Large-scale, clockwise circulation of winds

Large-scale, clockwise circulation of winds

29 February 2012 00:00 UTC

Large-scale, clockwise circulation of winds
Large-scale, clockwise circulation of winds

Large-scale, clockwise circulation of winds around a central region of high atmospheric pressure over western Europe.

Last Updated

22, October 2020

by Phil Chadwick (Meteorological Service of Canada ), Guido della Bruna (MeteoSvizzera Locarno ), Jochen Kerkmann and HansPeter Roesli (EUMETSAT)

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

Met-9, 29 February 2012, 06:00 UTC
Channel 05 (WV6.2)
Full Resolution
With isotachs 300 hPa (in m/s) (source: ECMWF / EUMeTrain)
Animation (28 Feb 20:00 UTC–29 Feb 08:00 UTC)

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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

Met-9, 29 February 2012, 07:00 UTC
Channel 12 (HRV)
Full Resolution (source: MeteoSwiss)
Animation (07:00–14:00 UTC, animated source: MeteoSwiss)