Large-scale, clockwise circulation of winds around a central region of high atmospheric pressure over western Europe.
22, October 2020
On 29 February 2012 western Europe was under the influence of a large, blocking high pressure system (anticyclone) with its centre over France. The polar jet and the embedded smaller scale cyclones and anticyclones were forced to move around this blocking 'Mother High'.
Looking at the Meteosat-9 WV6.2 image below, one can see a large number of interesting features like jet streaks, deformation zones, vorticity centres and wave clouds. This makes it is an excellent case for analysing these dynamic features, which is shown in the Interpretation Image , where green lines denote deformation zones, red Xs centres of vorticity maxima and blue Ns centres of vorticity minima. In any case, one has to realise that the WV surface is a three dimensional, multi-level surface. Thus the vorticity centres are really vorticity tubes that tilt in the vertical. Deformation zones are only lines where the moisture at a particular level intersects the deformation skin that encases an air mass. These deformation zones/lines slope to many different levels. From the deformation zone conceptual model, one can infer the location of many more paired and companion vorticity centres – enough to clutter the entire area of the map.
Of course, there are deformation zones associated with each and every one of the vorticity centres. Air mass flows tend to follow isentropic surfaces that slope upward to the north and downward to the south. In the water vapour imagery the northward pointing flows tend to get lighter in colour in spite of the competing effect of limb darkening. These tend to be warm conveyor belt flows. Southward directed flows tend to get darker in the WV imagery as they follow down the isentropic surfaces. These are typically cold advective flows.
You certainly notice that within the large anticyclone there are maybe only two identifiable vorticity minima as would be expected. On the perimeter of the anticyclone, vorticity must increase so that vorticity maxima must dominate the swirls that are evident. Since the circulation is nearly stationary with respect to the absolute frame of the earth, the vorticity minimum should also coincide with the contour high on isobaric charts. A very few streamline vectors have been added to give a sense of some of the flows.
The WV animation reveals the full details in the flow and need not be highlighted further. The stationary, terrain induced features are particularly striking in the WV animation. All of these things are evident in this most beautiful example of an anticyclone.
Figure 1: Meteosat-9 WV6.2 Image
Bands of vorticity maxima wrapped around a strong, cold low over the Atlantic (11 May 2006)
Water vapour vortex forming on deformation zone (10 April 2006)
Recognition and Impact of Vorticity Maxima and Minima in Satellite Imagery (training module)
The situation on 29 February 2012 is also interesting because of the orographic waves that developed over Norway and the Alps, which generated large high-level lee clouds on the lee side of the mountains. The high-level lee cloud over northern Italy and southern Switzerland (see animation below) reduced the incoming solar radiation and also delayed the arrival of foehn winds. However, in the afternoon the diurnal solar heating was strong enough to destroy the low-level inversion allowing the foehn (relatively weak) to descend into the valleys. Thus, despite the high-level cloud, record temperatures were reached for February with max temperature of 24.5 °C in Locarno.
Figure 2: Meteosat-9 HRV Image
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Severe convection in central Argentina in December 2020.
Nearly-record snowfall in the western Alps in December 2020
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