a cyclone and large-scale air mass subsidence associated with an anticyclone. Evidence of this forcing can be seen with the clouds in the cyclone ‘circle’ (i.e. area of influence) and almost no cloudiness in the anticyclone ‘circle’.
Meteosat-11 Airmass RGB imagery (Figure 2, left) is most suitable data to use to look at the synoptic systems, as it highlights the most important features at this scale — cold air masses (blue shades), warm air masses (green shades) and a separation area normally accompanied by jet streams associated with tropopause folding (red, elongated shapes). Overlaying the NWP mean sea level pressure (MSLP) values (Figure 2, right) confirms the information inferred by this RGB. It is obvious that multiple high/low centres exist inside two main pressure systems.
Looking into the solar spectre with the Natural Colour RGB confirms the presence (or absence) of clouds inside respective synoptic systems. This is a good product to consult in combination with the Airmass RGB, since the latter has a limited view of the lower parts of the troposphere. Also, the ‘natural view’ over any feature is assessed with more confidence with a human eye. Note: the distortion seen on the right-hand side of the image is an artefact called the bow-tie effect.
South of the line that separated two systems, i.e. over the Tyrrhenian Sea, one can see strong convective development. This is connected to a local low pressure system that appeared as a Mediterranean tropical-like cyclone, or Medicane (see Figure 1 marked with an M, top right, click to expand, and Figure 4).
The full dynamics of large synoptic systems and the Medicane over the Mediterranean Sea can be followed in the animated Airmass RGB imagery from 18–21 September (Figure 5). Note: the artificial black line across the observed domain is an image processing artefact.
Figure 5: Meteosat-11 Airmass RGB animation, 18 Sept 00:00 UTC–21 Sept 00:00 UTC. Download animation (MP4, 12 MB).